detector of ionization chamber-type in pulse mode … · in theory, the ionization chamber-type...

DETECTOR OF IONIZATION CHAMBER-TYPE IN PULSE MODE

FOR MEASUREMENTS OF RADON CONCENTRATION IN AIR

MARIAN ROMEO CALINa, MIHAELA ANTONINA CALINb, ILEANA RADULESCUa

a Horia Hulubei National Institute for Physics and Nuclear Engineering (IFIN HH),

Department of Life and Environmental Physics, 30 Reactorului Str., PO. BOX MG-6, 077125,

Bucharest – Magurele, Romania b National Institute of Research and Development for Optoelectronics INOE 2000,

409 Atomistilor Str., PO BOX MG-5, Magurele, Ilfov, 077125, Romania

Received July 5, 2016

This article deals with the development of a ionization chamber-type in pulse

mode capable of performing measurements of the radon from the natural background.

Its construction overcomes the problem of slow pulses deriving from long collection

times of the ions using a special electrode structure along with pulse-shaping

electronics that are optimized for high energy resolution and count rate. The system is

composed of: one ionization chamber, a high voltage source, an integrating

preamplifier, a data acquisition system and a computer. The paper also presents a

novel method for radon concentration measurements that is based on the alpha

counting with the ionization chamber and on a comparison of the measurements of

radon concentrations with those obtained with a specialized system Pylon AB 5.

Key words: Ionization chamber, Radon, Alpha detector, Alpha radiation, Radon

measurements, Equilibrium concentrations.

1. INTRODUCTION

Radon concentration in indoor air is known to be an increasing health

problem. Therefore, the measurement of air radioactivity has become vital, given

the extent of the social impact involved.

Statistically speaking, more than half of the annual effective dose received

by an individual from natural radioactive sources derives from breathing air that is

contaminated with the following series of noble gases: radon 222

Rn, thoron 220

Rn

and their decay products [1].

Rom. Journ. Phys., Vol. 61, Nos. 9–10, P. 1591–1603, Bucharest, 2016

1592 Marian Romeo Calin, Mihaela Antonina Calin, Ileana Radulescu 2

Following the classical physical principles a large set of instruments for

concentration measuring in the range 10–105 Bq×m

-3 have been created. Along the

typical practices for long-term integrating measurements we notice the exposure of

plastic film to the alpha decay of radon or its daughters, the discharge of an

electrically charged insulator or the adsorption of the radon gas on activated

charcoal and subsequent measurement of the gamma radiation from the decay of

the radon daughters with TLD or scintillation methods [2]. An ionization chamber that is filled with air represents an instrument that

combines a representative counting volume with spectral resolution. It is known

that a 5 MeV alpha particle generates approximatively 105 electron-ion pairs when

in a gas with atmospheric pressure which generates an electric pulse that has a

limited amplitude variation in the cases when the charge is completely collected.

This occurs for ionization chambers that are filled with rare gases such as: Ar, Kr

and Xe, that have electron affinity, and move to fast pulses that are easily managed

electronically [2].

Radioactivity can be the source or catalyzer of phenomena that are difficult

to handle on the long term, such as cancer and chromosomal variations that are

balanced only after many generations. It is known that little radiation doses impact

mostly the DNA molecules that are 106 times larger than other molecules in the

body. The specific literature has also observed that the incidence of malignant

diseases grows with the assimilated dose [3–6]. Given these aspects, we brought

forth and produced suitable instruments for the measurement of radon

concentration and for the contribution of alpha radioactive sources, both directly by

measuring radon concentration and indirectly, by measuring surface contamination

in order to determine air contamination [7].

In previous long-range alpha ray detectors, the intensity of the radian source

was high enough to determine a considerable a large number of overlapping pulses.

Along with instrumental noise and cosmic radiation, they led to a continuous

current that indicated the presence of alpha radiation, but didn’t allow the counting

of individual alpha events. In contrast, the proposed detector can notice even low-

intensity alpha sources, and therefore can be used to observe ionic pulses generated

by individual alpha disintegrations. This fact makes it an excellent alternative for

the detection of either radon or any other low-intensity source of alpha radiation. In

addition to this, unlike other conventional radon detectors, it exhibits real-time

measurement results. By ionizing one air molecule, an alpha particle looses

approximately 35 eV; as a result, a typical 5.5 MeV alpha diffusion of 222

Rn

produces over 157 × 103 atmospheric ions [6, 7]. An electron resulting from

ionization migrates rapidly to another air molecule, generating a fresh pair of ions.

The ion pairs can then be transported to an electrode where they are collected and

measured electronically (preamplifier, pulse counting circuit, etc.) [8]. Air

3 Detector of ionization chamber-type in pulse mode 1593

radioactivity is given mainly by the presence of radon, the average concentration of

which stands at 0.74 Bq/l in the underground air and 0.011 Bq/l above the ground

with a production rate of 0.0155 Bq 222

Rn/m2 ×sec [9, 10].

Therefore, we designed and constructed a detector of ionization chamber-

type in pulse mode capable of measuring both the radon concentration and the

contribution of alpha radioactive sources, by focusing directly on the measurement

of radon concentration.

2. MATERIALS AND METHODS

This article discusses a measuring system that incorporates a data acquisition

system and a dedicated calculation program, and is able to investigate the time and

amplitude distribution of the pulses generated by alpha radiation in the interior an

ionization chamber-type detector. In addition to this, we also argue on the

effectiveness of this approach.

In theory, the ionization chamber-type detector is a radon-detecting device

that is a part of the family of the long-range alpha detectors family. The logic on

which the ionization chamber operates is that despite a limited path in the

atmosphere, the alpha particles manage to ionize different types of air molecules.

These elements called atmospheric ions live long enough to be transported by the

large surrounding airflows. As a consequence, they can be observed at higher

lengths than the penetration lengths of primary alpha particles, thus ensuring a

reliable detection of radon through the electrostatic detection of individual pulses

arising from radon disintegration at a low level. By introducing special design

characteristics for the electrode system and electronics, the instrument covers a

range of radon concentrations ranging from about 0.4 to 4×104 Bq×m

-3 (aprox. 10

–2

to 103 pCi/l). The construction of the ionization chamber follows a few basic

physical considerations. The basic requirement on the geometrical size of the

chamber is that a large portion of the 5.5 MeV alpha particles from 222

Rn and the

6.0 MeV alpha particles from 218

Po should be completely stopped within the active

volume of the detector, i.e. the volume in which there is an appreciable electric

field [3, 4].

The detection of the pulses that alpha particles generate inside the ionization

chamber offers an opportunity for the measurement of the activity of the gases,

including atmospheric radon, and of the alpha sources inside the chamber. For an

internal radioactive gaseous source, the ionization chamber can theoretically

register the entire number of alpha particles that are emitted. Moreover, the

dimensions of the ionization chamber and the detection threshold influence the

detection efficiency given the fact that the alpha particles emitted by radionuclides

close to the chamber wall (or the collecting electrode) may not be kept entirely

within the working volume of the chamber and the amplitude of their pulses may


range below the detection threshold. Assuming an ionization chamber with a

diameter equal or larger than the alpha particle path, we will have relatively few

low-amplitude pulses, as the amount of alpha particles normal to the chamber wall

is very low even from radionuclides lying close to this wall.

In order to construct the ionization chamber-type detector capable of

measuring atmospheric radon, we had to overcome a series of aspects: Design and

construction of a detector that can deliver real-time results; Development of a

linkage procedure between the detector and specific electronic devices;

Construction of adequate electrodes capable of collecting the charge generated by

an alpha particle (several types of electrodes of different shapes and geometries

were developed and tested); Developing a system that can function in a wet

atmosphere; Improving its self-sufficient operation; Developing a technology for

the production of low-cost detectors that have reproducible characteristics; The

minimization of noise with the help of several devices (electronic, electric,

software and hardware devices etc.).

Fig. 1 – Pulsed-mode ionization chamber-type detector.

1 – exterior electrode; 2 – voltage electrode; 3 – collecting electrode; 4, 5 – PTFE insulators;

6 – coupling; 7 – inlet taps; 8 – filter-grating system; 9, 10 – clasping flanges.

In order to satisfy the above mentioned aspects, we constructed an ionization

chamber out of high quality stainless steel, developed for quantifying atmospheric

radon and the activity of alpha radiation sources. The 3-liter chamber was equipped

with both inlet taps and a special filter-grating system at the upper side. The

diagram of the detector is shown in Figure 1. In the first stages we conducted

preliminary measures in order to check the pulsed-mode functioning of the


ionization chamber. In this manner we obtained a measuring system that includes

the detector and the accompanying electronics including a computer that deals with

the acquisition and the processing of experimental data. Figure 1 shows ionization

chamber, a 3-liter device made like of stainless steel and Teflon (PTFE) insulators.

The chamber was equipped with inlet taps and a special filter-grating system at the

upper side. Detector has the following components: 1 – exterior electrode,

2 – voltage electrode, 3 – collecting electrode, 4–5 – PTFE insulators, 6 – coupling,

7 – inlet taps, 8 – filter-grating system, 9–10 – clasping flanges.

The electronic elements (subsystems) that accompany the pulsed-mode

detector are the following: preamplifier-integrator subsystem, zero-line adjustment

device, signal-filtering device, data-acquisition subsystem, mathematical

evaluation subsystem, low-voltage supply and high-voltage supply. The

preamplifier-integrator subsystem provides a voltage pulse that is proportional to

the electric charge deriving from the ionization that alpha particles generate in the

chamber. The design of this subsystem allows it to ensure minimal leakage current

at the inlet and minimal capacity between inlet and outlet. This two specifications

are related to the low ionization current that must be high enough to outstrip the

leakage current at the inlet and must have a low capacity to ensure a voltage pulse

that will be adequate for further processing. The preamplifier-integrator subsystem

was constructed using an ICH 8500A integrated circuit with a 10-15

A offset current

and an inlet-outlet noise capacity of 0.1 – 0.2 pF. To ensure constant integration, a

reaction circuit with 5×1011

Ω resistance was created in parallel with the noise

capacity between inlet and outlet. The zero-line adjustment device is built with the

purpose to offset the combined effects, of the leakage current at the preamplifier-

integrator inlet and of the current arising from the conductance of the insulators on

which the chamber electrodes were mounted. The device was symmetrically

supplied with +12V and –12V voltages, which also supplied the preamplifier-

integrator subsystem exiting through a potentiometer. The adjustable voltage and

preamplifier-integrator outlet signal along with the two supplying voltages and the

joint electrical mass were transmitted through a multiplex cable connecting the

preamplifier-integrator and the zero-line adjustment device. Given the fact that the

working signals are characterized by very low amplitudes on high impedances and

especially sensitive to the disturbances arising from voltage supply variations and

environmental noise, so the choice of an adequate filtering device depended on the

frequency range of the disturbing signals. The best solution which derived from

testing at different values of the circuit elements consisted in using a low-pass RC

filter with 106 kΩ resistance and a 1 µF shunt condenser at the exit of the ionization

chamber high-voltage supply; a second RC filter with 100 kΩ resistance and a

0.5 μF shunt condenser was used at the entrance to the data-acquisition system. The

data-acquisition system and the computer have to conduct a series of tasks such as:

amplifying the input signal before processing, ensuring the digital-analog

conversion of the input signal, ordering the input signals by time channels, starting


and stopping measurements after a certain number of channels. For this purpose,

we used a KEITHLEY acquisition plate which realizes digital-analog conversions

ensuring a 2 to 40,000 Hz and we employed a computer for the measurements and

processing of experimental data. Therefore, our measuring system includes a

specialized interface, a detection system, an atmospheric air drainage system, etc.

We designed a typical routine for signal processing from the ionization chamber-

type detector. This makes possible to specify the sampling frequency and

measuring time interval (by setting the number of sampled channels), exhibit the

resulting time spectrum and determine the number of pulses with an amplitude that

exceeds a preset threshold. In this way, the detection system is able to return the

count rate of the disintegrations that leads to alpha particle emission within a

specified energy range.

3. RESULTS AND DISCUSSION

Figure 2 shows the diagram of the proposed experimental setup. The analog

signal (proportional to the ionization current) emitted by the preamplifier-integrator

subsystem, was sampled at a certain frequency and turned into a digital signal by

the data acquisition system DAS-801. The sampled values are processed by a PC

and displayed as the time dependence of the ionization current. An ICH-8500A

chip and a parallel RC reaction circuit with 5×1011

Ω resistance and a 0.1 – 0.2 pF

capacity equal to the noise capacity between entrance and exit of chip, was used as

a preamplifier-integrator system. A special routine was designed for processing the

signals from the ionization chamber-type detector. It made it possible to set the

sampling frequency and measuring time interval (by setting the number of sampled

channels), display the resulting time spectrum and determine the number of pulses

the amplitude of which exceeded a preset threshold.

The first step in the determination of the the efficiency of the alpha particle

detection was the computation of the alpha particle spectrum for our specific

conditions. For a monoenergetic alpha transition such as the alpha disintegration of 222

Rn, the alpha spectrum will contain, besides the maximum energy transition line,

a continuous distribution of pulses from the alpha particles that have not been

stopped within the working volume of the chamber.

The computations have been conducted with the help of a Monte Carlo

routine for a cylindrical ionization chamber with 5 inch external diameter and a

thin wire collector; the chamber radius was approximately equal to the alpha

particle path in air at atmospheric pressure. Figure 3 exhibits the resulting

spectrum, in which the abscissa describes the energies in terms of maximum

energy of alpha transition from 222

Rn.


Fig. 2 – Block-diagram of the ionization chamber and associated equipment.

We notice the fact that the spectrum seems to concentrate towards maximum

alpha energy. Curve 1 of Fig. 4 depicts the efficiency of alpha particle detection in

terms of the detection threshold. For the case of detection thresholds that are equal

to 10–15 % of the maximum energy of 222

Rn alpha particle the obtained detection

efficiency is approximately or even higher if we take into account the main

progenies 218

Po and 214

Po. If the external electrode has a negative polarization, after

the alpha disintegration of 222

Rn and 218

Po, the progenies 218

Po and 214

Po generate

in balanced conditions positive ions, which are collected by the external electrode

[11, 12, 13].

Fig. 3 – Alpha particle distribution in terms of energy.

Given the fact that the collecting time is substantially lower than the time span

from collection to measuring, the alpha disintegrations of the radon progenies take

place on the chamber wall. Consequently, the alpha particles emitted by 218

Po and 214

Po have maximum energy and are detected inside the chamber with 100%


effectiveness. Curve 2 of Fig. 4 represents the effectiveness of alpha particle

detection for radon and its main progenies. In this case, the detection effectiveness

grows to 95% for thresholds around 15% of the maximum energy of alpha particles

emitted by radon [14–17]. We observe the fact that the maxim possible detection

threshold for a certain effectiveness increases with the chamber diameter due to an

increased percentage of alpha particles of maximum energy. In the case of alpha

particles deriving from a radioactive source, the detection effectiveness is virtually

as high as 100 % if the source is placed at least 4 to 5 cm away from the wall of the

ionization chamber.

Fig. 4 – Calculated effectiveness of alpha particle detection for ionization chamber.

Figure 5 shows a typical alpha spectrum obtained in the proposed setup by

radon and its progenies. We notice very few low-amplitude pulses. According to

experimental statistics based on about 1,500 pulses, only as few as 5% of the pulses

were lower in amplitude than the 25 nA (about 1 MeV) detection threshold

corresponding to a 95% effectiveness of alpha particle detection, which is in good

accordance with our earlier estimates.

It has been observed that, given a relatively mild variation of total detection

effectiveness with the detection threshold for low values of the latter, the detection

effectiveness does not decrease below 95% up to a detection threshold value of 0.3

(1.5 MeV). The measurement of the count rate of the alpha particles generated by

radon and its progenies can be associated to the volume activity of radon. For this

purpose, the subsequent set of elements need to be determined: The detection

efficiency for alpha particles generated by different radionuclides in the radon

disintegration chain; The time variation of the activities of these radionuclides; and

the relative weights of alpha transitions of different energies, in the disintegration

chains of these radionuclides.

We will consider these three aspects for the case of the 222

Rn isotope, which

is of fundamental theoretical and practical relevance for radioprotection. As


presented above the detection efficiency for the 222

Rn mother radionuclides is

different from that applying to its progenies, 218

Po and 214

Po, depending on the ion

dynamic of 218

Po and 214

Po in the ionization chamber. According to the results

found within approximately one hour of the 222

Rn uptake, a balance is reached, in

which the 218

Po and 214

Po ions arrive on the ionization chamber wall (the negative

electrode). As a result, 50% of the alpha particles emitted by the main progenies

are detected while the alpha particles emitted by 222

Rn within the entire volume of

the ionization chamber are detected with 90–100% effectiveness (Curve 1 in

Fig. 4).

We use the chain disintegration equation to compute the time variations of

the activities of all radionuclides descended from 222

Rn. We notice the fact that the

activity corresponding to the alpha count rate differs by less than 1% from the ideal

activity that corresponds to the situation in which 3 hours after intake all of the

three radionuclides considered would have the same halftimes as radon (after

2 hours the difference does not exceed 4%); the activities of the other progenies are

negligible even 250 hours after intake. Thus, measuring the count rate 2–3 hours

after radon intake satisfies the conditions both for radioactive balance and for the

ion dynamics of the radon progenies.

Fig. 5 – Experimental time spectrum of the ionization current.

With respect to the relative weights of the alpha transitions of different

energies in the disintegration chains of radon and its progenies, we notice the fact

that the alpha disintegrations of the three main radionuclides takes place mostly in

the basic states of the daughter nuclei (in the case of 222

Rn the most intense satellite

alpha line does not exceed the intensity of the main alpha line).

The detection dead time of the device is calculated from the formula:

n` = n/(1 + n ). Its values are of the order of = (0.223 0.023). The slope

0

0.05

0.1

0.15

0.2

0.25

1 94 187 280 373 466 559 652 745 838 931

I (p

A)

Channels (80 ms/channel)

Ionization current time spectrum (Rn Concentration=50 Bq/m3)


shown in Figure 6 indicates the comparable detection times for the three types of

detectors.

Fig. 6 – Measuring dead time of the ionization chamber-type detector.

3.1. COMPARISON OF MEASUREMENTS

The validation of these results was obtained by comparing them with other

radon measurements conducted with the Pylon AB-5 system. Table 1 shows the

results that derive from the comparison of radon concentration determined with the

ionization chamber and with the Pylon AB-5 system.

We notice the fact that the values measured in these two setups are very

similar and that the differences are smaller than the sum of their uncertainties

(Table 1) [18–20].

We acknowledge the fact that normal climatic conditions for measurements

such as temperature, pressure and humidity influence the transportation and

distribution of radon both in atmosphere and in the detector. The data acquired was

transferred to a computer with the help of a specialized software (Transfer Utility

1.1 (DTU)). The data have been refined with the MS Excel software and the

errors/deviations have been computed: S(n – 1) – experimental standard deviation,

S(n – 1) (%) – relative experimental standard deviation, S(med) – experimental

standard deviation of the mean, S(med) (%) – relative experimental standard

deviation; S (Poisson) – Poisson relative error, uncertainty, ε [%] = (S1/S2 –1) × 100 –

relative error and δ = S1 – S2 – difference measuring with two systems (must be less

than the amount uncertainty with two measuring systems, etc. (Table 1).

S (Poisson) is given for comparison with the value of the standard deviation on the

average value, for the purpose of statistical assessment of the average values versus

the total number taken [18–20].

Ta

ble

1

Th

e re

sult

s th

at d

eriv

e fr

om

th

e co

mp

aris

on

of

rad

on

con

cen

trat

ion d

eter

min

ed w

ith

th

e io

niz

atio

n c

ham

ber

and

wit

h t

he

Pylo

n A

B-5

syst

em

Mea

surin

g

Ion

iza

tio

n C

ha

mb

er i

n p

uls

e m

od

e (

S1

) P

YL

ON

AB

5 S

yst

em

(S

2)

Rel

ati

ve

Err

or

ε =

S1/S

2-1

ε [%

]

Del

ta

δ =

S1-S

2

[Bq

/m3]

Avera

ge

Rad

on

con

cen

trati

on

[Bq

/m3]

S(n

-1)

[Bq

/m3]

S(n

-1)

[%]

S(m

ed)

[Bq

/m3]

S(m

ed)

[%]

S(P

ois

son

)

[%]

Un

cert

ain

ty

[Bq

/m3]

Avera

ge

Rad

on

con

cen

trati

on

[Bq

/m3]

S(n

-1)

[Bq

/m3]

S(n

-1)

[%]

S(m

ed)

[Bq

/m3]

S(m

ed)

[%]

S(P

ois

son

)

[%]

Un

cert

ain

ty

[Bq

/m3]

M 1

1

4.8

0

12.2

8

15

1.4

5

1.5

6

1.3

4

0.5

4

13.2

0

12.3

5

13

2.1

2

2.2

3

1.0

1

0.6

7

0.1

2

1.6

0

M 2

1

0.8

0

10.4

5

10.6

5

1.6

7

1.3

7

1.5

6

1.2

4

10.3

0

8.7

8

12.4

5

1.1

7

2.4

5

1.3

5

0.9

7

0.0

4

0.5

0

M 3

1

3.2

0

7.3

4

16.6

8

3.2

3

6.2

3

3.2

3

1.5

6

12.8

0

9.6

7

12.4

7

3.3

6

4.6

7

3.2

3

1.4

7

0.0

3

0.4

0

M 4

1

6.2

0

6.6

7

10.2

3

3.3

4

3.2

3

4.1

2

1.6

7

17.5

0

9.0

5

10.0

3

3.2

5

3.4

8

4.5

6

1.3

4

0.0

7

1.3

0

M 5

2

4.1

0

7.6

7

9.2

3

3.6

7

3.2

4

3.7

8

2.1

2

25.2

0

11.2

4

12.2

5

2.4

6

2.5

7

4.1

2

1.7

8

0.0

4

1.1

0

M 6

3

0.5

0

4.4

6

7.7

8

3.3

4

4.1

2

5.4

3

2.1

6

29.9

0

10.3

2

10.1

4

1.5

6

4.6

7

6.3

2

2.1

2

0.0

2

0.6

0


4. CONCLUSIONS

Given the arguments presented above we can conclude that the measurement

of the count rate of the alpha particles emitted by radon and its progenies enables

the possibility of radon volume activity determination. Starting from the volume

activity of radon and using its chain disintegration scheme we can determine the

activity for all radon progenies.

The alpha sources found in the interior of the pulsed-mode ionization

chamber are captured with virtually 100 % effectiveness. Under this specification

and bearing in mind the disintegration scheme, the activities can be observed with a

high accuracy. By intercomparison experimental results we obtained very good

value method for measuring radon in atmospheric air.

Acknowledgements. This study was supported by the PNCDI II Program, Project No. PN 09

37 03 01/2015 by the Romanian Ministry for Education and Research.

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