ARTICLE IN PRESS

Applied Radiation and Isotopes 68 (2010) 1143–1149

Contents lists available at ScienceDirect

Applied Radiation and Isotopes

0969-80

doi:10.1

� Corr

Science

E-m

journal homepage: www.elsevier.com/locate/apradiso

Applicability study of using in-situ gamma-ray spectrometry technique for137Cs and 210Pbex inventories measurement in grassland environments

Junjie Li a,b,c,�, Yong Li c, Yanglin Wang a,b, Jiansheng Wu a,b

a The Key Laboratory for Environmental and Urban Sciences, Shenzhen Graduate School, Peking University, Shenzhen 518055, Chinab College of Urban and Environment Sciences, Peking University, Beijing 100871, Chinac Institute of Environment and Sustainable Development in Agriculture, The Chinese Academy of Agricultural Sciences, Beijing 100081, China

a r t i c l e i n f o

Article history:

Received 28 October 2008

Received in revised form

22 October 2009

Accepted 16 January 2010

Keywords:

Gamma-ray spectrometry

In-situ measurement137Cs and 210Pbex

Inner Mongolia

Grassland

43/$ - see front matter & 2010 Elsevier Ltd. A

016/j.apradiso.2010.01.030

esponding author at: The Key Laboratory fo

s, Shenzhen Graduate School, Peking Universi

ail address: menghu981@hotmail.com (J. Li).

a b s t r a c t

In-situ measurement of fallout radionuclides 137Cs and 210Pbex has the potential to assess soil erosion

and sedimentation rapidly. In this study, inventories of 137Cs and 210Pbex in the soil of Inner Mongolia

grassland were measured using an In-situ Object Counting System (ISOCS). The results from the field

study indicate that in-situ gamma-ray spectrometry has the following advantages over traditional

laboratory measurements: no extra time is required for sample collection, no reference inventories are

required, more economic, prompt availability of the results, the ability to average radionuclide

inventory over a large area, and high precision.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Fallout 137Cs and natural 210Pb are radionuclides that havebeen used to provide independent measurement of soil erosionand deposition rates and patterns. 137Cs is a man-made radio-nuclide associated with the nuclear weapon tests during the1950s and 1960s. Global fallout of 137Cs began in 1954, peakedbetween 1963 and 1964 and has decreased since this maximum;since the mid-1980s 137Cs fallout has often been below detectionlevels. Accidental events, such as the Chernobyl accident in April1986, can cause regional dispersal of measurable 137Cs that affectsthe total global deposition budget. This yearly pattern of falloutcan be used to develop a chronology of deposition horizons inlakes, reservoirs and flood plains. 137Cs is easily measured usinggamma-ray spectrometry (Ritchie and McHenry, 1973; Wallingand Quine, 1993). 210Pb is a naturally occurring radionuclide fromthe 238U decay series. It is derived from the decay of gaseous222Rn. Some 222Rn in the soil diffuses into the atmosphere anddecays to 210Pb and subsequent fallout of 210Pb provides an inputthat is not in equilibrium (excess) with its parent 226Ra (Robbins,1978). By measuring 210Pb and 226Ra in the soil, excess 210Pb canbe calculated and used to measure soil movement and to date

ll rights reserved.

r Environmental and Urban

ty, Shenzhen 518055, China.

sediment profiles. Gamma-ray spectrometry can be used tomeasure 210Pb and 226Ra (Joshi, 1987). The fallout radionuclideslaboratory measurement (LM) technique is used to estimate soilredistribution patterns for the last few decades.

Over the last decade, a number of studies on soil erosion andsedimentation have been conducted through laboratory measure-ments of fallout radionuclides, i.e. the LM analytical technique.The principle of LM is based on a comparison of measuredinventories (total activity per unit area) at individual samplingpoints with an equivalent estimate of the inventory representingthe cumulative atmospheric fallout input at the site, taking dueaccount of the different behavior of cultivated and no cultivatedsoils. Because long-term measurements of atmospheric fallout arerarely available, the cumulative input or reference inventory isusually established by sampling adjacent, stable sites, whereneither erosion nor deposition has occurred. Where sampleinventories of Cs-labelled soil are lower than the local referenceinventories, erosion may be inferred. Similarly, sample inven-tories in excess of the reference level are indicative of the additionof Cs-labelled soil by deposition. The magnitude and direction ofthe measured deviations from the local reference level provide aqualitative assessment of soil redistribution (Walling and Quine,1993; Walling and He, 1999). Laboratory techniques for measur-ing 137Cs involve the collection of soil or sediment cores from astudy site and their subsequent transfer to the laboratory forpreparation and analysis by gamma spectrometry. Such proce-dures are time consuming and there may be considerable delaybefore the results are made available. It is therefore difficult to

ARTICLE IN PRESS

J. Li et al. / Applied Radiation and Isotopes 68 (2010) 1143–11491144

obtain preliminary results, which could be used to guide thedevelopment of an ongoing sampling programme.

The selection of a reference site is central to the successfulexecution of a cesium-based erosion study. The reference site isused to establish the 137Cs inventory in the study region againstwhich the changes in inventory, both in disturbed sites anddepositional environments, can be assessed. The ideal referencesite will have experienced neither soil loss nor sediment deposi-tion, it will have been under continuous vegetation cover since thedeposition of 137Cs began in the early 1950s. The protected areassuch as parks, ceremonial areas, and burial grounds are commonlyused for reference sites. Reference sites should be as close aspossible to the disturbed sites that are to be sampled.

Laboratory measurement procedures for 137Cs measurement insoil erosion and sedimentation investigations involve the collectionof soil or sediment cores from a study site and their subsequenttransfer to the laboratory for preparation and analysis of 137Csactivity by gamma spectrometry. In cases where large numbers ofcores are collected and require analysis, the processing andlaboratory measurement will involve substantial effort. Further-more, an extended period of time will generally be required for themeasurements, because long count times are required for environ-mental samples containing relatively low levels of 137Cs activity.Appreciable delays in obtaining results may therefore arise and it isnot generally possible to obtain immediate data.

The time-integrated nature of the estimates of soil redistribu-tion and sediment-deposition rates obtained using the LM 137Cstechnique provides an average value for a period of ca. 45 years,which may be viewed as a limitation in cases where informationfor a shorter period is required. Another limitation is that in someenvironments where 137Cs inventories are small, the variability ofreference inventories can be large (Wallbrink and Murray, 1994).The net effect of this is to limit the precision of subsequentmanipulations between any reference and field measurements,since the precision of any estimated soil losses or gains is definedby the uncertainty surrounding the initial input reference terms. Itis probable that distribution of fallout 210Pb is similarly affected bythe same processes, and so the relationship between the twotracers may yield a more stable mean by which to define areference input than either nuclide in isolation. In view of theselimitations there is clearly a need to explore the potential forenhancing and extending 137Cs measurement technique by usingother methods to obtain 137Cs measurements within a shorterperiod of time and by using 210Pb radionuclide with behaviorsimilar to that of 137Cs. In contrast to the time-dependent fallout of137Cs, atmospheric fallout of 210Pb has been constant over the years(Crickmore et al., 1990; Nozaki et al., 1978).

The potential for using field portable in-situ gamma spectrometerto obtain on-site measurement of 137Cs inventories for soil andsediment is attracting increasing attention (Zombori et al, 1992; Tyleret al., 1996; He and Walling, 2000). In-situ measurement wouldappear to offer a number of significant advantages. These include,firstly, the potential for relatively rapid field measurement whichcould either replace the collection of cores for subsequent laboratorycounting or be used in a reconnaissance mode as a guide for theplanning of a more detailed sampling strategy, secondly, limiteddisturbance of the site under investigation; and thirdly, morerepresentative results by virtue of the greater surface area or massof soil or sediment involved in the measurement. Because of themuch larger sample mass associated with direct field measurement,the times required by in-situ 137Cs measurements will often besignificantly shorter than those required by laboratory detectors.Beside many other advantages, the main advantage of in-situmeasurement technique is that it eliminates the problem of locatingsuitable reference sites which is the main problem in applying LMtechnique, because in many cases it is almost impossible to find a

suitable reference site, which is an essential part of LM techniques.Since the pioneering work of Beck et al. (1972), the use of in-situgamma spectrometry measurement for documenting levels ofenvironmental radioactivity in soils have become well established(Miller and Helfer, 1985; Helfer and Miller, 1988; Zombori et al.,1992; Lettner et al., 1996; Tyler et al., 1996; He and Walling, 2000).Several direct measurements of 137Cs and 7Be have been conductedby using in-situ gamma spectrometry in Russia (He and Walling,2000; Ramzaev et al., 2005; Golosov et al., 2000). An empiricalrelationship between in-situ measurement of 137Cs activity and total137Cs inventories had been established for soils from a cultivated fieldand for flood plain sediments, based on information on the verticaldistribution of 137Cs in the soil and sediment provided by the forwardscattering ratio derived by the field measured spectra. This relation-ship has been used to estimate 137Cs inventories from in-situmeasurements of 137Cs activity at other locations (He and Walling,2000). The results of a survey for Post-Chernobyl 137Cs contaminationand natural radiation (40K, Uranium and thorium series) in Belgiumclearly illustrate that the in-situ technique provides a means for thecomplete characterization of the gamma radiation field at a givenlocation. It allows a fast, accurate and sensitive determination ofradionuclides in the soil. About sixty measurements, equallydistributed across Belgium (30,507 km2) were executed in 1995–1997, with 137Cs values ranging from 0.5 to 6.6 kBq m�2. In-situmeasurement is, however, not without limitations. These include theneed to take account of the field of view of the detector and of thevariability of detector response across this field. More importantly, in-situ measurement of 137Cs activity obtained using field portabledetectors will be influenced by the attenuation of 137Cs gamma raysby the soil or sediment matrix. Information of the depth distributionof 137Cs in soil or sediment profiles is, therefore, an essentialrequirement in deriving a reliable estimate of the total 137Cs inventoryat a measurement point (Beck et al., 1972; Miller and Helfer, 1985).This latter problem is particularly significant in the case of bomb-derived 137Cs. Because of its relatively long half-life and an extendedperiod of atmospheric deposition, it will generally have penetratedrelatively deeply in soil profiles, due to mixing of the soil by tillage,natural download translocation mechanisms, and burial of surfacehorizons by accreting soil or sediment at depositional sites. Thesefactors are likely to be practically significant at locations where soilerosion of flood plain sedimentation is under investigation (Zapata,2002). Despite the widespread use of in-situ measurement of 137Cs, itsuse for 210Pbex has been less widely recognized and exploited. In thisstudy in-situ measurement technique is proved to be valid for 210Pbex

and could be a gateway in this direction, but there is still a need tofurther promote work in this direction for confirmation of the validityof this technique.

Here we conducted a more detailed investigation, comparingthe in-situ and LM technique for 137Cs and 210Pbex in a grassland ofInner Mongolia. In this study, portable gamma spectrometry hasbeen used. Soil samples collected from selected points were alsomeasured in the laboratory. This study was conducted to confirmthe important factors that influence measurement results, to findthe effective measurement area of gamma-ray detectors and toestimate the influence of the profile distribution of radionuclideson the inventory of radionuclides when using in-situ measurement.The paper discusses the applicability of using portable gammaspectrometry to measure 137Cs and 210Pbex in the soil.

2. Materials and methods

2.1. Study site

This study was undertaken at the Ecosystem Research Stationof the Chinese Academy of Sciences (CAS) in the Inner Mongolia

ARTICLE IN PRESS

J. Li et al. / Applied Radiation and Isotopes 68 (2010) 1143–1149 1145

Grassland region (431260 441390N, 1151320 1171120E) that is a partof Xilin River Basin. It is one of the regions in the north of Chinathat is most seriously affected by wind erosion. It belongs to amedium temperate climatic zone. The geography is characteristicof typical low mound-plains. The major soil type in this region iscalcic chestnut.

Since 1980, the site has been fenced off to forbid grazing and isideal for studying the rate of soil loss by wind erosion anddifferent grazing intensities and assessing the impact of grasslandmanagement practices. The north-west wind is the main factorcausing soil loss and degeneration in the area.

The elevation varies from 1260 to 1285 m. The average annualtemperature and precipitation at the study site are 2.30 1C and322 mm, respectively. A windy period between March and May iscaused by high air pressure gradients between the Siberianmidland and East Asia during the spring time. Between Octoberand April the climate is cold and dry, while the growing seasonfrom May until September is short and characterized by heavyrainfall events.

The short vegetation growth period (o150 d) of S. grandis andL. chinensis grass starts at the end of April. Moderate grazing in thestudy site resulted in relatively good grass conditions comparedto the heavily grazed areas in adjacent regions.

For this study, a whole slope was chosen in the experimentstation, according to the lee side and luv side from northwest tosoutheast direction and in-situ 137Cs and 210Pbex measurementswere made at five sites on the northwest–southwest slopeincluding areas at the of bottom, middle, upper, summit and lee.The slope length is about 1000 m. The elevation of in-situmeasurement sites is shown in Fig. 1.

2.2. Measurement procedures

For measurement two different procedures were used simul-taneously to compare the results, in-situ measurement and LM.

2.2.1. In-situ measurement procedure

In-situ Object Counting System (ISOCS) from Canberra whichconsists of a BE5030 HPGe portable detector and a data collection/processing system, was used for in-situ measurement. The dataprocessing system is comprised of a MCA (Inspector 2000), aportable PC and Genie-2000 software. This technique involves thecollection of a gamma spectrum and its subsequent analysis todetermine the activity of 137Cs and 210Pb within the soil. Thewhole system is mounted on a mobile cart. A shielded HPGe

Distance from foot of the hillslope (m)200

Ele

vatio

n (m

)

1270

1275

1280

1285measurement site

400 600 800

Fig. 1. In-situ measurement sites of 137Cs and 210Pbex inventories along NW wind

direction in Inner Mongolia grassland.

detector cooled by liquid nitrogen is fixed at 1 m above the groundlevel. Shielding and collimators were used to reduce gamma rayscoming from the ambience environment. The collimators were setto 301, 901 and 1801 fields of view. The collimator angle was 901and the measurement area in the fields of view was 3.14 m2. Fig. 2is the sketch map of the in-situ detector at the measurementlocation. Some gamma photons coming from the soil wereobstructed by the cart and could not reach the detector. As aresult, counts given by the detector were reduced, whichintroduces some uncertainty into the calculation. Thisobstruction of gamma rays by the cart was taken into accountin the final calculation. For example, the measurement area of 901collimator was 3.14 m2, but after subtracting the area of thedetector that was obstructed by the cart, the real measurementarea of detector was 2.91 m2.

In the in-situ measurement, the energy peak of 137Cs and 210Pbwere 661.6 and 46.54 keV, respectively. The gamma emission rateof 226Ra was relatively low and could not be measured directly.Therefore, the activity of its two daughters, 214Pb and 214Bi wasmeasured and their mean value was considered to be the activityof 226Ra (Nir-El et al., 1999). Thus, the important assumption isthat these two radon daughters are in equilibrium with the parent226Ra. Excess 210Pb was determined by subtracting the concen-tration of 210Pb from that of its parent, 226Ra. The measurementtime was 3600 s and the activity is given in Bq m�2.

2.2.2. Energy calibration of the gamma spectrometer

Energy calibration of the gamma spectrometer was undertakenby using 152Eu standard point source. Eleven energy peaks ofgamma rays were selected: 121.78, 244.69, 344.27, 411.11,443.97, 778.89, 867.32, 964.01, 1085.78, 1112.02 and1407.95 keV. The relation between energy, channel and FWHMis as follows:

EðkeVÞ ¼ 0:569þ0:206C ð1Þ

F ¼ 0:817þ0:060� E1=2 ð2Þ

where E is the energy, C the channel, F the FWHM.

2.2.3. Efficiency calibration of the ISOCS

Efficiency calibration is the main difficulty in radionuclidemeasurement using in-situ gamma spectrometry because thesetwo nuclides penetrate to different depths in uncultivated soils,thereby producing an activity ratio that varies with depth(Wallbrink and Murray, 1996). If baseline data is not available,it is difficult to make sure the profile distribution of the

Detector

45 Air

100cm

Soil

Fig. 2. Diagram of in-situ measurements of fallout radionuclides and sampling

points for laboratory calibration of FRN. Note: � Core sampling points; & Profile

sampling points.

ARTICLE IN PRESS

J. Li et al. / Applied Radiation and Isotopes 68 (2010) 1143–11491146

radionuclide in the soil. Therefore, use of the ISOCS efficiencysoftware to calibrate soil geometry is the main problem. Forefficiency calibration, an assumed depth in the operator interfacecan be used to generate the efficiency calibration curve. Togenerate a new efficiency calibration curve, the depth parametercan be changed and the sample spectrum reanalyzed using thisnew curve in order to test the impact of these assumptions on theresults (Nir-El et al., 1999).

The equation of the efficiency curve is fitted by the efficiencyvalue using a double logarithm polynomial. The calibrationequation is as follows:

ln e¼�48:28þ20:52ln E�2:906ðln EÞ2�1:208

� 10�1ðlnEÞ3þ5:329� 10�2

ðln EÞ4�2:988� 10�3ðln EÞ5 ð3Þ

where E is the energy, e the efficiency.

2.2.4. Laboratory measurement procedure

The laboratory Gamma spectrometry system (Canberra) con-sists of a BE5030 HPGe detector surrounded by a 10-cm-thickbackground 747E top opening lead shield and is coupled to anamplifier and multichannel analyzer (DSA-1000). It can measuregamma rays having energy from 3 keV to 3 MeV. The computersoftware Genie-2000 was applied to analyze the spectrum.

The soil samples for laboratory measurement were alsocollected from those sites where in-situ measurements weremade (Fig. 2). In order to confirm the profile distribution of theradionuclides, 12 samples at incremental depths were collectedfrom the middle site (0–2, 2–4, 4–8, 8–12, 12–16, 16–20, 20–40,40–60, 60–90, 90–120, 120–150 and 150–170 mm) by inserting arectangular iron frame (length 50 cm, width 19 cm) into the soil.The other four sites were sampled by inserting perpendicularly an8 cm diameter steel auger 15 cm in the soil. The samples weretransported to the laboratory for measurement. In the laboratory,all the collected soil samples were dried, disaggregated andpassed through a 2 mm mesh sieve. The sieved samples were putin a 101 mm�25 mm column box made of Plexiglas, sealed andset aside for more than 28 days. The gamma spectrometry wasundertaken using a Canberra HPGe detector; the counting timewas set at 36,000–86,400 s, sufficient to provide a typicalanalytical precision of 74.7%. The four samples from each sitewere measured in the laboratory and their mean value was takenas the inventory for their respective sites.

Table 1Specifications of the gamma spectrometers (1332.5 keV for 60Co).

Characteristics In-situ detector Laboratory detector

High voltage (V) 5000 4500

Resolution/FWHM (keV) 2.060 1.643

Relative efficiency (%) 50.00 50.92

Peak symmetry (FWTM/FWHM) o2.0 o2.0

Table 2137Cs and 210Pb inventory of in-situ measurement using different collimators in the sa

Collimator (deg) 137Cs

cps Total activity7uncertainty (Bq) Relative uncertain

30 0.09 6.36�10377.80�102 12.26

90 0.32 6.31�10374.02�102 6.37

180 0.76 1.11�10675.92�104 5.33

The activity of 137Cs and 210Pbex (Bq kg�1) was measured in thelaboratory. The activities of 137Cs and 210Pbex (Bq m�2) for bulkcores were calculated using

Aa ¼ AMT=S ð4Þ

where Aa is the areal activities of 137Cs and 210Pbex (Bq m�2), MT

the total mass of the bulk core (kg), A the activity of 137Cs and210Pbex of the sub-sample of the bulk core analyzed (Bq kg�1), S

the corer area (m2).The activity of the sub-sample of the bulk core was analyzed

and then converted into areal activities.

2.2.5. Total uncertainty of the measurement

In this section, the methodology for calculating the totaluncertainty of in-situ measurement using gamma-ray spectro-metry is presented. Some uncertainties that affect the accuracy ofin-situ measurement are constant. Others are function of one ortwo parameters. The computation of the total uncertainty of thiscalibrator is made in two parts as follows:

(1)

me loca

ty (%)

First, the random uncertainties for errors that are constantor depend on the same parameters are calculated. Therandom uncertainty is given by

uc ¼ c

�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiur

100

� �2

þus

s

� �2

þuv

v

� �2

þue0

e0� �2

þuy

y

� �2

þuk

k

� �2

sð5Þ

where c is the activity per unit volume (area or mass) ofthe sample, ur the user-defined random uncertainty (%), s

the net peak area, us the uncertainty of the net peak area s, v

the sample quantity, uv the uncertainty of the samplequantity v, e0 the effective efficiency, u0e0 the uncertainty ofthe effective efficiency, y the branching ratio, uy theuncertainty of the branching ratio y, k the composite decaycorrection factor, uk the uncertainty of the composite decaycorrection factor k.

(2)

Second, the total uncertainty of the measurement is the sumof the random uncertainties obtained in item 1 and thesystematic uncertainty. Hence

ucðTÞ ¼ ucþusysc

100ð6Þ

where uc(T) is the total uncertainty of the activity c, usys theused defined systematic uncertainty (%).

2.3. Specifications of the two gamma spectrometers

The specifications of the two gamma spectrometers are shownin Table 1.

tion.

210Pb

cps Total activity7uncertainty (Bq) Relative uncertainty (%)

0.04 4.77�10474.11�104 86.16

0.52 4.20�10471.37�104 32.62

0.73 4.87�10679.07�105 18.62

ARTICLE IN PRESS

Table 3Comparisons between in-situ and laboratory measurements of 137Cs and 210Pbex inventories, calculated following the presupposed profile distribution of the radionuclides

in soil.

Element Site In-situ (Bq m�2) Relative uncertainty (%) Laboratory (Bq m�2) Relative uncertainty (%) Variation (%)

137Cs Site-1 21667138 6.37 28907380 13.15 �33.4

Site-2 20927140 6.69 25447341 13.40 �21.6

Site-3 20167133 6.60 30047416 13.85 �49.0

Site-4 23517158 6.72 27547374 13.58 �17.1

Site-5 21397140 6.55 31287234 7.48 �46.2

210Pbex Site-1 12,26774705 38.36 644373003 46.61 47.5

Site-2 10,75373323 30.91 457872683 58.61 57.4

Site-3 13,44373351 24.93 525372943 56.03 60.9

Site-4 10,78673625 33.61 540872739 50.65 49.9

Site-5 718173359 46.77 574072577 44.90 20.1

Note: The results of the laboratory measurements are the mean of the four sampling sites. The uncertainty of the results is the combined uncertainty of the four sampling

sites (the measurement area is 2.91 m2).

137Cs (Bq kg-1)0

Dep

th (m

m)

90

60

40

20

16

12

8

4

2

20 40 60 80 100 120 140

Fig. 3. The depth distribution of 137Cs in soil.

210Pbex (Bq kg-1)

0

Dep

th (m

m)

170150120

906040201612842

100 200 300 400 500

Fig. 4. The depth distribution of 210Pbex in soil.

J. Li et al. / Applied Radiation and Isotopes 68 (2010) 1143–1149 1147

3. Results

The total activity of 137Cs and 210Pb by in-situ measurementusing different collimators in the same location is shown inTable 2. The measurement area using 301, 901 and 1801collimators are 0.23, 2.91 and 313.77 m2, respectively.

The results shown in Table 2 indicate that by increasing thecollimator angle, the measurement area, and the count rate (cps)of the detector and relative uncertainty increases. The totalactivity of 137Cs and 210Pb using a 1801 collimator was the highestwith the best precision and the 301 collimator was the lowest.

3.1. 137Cs and 210Pbex inventory calculated by the presupposed depth

distribution of the radionuclides in soil

Table 3 shows the comparisons between in-situ and laboratorymeasurements of 137Cs and 210Pbex inventories calculatedfollowing the presupposed profile distribution of theradionuclides in soil. The measurement conditions were thesame in both cases, i.e. the detector was 1 m above the ground,the collimator angle was 901, the measurement time was 3600 sand the assumed parameter of the radionuclide distribution depthin the efficiency calibration was 150 mm.

Table 3 indicates that the in-situ 137Cs inventory(range=2016–2351 Bq m�2, mean value=2153 Bq m�2) is lowerthan that measured in the laboratory (range=2544–3128 Bq m�2,mean value=2864 Bq m�2). The relative uncertainty of in-situmeasurements is evidently higher than laboratory measurements.There are three sites where the variation between in-situ andlaboratory measurements is larger than 30%.

The relative uncertainty of the results for 210Pbex in-situmeasurements is evidently higher than laboratory measurementsat all the selected sites except site-5. The on-site 210Pbex valuesvary from 7181 to 13,443 Bq m�2 (mean=10,886 Bq m�2) whilelaboratory values range from 4578 to 6443 Bq m�2(mean=5484Bq m�2). The variation between the two methods is high and attwo sites it is more than 50%.

3.2. 137Cs and 210Pbex inventory calculated using the actual depth

distribution of the radionuclides in soil

137Cs and 210Pbex inventories of depth incremental samples(collected from site-4) measured in the laboratory are depicted inFigs. 3 and 4.

Table 4 indicates that in-situ 137Cs values are close to theresults measured in the laboratory. The absolute value of variationis quite small. The range of the on-site 137Cs inventory is 2652–3092 Bq m�2 (mean=2832 Bq m�2) which is close to thelaboratory range of 2544–3128 Bq m�2 (mean=2864 Bq m�2). IfTables 3 and 4 are compared, it can be seen that 210Pbex valuescalculated by the actual profile distribution are smaller at all thesites. They range from 5430 to 10,704 Bq m�2.

ARTICLE IN PRESS

Table 4Comparison between in-situ and laboratory measurements of 137Cs and 210Pbex inventories calculated by actual profile distribution of radionuclides in soil.

Element Site In-situ (Bq m�2) Relative uncertainty (%) Laboratory (Bq m�2) Relative uncertainty (%) Variation (%)

137Cs Site-1 28507182 6.39 28907380 13.15 �1.4

Site-2 27527184 6.69 25447341 13.40 7.6

Site-3 26527175 6.60 30047416 13.85 �13.3

Site-4 30927207 6.69 27547374 13.58 10.9

Site-5 28147185 6.57 31287234 7.48 �11.2

210Pbex Site-1 961273956 41.16 644373003 46.61 33.0

Site-2 828372795 33.74 457872683 58.61 44.7

Site-3 10,70472818 26.33 525372943 56.03 50.9

Site-4 838073049 36.38 540872739 50.65 35.5

Site-5 543072825 52.03 574072577 44.90 �5.7

Note: The results of the laboratory measurements are the mean of the four sampling sites. The uncertainty of the results is the combined uncertainty of the four sampling

sites (the measurement area is 2.91 m2).

J. Li et al. / Applied Radiation and Isotopes 68 (2010) 1143–11491148

4. Discussion

The results of the study show that the angle of collimation hasan evident influence on in-situ measurement of environmentalradionuclides. When a 301 collimation is installed in a detector,the area of detection is smaller, the count rate of the detector islow, the uncertainty is high and the precision is low. When a 901collimation is used, the area of detection increases, the count rateof the detector is higher, the uncertainty decreases and theprecision increases. When the collimation is 1801 the uncertaintyis the lowest and the precision is the highest. For 1801 collimationthe area detected is largest but the actual area detected is not easyto confirm. Some reports suggest that in cases of 1801 collimation,the field view of the detector is a circle of ca. 20 m in diameter(Tyler et al., 1996; Ramzaev et al., 2005) but there is not sufficientexperimental evidence. Evidently, 1801 collimation is fit for thequalitative measurement of radionuclides on a large spatial scale(the diameter is larger than 10 m), but is not fit for thequantitative measurement of radionuclides in microscale varia-bility (Chesnokov et al., 1999; Tyler et al., 1996).

The depth distribution of radionuclides in soil is anotherimportant factor influencing the results of in-situ measurement.Because the depth distribution of radionuclides in soil is variablepresupposed profile distributions of radionuclides in soil willunderestimate or overestimate the inventory of environmentalradionuclides. Fig. 3 reveals that 137Cs activity peaks exists at adepth of 4–8 mm. 137Cs activity increases from the surface to 4–8 mm and then declines to nearly zero below 90 mm in depth. Thedepth distribution of 210Pbex shows an exponential decay (Fig. 4).There is almost no 210Pbex below 90 mm. Thus, for in-situmeasurement, the depth distribution of radionuclides for effi-ciency calibration is assumed to be 150 mm and it contributes tothe source of measurement error. ISOCS calibration software hasthe ability to vary the assumed depth distribution parameter,generate a new efficiency curve and re-analyze the samplespectrum accordingly. The results of the study indicate that theassumed depth distribution in soil obtained in advance for the137Cs inventory from in-situ measurement is in agreement withthe 137Cs inventory obtained from the cores analyzed in thelaboratory. The 210Pbex inventory from in-situ measurement is 1.5times higher than the 210Pbex inventory obtained from the coresanalyzed in the laboratory. This may be due to the short countingtime. However, further study is required to ascertain the reason.

In comparison with laboratory measurements, the precision ofin-situ measurements is much higher. In-situ measurements canreduce the measurement error arising from field sampling, andless counting time is required. Just 1 h of counting time issufficient to obtain a measurement precision comparable to alaboratory analysis requiring from 10 to 48 h.

5. Conclusions

This study indicates that the precision of radionuclideinventories obtained by in-situ measurements is better than thatobtained by laboratory measurements. Obviously, it is an effectivetracing technique for assessing soil erosion and soil redistributionrapidly and more accurately.

Acknowledgements

We would like to thank the National Natural Science Founda-tion of China (NSFC No. 40671097, 40771131, 40635028) andIAEA Project (IAEA/RAS5043, TC CPR5015, Research Contract No.12323) for the financial supports. And I would like to express myappreciation for the training workshop of gamma spectrometryprovided by the Abdus Salam International Centre for TheoreticalPhysics.

References

Beck, H.L., DeCampo, J., Gogolak, C., 1972. In situ Ge (Li) and NaI (Ti) gamma-rayspectrometry. USDOE Report HASL-258, US Department of Energy, Washing-ton.

Chesnokov, A.V., Fedin, V.N., Ivanov, O.P., Liksonov, V.I., Potapov, V.N., Shcherbak,S.B., Smirnov, S.V., Urutskoev, L.I., Govorun, A.P., 1999. Method and device tomeasure 137Cs soil contamination in-situ. Nucl. Instrum. Methods Phys. Res.Sect. A 420, 336–344.

Crickmore, M.J., Tazioli, G.S., Appleby, P.G., Oldfield, F., 1990. The Use of NuclearTechnique in Sediment Transport and Sedimentation Problems, IHP-III Project5.2 SC-90/WS-49. UNESCO, Paris.

Golosov, V.N., Walling, D.E., Kvasnikova, E.V., Stukin, E.D., Nikolaev, A.N., Panin,A.V., 2000. Application of a field-portable scintillation detector for studying thedistribution of 137Cs inventories in a small basin in Central Russia. J. Environ.Radioact. 48, 79–94.

He, Q., Walling, D.E., 2000. Calibration of a field-portable gamma detector to obtainin situ measurements of the 137Cs inventories of cultivated soils and floodplainsediments. Appl. Radiat. Isot. 52, 865–872.

Helfer, I.K., Miller, K.M., 1988. Calibration factors for Ge detector used for fieldspectrometry. Health Phys. 55, 15–29.

Joshi, S.R., 1987. Non-destructive determination of lead-210 and radium-226 insediments by direct photon analysis. J. Radioanal. Nucl. Chem. Artic. 116, 169–182.

Lettner, H., Andrasi, A., Hubmer, A.K., Lovranich, E., Steger, F., Zombori, P., 1996. Insitu gamma-spectrometry intercomparison exercise in Salzburg, Austria. Nucl.Instrum. Methods Phys. Res. Sect. A 369, 547–551.

Miller, K.M., Helfer, I.K., 1985. In situ measurements of 137Cs inventory in naturalterrain. Symposium of the Health Physics Society, 243–251.

Nir-El, Y., Lavi, N., Ne’eman, E., Brenner, S., Haquin, G., 1999. In situ gamma-rayspectrometric measurement of uranium in phosphate soils. J. Environ.Radioact. 45, 185–190.

Nozaki, Y., DeMaster, D.J., Lewis, D.M., Turekain, K.K., 1978. Atmospheric 210Pbfluxes determined from soil profiles. J. Geophys. Res. 83, 4047–4051.

Ramzaev, V., Yonehara, H., Hille, R., Barkovsky, A., Mishine, A., Sahoo, S.K., Kurotaki,K., Uchiyama, M., 2005. Gamma-dose rates from terrestrial and Chernobylradionuclides inside and outside settlements in the Bryansk Region, Russia in1996-2003. J. Environ. Radioact. 85, 1–23.

ARTICLE IN PRESS

J. Li et al. / Applied Radiation and Isotopes 68 (2010) 1143–1149 1149

Ritchie, J.C., McHenry, J.R., 1973. Determination of fallout Cs-137 and naturalgamma-ray emitting radionuclides in sediments. Int. J. Appl. Radiat. Isot. 24,575–578.

Robbins, J.A., 1978. Geophysical and geochemical applications of radioactive lead.In: Niagru, J.O. (Ed.), The Biochemistry of Lead in the Environment, vol. 1A.Elsevier, Amsterdam, pp. 285–393.

Tyler, A.N., Sanderson, D.C.W., Scott, E.M., 1996. Estimating and accounting for137Cs source burial through in-situ gamma spectrometry in salt marshenvironments. J. Environ. Radioact. 33 (3), 195–212.

Wallbrink, P.J., Murray, A.S., 1996. Determining soil loss using the inventory ratioof excess lead-210 to Cesium-137. Soil. Sci. Soc. Am. J. 60, 1201–1208.

Walling, D.E., Quine, T.A., 1993. Use of Caesium-137 as a Tracer of Erosion andSedimentation: Handbook for the Application of the Caesium-137 Technique.

UK Overseas Development Administration, Department of Geography, Uni-versity of Exeter, Exeter.

Walling, D.E., He, Q., 1999. Improved models for estimating soil erosion rates fromcaesium-137 measurements. J. Environ. Qual. 28, 611–622.

Wallbrink, P.J., Murray, A.S., 1994. Fallout of 7Be over south eastern Australia.Journal of Environmental Radioactivity 25, 213–228.

Zapata, F., 2002. Handbook for the Assessment of Soil Erosion and Sedimentationusing Environmental Radionuclides. Kluwer Academic Publishers, Dordrecht.

Zombori, P., N�emeth, I., Andrasi, A., Lettner, H., 1992. In-situ gamma-spectrometricmeasurement of the contamination in some selected settlements of Byelor-ussia (BSSSR), Ukraine (UkrSSR) and the Russian Federation (RSFSR). J. Environ.Radioact. 17, 97–106.