methodology of tritium determination in aqueous …3 laboratory for low-level radioactivities,...

Chapter

METHODOLOGY OF TRITIUM

DETERMINATION IN AQUEOUS SAMPLES BY

LIQUID SCINTILLATION COUNTING

TECHNIQUES

Ivana Stojković 1*, Nataša Todorović 2, Jovana Nikolov 2, Ines

Krajcar Bronić 3 and Jadranka Barešić 3 1 Faculty of Technical Sciences, University of Novi Sad, Novi Sad, Serbia

2 Faculty of Sciences, University of Novi Sad, Novi Sad, Serbia 3 Laboratory for Low-level Radioactivities, Ruđer Bošković Institute, Zagreb, Croatia

ABSTRACT

Liquid scintillation counting (LSC) is the most commonly used technique for measurement

of tritium activity concentration in aqueous samples, therefore, this chapter offers an

overview of advances in measurement methods and applications of different LSC methods

for tritium determination, presents development and optimization of those methods as well

as comparison of their advantages and limitations. With respect to low levels of tritium in

the environment, although LSC technique in general is adequate for low-level

measurements of environmental samples, certain conditions have to be met in order to

obtain accurate and reliable results of tritium activity measurements, as discussed in this

chapter. LSC methodology is convenient for aqueous samples since it assumes their mixing

with liquid scintillation cocktails.

Ultra-low background liquid scintillation spectrometer Quantulus 1220TM has been used in

all presented experiments. Presented methods differ in sample preparation techniques:

electrolytic enrichment, distillation, direct mixing of sample with scintillation cocktail

(without any sample pretreatment), and sample combustion, which altogether result in

different detection limits and range of application. Each of described methods is discussed

in terms of its application, advantages, cost and limits of detection. Relevant optimal

parameters were investigated and established for each method. Presented results enable

evaluation of influence that scintillation cocktail/vial/blank sample selection have on figure

of merit value and obtained tritium activity concentrations. Optimization of sample-to-

cocktail ratio, choice of appropriate scintillation cocktail based on the comparison of their

* Corresponding Author Email: [email protected]

I. Stojković et al. 2

efficiency, background and minimal detectable activity determination were performed.

Additionally, the effect of chemiluminescence and photoluminescence and different

combinations of scintillator/vial were analyzed.

Quench phenomena and their origins were described in brief as well, since they can

seriously decrease detection efficiency and, consequently, obtained tritium activity

concentrations during counting. Analysis of quench influence on detection efficiency

resulted in quench correction curve that is presented also, with discussion on how various

quenchers with different strengths affect LS measurements. From the presented results it

could be concluded that different scintillation cocktails do not behave similarly in the

presence of various quenchers and have different resistance, which indicates that quench

correction curve is applicable only for specific experimental conditions, i.e. for a certain

sample composition and LS counter. It should be developed/determined in each laboratory

for each specific application.

Liquid scintillation counting is irreplaceable in environmental tritium monitoring and

research. Furthermore, one can choose the most adequate method for certain application

that compromises the laboratory requirements and limitations among the presented sample

preparation techniques, based on comparative study of their effectiveness, cost and

minimum detectable activity concentration typical for each of presented methods.

Keywords: Liquid Scintillation Counting (LSC), optimization, Quantulus 1220TM

INTRODUCTION

Tritium is continuously distributed throughout man’s environment, being a radionuclide

generated both in interactions of cosmic rays in the atmosphere and as a result of human activity

(including thermonuclear bomb tests, operation of nuclear reactors, and manufacture of nuclear

weapons as well as various industrial and medical applications). The fact that its occurrence is

mainly in the form of tritiated water (HTO) demands regular and precise control of tritium

release from nuclear power plants into the environment, which is supported by international

legislations and national regulations [DOE Handbook, 2008]. Tritium monitoring is important

around nuclear power plants in whose vicinity exist water wells that supply the population with

drinking water [Jean-Baptiste et al., 2007; Hanslik et al., 2005; Hanslik et al., 2009;

Bolsunovsky et al., 2003], thus preventing possibility of internal exposure through ingestion of

drinking waters with elevated levels of tritium. There is a concern that tritium, mostly released

into the natural water recipients, rivers, lakes and seas, might enter the water wells of drinking

water [Grahek et al., 2016]. Like most other radionuclides, tritium is carcinogen, mutagen and

teratogen [Ravikumar and Somashekar, 2011b]. Namely, exposure to tritium has been clinically

proven to cause cancer, genetic mutations and birth defects in laboratory animals. In studies

conducted by Lawrence Livermore Laboratory, a comprehensive review of the carcinogenic,

mutagenic and teratogenic effects of tritium exposure revealed that tritium packs 1.5 to 5 times

more relative biological effectiveness (RBE), or biological change per unit of radiation, than

gamma radiation or X-rays [Straume, 1993; Hill and Johnson, 1993]. Beside water samples,

tritium has been monitored in environmental samples as well, which requires development of

technologies for tritium and deuterium separation, for example by cryogenic distilation [Varlam et

al., 2011]. Tritium monitoring in drinking or surface waters, as well as in groundwater has

found application not only in health and safety considerations [Renne et al., 1975] but also

carries valuable information in hydrogeological and hydrological studies, complementing

geochemistry and physical hydrogeology investigations, and is one of the most important

Methodology of tritium determination in aqueous samples by LSC techniques

3

transient tracers used in hydrological research [Michel, 2005; Horvatinčić et al., 1986; Rank,

1993, Rank et al., 1998]. Tritium level also provides information on groundwater dynamics and

recharge rates [Ravikumar and Somashekar, 2011a].

The natural production by cosmic rays results in a steady-state inventory on the Earth’s

surface of about 3.5-4.5 kilograms of tritium [Michel, 2005]. The average natural (cosmogenic)

concentration of tritium in environmental waters has been estimated to range from 0.12 to 0.9

Bq l-1 [Mook, 2001; Palomo et al., 2007; Baeza et al., 2001]. It is interesting to mention that

generated maximum tritium activity concentrations due to fallout from nuclear weapons testing

in the atmosphere was reached in 1963 of nearly 470 Bq l-1 in rainwater [Mook, 2001], and it

has been decreasing since cessation of nuclear tests [NCRP Report No.62, 1979] at a rate

approximately equal to its half-life [José Madruga et al., 2009]. The level of tritium in

environment (“background”) currently ranges between 1 and 4 Bq l-1 [Mook, 2001; Palomo et

al., 2007; Baeza et al., 2001; Hanslik et al., 2005; Bolsunovsky et al., 2003; Varlam et al.,

2009], with tendency to come to its natural cosmogenic level. However, routine releases and

accidental spills of tritium from nuclear power plants increase its natural concentration, thus

pose a growing health and safety concern.

European Commission determined the upper limit for tritium in drinking water to be 100

Bq l-1 European Commission, 1998, which is not value based on health effect relative to its

consumption but rather on monitoring value that would indicate leakage or release from power

plant that needs further check if other radionuclides are present in water. Activity concentration

limit recommended by WHO based on health concerns is 10 000 Bq l-1 (for a 70 kg man who

drinks 2 l of water per day) World Health Organization WHO, 2011.

Common methods for determination of low-level tritium activity concentrations are liquid

scintillation counting (LSC) and gas proportional counting (GPC) [Obelić et al., 2004; Gröning

et al., 2009; Barešić et al., 2010; Forkapić et al., 2011]. While the GPC technique requires

chemical conversion of liquid water to an appropriate counting gas [Obelić et al., 2004; Gröning

et al., 2009], in the LSC technique a water sample is directly combined with an appropriate

aqueous scintillation cocktail, required pre-treatment is minimal while the counting efficiency

and precision are higher than that of GPC [Noakes and Filippis, 1988]. Direct GPC technique

provides 2-3 times lower detection limits than LSC direct method [Barešić et al., 2010; Gröning

et al., 2009]. However, if samples are electrolytically enriched, LSC system assures lower

detection limits and with better precision is more suitable for most natural water samples

including precipitation and groundwater samples [Gröning et al., 2009; Barešić et al., 2010].

The purpose of the chapter is to provide detailed survey on various methods for measurement

of tritium activity concentration by liquid scintillation counting technique and present results

of their development and optimization, thus enabling comparison of their advantages,

limitations and range of application.

1. LIQUID SCINTILLATION COUNTING METHODOLOGY

Precise measurements of naturally-occurring tritium activity concentrations are fairly

difficult – one problem is very low-energy beta disintegration of tritium, particularly

disadvantageous for efficient detection, and the other one is quite low specific activity of

samples because of rapid mixing of generated tritium in nature with extensive amounts of water

and hydrogen gas in the atmosphere [Janković et al., 2012]. Advances in liquid scintillation


technology, together with new and emerging sample preparation techniques, now enable

researchers to consider LSC as an alternative environmental sample radionuclide counting

method, or as a potentially useful screening tool [L’Annunziata, 2012]. During the 1980s, a

new generation of commercially manufactured LS counters offered 1) significant reduction of

background level thus enabling lower minimum detectable activity (𝑀𝐷𝐴) to be achieved, and

2) spectrometric and sophisticated data processing by incorporation of multichannel analyzers

(MCAs) and microcomputer technology into modern LS counters.

The main advantages of LSC methods in measurements of low-energy beta activities such

as tritium’s are its rapidity, sensitivity and simplicity, low detection limits that can be achieved,

convenient even for natural tritium concentration determination. One main problem of liquid

scintillation spectroscopy in general is quench presence in samples which can alter detection

efficiency and thus affect obtained activity concentrations. However, reliable results can be

assured if corrections of possible quench occurrence are established for each method,

instrument and additional equipment (LS cocktail and vial used, for example), and laboratory

conditions in which sample preparation and their counting take place.

The development of modern LS counters and use of safer scintillation cocktails enabled

reliable tritium activity determination almost to the limit of 0.6 Bq l-1 without any pretreatment

of samples. Below this limit determination becomes more demanding as it is necessary to have

instruments that have very low background such as Tri-Carb 2770/3180 or Quantulus 1220 and

dispose of equipment for tritium enrichment [Morgenstern and Taylor, 2009; Varlam et al.,

2009; Jakonić et al., 2014b]. The detection limits of ultra-low background LS counters are

around 1-3 Bq l-1 for non-enriched water samples [Hanslik et al., 2005; Palomo et al., 2007;

Varlam et al., 2009; Jakonić et al., 2014b]. Typical detection limits for electrolytically enriched

samples are 0.03-0.06 Bq l-1 [Barešić et al., 2010], while it can go from 0.01 Bq l-1 down to

0.001 Bq l-1 with enrichment factors 26 and 175, respectively [Morgenstern and Taylor, 2009].

Routine measurements must provide reliable 3H determination in a wide range of activities in

order to improve assessment of environmental impacts, since 3H concentration in water released

directly into environment from nuclear power plants can vary in a wide range depending on

dilution phenomenon and sampling distance from the source [Grahek et al., 2016].

1.1. QUANTULUS 1220TM

Results of all presented experiments were obtained using Ultra Low Level Liquid Scintil-

lation Spectrometer Wallac Quantulus 1220TM manufactured by PerkinElmer (Finland, 2002).

This instrument has design and construction according to the concept of total optimization,

which enables the accurate measurement of low levels of radioactivity found, for example, in

radiocarbon dating and in the analysis of environmental radioactivity Quantulus 1220

Instrument Manual, 2002. Possibility of ultra-low radioactivity measurements is due to active

and passive background reduction systems set around the vial chamber. The passive shield is

made of massive asymmetric lead shield of approximately 630 kg that reduces cosmic and

environmental gamma radiation together with soft muon component (with graded cadmium

that absorbs thermal neutrons and copper inner linings that absorbs secondary X-rays induced

by cosmic radiation in lead). The active anticoincidence shield is based on a mineral oil

scintillator with additional pair of photomultiplier tubes (PMTs) that absorb high-energy

gamma radiation and hard cosmic ray component. These PMTs work in anticoincidence with


5

the pair of PMTs that surrounds sample chamber. Low-activity materials were used in the

construction of the Quantulus. There is also a delayed coincidence circuit (DCOS) inside the

Quantulus, which is useful for the correction of chemiluminescence events such as emission of

single photon Quantulus 1220 Instrument Manual, 2002.

The Quantulus 1220 incorporates two dual programmable multichannel analyzers (MCA),

one is used for active shield and the second one is used for spectra record. Those MCA’s are

divided in two halves, resolution of 1024 channels each. The logic signal can be selected to

trigger or inhibit analogue to digital conversion or to select the MCA memory half. The system

is provided with two pulse analysis circuits that are accessible for the users: a pulse shape

analysis (PSA) and pulse amplitude comparator (PAC) circuit. PSA allows 𝛼/𝛽 spectra

separation based on duration of electronic pulses, which is useful in simultaneous 𝛼/𝛽

measurements since these spectra are stored in different halves of MCA’s, α-MCA and β-MCA.

PSA adjustment is carried out by recording spectra of pure alpha emitter and pure beta emitter,

looking for optimal PSA level where there is equal and minimal interference of alpha events

into β-MCA, and beta events into α-MCA [Todorović et al., 2012]. Optimization of PAC

parameter can further decrease background component, which is mostly used during 14C

measurements.

The tritium configuration of the MCA’s setting eliminates the random noise of phototubes,

inhibits the coincidence pulse from the guard and the sample, and monitors the random

coincidence by DCOS in a half of the MCA, the whole sample spectrum being recorded in the

other half of the MCA Quantulus 1220 Instrument Manual, 2002. A delayed coincidence

circuitry combines another pulse stream with the prompt coincidence one in order to monitor

the random coincidence signals which come from chemiluminescence. The delayed

coincidence spectrum can correct the normal coincidence spectrum by subtracting it channel

by channel.

It is also necessary to mention one other advantage of Quantulus, the automatic

measurement of quench indicating parameter of each counted sample, SQP(E) parameter,

Spectral Quench Parameter of the External Standard. It represents channel number of 99.5% of

spectrum generated by external standard 152Eu stored in Quantulus. Samples with higher quench

level have lower SQP(E) parameters, which is a consequence of spectra shifting towards lower

channels in the presence of quench.

Spectra were acquired and evaluated by WinQ and EasyView software, respectively.

2. SAMPLE PREPARATION TECHNIQUES – OPTIMAL PARAMETERS

STUDY OF DIFFERENT LSC METHODS

As mentioned, environmental tritium concentration level ranges in the instrument’s

detection limit, therefore it is necessary to examine performance of different methods and to

establish detection limits in which determination in routine measurements is reliable and

accurate. In the following sections a review of sample preparation techniques, description of

methodologies, instruments, as well as some results of intercomparison measurements are

provided. The primary objective of all sample preparation methods is to obtain a stable

homogeneous solution suitable for analysis by liquid scintillation counting. There are no

absolutes in sample preparation; whichever method produces a sample that lends itself to

accurate and reproducible analysis is acceptable [L’Annunziata, 2012].


Methods permitting 3H separation or at least 3H concentration prior to counting are physical

in character because the mass differences of HTO and H2O produce greater differences in

physical than in chemical properties [Jacobs, 1968]. Several methods have been developed for 3H enrichment, the most commonly used technique for the analytical purposes is electrolysis.

2.1. ELECTROLYTIC ENRICHMENT

Tritium (3H) activity concentration of natural waters (precipitation, groundwater, surface

waters) has recently become too low to be directly measured by low-level liquid scintillation

(LSC) techniques. However, it has long been known that the tritium activity concentration can

be increased to an easily measurable level by applying electrolytic enrichment [Kaufman and

Libby, 1954]. An electrolyte must be added to water to make it conductive. Due to the

difference in masses of 1H, 2H and 3H, the dissociation energies of these isotopes are different

so the dissociation of lighter molecules appears at a lower dissociation energy compared to that

of heavier molecules [Gat, 1980]. Thus, during the electrolysis the lighter molecule H2O

dissociates more rapidly compared to the heavier HTO molecule leading to an enrichment of

HTO molecule in the remaining water phase. The electrolytic enrichment process therefore not

only dissociates water into their respective gasses hydrogen and oxygen with the observed

depletion in the quantity but also relatively enriches HTO molecules with respect to H2O

molecule in the liquid phase, and the tritium activity concentration in the remaining electrolyte

increases [Cameron, 1967]. The reduction of sample quantity by a factor of 10 to 100 or more

can be achieved. The initial sample mass is usually either 250 ml or 500 ml, but 1 l and 2 l

initial volume is also possible [Morgenstern and Taylor, 2009]. The larger the initial water

volume, the larger the enrichment factor, i.e., the ratio of the final to initial tritium activity

concentration of a sample. Usual tritium enrichment factors range from about 3 [Janković et

al., 2012] to 18 [Gröning et al., 2009] for 250 ml initial sample volume, from 18 [Barešić et al.,

2011] to 28 [Gröning et al., 2009] for 500 ml initial volume, and can reach 75 or even 175 for

1 l and 2 l initial sample volume, respectively [Morgenstern and Taylor, 2009]. The cells for

electrolysis can be connected in a raw, so one electrolysis run enables simultaneous enrichment

of a series of unknown samples and tritium-free and standard samples under identical

conditions, and therefore enables also a quantitative determination of the enrichment factor.

The sample preparation process has three main phases: primary distillation of water,

electrolysis and secondary distillation. The details about the systems (cell design, cell material,

etc.) and the technical procedure may vary from systems to system. As an example of the tritium

enrichment system, here we describe in details the system at the Ruđer Bošković Institute (RBI)

that was implemented in 2008, following the design of the electrolysis system developed at the

IAEA Isotope Laboratory [Rozanski and Gröning, 2004; Gröning et al., 2009] and implemented

also at the Jožef Stefan Institute in Ljubljana, Slovenia [Kožar Logar and Glavič-Cindro, 2008].

The first step of sample pretreatment is primary distillation. Any impurity in the water that

is to be subjected to electrolysis can cause corrosion of the cell anodes and prevent the cathodes

from developing efficient hydrogen isotope separation [Morgenstern and Taylor, 2009].

Adequacy of purification by distillation can be checked by a water conductivity check. The

required purity depends on the geometry of the cells, the electrode material and the applied

voltage. For the system implemented at RBI the required conductivity of distilled samples is

<50 µS/cm. In case of conductivity >50 µS/cm, samples have to be distilled again [Barešić et

al., 2011].


7

a

b c

d

Figure 1. The equipment for electrolytic enrichment of water with tritium at the Ruđer Bošković

Institute, Zagreb, Croatia. a) General view of the experimental room: right – the electrolysis

unit, left – primary distillation units, place for cleaning and washing, and the shelf for sample

keeping. b) the electrolysis unit: left – cooling unit and control of electric power, right –

refrigerator with 20 cells. c) Anode (stainless steel) and cathode (mild steel) of an electrolysis

cell. d) Gas bubblers filled with silicone oil, marked by the same number as the cells, enable

visual check of the electrolysis going on.


The process of electrolytic enrichment of water with tritium requires special equipment

placed in an adequate environment. The equipment can be designed and produced by the

laboratory or can be purchased. The enrichment equipment installed at RBI was produced by

the Faculty of Physics and Applied Computer Sciences, AGH University of Science and

Technology, Krakow, Poland, and is very similar to that in the IAEA Isotope Laboratory

[Rozanski and Gröning, 2004; Gröning et al., 2009, Barešić et al., 2010, 2011]. Figure 1a shows

the photograph of the whole system in an experimental room devoted to electrolysis and some

details of the system are shown in Figures 1 b,c,d.

The main part of the system of electrolytic enrichment consists of 20 cells of 500 ml volume

placed in a refrigerator (Figure 1b) that cools the cells and keeps them at 2–5 °C low

temperature to prevent the loss of tritiated water molecules by evaporation during the

electrolysis. Anodes are made of stainless steel, and cathodes of mild steel (Figure 1c) enabling

achievement of high tritium enrichment factors, high tritium retention factors and good

reproducibility [Morgenstern and Taylor, 2009]. Each cell is filled with 500 ml of previously

distilled water sample and 1.50 – 1.55 g of Na2O2 as electrolyte is added. Gas produced in each

cell passes through a glass bubbler filled with silicone oil (Figure 1d) for visual checking of the

process and then it is led by a ventilation system into the open atmosphere. Each enrichment

run contains 15 unknown samples, 3 spike waters (water of known tritium activity

concentration) used for monitoring the electrolysis performances and 2 tritium-free (“dead

water”) samples used for system control. The position of the three spike waters and 2 dead

waters are shifted in the subsequent electrolysis runs and after 20 runs each cell has been

subjected to equal numbers of spike and dead waters. Enrichment procedure lasts for 8 days,

i.e., after predefined 1420 Ah it stops automatically.

Each electrolysis run is characterized by the tritium enrichment factor 𝐸 (symbol 𝑍 is used

sometimes in literature) and the enrichment parameter 𝑃. The enrichment factor 𝐸 represents a

ratio between the final and initial 3H activity concentrations, 𝐴f/𝐴i, while the tritium

enrichment parameter 𝑃 indicates the retention of the original tritium content during the

electrolytic process [Gröning et al., 2009, Rozanski and Gröning, 2004]. In an ideal case 𝑃

would have the value of 1 (100 % retention of tritium). However, in a real case 𝑃 value above

0.9 indicates sufficient performance [Gröning et al., 2009]. The general relation between the

enrichment factor 𝐸 and the enrichment parameter 𝑃 can be described as [Morgenstern and

Taylor, 2009]:

𝐸 = 𝐴f

𝐴i = (

𝑊i

𝑊f)

𝑃 . (1)

Here, 𝑊i and 𝑊f are the initial and final mass of the sample.

The values of 𝐸 and 𝑃 for each electrolysis run can be calculated using the initial and final

mass of water in cells and individual count rates of spike water before and after enrichment

[Rozanski and Gröning, 2004].

The mean enrichment factor determined from the three spikes, 𝐸spike , in each run can be

obtained as:

𝐸spike = 1

3 ∑ (

𝑁spike, j

𝑁spike,BE

)3j=1 . (2)

Here, 𝑁spike,BE represents the net count rate of the spike before enrichment, and 𝑁spike, j the

net count rate of the spike j (j = 1, 2, 3) after enrichment.

The enrichment parameter 𝑃 is obtained as:


9

𝑃 =1

3 ∑ ((𝑊i,spike − 𝑊f,spike)

j× ln𝐸spike (ln (

𝑊i,spike

𝑊f,spike

)𝑗

× ⁄𝑄

2.975)3

j=1 , (3)

where 𝑊i,spike and 𝑊f,spike are the initial and final masses of the spike j (j = 1, 2, 3), 𝑄 is the

number of Ah for the electrolysis run, and 2.975 is the Faraday constant (Ah/g).

The value of the enrichment factor for each individual sample, 𝐸sample, can be obtained as:

𝐸sample = exp (𝑄

2.975× 𝑃 ×

ln(𝑊i,sample 𝑊f,sample⁄ )

𝑊i,sample−𝑊f,sample

) , (4)

where 𝑊i,sample and 𝑊f,sample are the initial and final mass of the sample. The mass of water is

determined gravimetrically by weighing the empty cells, the cells with the sample water before

and after the electrolysis.

2008 2009 2010 2011 2012 2013 2014 2015 2016 201716

18

20

22

24

26

28

30

32

34

E s

pik

e

date of the electrolysis run start

Figure 2. The mean enrichment factor 𝐸spike (eq. 2) for the 50 electrolysis runs of the Ruđer

Bošković Institute system, period 2008 – 2017.

Quality control of the electrolysis system includes constant monitoring of 𝐸 and 𝑃 values.

The mean enrichment factor 𝐸spike (eq. 2) and the enrichment parameter 𝑃 (eq. 3) of the RBI

electrolysis system are shown in Figures 2 and 3, respectively. Increase of 𝑃 values during first

6 runs is obvious and is explained by consolidation of iron/mild steel electrolytic cells. Namely,

the cathode surfaces have to be brought to a state of high catalytic efficiency, and this is usually

done simply by running-in the cells over a certain period [Morgenstern and Taylor, 2009]. Low

𝑃 values (<0.92, Figure 3) obtained during the consolidation runs are related to the relatively

low 𝐸spike values (<20, Figure 2). In further analysis the first 6 consolidating electrolysis runs

are not taken into account. It is also interesting to note the two 𝑃 values higher than the

theoretical value of 1, related with the higher 𝐸spike values (>30) in 2012, and these values

cannot be explained even after careful investigation of the two electrolysis runs. After

stabilization of the systems, the average enrichment factor of the RBI system is 𝐸spike,RBI =

25 ± 3, and it ranges from 20.4 to 32. The mean enrichment parameter for the same set of


electrolysis runs, excluding the two values >1, is 𝑃RBI = 0.953 ± 0.013, with the range from

0.927 to 0.979 (shaded area in Figure 3). The 𝑃 values close to the unity show a high tritium

retention and relatively low scatter.

2008 2009 2010 2011 2012 2013 2014 2015 2016 20170.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

date of the electrolysis run start

P

Figure 3. The enrichment parameter 𝑃 (eq. 3) or the 50 electrolysis runs of the Ruđer Bošković

Institute system, period 2008 – 2017. The shaded area encompasses 𝑃 values in the range 0.927

– 0.979, mean value 0.953, which are characteristic for the RBI system after its consolidation.

cell number

Figure 4. The enrichment factor 𝐸 (eq. 4) for each individual cell of the RBI electrolysis system

for electrolysis runs after stabilization, 2009 – 2012. The box plots show the usual features of

the distribution of 𝐸 values in each cell: the median value and 25th and 75th percentiles in a

shaded box, minimal and maximal values (symbol ), and the mean value (symbol ).

c#1

c#2

c#3

c#4

c#5

c#6

c#7

c#8

c#9c#

10c#

11c#

12c#

13c#

14c#

15c#

16c#

17c#

18c#

19c#

20

16

18

20

22

24

26

28

30

32

34

36

38

E


11

For each sample in each electrolysis run, the individual tritium enrichment factor has to be

determined (eq. 4). The system showed variations in the enrichment factor 𝐸sample among

individual enrichment cells within a single electrolysis. An analysis of the individual 𝐸 values

in individual cells for electrolysis runs after stabilization of the systems is shown in Figure 4.

The highest individual 𝐸 value of 37.7 is obtained in cell no. 10 during the electrolysis run 28

in 2012, that was characterized by high values both 𝐸spike and 𝑃 values, as discussed earlier in

this chapter. The highest mean 𝐸 value of 25.7 ± 2.9 was obtained in cell no. 3 and the lowest

21.7 ± 2.0 in cell no. 15.

To check whether the position of a sample in cell no. 3 or no. 15 could influence the final

result of tritium activity concentration, we performed electrolysis of the same sample in the

same electrolysis run in both cells (Table 1). The obtained final result was the same in both

cases, showing that the enrichment factors for both cell were determined correctly and that the

position of a sample in the particular cell does not affect the final tritium activity concentration.

Table 1. Comparison the tritium activity concentrations of the same sample (sample

code T-4453) after electrolytic enrichment (run #31, February 2013) in two electrolytic

cells with different mean tritium enrichment factors.

cell. no 𝐸sample 𝐴sample [Bq l-1]

3 28.7 0.83 ± 0.16

15 22.6 0.84 ± 0.20

After electrolysis enriched samples are very basic and sometimes coloured due to spending

of cathode material and they have to be distilled again. A quantity of 6-8 g of PbCl2 is added

into each sample prior to secondary distillation.

Once the distilled enriched water is obtained, the measurement process in liquid

scintillation counter can start. The sample preparation for measurement is the same as after

other pretreatment techniques described in this chapter. At RBI, low-diffusion polyethylene 20

ml vials are used for scintillation cocktails of 8 ml of sample and 12 ml of scintillator Ultima

Gold LLT. Each measurement run consists of 20 enriched samples (3 of them are spike waters,

2 are tritium-free waters and 15 unknown samples) and 4 non-enriched samples: 2 tritium-free

samples, a spike and a referent sample of known activity for calibration. Samples are usually

measured in 8 cycles of 50 minutes, resulting in an average of 400 min measurement of each

sample. By optimization of the measuring conditions in Quantulus 1220, the interval between

25th and 187th channel was determined as the best region of interest [Obelić et al., 2004]. This

region comprises 92 % of the tritium spectrum and 82 % of the background spectrum, resulting

in an efficiency of 23.6 %.

If the standard and a sample are prepared in the same way, and are both measured under

identical conditions, the measurement efficiency is the same for all samples. The tritium activity

concentration 𝐴sample in the analyzed sample is calculated as [Rozanski and Gröning, 2004]:

𝐴sample = 𝑁sample 𝐴standard

𝑁standard 𝐸sample

𝐷 , (5)

where 𝑁sample and 𝑁standard are the net count rates (usually min-1) of a sample and the standard,

respectively, 𝐴standard is the tritium activity concentration of the standard used, 𝐸sample is the


enrichment factor for the sample, calculated by eq. (4), and 𝐷 is the factor taking into account

decay of tritium in the sample from the day of sampling to the date of measurement.

The electrolytic enrichment of water with tritium is by no mean a complex procedure,

although the basic principles seem to be very simple. The analytical challenges are growing as

the environmental tritium activities in precipitation and groundwater are decreasing.

Laboratories are involved in constant improvement of the process to reach better performances

of the system, higher operational safety standards and better protection against tritium cross-

contamination during sample preparation [Gröning et al., 2009]. Since during the electrolysis

a large amount of highly explosive hydrogen/oxygen gas mixture is produced, several security

and safety features have to be implemented in each tritium laboratory, such as automatic

stopping the electrolysis after a predefined number of Ampere-hours, temperature control of

the refrigerated system and automatic stopping in case of either too high or too low temperature,

stopping in case of the failure of the cooling system, control of gas flow by glass bubblers filled

with silicone oil, efficient ventilation system. Finally, good quality control of the system,

including monitoring of 𝐸 and 𝑃 values, as well as monitoring of individual performance of

each cell, special care to avoid any contamination or memory effect of the cells, and

participation in various international intercomparison studies, can result in a reproducible,

accurate and precise results of tritium activity concentration down to very low detection limits.

2.2. DISTILLATION

The physical basis of tritium enrichment by distillation process lies in the fact that the rates

of escape of atoms and molecules from a liquid surface are generally inversely proportional to

the square roots of their masses [Moeller, 1954]. The separation factor of HTO and H2O is

rather small, so multistage distillation techniques are necessary for satisfactory results

[Verhagen and Sellschop, 1965]. This procedure is simple and inexpensive, and detection limits

that can be achieved suggest that its application is limited to environmental samples monitoring

around nuclear facilities, public exposure to tritium and dose assessment studies. The major

drawbacks to distillation as an enrichment process are the relatively long times required for

enrichment, the poor reproducibility and the modest enrichment factors attained [Jacobs, 1968].

There is also a marked decrease in the degree of separation as the temperature is increased; the

separation factor is only about 1.036 at 100°C [Jacobs, 1968]. The optimum fractionation of

tritium with respect to water, for identical times of distillation, is achieved at a pressure of 100

to 120 mm Hg (for which water boils at 52-55°C) [Zel'venskii et al., 1965]. Distillation of water

samples represents purification procedure, especially convenient if water samples are colored

or their origin suggests organics or interfering chemicals are present [L’Annunziata, 2012].

This section will illustrate implementation of ASTM method based on distillation of

samples [ASTM International D 4107–08, 2006] in Laboratory for low radioactivity

measurements at the Department of Physics, University of Novi Sad, Serbia (DP-UNS)

[Nikolov et al., 2013; Jakonić et al., 2014b].

A 100 ml drinking water sample aliquot is treated with 0.5 g of sodium hydroxide and 0.1

g of potassium permanganate (the alkaline treatment is to prevent other radionuclides such as

radioiodine and radiocarbon from distilling over with the tritium, the permanganate treatment

is to oxidize trace organics that could cause quenching interferences). A middle fraction of the

distillate is collected for tritium analysis (early and late fractions are more apt to contain

interfering materials and are therefore discarded). The collected distillate fraction is thoroughly


13

mixed and a portion (up to 10 ml) is mixed with LS cocktail, after dark adapting, it is counted

in LSC system for tritium beta particle activity.

Calibration of LS counter assumed preparation of the following samples:

- RWTS – raw water tritium solution: tritium standard solution added to background raw

water that hasn’t been distilled (RW)

- DRW – distilled raw water, used for background determination

- DWTS – distilled water tritium standard solution: RW was distilled and tritium standard

solution was added so that tritium activity concentration is equal to the one in RWTS

- DRWTS - distilled raw water tritium standard: RWTS added with sodium hydroxide and

potassium permanganate, and distilled afterwards (first 10 ml of distillate was discarded, so

that the middle distillate was collected)

Three aliquots of DRWTS, DWTS and DRW were prepared, mixed with LS cocktail in

polyethylene 20 ml vials (PE vials), dark-adapted and counted on LS counter Quantulus, with

obtained count rates as 𝑅DRWTS [s−1], 𝑅DWTS [s−1] and 𝑅DRW [s−1], respectively. Detection

efficiency was calculated as follows:

𝜀 =𝑅DWTS−𝑅DRW

𝐴DWTS , (6)

where 𝐴DWTS [Bq] was the activity of DWTS. Recovery correction factor, 𝐹, was obtained as:

𝐹 =𝑅DRWTS−𝑅DRW

𝜀 𝐴RWTS , (7)

where 𝐴RWTS [Bq] was the activity of RWTS. Tritium activity concentration of analyzed

sample, 𝐴 [Bq l−1], was calculated as:

𝐴 =𝑅S−𝑅DRW

𝜀 𝐹 𝑉 𝑒−𝜆𝑡 , (8)

where 𝑅S [s−1] is sample’s aliquot gross count rate, 𝑉 [l] is sample volume, 𝜆 [d−1] =ln 2

𝑡1/2 is

decay constant for tritium, 𝑡1/2 = 4500 d is half-life of tritium, 𝑡[d] is elapsed time between

sampling and counting.

Counts

(ar

bit

rary

unit

s)

5 Channel number

Figure 5. Low-energy beta region, 1-250 channel: spectra of different tritium activity

concentration (EasyView).


The critical activity concentration, 𝐿𝐶 [Bq l−1], was calculated for each sample as follows:

𝐿𝐶 =1.65√ 𝑅DRW 𝑡S( 1+

𝑡S

𝑡DRW)

𝜀 𝑡S 𝐹 𝑉 𝑒−𝜆𝑡 , (9)

where 𝑡S [s] and 𝑡DRW [s] were counting times of sample and background, respectively. The

measured activity concentration, may be compared to 𝐿𝐶 to determine whether tritium is clearly

present in the sample. When the detection criterion was used, a priori minimum detectable

activity concentration, 𝑀𝐷𝐴 [Bq l−1], was determined as:

𝑀𝐷𝐴 =2.71+3.29√ 𝑅DRW 𝑡S( 1+

𝑡S

𝑡DRW)

𝜀 𝑡S 𝐹 𝑉 𝑒−𝜆𝑡 . (10)

Figure of merit, 𝐹𝑂𝑀 [s], was used as evaluation parameter of method’s performance:

𝐹𝑂𝑀 =𝜀2

𝑅DRW . (11)

Low diffusion polyethylene 20 ml vials were used because of their lowest background

level. Spectra generated by tritium are displayed in Figure 5, where the tritium window was

fixed between channels 1-250 [Nikolov et al., 2013. The default MCA tritium configuration

of WinQ software was used for measurements, it eliminates noise from photomultiplier tubes

as well as coincident signals from shield and sample.

0 2 4 6 8 10 12 14 16 18 20

5

10

15

20

25

30

OptiPhase HiSafe 3

OptiPhase HiSafe 2

DW

TS

co

un

t ra

te [

s-1

]

V [ ml ]

Figure 6. Count rate dependence on different DWTS sample volume (with total

sample+cocktail volume of 20 ml), distillation method for 3H analysis [Jakonić et al., 2014b].

Different sample : scintillation cocktail volume ratios were explored with highest detection

efficiency values demanded, i.e. highest count rates of DWTS samples demanded, Figure 6.

DWTS was mixed in different ratios with cocktail in 20 ml vials. Optimal sample : cocktail

volume ratio was determined to be 9:11 for OptiPhase HiSafe 3, and 8:12 for OptiPhase HiSafe

2, as it can be seen in Figure 6. However, since environmental samples can induce phase


15

separation [Schonhofer, 1994; Pujol and Sanchez-Kabeza, 1999], we have decided to prepare

all samples in more conservative ratio, 8:12. In general, increment of cocktail’s volume

increases the counting efficiency and background as well (since the number of cosmic

interactions in cocktail is growing), which causes diminishing of the 𝐹𝑂𝑀 values [Komosa and

Slepecka, 2010].

Maximum efficiency for measurements of tritium activity concentration can be 50-60%

[Komosa and Slepecka, 2009]. Measurement parameters for two cocktails used are given in

Table 2. Ultima Gold Low Level Tritium (Ultima Gold LLT) is a better choice of cocktail than

OptiPhase HiSafe 3 since it has higher efficiency and lower detection limit. OptiPhase HiSafe

3 contains surfactants (organic phosphates) which lower its efficiency [Komosa and Slepecka,

2009]. From obtained 𝑀𝐷𝐴’s, it can be concluded that distillation method is convenient for

monitoring purposes of drinking and surface waters and dose assessment in the environment.

𝑀𝐷𝐴 values are in agreement with previous findings for the same volume ratio and for the

distillation preparation procedure [Palomo et al., 2007].

Table 2. Comparison of two cocktail’s performance (distillation method for 3H

determination, PE vials, 8:12 sample:cocktail ratio).

Scintillation cocktail Blank [s-1] Efficiency

[%]

Recovery

factor

𝑀𝐷𝐴 [Bq l-1]

(𝑡DRW = 600 min)

OptiPhase HiSafe 3 0.025 30.5 (6) 1.0016 1.7

UltimaGold LLT 0.023 35.8 (7) 0.9891 1.4

The selection of the optimal performing LS cocktail should be based on well balanced

compromise between cocktail performance, laboratory requirements, additional considerations

about handling/storage characteristics, cocktail expense, and waste treatment implications

[Elliot and van Mourik, 1987; Medeiros et al., 2003]. As a consequence, no specific cocktail

can be identified as fit for all purposes; the manufacturers of liquid scintillation cocktails are

forced to compromise in designing their cocktails, enforcing one performance aspect (e.g.

higher sample load/compatibility) at the expense of another (e.g. counting efficiency or

stability) [Verrezen et al., 2008]. Quenching is the most important factor responsible for a

reduction in counting efficiency for a given sample/cocktail mixture in LSC experiments, and

thus the reliability (uncertainty) of the calculated result and the sensibility (detection limit) of

the method, whereby the use of a liquid scintillation cocktail with an inherently high resistance

to quenching is important [Verrezen et al., 2008]. Reports in [DeVol et al., 2007] confirm the

response of a cocktail to one quench agent relative to that of another is not consistent from

cocktail to cocktail. For example, better resistance to quench presence was confirmed for

Ultima Gold LLT compared to OptiPhase HiSafe 3, especially in the presence of higher levels

of quench (SQP(E)<600), as shown in [Stojković et al., 2015]. Ultima Gold LLT in general, is

a cocktail most adequate for low level measurements, with minimal background and provides

better alpha/beta separation significant for environmental samples.

Method validation. Tritium analysis by liquid scintillation counting in aqueous samples has

been routinely carried out in two laboratories, Laboratory for low radioactivity measurements

at the Department of Physics, University of Novi Sad, Serbia (DP-UNS) where samples are


distilled before counting and Laboratory for measurement of low-level radioactivities (Ruđer

Bošković Institute, RBI, Zagreb, Croatia), where samples are electrolytically enriched.

Intercomparison measurements have been carried out in order to validate both methods [Krajcar

Bronić et al., 2012], as presented in Figure 7. Correlation coefficient has been obtained to

1.05(3), which confirms good performance and high accuracy of both applied methods. It can

be noticed that ASTM method gave greater measurement uncertainties as it is less precise than

electrolytic enrichment, as expected.

0 1 2 3 4 5

0

1

2

3

4

5

6

RBI - A [Bq l-1]

DP

-UN

S -

A [

Bq

l-1]

linear fit of data

y = 1.05(3) x , R2=0.98693

95% confidence bands

Figure 7. Intercomparison of RBI (electrolytic enrichment procedure) and DP-UNS (distillation

procedure) results.

2.3. DIRECT METHOD

Laboratory for low radioactivity measurements DP-UNS had developed another, more

rapid method for tritium determination that assumes direct mixing of water samples with

scintillation cocktails [Jakonić et al., 2014b]. This direct method has been proposed and tested

by [Pujol and Sanchez-Kabeza, 1999]. The complete establishment of this method involved

investigation of the influence of types of vials, different commercially available cocktails and

optimal sample : cocktail ratios on parameters for measurements of tritium activity

concentration.

For the following measurements, MCA configurations of Quantulus were chosen

manually, since the method demands monitoring of chemiluminescence counts, 𝑟𝑞

(configuration setup for counting is published in detail in [Grahek et al., 2016]), and PSA setup

was adjusted for the optimal value that provides accurate alpha/beta spectra separation with the

smallest alpha-to-beta and beta-to-alpha spillover factors. Optimal PSA parameter was

determined with 241Am and 90Sr/90Y standard solutions. It should be mentioned that chemical

properties of scintillation cocktail applied can affect optimal PSA [Stojković et al., 2015],


17

therefore, its value should be adjusted individually for each of the cocktails used. Tritium

spectra generated the same shape as presented in Figure 5

First of all, detection efficiency should be determined based on the measurements of active

samples, i.e. distilled water spiked with tritium solution of known activity,

𝜀 =𝑠

𝑉 𝐴S , (12)

where 𝑠 [s-1] represents count rate of prepared 3H standard, 𝐴S [Bq l-1] represents its activity

concentration, and 𝑉 [l] its volume. The activity concentration of any sample, 𝐴 [Bq l-1], should

be determined via expression:

𝐴 =𝑟−(𝑏+𝑟𝑞)

𝜀 𝑉 , (13)

where 𝑟 [s-1], 𝑏 [s-1] and 𝑟𝑞 [s-1] are count rates of sample, background and chemiluminescence

spectra in channels 1-250, respectively. Chemiluminescence counts can be significant mostly

in highly active samples, according to experience [Jakonić et al., 2014b]. Detection limit is

dependent on time of background measurement, 𝑡 [s]:

𝑀𝐷𝐴 =2.71+4.65 √(𝑏+𝑟𝑞) 𝑡

𝜀 𝑉 𝑡 . (14)

0 2 4 6 8 10 12 14 16 18 200

1

2

3

4

5

6

7

OptiPhase HiSafe 3

MD

A [

Bq

l -1

]

V [ml]

Figure 8. MDA achieved in different sample volumes (for total sample+OptiPhase HiSafe 3

cocktail volume 20 ml in polyethylene vials).

The following part of this section will present the results of method’s optimization, with

the purpose to introduce development of the direct method step-by-step. As mentioned, samples

do not need any pretreatment procedure, but direct mixing of water sample with scintillation

cocktail in previously established optimal ratio. Therefore, the first parameter to be determined

was optimal sample : scintillation cocktail volume ratio. It is known that background values are

dependent on cocktail’s volume. Background level slightly increases with increment of

cocktail’s volume since higher cocktail density. Also, increased cocktail volume has influence

on an efficient interaction with cosmic rays and increment of the amount of organic phosphates


with likely a trace amount of uranium in chemical composition of cocktails [Komosa and

Slepecka, 2009]. In Figure 8, optimal ratio has been explored, with the conclusion that the

lowest 𝑀𝐷𝐴 (1.55 Bq l-1) was reached at 9:11 ratio, the highest holding capacity of used

cocktail for distilled water. The same conclusion followed from the measurements of active

DWTS samples from Figure 6, and from other authors [Ravikumar and Somashekar, 2011b;

Pujol and Sanchez-Cabeza, 1999; Varlam et al., 2001; Eriksen et al., 2002]. In direct method,

8:12 volume ratio was chosen to avoid possible phase separation and heterogeneity in sample-

scintillator mixture [Ravikumar and Somashekar, 2011a], furthermore, for OptiPhase HiSafe 3

and Ultima Gold LLT cocktails it is known that its performance and efficiency decrease at

sample loads exceeding 50% [Verrezen et al., 2008].

Table 3. Measurement parameters as performance indicators of the direct LSC method

of tritium activity determination (8:12 sample : cocktail volume ratio).

Vial type Scintillation

cocktail

Blank

[s-1]

SQP(E)

of

blank

Detection

efficiency

[%]

𝐹𝑂𝑀

[s]

𝑀𝐷𝐴

[Bq l-1]

(𝑡 = 600

min)

PE vials: Low-

Diffusion

Polyethylene

Vials - 20ml

Anti-Static

Ultima Gold uLLT 0.022 783.0 32.45 (7) 4.79 1.43

Ultima Gold LLT 0.023 793.2 36.45 (15) 5.78 1.30

OptiPhase HiSafe 2 0.024 800.3 35.1 (5) 5.13 1.38


Glass vials:

High

Performance

Glass Vial -

20ml

Ultima Gold uLLT 0.153 787.5 31.72 (10) 0.66 3.81

Ultima Gold LLT 0.190 792.8 31.27 (10) 0.51 4.30



Studies have shown that in the same type of vial without quenching, the background count rate

values did not differ significantly for several different cocktails used [Komosa and Slepecka,

2009]. Results of experiments with both, polyethylene and glass vials that were tested in order

to determine their influence on background level together with different scintillation cocktails

that could impact background level and efficiency as well, are presented in Table 3. The

evaluation of the overall performance of a cocktail involves the quantification of several

factors: besides sample load capacity/compatibility, the effect of the sample load on 3H

counting efficiency, figure of merit (𝐹𝑂𝑀) and sample/counting efficiency stability over time,

as well as quench resistance estimation. Low-diffusion PE vials are more suitable for

measurements of tritium activity concentration since their lowest background level. Glass vials

induce higher count rate in low-energy window, most likely caused by 40K; in addition to that,

a peak connected with 6 keV X-ray emission from Ar, a decay product of 40K, can be observed

in background spectrum of glass vials [Komosa and Slepecka, 2009; Chalupnik et al., 1996].

Obtained parameters in Table 3 are in agreement with manufacturer’s claims [PerkinElmer

Scintillation Cocktails & Consumables]. Highest 𝐹𝑂𝑀 values and lowest 𝑀𝐷𝐴 achieved for

600 minutes of measurement indicate best performance for PE vials and Ultima Gold LLT


19

cocktail, which is the reason why those cells are shaded in the Table 3. OptiPhase HiSafe 2

produces white emulsion samples due to much higher amount of both scintillators, namely 2.5

times more PPO and 25 times more bis-MSB than for HiSafe 3 [PerkinElmer Scintillation

Cocktails & Consumables]. Such features are in favor of the manufacturer claims that the

HiSafe 2 scintillation cocktail is suitable for aqueous and non-aqueous solvents, probably more

adequate for those around neutral pH values and with low ionic strength, while HiSafe 3 is for

solutions with strong ionic strength that delivers transparent samples in all examined volume

ratios. Ultima Gold LLT and Ultima Gold uLLT are safer multipurpose LS cocktails for a wide

range of aqueous and non-aqueous samples, with the lowest background, most suitable for

tritium monitoring and research. Ultima Gold LLT meets additional requirements for a low-

level counting cocktail such as long-term stability and subambient temperature stability, and

can accept the important mineral acid species normally encountered in alpha/beta counting

applications [L’Annunziata, 2012]. Also, the sample stability of the cocktail and counting

efficiency become important in some specific cases where LSC vials have to be stored for

longer periods of time after preparation before being counted or recounted [Verrezen, 2008].

Calibration procedure of the LS counter was performed with set of prepared samples with 3H solution of known activities in 8:12 volume ratio. Detection efficiency, presented in Table

3, has been obtained as the slope of the regression lines for each cocktail, with excellent

correlation coefficients, as presented in Figure 9.

0 50 100 150 200 250 300 350 400

0

20

40

60

80

100

120

140

160

y = 0.3645(12) x , R2=0.99994

y = 0.3182(10) x , R2=0.99995

y = 0.3245(7) x , R2=0.99998

A [ Bq ]

Ultima Gold LLT

OptiPhase HiSafe 3

Ultima Gold uLLT

Co

un

t r

ate

[s-1

]

Figure 9. Example of system calibration with different cocktails for direct LSC mesurements

(PE 20 ml vials, 8:12 sample:cocktail volume ratio).

Figure 10 shows 𝑀𝐷𝐴’s behavior vs. counting time for Ultima Gold uLLT cocktail and

data parameterization by the exponential fit. Longer measurements assure better counting

statistics and provide possibility of lower 𝑀𝐷𝐴’s to be achieved. If net count of a sample is the

difference between its measured rate 𝑟′ and background 𝑏, 𝑟 = 𝑟′ − 𝑏, its measurement

uncertainty 𝜎𝑟, and the content of tritium is low (𝑟′ ≈ 𝑏), the relation that has been obtained

and published in [Eriksen et al., 2002] is in the form of binomial series expansion:


(𝜎𝑟

𝑟)

2= [𝑟′ (1 −

𝑏

𝑟′)]

−2 𝑟′+𝑏

∆𝑡=

1

𝑟′ 2 𝑟′+𝑏

∆𝑡∑ 𝑘𝑘=1 (

𝑏

𝑟′ )

𝑘−1 . (15)

This expression means that counting period ∆𝑡 should be set dependent on accuracy level, i.e. 𝜎𝑟

𝑟 , that has to be reached and it is clearly the function of count rate 𝑟′. If low counting rates

are considered, very long counting times are required [Eriksen et al., 2002]. From the

experiments given in Table 4, there is also a difference between several measurements in short

intervals and one long measurement even if the total counting time is equal. Several short

measurements provide greater measurement uncertainties, but on the other hand, it is an

opportunity to monitor system’s performance and to detect irregularities or eventual

fluctuations in count rates.

0 100 200 300 400 500 600 700 800 900 1000 1100 12000

1

2

3

4

5

6

7

Ultima Gold uLLT

MD

A [

Bq

l-1]

t [min] Figure 10. Minimal detectable activity dependence on measurement time for 8:12

sample:cocktail volume ratio in PE vials:

𝑦 = 0.86(21) + 5.5(4) exp [−𝑥

69(9) ] + 2.2(4) exp [−

𝑥

465(185) ] ; 𝑅2 = 0.9973 .

Here some findings about possible interference of chemiluminescence and

photoluminescence occurrence in energy range of 3H will be discussed. Both blank and active

samples have been counted on Quantulus immediately after their preparation. For samples with

very low activity (blank samples) there was some indication for chemical reactions taking place

during first 3 hours after their preparation, and 𝑟𝑞 spectrum in first 6 hours dominated

background count rates, but was also rapidly decreasing until constant close-to-zero value.

Therefore it was determined that maximal waiting time for background and samples with very

low activity should be 3 hours, and estimated half-life of chemical reactions was approximately

16.4 minutes [Jakonić et al., 2014b]. Chemiluminescence spectrum is always necessary to

control, but despite conclusions that 1 day waiting time is adequate for Quantulus performances

[Pujol and Sanchez-Cabeza, 1999], in case of counting samples by direct LSC method it is not

necessary since chemiluminescence spectrum is monitored and taken into account in equation

(eq. 13). Additionally, exploring the effect on count rate after a few minutes of direct exposure

of one sample to sunlight, it was determined that reactions of photoluminescence due to

activation of cocktail by ultraviolet light can significantly increase count rate (they are more


21

intense than chemiluminescence) but have a relatively rapid decay rate, so it was concluded

that these do not affect spectrum after waiting time of about 2 hours [Jakonić et al., 2014b].

Table 4. Optimization of measurement time for direct LSC method (8:12

sample:cocktail volume ratio in PE vials, two cocktails used, Ultima Gold LLT and

OptiPhase HiSafe 3).

Referential

𝐴 [Bq l-1]

𝐴 [Bq l-1]

10 cycles of 30 min measurements

𝐴 [Bq l-1]

1 cycle of 300 min measurement

64.93 59 (4) * 59.5 (13) *

- 10.8 (26) * 12.7 (8) *

- 23.6 (18) * 23.4 (9) *

28.35 25.2 (28) ** 25.6 (10) **

36.52 31.4 (23) ** 31.1 (11) **

* Ultima Gold LLT used

** OptiPhase HiSafe 3 used

Therefore, optimization of direct method led to conclusions that 8 ml of sample should be

mixed with 12 ml of the scintillation cocktail (Ultima Gold LLT showed best performance) in

a 20 ml low diffusion polyethylene vial. Measurement time and number of repetitions should

be adapted to required measurement uncertainty. Direct method is ideal for routine control

releases from the nuclear power plants or in case of nuclear accidents, where it is necessary to

have rapid, simple, inexpensive and reliable methods for the determination of radionuclides.

2.4. SAMPLE COMBUSTION

The following part of the chapter will present another possibility of sample preparation for

LSC analysis, sample combustion by Model 307 PerkinElmer Sample Oxidizer. This system

has the ability to ensure consistently reproducible, high quality single and dual labeled 3H and 14C samples. During the combustion, the organic portion of the sample in an oxygen-rich

atmosphere is completely converted to water (hydrogen present is oxidized to H2O) and carbon

dioxide (carbon present is oxidized to CO2), while the inorganic portion remains in the particle

trap [L’Annunziata, 2012]. Advantages of sample combustion are: rapid sample processing

time, sample can be wet, dry or freeze-dried, any sample containing H and/or C can be

combusted, sample sizes up to 1.5 g are possible, excellent radioactive recovery (>97%),

memory effect <0.08%, no loss of radioactivity by volatilization, no chemiluminescence

interference and no color quench interference [Oxidizer Application Note, 2002].

Disadvantages of sample combustion are: initial capital investment, it is only suitable for 3H

and 14C, need a gas supply (oxygen and nitrogen), must be operated in a fume hood and reagents

are corrosive and flammable [Oxidizer Application Note, 2002].

The sample is placed in the ignition basket, a platinum coil capable of developing high

temperatures, and is combusted in a continuous flow of oxygen, forming water and carbon

dioxide thus separating samples for 3H and 14C analysis. The combustion time is set by the

operator and is determined by the sample material and size. Here only the tritium collection

system will be described. All isotopes of hydrogen, including tritium, are oxidized to tritiated


water in the form of a steam. The steam is condensed in an air cooled condenser and the tritiated

water is collected in the tritium counting vial. Any uncondensed water is collected in the tritium

exchange column. After the sample is completely combusted, steam is injected into combustion

chamber as part of the combustion cycle to sweep tritiated water from the combustion flask and

condenser into the tritium counting vial. At the end of the cycle, the water in the exchange

column is flushed down into the tritium counting vial with the tritium scintilator Monophase S,

which is adequate cocktail for Model 307 Oxidizer. Oxidation eliminates color quenching and

reduces background counts and variation in chemical quenching. The system uses nitrogen or

air to dispense the scintillators and absorption chemicals and to flush the combustion products

from the trapping system into the counting vials [Perkin Elmer Operation Manual, 2003].

The water collection is accomplished in multiple stages. The first stage is an air cooled

condenser, where the steam simply condenses on the walls of the condenser. After the

combustion cycle is complete, the water is flushed from the condenser by a steam and

nitrogen/air flush through the flask. Some traces of uncondensed water vapor will be trapped

in the exchange column. The exchange column, containing water, exchanges the water vapor

in the long spiral tubing. This water is flushed out with a water absorbing chemical into the

collection vial. At the end of the oxidation cycle, the trapped and condensed water is flushed

from the exchange column with the counting solution. This multiple stage tritium collection

system ensures a consistently high percentage of radionuclide recovery [Perkin Elmer

Operation Manual, 2003].

Counts

(ar

bit

rary

unit

s)

5

Channel number

Figure 11. Spectra of sample set with the same tritium activity concentration and different

amounts of distilled water added to 13 ml of Monophase S cocktail (EasyView).

Quench correlation curve establishment was done in the following way: 13 ml of

Monophase S was injected into the 7 tritium collection polyethylene vials along with the system

process. One of them was blank vial with no addition of distilled water. The other six calibration

vials contained increasing volumes of distilled water, from 0-5 ml, which meant that their

quench level had been increasing as well. Exactly the same amount of a known quantity of


23

tritium solution SPEC-CHECH with referential activity ~2833 Bq was injected into each of the

six calibration vials marked. The vials were shaken vigorously after recapping to ensure a

uniform suspension of Monophase S and water. After stabilization and dark adaptation, each

of the six vials was counted on Quantulus for 20 minutes in three cycles, their spectra are shown

in Figure 11 and the counting window was set between channels 1-600. The default MCA

tritium configuration of WinQ software was used for measurements. Quench indicating

parameter (QIP) i.e. SQP(E) parameter, was recorded for each vial. The three SQP(E) values

and net tritium count rates for each of the six vials were averaged. Detection efficiency should

be determined for each vial as the ratio of average count rate of standard 𝑠 [s-1], and spiked

tritium activity 𝐴S [Bq]:

𝜀[%] =𝑠

𝐴S∙ 100 . (16)

The obtained counting efficiency vs. SQP(E) parameter is given in Figure 12 where it can

be seen that efficiency was approximately 35% in range SQP(E) from 860 to 800.

800 810 820 830 840 850 860

33

34

35

36

37

38

[%] = 0.003(8)*SQP(E)+32(7)

Average SQP(E) parameter

Eff

icie

ncy

[%

]

Figure 12. Quench correction curve for tritium activity determination via Model 307 Sample

Oxidizer method (combusted sample is added to 13 ml of scintillation cocktail Monophase S

in PE vials).

Samples for 3H analysis were prepared according to operating procedures. The sample to

be oxidized should be injected into combusto-cone: in sample volume of 0.8 ml water, 0.4 ml

of Combustaid was added to help moderate the burning process [Nikolov et al., 2013]. In all

samples Oxidizer automatically added the adjusted volume of 13 ml of scintillation cocktail

Monophase S in polyethylene vials. The activity concentration of 3H 𝐴 [Bq l-1] was calculated

according to the following formula:

𝐴 =𝑎−𝑏

𝜀 𝑐 𝑉 , (17)

where 𝑎 [s-1] – the count rate of the sample [s-1], 𝑏 [s-1] – count rate of the background, 𝑐 – the

trapping efficiency (or recovery factor), 𝑉 [l] – the volume of the sample analyzed, 𝜀 – the

efficiency of the sample measurement. The trapping efficiency was determined according to

the detailed instruction provided in Oxidizer Manual [Perkin Elmer Operation Manual, 2003],


results of these experiments gave 𝑐 = 0.97. The trapping efficiency could be quickly tested by

combusting a known activity of 3H and comparing those samples with non-combusted samples

with the same activity [Vartti, 2009].

The lower limit of detection 𝐿𝐿𝐷 [Bq l-1] was calculated using the formula:

𝐿𝐿𝐷 =4.65 √𝑏

𝜀 𝑐 𝑉 𝑡 , (18)

where 𝑡 [s] was the counting time. With a 300 minute counting time, a background of

(0.023±0.004) s-1, detection efficiency of 35%, the trapping efficiency of 97% and a sample

volume of 0.8 ml, the 𝐿𝐿𝐷 was 1.12 Bq l-1 [Nikolov et al., 2013], while for 600 minutes of

measurement the 𝐿𝐿𝐷 of 0.55 Bq l-1 could be achieved. 𝐿𝐿𝐷 dependence on duration of

counting is plotted in Figure 13.

0 100 200 300 400 500 600 700 800 900 10000

1

2

3

4

5

6

LL

D [

Bq

l-1]

t [min]

Model 307 Sample Oxidizer

Figure 13. Detection limit vs. counting time for Model 307 Sample Oxidizer method for tritium

activity determination (combusted sample is added to 13 ml of Monophase S in PE vials):

𝑦 = 0.29(8) + 2.5(5) exp [−𝑥

253(60) ] + 10.9(5) exp [−

𝑥

48(5) ] ; 𝑅2 = 0.9993 .

Flame combustion, operated in either manual or robotic format, provides a very powerful

tool to rapidly process many different sample types with a high degree of precision and

accuracy. The technique requires a minimal amount of time and sample handling and eliminates

color quench and chemiluminescence interferences [Oxidizer Application Note, 2002].

Application of sample combustion is ideal for monitoring program around nuclear facilities and

in case of nuclear emergencies where rapid and effective procedures are required. When

compared to direct counting and electrolytic enrichment, Sample Oxidizer methods provides

satisfactory and comparable results for environmental monitoring [Nikolov et al., 2013].

It is also worth to mention that combustion techniques are not popular for tritium

determination in water samples, their advantage is that any chemical procedure such as

distillation or electrolytic enrichment is avoided during sample preparation. Additionally, direct

method for tritium activity measurements is cheaper and simpler technique for water analysis.

Combustion techniques have become far more interesting and useful when applicable to soil or

other matrices rather than water samples. Organically bound tritium, OBT, more significant

tritium fraction with respect to understanding tritium behavior in the environment, could be

determined via this technique in terrestrial biological materials, aquatic biological materials


25

and soil samples [Kim et al., 2013; Lehto and Hou, 2011]. The environmental quantification

and behaviour of OBT are still not well known and have been explored among tritium research

community in the past few years [Yankovich et al., 2011], therefore combustion techniques

will certainly be significant tool for further OBT studies in soil, vegetation, animals, foodstuff

and other matrices.

3. QUENCH CORRECTIONS

Although environmental samples prepared for liquid scintillation do not present self-

absorption problems, dissolved minerals in natural water may absorb some light produced in

the scintillator in a process called quenching which reduces detector efficiency [Pujol and

Sánchez-Cabeza, 1997]. Quench effects during the transfer of nuclear decay energy to the

scintillation cocktail solvent process can cause a serious reduction in counting efficiency in

LSC experiments for a given sample/cocktail mixture.

It has been demonstrated that the lower the energy of the decay, the greater is the effect of

quench on the counting efficiency for beta-emitting radionuclides [Horrocks, 1974;

L’Annunziata, 2012], which means that tritium spectrum is most greatly affected by quench

presence since it emits beta particles with lowest energy, 𝐸max = 18.6 keV. Quench influences

tritium spectra in two ways: the endpoint or maximum intensity of the pulse height spectrum is

being reduced - shifted towards lower channels in multichannel analyzers i.e. towards lower

energies, whilst area under spectrum is being diminished which means decreased total number

of counts that are recorded. These effects significantly reduce efficiency detection.

There are mainly four different types of quenching that may occur in the mixture:

absorption or physical quenching, chemical quenching, photon or color quenching and solvent

dilution quenching [Varlam et al., 2001]. An important step in LSC is to determine the amounts

of some or all of these quenching modes and to make correction or adjustments based on the

determined amounts [Thomson, 2002]. Chemical quenching is due to chemical substances that

interfere in the energy transference from the solvent to the solute [Manjón et al., 2002], they

absorb part of nuclear decay energy before it is transferred to scintillation cocktail solvent. The

color of the sample can interfere the light transmission to the photocathode, giving place also

to quenching [Villa et al., 2003, 2004], it is present in sample when its color is visible. Color

quenching assumes that some photons are absorbed in vial before they reach PMT. Both

chemical and color quench have the consequence that photomultiplier tubes detect less photons

than compared to unquenched samples which means that the counting efficiency is being

reduced. Color quench can be easily noticed and common ways are to decolorize or bleach

samples prior to LS analysis if possible, leaving only chemical quench in sample. Since

chemical quenching is always present in LSC samples, dealing with this problem is necessary

for the appropriate use of every LS counter.

Different methods have been developed in order to correct for quenching: (1) internal

standard method, (2) sample spectrum method, (3) external standard method, and (4) direct

DPM method; the overview of all methods, their advantages and range of usage is provided in

detail in [L’Annunziata, 2012]. Also, there are more sophisticated LSC methods as well, TDCR

for example (Triple-to-Double Coincidence Ratio), used for the measurement of absolute

activity of beta-emitting radionuclides [Priya et al., 2013]. The method is based on a free

parameter model describing the process of light emission and detection in a scintillation


counter. It is to be noted that the TDCR value is neither a quenching indicator nor an efficiency

value. The TDCR value as a quenching indicator is only applicable if the relation between the

TDCR value and the efficiency is unambiguous. This is the case with pure beta emitters but not

radionuclides decaying by electron capture (EC) or radionuclides with complex decay schemes,

where up to three efficiency values can correspond to the same TDCR value Wanke et al.,

2012.

In the following paragraphs, external standard method will be briefly discussed and

demonstrated. Although quenching varies with samples, it is possible to compensate it and to

determine the sample activity accurately if the counting efficiency of the measured sample is

known. The idea of the external method is to equip LS system with a calibration curve - this

means to establish the tritium counting efficiency dependence on the some quench-indicating

parameter (QIP), which could be applied for each measured sample. During sample analysis,

the value of the QIP is used to determine the reduction of radionuclide counting efficiency,

based on the created quench correction curve and subsequently for the determination of

radionuclide activity. This procedure does not correct for other effects such as

chemiluminescence. With the development of the multichannel analyzer (MCA), sample

spectrum QIPs have become more sophisticated, as all of the channels of the MCA can be used

simultaneously to measure quench [L’Annunziata, 2012].

There are several QIPs in liquid scintillation analysis. An example is Spectral Quench

Parameter of External standard, SQP(E), which is a robust and widely used parameter in

monitoring and correcting for sample quench levels in the determination of detection

efficiencies [L’Annunziata, 2012]. Quantulus 1220™ has the ability to automatically record

SQP(E) value of samples, therefore, the simplicity of this method makes it very popular and

widely used.

Determination of SQP(E) parameter with the Quantulus counter is carried out by

generating of two spectra: the first one is being recorded when the sample is exposed to γ-

radiation from some external source of 226Ra or 152Eu stored inside Quantulus [Quantulus 1220

Instrument Manual, 2002]. Such spectrum contains both Compton electron events and the

sample events. The second spectrum is being recorded from the sample alone during the same

counting time. The net external standard spectrum is obtained after subtracting the first and the

second spectra. The end point SQP(E) is determined by the MCA (with 1024 logarithmic

channels) as the channel below which 99.5% of all the signal of the net external standard

spectrum is found. Such an approach gives a quick method of adjusting counting efficiency,

which is very practical for environmental measurements [Minne et al, 2008]. SQP(E) is a

relative number that has a meaning when compared to the SQP(E) of some other sample. One

possible drawback of external standard method via SQP(E) parameter monitoring is the fact

that Compton electrons produced by external standard are energetic beta particles and do not

have the same behavior of weak beta particles in the analyzed sample itself, hence, a small

amount of quenching in tritium sample may remain undetected [Varlam et al., 2001].

The external standard method for quench correction can be the most reliable when very

accurate DPM values are required, because the users can control all aspects of the preparation

of the quenched standards in order to represent the chemistry of their experimental samples

most closely [L’Annunziata, 2012]. Quench set of sealed standards in vials can be purchased

for the radionuclide and scintillation cocktail of interest, or made in laboratory according to the

following demands: ISO 9698 recommendations for calibration, the sample, the blank sample


27

as well as the calibration source should be put in the same type of vial in the same geometry,

keeping the same ratio between sample and scintillation cocktail, the detection equipment

should maintain the same temperature while the value of quench parameter should be included

in the calibration curve [ISO 9698, 2010]. This means that the quench curves are specific to the

LS counter and to the sample composition. The addition of any chemical changes the

composition of a liquid scintillation sample and, consequently, the cross section for photon

interaction is changed, which also has an influence on the measured SQP(E) value. Selection

of appropriate chemical quenching agent should be done so that the properties of aqueous

sample and sample-cocktail mixture are preserved or not significantly influenced [Varlam et

al., 2015]. Quench set of vials represents identical vials with the same amount of known

radionuclide activity but with varying levels of quench (i.e. with gradually increased amounts

of quenching agent). From the count rates of each tritium standard and SQP(E) value measured

by the LSC, a standard curve of counting efficiency correlated to SQP(E) is plotted. When a

sample of unknown activity is analyzed in the LSC, the instrument will determine the SQP(E)

value of the sample, and extract the counting efficiency from the obtained calibration curve.

One example of quench curve established for the use of OptiPhase HiSafe 3 cocktail, obtained

with the addition of nitromethane as a quenching agent is displayed in Figure 14.

450 500 550 600 650 700 750 8000

5

10

15

20

25

30OptiPhase HiSafe 3, addition of CH

3NO

2

y = -0.53(38) + 0.023(4)*exp [0.00944(24) x]

R2

= 0.99997

Eff

icie

ncy

[%

]

SQP(E)

Figure 14. Example of quench correction curve for direct LSC method of tritium activity

determination (8:12 sample : cocktail volume ratio in 20 ml PE vials).

Investigations on the behavior of common chemical agents that induce chemical quench

with different quench strengths (nitromethane, nitric acid, acetone, saturated solutions of

chloroform, carbon tetrachloride, dichloro-methane) have been conducted and published in

[Varlam et al., 2015; Jakonić et al., 2014a]. For color quench agents, sodium chromate can be

used [Manjón et al., 2002] or yellow dye, if waters are expected to be colored. The important

task for laboratory routine measurements is to select appropriate quenching agent for the


preparation of quench set of vials and to examine their stability. Here are some of the findings

concerning that problem.

Nitromethane is most common choice, since it is the strongest quencher with very good

solubility, and very small amounts are required to achieve wide range of quench levels in

samples. The conductivity and pH values of samples with added nitromethane are near to

environmental samples [Varlam et al., 2015]. The use of nitromethane in combination with

OptiPhase HiSafe 2 demands storing the set of prepared quenched samples for at least two

weeks after its preparation, probably in order for the reduction reactions induced by sodium

borohydride to take place, as for the set of nitromethane quenched samples prepared with

OptiPhase HiSafe 3, the samples become stable after one day of preparation [Jakonić et al.,

2014a].

Carbon tetrachloride is also often used as a quenching agent for the purpose of quench

curve establishment [Verrezen et al., 2008; Komosa and Slepecka, 2009; Ravikumar and

Somashekar, 2011a], it is not as strong as nitromethane, but known to be stronger quencher

than water by two orders of magnitude [Grau Carles, 2006].

Nitric acid is not very good choice since it generates very mild quench; samples are not

always stable due to separation of some cocktail components in acid media, and their extremely

low pH and high conductivity values are different with respect to environmental LSC samples

[Varlam et al., 2015]. Application of nitric acid in combination with OptiPhase HiSafe 2 is not

possible at all, since coagulations always take place, whilst as addition to OptiPhase HiSafe 3

there is the limit of the maximum added volume, since at higher concentrations HNO3 causes

nitrification of the aromatic components in the scintillator [Jakonić et al., 2014a].

Quenched samples with acetone are stable, acetone is miscible with water and organic

solutions at all concentrations, but since it is moderate quencher, its usage for quench curve

establishment is appropriate when no serious quench is expected in samples [Jakonić et al.,

2014a].

The overall conclusion is that cocktail’s response is different in presence of various

quenchers, therefore selection of quenching agent should be made after comprehensive study

of sample matrix that is going to be analyzed so that expected mechanisms of quench in real

samples are as similar to the ones in quench set of vials as possible. Once the quench curve is

obtained, it is possible to store it in LS counter so that it can be automatically applied it in

further routine measurements.

SUMMARY AND CONCLUSION

Liquid scintillation counting has proven to be irreplaceable in environmental tritium

monitoring where low instrument background can enable natural tritium level determination

and significantly affect its precision as well. The aim of this chapter was to present

comprehensive and thorough overview of methods for tritium activity determination in water

samples within LSC techniques, based on some results of characteristic parameters obtained

during their establishment in two laboratories, DP-UNS and RBI. Both laboratories have used

Quantulus 1220TM as LS counter, equipped for ultra low-level measurements in environmental

studies.

Presented sample preparation procedures encompassed large variety of techniques, from

absence of any pretreatment to distillation, combustion, and electrolytic enrichment. For


29

environmental monitoring, Direct counting, Electrolytic enrichment and Sample Oxidizer

methods gave comparable results and acceptable limits of detection, as shown by the

intercomparison results published in [Nikolov et al., 2013]. Still, different approaches to sample

preparation prior to counting resulted in variable detection limits and precision, thus, these

methods have found different range of applications.

Tritium determination by electrolytic enrichment requires expensive equipment, multi-

stage procedure and long time (approximately 8 days) for sample preparation. However, this

method among all presented ones, has highest precision and lowest minimal detectable activity

which varies between 0.03-0.05 Bq l-1 for total counting time 300-500 min, which makes it

applicable in broad spectrum of research. Measurements of tritium activity concentration are

regularly used in hydrogeological, hydrological and oceanic processes study in the

environment, groundwater movement and dating research since natural low levels of tritium

can be quantified.

On the other hand, tritium releases associated with global fallout, nuclear accidents or

nuclear industry, for example nuclear fuel cycle activities (fuel enrichment, fuel fabrication,

power generation, and fuel reprocessing) can be easily monitored via inexpensive, rapid and

simple methods such as direct method or sample combustion. These are ideal techniques in

environmental monitoring or nuclear emergencies since sample preparation takes few minutes

and tritium assessment in sample can be made in few hours of counting. Sample combustion

requires also additional equipment, with additional drawback of working with flammable

substances. Minimal detectable activity achieved for Sample Oxidizer method is 1.12 Bq l-1 for

300 min of counting. Direct method is perhaps better solution than sample combustion, it is

simple and does not require any equipment, with 𝑀𝐷𝐴 in range 1.3-2.0 Bq l-1 obtained for 600

minutes of measurement, depending on the scintillation cocktail used. However, the Sample

Oxidizer method is the only one applicable to organic/biological samples when OBT

(organically bound tritium) has to be extracted for the monitoring purposes.

Samples can be subjected to distillation prior to counting, which is a procedure for

purification of samples, especially if water samples are colored or their origin suggests organics

or interfering chemicals are present. It is often recommended in routine tritium assessment in

drinking waters, together with dose evaluation and health hazard studies. For example, for

samples prepared with OptiPhase HiSafe 3 cocktail, 𝑀𝐷𝐴 is 1.4 Bq l-1 for 600 minutes of

measurement, which is far below the upper limit for tritium level permitted via legislations.

All methods were presented together with necessary calculations, demonstrated calibration

procedures and results of optimization experiments. Conservative sample-to-cocktail volume

ratio 8:12 has been adopted. PE vials (Low-diffusion Polyethylene Vials – 20 ml Anti-Static)

were used since their lower background level. As concluded, cocktail choice depends on the

balanced compromise between laboratory requirements (acceptable measurement uncertainty,

required detection limit, sample volume used for routine measurement and its

chemical/physical properties, waste treatment regulations etc.) and cocktail performance

(efficiency detection, intrinsic background contribution, quench resistance, sample stability

over time, its stability and load capacity) [Verrezen et al., 2008]. Ultima Gold LLT in general,

is a cocktail most adequate for low level measurements, with minimal background and provides

better alpha/beta separation significant for environmental samples. Quench interferences are

very common during tritium determination, therefore, any laboratory that is caring out routine

measurements of tritium activity concentration should establish quench correction curve


according to sample matrix that is going to be analyzed so that expected mechanisms of quench

in real samples are as similar to the ones in quench set of vials as possible.

FUNDING

The authors acknowledge the financial support of the Provincial Secretariat for Higher

Education and Scientific Research within the project „Radioactivity in drinking water and

cancer incidence in Vojvodina” no. 114-451-2405/2016, the Bilateral Scientific Project

between Croatia and Serbia for 2016-2017: „Optimisation of methods for radioactivity

measurements (3H, 14C, 90Sr, 222Rn) in environmental samples“ and the Ministry of education,

science and technological development of the Republic of Serbia within projects no. OI171002

and III43002.

REFERENCES

ASTM International D 4107–08, Standard Test Method for Tritium in Drinking Water, ASTM

International. 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428–

2959, United States.

Baeza, A., Brogueira, A.M., Carreiro, M.C., Garcia, E., Gil, J.M., Miro, C., Sequiera, M.M.,

Teixeira M.M. 2001. “Spatial and temporal evolution of the levels of tritium in the Tagus

river in its passage through Caceres (Spain) and Alentejo (Portugal).” Water Research

35:705-714.

Barešić, J., Horvatinčić, N., Krajcar Bronić, I., Obelić, B. 2010. “Comparison of two techniques

for low–level tritium measurement – gas proportional and liquid scintillation counting.”

Proceedings of 3rd European IRPA Congress, Helsinki, Finland; 01/2010.

Barešić, J., Krajcar Bronić, I., Horvatinčić, N., Obelić, B., Sironić, A., Kožar-Logar, J. 2011.

“Tritium activity measurement of water samples using liquid scintillation counter and

electrolytic enrichment.” In: Proceedings of the 8th Symposium of Croatian Radiation

Protection Association. Krajcar Bronić, I., Kopjar, N., Milić, M., Branica, G. (Eds.). Krk,

Croatia, 13.-15.4.2011., HDZZ, Zagreb, 2011 p. 461-467.

Bolsunovsky, A.Ya., Bondareva, L.G. 2003. “Tritium in surface waters of the Yenisei River

Basin.” Journal of Environmental Radioactivity 66:285–294.

Cameron, J. F. 1967. “Survey of systems for concentration and low background counting of

tritium in water.” In: Radioactive dating and methods of low-level counting. IAEA, Vienna

ST/PUB/152, pp. 543-573.

Chalupnik, S., Lebecka, J., Mielnikow, A., Michalik, B. 1996. “Determining radium in water:

comparison of methods.” In: Liquid Scintillation Spectrometry 1994. Cook, G.T.,

Harkness, D.D., MacKenzie, A.B., Miller, B.F., Scott E.M., (Eds.). Radiocarbon,

University of Arizona, Tucson, p. 103-109.

DeVol, T.A., Theisen, C.D., DilPrete, D.P. 2007. “Effect of Quench on Alpha/Beta Pulse Shape

Discrimination of Liquid Scintillation Cocktails.” Operational Radiation Safety. Health

Physics 92(5):S105-S111.

DOE HANDBOOK. 2008. Tritium handling and safe storage. U.S. Department of Energy,

Washington, D.C. 20585, DOE–HDBK–1129–2008.

https://www.researchgate.net/publication/265641016_Radiation_detection_technologies__Poster_presentations_Barei_et_al._Comparison_of_two_techniques_for_low-level_tritium_measurement__gas_proportional_and_liquid_scintillation_counting_Comparison_of_two_techniques_for_low-level_tritium_measurement__gas_proportional_and_liquid_scintillation_counting?ev=prf_pub

https://www.researchgate.net/publication/265641016_Radiation_detection_technologies__Poster_presentations_Barei_et_al._Comparison_of_two_techniques_for_low-level_tritium_measurement__gas_proportional_and_liquid_scintillation_counting_Comparison_of_two_techniques_for_low-level_tritium_measurement__gas_proportional_and_liquid_scintillation_counting?ev=prf_pub


31

Elliot, J.C., van Mourik, B. 1987. “A performance comparison of select alkylbenzene and

detergent based liquid scintillation cocktails.” Applied Radiation and Isotopes 38 (8):629–

633.

Eriksen, D., Martini, V., Hartvig, S.K. 2002. “Assessment of tritium detection limits at Institute

for Energy Technology (IEF) with Quantulus low–level liquid scintillation spectrometer.”

In: LSC 2001, Advances in Liquid Scintillation Spectrometry. Mobius, S., Noakes, J.E.,

Schonhofer, F. (Eds.). Radiocarbon, University of Arizona, Tucson, p. 203–211.

European Commission. 1998. European drinking water directive 98/83/EC of 3 November

1998 on the quality of water intended for human consumption. Official Journal Legislation.

p. 330.

Forkapić, S., Nikolov, J., Todorović, N., Mrdja, D., Bikit, I. 2011. „Tritium Determination in

Danube River Water in Serbia by Liquid Scintillation Counter.“ World Academy of

Science, Engineering and Technology 52:520-523.

Gat, J. R. 1980. “The isotopes of hydrogen and oxygen in precipitation.” In: Handbook of

Environmental Isotope Geochemistry, vol. 1. The Terrestrial Environment (ed. by P. Fritz

& J.-Ch. Fontes), 21–48. Amsterdam: Elsevier. Cameron, 1967.

Grahek, Ž., Breznik, B., Stojković, I., Coha, I., Nikolov, J., Todorović, N. 2016. „Measurement

of tritium in the Sava and Danube Rivers.” Journal of Environmental Radioactivity 162-

163:56-67.

Grau Carles, A. 2006. “Synergic quenching effects of water and carbon tetrachloride in liquid

scintillation gel samples.” Applied Radiation and Isotopes 64:1505-1509.

Gröning, M., Auer, R., Brummer, D., Jaklitsch, M., Sambandam, C., Tanwee, A., Tatzber, H.

2009. “Increasing the performance of tritium analysis by electrolytic enrichment.” Isotopes

in Environmental and Health Studies 45:118–125.

Hanslık, E., Ivanovova, D., Jedinakova–Krıžova, V., Juranova, E., Šimonek, P. 2009.

“Concentration of radionuclides in hydrosphere affected by Temelın Nuclear Power Plant

in Czech Republic.” Journal of Environmental Radioactivity 100:558–563.

Hanslik, E., Jedinakova-Krızova, V., Ivanovova, D., Kalinova, E., Sedlaova, B., Šimonek, P.

2005. “Observed half-lives of 3H, 90Sr and 137Cs in hydrosphere in the Vltava River basin

(Bohemia).” Journal of Environmental Radioactivity 81:307-320.

Hill, R.L., Johanson, J.R. 1993. “Metabolism and dosimetry of tritium.” Health Physics 65:628-

647.

Horrocks, D.L. 1974. Applications of liquid scintillation counting. Academic Press, New York

and London.

Horvatinčić, N., Krajcar Bronić, I., Pezdič, J., Srdoč, D., Obelić, B. 1986. “The distribution of

radioactive (3H, 14C) and stable (2H, 18O) isotopes in precipitation, surface and

groundwaters of NW Yugoslavia.” Nuclear Instruments and Methods B17:550–553.

Instrument Manual. 2002. Wallac 1220 Quantulus Ultra Low Level Liquid Scintillation

Spectrometer. PerkinElmer Life Sciences, Wallac Oy, P.O. Box 10, FIN-20101 Turku,

Finland.

International Organization for Standardization. 2010. ISO 9698—water quality: determination

of tritium activity concentration—liquid scintillation counting method. 2nd edn.

Jacobs, D.G. 1968. “Sources of tritium and its behavior upon release to the environment.”

Report No. TID–24635, U.S. Atomic Energy Commission, Washington.

Jakonić, I., Nikolov, J., Todorović, N., Tenjović, B., Vesković, M. 2014a. “Study on quench


effects in liquid scintillation counting during tritium measurements.” Journal of

Radioanalytical and Nuclear Chemistry 302:253–259.

Jakonić, I., Todorović, N., Nikolov, J., Krajcar Bronić, I., Tenjović, B., Vesković, M. 2014b.

“Optimization of low–level LS counter Quantulus 1220 for tritium determination in water

samples.” Radiation Physics and Chemistry 98:69–76.

Janković, M., Todorović, D., Todorović, N., Nikolov, J. 2012. “Natural radionuclides in

drinking waters in Serbia.” Applied Radiation and Isotopes 70:2703-2710.

Jean-Baptiste, P., Baumier, D., Fourre, E., Dapoigny, A., Clavel, B. 2007. “The distribution of

tritium in the terrestrial and aquatic environments of the Creys-Malville nuclear power

plant (2002-2005).” Journal of Environmental Radioactivity 94:107-118.

José Madruga, M., Manuela Sequeira, M., Rita Gomes, A. 2009. “Determination of tritium in

waters by liquid scintillation counting.” In: LSC 2008, Advances in Liquid Scintillation

Spectrometry. Eikenberg, J., Jäggi, M., Beer, H., Baehrle, H., (Eds.). Radiocarbon,

University of Arizona, Tucson, p. 353–359.

Kaufman, S., Libby, W.F. 1954. “The natural distribution of tritium.” Physical Review

93(6):1337–1344.

Kim, S.B., Baglan, N., Davis, P.A. 2013. “Current understanding of organically bound tritium

(OBT) in the environment.” Journal of Environmental Radioactivity 126:83-91.

Komosa, A., Slepecka, K. 2009. “Study on quenching effects for 14C and 3H measurement

parameters using a Quantulus spectrometer.” In: LSC 2008, Advances in Liquid

Scintillation Spectrometry. Eikenberg, J., Jäggi, M., Beer, H., Baehrle, H., (Eds.).

Radiocarbon, University of Arizona, Tucson, p. 161–172.

Komosa, A., Slepecka, K. 2010. “Effect of liquid scintillation cocktail volume on 3H and 14C

measurement parameters using a Quantulus spectrometer.” Nukleonika 55(2):155–161.

Kožar Logar, J., Glavič-Cindro, D. 2008. “Establishment of low-level tritium laboratory.” In:

LSC 2008, Advances in Liquid Scintillation Spectrometry. Eikenberg, J., Jäggi, M., Beer,

H., Baehrle, H., (Eds.). Radiocarbon, University of Arizona, Tucson, p. 241–249.

Krajcar Bronić, I., Todorović, N., Nikolov, J., Barešić, J. 2012. “Intercomparison of low–level

tritium and radiocarbon measurements in environmental samples.” In: Ristić, G. (Eds.),

Proceedings 1st International conference on radiation and dosimetry in various fields of

research, RAD 2012, Niš, Serbia, p. 279–282.

L’Annunziata. M.F. 2012. Handbook of Radioactivity Analysis. Academic Press, 3rd edition.

Lehto, J., Hou, X. 2011. Chemistry and Analysis of Radionuclides, Laboratory Techniques and

Methodology. WILEY–VCH Verlag & Co. KgaA, Germany.

Manjón, G., Absi, A., Villa, M., Moreno, H.P., García-Tenorio, R. 2002. “Efficiency,

background and interference in colored samples using a LSC Quantulus 1220.” In: LSC

2001, Advances in Liquid Scintillation Spectrometry. Möbius, S., Noakes, J.E., Shönhofer,

F. (Eds.), Radiocarbon, University of Arizona, Tucson, p. 83–91.

Medeiros, R.B., Godinho, R.O., Mattos, M.F.S.S. 2003. “Comparison of the efficacy of

biodegradable and non-biodegradable scintillation liquids on the counting of tritium- and

[14C]-labeled compounds.” Brazilian Journal of Medical and Biological Research 36

(12):1733–1739.

Michel, R.L. 2005. Tritium in the hydrologic cycle. In: Aggarwal, P.K., Gat, J.R., Froehlich,

K.F.O., (Eds.) Isotopes in the Water Cycle: Past, Present and Future of a Developing

Science. IEA, p. 53–66.

http://www.sciencedirect.com/science/article/pii/S0265931X13001604



http://www.sciencedirect.com/science/journal/0265931X/126/supp/C


33

Minne, E., Heynen, F., Hallez, S. 2008. “Possible overestimation of the external standard

quench parameter on Wallac 1220 Quantulus™ with high energetic beta-emitters.” Journal

of Radioanalytical and Nuclear Chemistry 278 (1):39–45.

Moeller, T. 1954. Inorganic Chemistry. pp. 38-52, John Wiley & Sons, Inc., New York.

Mook, W.G. 2001. Environmental Isotopes in the Hydrological Cycle, Principles and

Applications, Volumes I-IV, Technical Documents in Hydrology No. 39, IAEA-UNESCO,

Paris 2001.

Morgenstern, U., Taylor, C.B. 2009. „Ultra low-level tritium measurement using electrolytic

enrichment and LSC.“ Isotopes in Environmental and Health Studies 45:96–117.

National Council on Radiation Protection and Measurements. NCRP Report No.62, 1979.

Tritium in the environment. Washington.

Nikolov, J., Todorović, N., Janković, M., Voštinar, M., Bikit, I., Vesković, M. 2013. “Different

methods for tritium determination in surface water by LSC.” Applied Radiation and

Isotopes 71:51–56.

Noakes, J.E., Filippis, S.De. 1988. “Tritium monitoring of nuclear power plants by liquid

scintillation counting.” In: 2nd International Seminar for Liquid Scintillation Analysis,

Packard Japan K.K., Tokyo, p. 123-128.

Obelić, B., Krajcar Bronić, I., Horvatinčić, N., Barešić, J. 2004. “Comparison of different

methods of environmental radioactivity measurements at Zagreb Radiocarbon and Tritium

Laboratory.” IRPA 11 Full Papers, ISBN 84-87078-05-2. IRPA, 2004. 6c21-1-7.

Oxidizer Application Note. 2002. A Comparison of Sample Oxidation and Solubilization

Techniques By Jock Thomson, PerkinElmer Life Sciences, Inc. 2002.

Palomo, M., Penalver, A., Aguilar, C., Borrull, F. 2007. “Tritium activity levels in

environmental water samples from different origins.” Applied Radiation and Isotopes 65

(9):1048–1056.

PerkinElmer Operation Manual. 2003. Model 307 PerkinElmer Sample Oxidizer, Manual

Reorder No. 1694057. Printed in U.S.A., Publication No. 1694057 Rev. L, PerkinElmer,

Inc., 2003.

PerkinElmer Scintillation Cocktails & Consumables. 2007. http://www.perkinelmer.com/lab-

solutions/resources/docs/BRO_ScintillationCocktailsAndConsumables.pdf. Accessed

July, 2017.

Priya, S., Murali, M.S., Mary, G., Radhakrishnan, K., Gopalakrishnan, R.K., Goswami, A.

2013. “Validation of chemical separation method for the determination of 63Ni using TDCR

technique in steel samples of APSARA reactor.” Journal of Radioanalytical and Nuclear

Chemistry 298:1551-1557.

Pujol, L., Sánchez-Cabeza, J.A. 1997. “Role of quenching on alpha/beta separation in liquid

scintillation counting for several high capacity cocktails.” Analyst 122:383-385.

Pujol, Ll., Sanchez–Cabeza, J.A. 1999. “Optimisation of liquid scintillation counting

conditions for rapid tritium determination in aqueous samples.” Journal of Radioanalytical

and Nuclear Chemistry 242 (2):391–398.

Rank, D. 1993. Environmental tritium in hydrology: present state. In: Liquid Scintillation

Spectrometry 1992. Noakes, J.E., Schonhofer, F., Polach, H.A., (Eds.). Radiocarbon,

University of Arizona, Tucson, p. 327–334.

Rank, D., Alder, L., Froehlich, K., Rozanski, K., Stichler, W. 1998. “Hydrological parameters

and climatic signals derived from long–term tritium and stable isotope time series of the


River Danube.” In: Isotope Techniques in the Study of Environmental Change. IAEA,

Vienna, 1997, p. 191–205.

Ravikumar, P., Somashekar, R.K. 2011a. “Environmental Tritium (3H) and hydrochemical

investigations to evaluate groundwater in Varahi and Markandeya river basins, Karnataka,

India.” Journal of Environmental Radioactivity 102:153-162.

Ravikumar, P., Somashekar, R.K. 2011b. “Natural 3H radioactivity analysis in groundwater

and estimation of committed effective dose due to groundwater ingestion in Varahi and

Markandeya river basins, Karnataka State, India.” Journal of Radioanalytical and Nuclear

Chemistry 288:271–278.

Renne, D.S., Sandusky, W.F., Dana, M.T. 1975. “An Analysis of Tritium Releases to the

Atmosphere by a CTR.” Report No. BNWL–SA–5481, 3rd ERDA Environmental

Protection Conference in Chicago, Illinois, 1975, Battelle Pacific Northwest Laboratories,

Richland, Washington.

Rozanski K., Gröning M. 2004. “Tritium assay in water samples using electrolytic enrichment

and liquid scintillation spectrometry.” In: Quantifying uncertainty in nuclear analytical

measurements, IAEA-TECDOC-1401; 2004. p. 195-217.

Schonhofer, F. 1994. “Low–level measurements of radioactivity in the environment techniques

and applications.” In: Garcia–Leon, M., Garcia–Tenorio, R., (Eds.), World Scientific,

Singapore, p. 155.

Stojković, I., Tenjović, B., Nikolov, J., Todorović, N. 2015. “Radionuclide, scintillation

cocktail and chemical/color quench influence on discriminator setting in gross alpha/beta

measurements by LSC.” Journal of Environmental Radioactivity 144:41–46.

Straume, T. 1993. “Tritium risk assessment.” Health Physics 65:673-682.

Thomson, J. 2002. “Quench and quench curves.” p. 65-73, In: LSC 2001, Advances in Liquid

Scintillation Spectrometry. Möbius, S., Noakes, J.E., Shönhofer F., (Eds.), Radiocarbon,

The University of Arizona, Tucson, p. 456.

Todorović, N., Nikolov, J., Tenjović, B., Bikit, I., Vesković, M. 2012. “Establishment of a

method for measurement of gross alpha/beta activities in water from Vojvodina region.”

Radiation Measurements 47:1053–1059.

Varlam, C., Faurescu, I., Vagner, I., Faurescu, D., Duliu, O.G. 2015. “Nitromethane and other

quenching agents used to determine the tritium activity concentration by liquid scintillation

spectroscopy.” Journal of Radioanalytical and Nuclear Chemistry 303:789–795.

Varlam, C., Ionita, G., Stefanescu, I., Steflea, D. 2001. “Comparative study between external

standard method and internal standard method for low–level tritium measurements.”

Proceedings of the International Conference Nuclear Energy in Central Europe, Portorož,

Slovenia 2001.

Varlam, C., Stefanescu, I., Duliu, O.G., Faurescu, I., Popescu, I. 2009. “Applying direct liquid

scintillation counting to low level tritium measurement.” Applied Radiation and Isotopes

67:812–816.

Varlam, C., Stefanescu, I., Faurescu, I., Vagner, I., Faurescu, D., Bogdan, D. 2011.

“Establishing routine procedure for environmental tritium concentration at ICIT.”

Romanian Journal of Physics 56 (1–2):233–239.

Vartti, Vesa-Pekka. 2009. “Optimizing the counting conditions for carbon-14 for the sample

oxidizer-liquid scintillation counter method.” In: LSC 2008, Advances in Liquid

Scintillation Spectrometry. Eikenberg, J., Jäggi, M., Beer, H., Baehrle, H., (Eds.),

Radiocarbon, University of Arizona, Tucson, p. 293–298.


35

Verhagen, B.Th., Sellschop, J.P.F. 1965. “Enrichment of Low-Level Tritium by Thermal

Diffusion for Hydrological Applications.” In: Proceedings of the Third International

Conference on the Peaceful Uses of Atomic Energy, Geneva, 1964, Vol. 12, p. 398-405,

United Nations, New York, 1965.

Verrezen, F., Loots, H., Hurtgen, C. 2008. “A performance comparison of nine selected liquid

scintillation cocktails.” Applied Radiation and Isotopes 66:1038–1042.

Villa, M., Manjon, G. 2004. “Low–level measurements of tritium in water.” Applied Radiation

and Isotopes 61:319–323.

Villa, M., Manjon, G., Garcıa-Leon, M. 2003. “Study of colour quenching effects in the

calibration of liquid scintillation counters: the case of 210Pb.” Nuclear Instruments and

Methods A 496:413–424.

Wanke, C., Kossert, K., Nahle, O.J. 2012. “Investigations on TDCR measurements with the

HIDEX 300 SL using a free parameter model.” Applied Radiation and Isotopes 70:2176–

2183.

World Health Organization WHO. 2011. Guidelines for Drinking-water Quality, 4th edition.

World Health Organization, WHO Press, Genève, Switzerland, ISBN: 978 92 4 154815 1.

Yankovich, T.L., Kim, S.B., Baumgartner, F., Galeriu, D., Melintescu, A., Miyamoto, K.,

Saito, M., Siclet, F., Davis, P. 2011. “Measured and modeled tritium concentrations in

freshwater Barnes mussels (Elliptio complanata) exposed to an abrupt increase in ambient

tritium levels.” Journal of Environmental Radioactivity 102:26–34.

Zel'venskii, Y.D., Nikolaev, D.A., Tatarinskii, V.S., Sbalygin, V.A. 1965. “Concentration of

Water Samples for the Determination of the Tritium Content.” Soviet Atomic Energy

18(4):466-472.