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In-situ measurements of the radioactive fallout deposit
Nuclear Instruments and Methods m Physics Research North-Holland
A300 (1991) 611-615
611
In-situ measurements of the radioactive fallout deposit M. Korun, R. Martincic and B. Pucelj of Ljubljana, Ljubljana,
J Stefan Institute, University
Received
6 July 1990
and m revised form
16
October
Yugoslavia 1990
An improved method to determine radionuclide concentrations m soil and the radioactive fallout deposit is presented. The approach is based on in-situ gamma-ray spectrometric measurements performed with a portable high-resolution gamma spectrometer and on calculations of the depth distribution based on the energy dependence of the attenuation of gamma rays in soil . The results are compared with laboratory analysis of collected soil samples. 1. Introduction Determination of the deposition of radioactive fallout by soil sampling and subsequent measurements in the laboratory is time consuming and sometimes rather unreliable because of difficulties to collecting and preparing representative samples. These difficulties are usually overcome by the use of special sampling devices and preparation of composite samples which represent larger areas. Considerable time is also required to assess the depth distribution of the contaminants . Methods to determine the radionuclide concentration m soil and on the soil surface in a short period of time by field gamma spectrometry have been developed [1,2]. However, these methods require that assumptions about the depth distribution of radionuclei have to be made in advance [2,3]. An improved approach for assessing radioactive fallout deposit was developed in order to avoid this inconvenience. Information about the depth distribution is obtained on the basis of the energy dependence of the attenuation of gamma rays in the soil . 2. The model In the proposed model, an exponential decrease of the concentration of radionuclides in soil is assumed, as is common in studies of depth distribution : S.(z) = Sm( ho) e where S.(z) represents the activity concentration per unit mass of soil (Bq/kg) at the depth z, 1/a is the relaxation length and Sm(h 0) is the activity at the surface. The exponential model closely resembles the actual radionuclide distribution in the soil in the case of a single deposition and undisturbed soil . In the case of 0168-9002/91/$03 .50 ~ 1991 -
repeated depositions, the depth distribution is the sum of exponential distributions each with a differenet relaxation length . Parameters to the exponential model correspond to average values . If soil is disturbed in some way, the exponential model is no longer valid. However, if the soil is well mixed to a depth of several attenuation lengths, the estimated relaxation length will be large and the assumption of a homogeneous radionuclide distribution, which is just a special case of the exponential distribution, becomes correct. In the proposed model a uniform composition, a constant density of the soil and an inactive layer of constant thickness d 0 over the soil, which in practice may represent vegetation, water, snow, etc., is assumed (fig. 1) . The total amount of contaminant per unit area S, (Bq/M Z ), which is equivalent to the integral of the concentration over an "infinite" depth of the soil, is then expressed as Sa-Sm(h0)
a,
(2)
where o, is the soil density . If in the gamma spectrum the gamma-ray peak at energy E is identified, the net count rate R (counts/s) in the peak can be calculated as follows: R = p,bCS,(z) dz h0
X
J0
x27rvl(z, r) ee -",d, e -"- d, dr .
(3)
The Ge detector is placed at a height h 0 above the ground (fig . 1), the calibration function n(z,r) represents the probability that the gamma rays are recorded in the total absorption peak if they were emitted at the point (z,r), and three exponential factors determine the attenuation of the gamma rays in the soil, the inactive
Elsevier Science Publishers B.V . (North-Holland)
612
M Korun et al. /Measurements of radioactive fallout
layer and air, respectively . The factor b is the branching ratio, p. s, ft, and Pa are linear attenuation coefficients and d ç , d, and d a are the paths of gamma rays through the soil, inactive layer and air, respectively . They are expressed as z - ho r2+z2 z do zz d, = z r2 + >
dependence was chosen because it has the same form as the exponentional factors which define the attenuation of gamma rays . Consequently, the anisotropy of the efficiency behaves on integration as an additional absorption layer . The efficiency il(ho, 0) and the parameters A and B are functions of energy . They are determined by the calibration procedure. After the integration, the count rate R may be written in a form analogous to the standard expression for the specific acitvity of a sample measured in the laboratory,
d,=
da= ho
z
do
(4)
r2+z2
Usually it is assumed that Ge detectors have an isotropic efficiency. But for detectors with thin windows this assumption is not correct at low gamma energies . Taking into account that fact, the calibration function rl(z,r) may be written in the form
R - p,bSm (h o )E(a,E),
(6)
where the function E(a,E) is analogous to the product of the mass of the sample and the detector efficiency and contains information about the relaxation length, attenuation of gamma rays and the efficiency of the detector . It can be expressed in the following way :
2 ho = rl( ho, 0) [A+(1 _A) e B e _(B/=) zz+rzl , z 2 +r 2
h2
=21T aa°n(ho,0){A
where the quantity i1(ho, 0) is the absolute efficiency for a point source placed on the axis of the detector at a distance h o . The factor h 21(r 2 + z2 ) introduces the inverse square law into the calibration function and the function in square brackets introduces the anisotropy of the detector efficiency . The parameter A represents the strength and B the angular dependence of the anisotropy. If A = 1 or B = 0 the detector efficiency is isotropic. This particular expression for the angular
e'P/u,
X [Ei(-(1 +a/gs)p) - e-Pw,Ei(-p) ] +(1 -A) e( "/i',)P'+B
X [Ei(-(1 +a/tt,)p') -e-'P'/P,Ei(-p') ] where p = Paho + 1a,do and p' = p + B . Fi p., and ha are linear attenuation coefficients for the soil, the inac-
Ge DETECTOR r ho Air da do
Inactive layer a zI l
dr
Soil
Fig. 1. Schematic layout of the in-situ measurement of the radioactive fallout deposit
M Korun et al. / Measurements of radioactive fallout
tive layer and air, respectively, and Ei represents the exponential integral [4]. If one assumes a value for the relaxation length, it is possible to calculate S.(h o) using eq. (6) and standard software for activity calculations . The total deposit Sa is then calculated using eq . (2). If a radionuclide emits only one gamma line then that is about as much as can be done . However, for radionuclides with more gamma lines this method can also be used to determine the distribution of radioisotopes in the soil . For low-energy gamma rays only the surface of the soil contributes to the measured lines, in contrast to the case of more penetrating high-energy gamma rays, where the information about the concentration comes from a thicker layer of the soil .
For a homogeneous distribution, h can be evaluated in the following form : h = { AF( p) + (1 - A) eBF( p') }
/ {tL,A[e-P+pEi(-p)] +(1-A)eB[e-° ' +Ei(-p') ]}, where F(p)=e-P(I-p) -( -p) +p'[l'-F+ln(p)-Ei(-p)] and 7 is the Euler constant (0 .577 - - - ). If the supposition about homogeneity is justified, all gamma lines of a certain radioisotope give consistent values for Sm (h o ) . A nonzero slope of the S_(h o ) :h curve indicates a nonzero relaxation length and an inhomogeneous distribution . In that case eqs. (6) corresponding to different gamma lines of the same radionuclide are treated as a system of equations and solved for Sm (h o ) and a by an iteration procedure. A first approximation for a is extracted from the average slope of the S.(ho) : h curve and a starting value for_ S.(ho) is obtained by extrapolating the S_(ho) : h curve to the zero thickness value (fig . 2) . New a values for each gamma line are obtained from eqs. (6) using the starting value for S.(ho) and the weighted average á is then used as a new approximation for a. Subsequently a set of S.(ho ) is obtained from eqs. (6) and a weighted average is calculated as the next approximation for S_(ho) . Only one step to the iteration procedure is enough for all practical purposes . For this evaluation two additional databases to the usual nuclide library are needed : the attenuation coeffi-
3. Evaluation The analysis of the spectrum begins with a peak search, followed by nuclide identification and interference corrections, if any. The next step in the determination of the distribution of radio-isotopes in the soil is a check for homogeneity. Assurrung a homogeneous distribution (a = 0), the activities at the surface S_(ho) are calculated according to eq. (6). S_(ho) obtained from the gamma lines of the same radioisotope are treated as a function of the average depth of the soil layer h from which the gamma rays are emitted. h is defined as : h=
RJ
(z-h o ) dR(r, z),
where dR(r, z) stands for the count rate of gamma rays coming from the volume element 21T dr dz (fig . 1) . 500
613
Activity at the surface [Bq/kg] --
400 300 ik32
keV
Cs-137 Cs-134
200
605 keV
100 0
Fig
2 Sm(ho) h
0
Bel keV
Th-228 2
58
4
Average depth
[g/clnz ]
curves presented for 228 Th (homogeneous distribution) and for
134
6
Cs
8
and
137CS
(mhomogeneous distribution).
61 4
M Koran et al. / Measurements of radioactive fallout
Table 1 Comparison of the activities of homogeneously distributed radionuclei Type of measurement
Isotope
Location no. 2 specific activity
Location no . 3 specific activity
38±1 38+1 43+2
47+1 44+2 45+3
73+6 60++2 71+7
37+4 40+8 33+2
44+2 37+6 39+2
55+5 48+8 60+2
[Bq/kg]
Laboratory measurement
226
In-situ measurement no . 1
226 Ra
In-situ measurement no . 2
226 Ra
In-situ measurement no . 3
226
228
Location no . I specific activity
Ra
Ra 228 Th
228 228
228 228
228 228
Ra Th
Ra Th Ra Ra Th
[Bq/kg]
ments of soil samples and the analysts of the to situ recorded spectrum was performed. The in-situ measurements were performed by a portable semiconductor
gamma-ray detector with a beryllium window and a relative efficiency of 25%, connected to a ND SIX portable analyzer . The analysis of the spectra was performed on an ND9900 computer . The comparison between the laboratory measurements of soil samples and the in-situ measurements is
[Bq/kg]
presented in tables 1 and 2. The errors of the in-situ measurements stem from statistical errors only .
The measurements were made at points where the homogeneity of the deposition had been demonstrated in previous studies. The soil samples were taken from depths 0-1, 1-2, 2-3, 3-5, 5-7 and 7-10 cm on a 20
52+33 48+7 51+2
cmz surface. The measurement at location 2 was re-
peated three times at the same point in order to check the reproducibility .
49+22 44+88 53+3
The composition of the soil was approximated with a mixture of 80% S'O2 and 20% H2O, the average soil density was measured and the thickness of the inactive
layer determined by the measured average thickness of .The concentrations of rathe grass layer (0 .1 g/cm2) dionuclides obtained at the surface and the relaxation
cients as functions of energy and the detector database .
lengths depend on the water content of the soil . How-
The latter contains the absolute efficiency of the detector and the parameters of the angular dependence of the
ever, the deposition remains unaffected provided that the water content is constant with depth. An excess of
efficiency as functions of the energy .
moisture at the surface may be taken into account by the thickness of the inactive layer. The measuring time for obtaining one in-situ gamma
4. Results
spectrum was between one and two hours while, in the
laboratory the measurements took four days since up to
In order to verify the reliability of the described
method, a comparison
between laboratory
six samples were measured to determine the relaxation length . By using a detector which enables detection of
measure-
Table 2 Comparison of the data for mhomogeneously distributed radionuclei Type of measurement
Isotope
Laboratory measurement
125
In-situ measurement no. 1
125
Sb Cs 137 Cs
In-situ measurement no . 2
125
In-situ measurement no . 3
Location no . 1 Deposit
Location no . 2
[kBq/m2 ]
Relaxation length [cm]
0.31+0.1 0.1 2.2 +_ 0.2 163 ±0 .8
2.4+01 _ 2.9±01
035+0.03 2 6 +_ 0.2 176 +0 .8
1 9+0.3 0.3 24+0 .3
015+0.05 1.27+0.7 0.7 9.97+0.5
2.1+0 0.5 .5 3.0+0 .5
2.2 +_ 0.2 15 .9 +07
29+0 _ .2 34+0 .1
0.41+0.2 2.85±0.5 13 .1 +0 .3
26±0 .1 1 5+0.2
0.25+02 1.86±0.2 9.5 +0 .4
41±03 2.3+0 .1
134 Cs 137
2.3 +0 .2 18 .2 +0 .1
3.2+02 5 .6+0 .2
125
0.2 +01 1 .94+0.2 18 .7 +0 .8
2.4+01 6.1+0 .2
Sb Cs 137 Cs 134
134
Sb Cs
134
Sb
Cs 137 Cs
Deposit
Location no . 3
[kBq/m 2]
Relaxation length [cm]
Deposit [kBq/m2 ]
Relaxation length [cm]
M. Koran et al. / Measurements of radioactwe fallout
X-rays it was even possible to determine the deposit of 137 Cs . 5. Conclusion The comparison of the results from laboratory measurements and the results from in-situ measurements shows a reasonable agreement of specific activities of radionuclides which are homogeneously distributed in the soil . For inhomogeneously distributed radionuclei, the agreements of deposits are good, but the calculated relaxation lengths slightly overestimate the actual depth distribution of the radionuclides in the ground, probably due to the oversimplifications in the model. In conclusion, the method has its advantages and disadvantages. Time-consuming sampling and sample preparation are avoided, measuring time is small compared to measuring time in the laboratory, where several samples have to be measured in order to determine the relaxation length and the deposit, and finally the problem of the representativeness of soil samples is mini-
61 5
mized. It is estimated that the data from in situ measurements are representative for about 30 mz area . On the other hand the method cannot be applied independently for radionuclides which emit gamma rays at one energy only without some previous knowledge about relaxation lengths. Our experience is that in-situ measurements could be superior to laboratory measurements with respect to the determination of radionuclides to the soil, especially in emergency conditions. References [1] H.L . Beck, J. DeCampo and C.V . Gogolak, HASL-258 (1972) . [21 C. Munth, H V61kle and O. Huber, Nucl. Instr. and Meth . A243 (1986) 549. [31 U. Tveten (ed.), NKA Project AKTU-200, Environmental Consequences of Releases from Nuclear Accidents, Final Report (1990) . [4] M Abramowitz and I.A. Stegun, Handbook of Mathematical Functions (Dover, New York, 1965).
A300 (1991) 611-615
611
In-situ measurements of the radioactive fallout deposit M. Korun, R. Martincic and B. Pucelj of Ljubljana, Ljubljana,
J Stefan Institute, University
Received
6 July 1990
and m revised form
16
October
Yugoslavia 1990
An improved method to determine radionuclide concentrations m soil and the radioactive fallout deposit is presented. The approach is based on in-situ gamma-ray spectrometric measurements performed with a portable high-resolution gamma spectrometer and on calculations of the depth distribution based on the energy dependence of the attenuation of gamma rays in soil . The results are compared with laboratory analysis of collected soil samples. 1. Introduction Determination of the deposition of radioactive fallout by soil sampling and subsequent measurements in the laboratory is time consuming and sometimes rather unreliable because of difficulties to collecting and preparing representative samples. These difficulties are usually overcome by the use of special sampling devices and preparation of composite samples which represent larger areas. Considerable time is also required to assess the depth distribution of the contaminants . Methods to determine the radionuclide concentration m soil and on the soil surface in a short period of time by field gamma spectrometry have been developed [1,2]. However, these methods require that assumptions about the depth distribution of radionuclei have to be made in advance [2,3]. An improved approach for assessing radioactive fallout deposit was developed in order to avoid this inconvenience. Information about the depth distribution is obtained on the basis of the energy dependence of the attenuation of gamma rays in the soil . 2. The model In the proposed model, an exponential decrease of the concentration of radionuclides in soil is assumed, as is common in studies of depth distribution : S.(z) = Sm( ho) e where S.(z) represents the activity concentration per unit mass of soil (Bq/kg) at the depth z, 1/a is the relaxation length and Sm(h 0) is the activity at the surface. The exponential model closely resembles the actual radionuclide distribution in the soil in the case of a single deposition and undisturbed soil . In the case of 0168-9002/91/$03 .50 ~ 1991 -
repeated depositions, the depth distribution is the sum of exponential distributions each with a differenet relaxation length . Parameters to the exponential model correspond to average values . If soil is disturbed in some way, the exponential model is no longer valid. However, if the soil is well mixed to a depth of several attenuation lengths, the estimated relaxation length will be large and the assumption of a homogeneous radionuclide distribution, which is just a special case of the exponential distribution, becomes correct. In the proposed model a uniform composition, a constant density of the soil and an inactive layer of constant thickness d 0 over the soil, which in practice may represent vegetation, water, snow, etc., is assumed (fig. 1) . The total amount of contaminant per unit area S, (Bq/M Z ), which is equivalent to the integral of the concentration over an "infinite" depth of the soil, is then expressed as Sa-Sm(h0)
a,
(2)
where o, is the soil density . If in the gamma spectrum the gamma-ray peak at energy E is identified, the net count rate R (counts/s) in the peak can be calculated as follows: R = p,bCS,(z) dz h0
X
J0
x27rvl(z, r) ee -",d, e -"- d, dr .
(3)
The Ge detector is placed at a height h 0 above the ground (fig . 1), the calibration function n(z,r) represents the probability that the gamma rays are recorded in the total absorption peak if they were emitted at the point (z,r), and three exponential factors determine the attenuation of the gamma rays in the soil, the inactive
Elsevier Science Publishers B.V . (North-Holland)
612
M Korun et al. /Measurements of radioactive fallout
layer and air, respectively . The factor b is the branching ratio, p. s, ft, and Pa are linear attenuation coefficients and d ç , d, and d a are the paths of gamma rays through the soil, inactive layer and air, respectively . They are expressed as z - ho r2+z2 z do zz d, = z r2 + >
dependence was chosen because it has the same form as the exponentional factors which define the attenuation of gamma rays . Consequently, the anisotropy of the efficiency behaves on integration as an additional absorption layer . The efficiency il(ho, 0) and the parameters A and B are functions of energy . They are determined by the calibration procedure. After the integration, the count rate R may be written in a form analogous to the standard expression for the specific acitvity of a sample measured in the laboratory,
d,=
da= ho
z
do
(4)
r2+z2
Usually it is assumed that Ge detectors have an isotropic efficiency. But for detectors with thin windows this assumption is not correct at low gamma energies . Taking into account that fact, the calibration function rl(z,r) may be written in the form
R - p,bSm (h o )E(a,E),
(6)
where the function E(a,E) is analogous to the product of the mass of the sample and the detector efficiency and contains information about the relaxation length, attenuation of gamma rays and the efficiency of the detector . It can be expressed in the following way :
2 ho = rl( ho, 0) [A+(1 _A) e B e _(B/=) zz+rzl , z 2 +r 2
h2
=21T aa°n(ho,0){A
where the quantity i1(ho, 0) is the absolute efficiency for a point source placed on the axis of the detector at a distance h o . The factor h 21(r 2 + z2 ) introduces the inverse square law into the calibration function and the function in square brackets introduces the anisotropy of the detector efficiency . The parameter A represents the strength and B the angular dependence of the anisotropy. If A = 1 or B = 0 the detector efficiency is isotropic. This particular expression for the angular
e'P/u,
X [Ei(-(1 +a/gs)p) - e-Pw,Ei(-p) ] +(1 -A) e( "/i',)P'+B
X [Ei(-(1 +a/tt,)p') -e-'P'/P,Ei(-p') ] where p = Paho + 1a,do and p' = p + B . Fi p., and ha are linear attenuation coefficients for the soil, the inac-
Ge DETECTOR r ho Air da do
Inactive layer a zI l
dr
Soil
Fig. 1. Schematic layout of the in-situ measurement of the radioactive fallout deposit
M Korun et al. / Measurements of radioactive fallout
tive layer and air, respectively, and Ei represents the exponential integral [4]. If one assumes a value for the relaxation length, it is possible to calculate S.(h o) using eq. (6) and standard software for activity calculations . The total deposit Sa is then calculated using eq . (2). If a radionuclide emits only one gamma line then that is about as much as can be done . However, for radionuclides with more gamma lines this method can also be used to determine the distribution of radioisotopes in the soil . For low-energy gamma rays only the surface of the soil contributes to the measured lines, in contrast to the case of more penetrating high-energy gamma rays, where the information about the concentration comes from a thicker layer of the soil .
For a homogeneous distribution, h can be evaluated in the following form : h = { AF( p) + (1 - A) eBF( p') }
/ {tL,A[e-P+pEi(-p)] +(1-A)eB[e-° ' +Ei(-p') ]}, where F(p)=e-P(I-p) -( -p) +p'[l'-F+ln(p)-Ei(-p)] and 7 is the Euler constant (0 .577 - - - ). If the supposition about homogeneity is justified, all gamma lines of a certain radioisotope give consistent values for Sm (h o ) . A nonzero slope of the S_(h o ) :h curve indicates a nonzero relaxation length and an inhomogeneous distribution . In that case eqs. (6) corresponding to different gamma lines of the same radionuclide are treated as a system of equations and solved for Sm (h o ) and a by an iteration procedure. A first approximation for a is extracted from the average slope of the S.(ho) : h curve and a starting value for_ S.(ho) is obtained by extrapolating the S_(ho) : h curve to the zero thickness value (fig . 2) . New a values for each gamma line are obtained from eqs. (6) using the starting value for S.(ho) and the weighted average á is then used as a new approximation for a. Subsequently a set of S.(ho ) is obtained from eqs. (6) and a weighted average is calculated as the next approximation for S_(ho) . Only one step to the iteration procedure is enough for all practical purposes . For this evaluation two additional databases to the usual nuclide library are needed : the attenuation coeffi-
3. Evaluation The analysis of the spectrum begins with a peak search, followed by nuclide identification and interference corrections, if any. The next step in the determination of the distribution of radio-isotopes in the soil is a check for homogeneity. Assurrung a homogeneous distribution (a = 0), the activities at the surface S_(ho) are calculated according to eq. (6). S_(ho) obtained from the gamma lines of the same radioisotope are treated as a function of the average depth of the soil layer h from which the gamma rays are emitted. h is defined as : h=
RJ
(z-h o ) dR(r, z),
where dR(r, z) stands for the count rate of gamma rays coming from the volume element 21T dr dz (fig . 1) . 500
613
Activity at the surface [Bq/kg] --
400 300 ik32
keV
Cs-137 Cs-134
200
605 keV
100 0
Fig
2 Sm(ho) h
0
Bel keV
Th-228 2
58
4
Average depth
[g/clnz ]
curves presented for 228 Th (homogeneous distribution) and for
134
6
Cs
8
and
137CS
(mhomogeneous distribution).
61 4
M Koran et al. / Measurements of radioactive fallout
Table 1 Comparison of the activities of homogeneously distributed radionuclei Type of measurement
Isotope
Location no. 2 specific activity
Location no . 3 specific activity
38±1 38+1 43+2
47+1 44+2 45+3
73+6 60++2 71+7
37+4 40+8 33+2
44+2 37+6 39+2
55+5 48+8 60+2
[Bq/kg]
Laboratory measurement
226
In-situ measurement no . 1
226 Ra
In-situ measurement no . 2
226 Ra
In-situ measurement no . 3
226
228
Location no . I specific activity
Ra
Ra 228 Th
228 228
228 228
228 228
Ra Th
Ra Th Ra Ra Th
[Bq/kg]
ments of soil samples and the analysts of the to situ recorded spectrum was performed. The in-situ measurements were performed by a portable semiconductor
gamma-ray detector with a beryllium window and a relative efficiency of 25%, connected to a ND SIX portable analyzer . The analysis of the spectra was performed on an ND9900 computer . The comparison between the laboratory measurements of soil samples and the in-situ measurements is
[Bq/kg]
presented in tables 1 and 2. The errors of the in-situ measurements stem from statistical errors only .
The measurements were made at points where the homogeneity of the deposition had been demonstrated in previous studies. The soil samples were taken from depths 0-1, 1-2, 2-3, 3-5, 5-7 and 7-10 cm on a 20
52+33 48+7 51+2
cmz surface. The measurement at location 2 was re-
peated three times at the same point in order to check the reproducibility .
49+22 44+88 53+3
The composition of the soil was approximated with a mixture of 80% S'O2 and 20% H2O, the average soil density was measured and the thickness of the inactive
layer determined by the measured average thickness of .The concentrations of rathe grass layer (0 .1 g/cm2) dionuclides obtained at the surface and the relaxation
cients as functions of energy and the detector database .
lengths depend on the water content of the soil . How-
The latter contains the absolute efficiency of the detector and the parameters of the angular dependence of the
ever, the deposition remains unaffected provided that the water content is constant with depth. An excess of
efficiency as functions of the energy .
moisture at the surface may be taken into account by the thickness of the inactive layer. The measuring time for obtaining one in-situ gamma
4. Results
spectrum was between one and two hours while, in the
laboratory the measurements took four days since up to
In order to verify the reliability of the described
method, a comparison
between laboratory
six samples were measured to determine the relaxation length . By using a detector which enables detection of
measure-
Table 2 Comparison of the data for mhomogeneously distributed radionuclei Type of measurement
Isotope
Laboratory measurement
125
In-situ measurement no. 1
125
Sb Cs 137 Cs
In-situ measurement no . 2
125
In-situ measurement no . 3
Location no . 1 Deposit
Location no . 2
[kBq/m2 ]
Relaxation length [cm]
0.31+0.1 0.1 2.2 +_ 0.2 163 ±0 .8
2.4+01 _ 2.9±01
035+0.03 2 6 +_ 0.2 176 +0 .8
1 9+0.3 0.3 24+0 .3
015+0.05 1.27+0.7 0.7 9.97+0.5
2.1+0 0.5 .5 3.0+0 .5
2.2 +_ 0.2 15 .9 +07
29+0 _ .2 34+0 .1
0.41+0.2 2.85±0.5 13 .1 +0 .3
26±0 .1 1 5+0.2
0.25+02 1.86±0.2 9.5 +0 .4
41±03 2.3+0 .1
134 Cs 137
2.3 +0 .2 18 .2 +0 .1
3.2+02 5 .6+0 .2
125
0.2 +01 1 .94+0.2 18 .7 +0 .8
2.4+01 6.1+0 .2
Sb Cs 137 Cs 134
134
Sb Cs
134
Sb
Cs 137 Cs
Deposit
Location no . 3
[kBq/m 2]
Relaxation length [cm]
Deposit [kBq/m2 ]
Relaxation length [cm]
M. Koran et al. / Measurements of radioactwe fallout
X-rays it was even possible to determine the deposit of 137 Cs . 5. Conclusion The comparison of the results from laboratory measurements and the results from in-situ measurements shows a reasonable agreement of specific activities of radionuclides which are homogeneously distributed in the soil . For inhomogeneously distributed radionuclei, the agreements of deposits are good, but the calculated relaxation lengths slightly overestimate the actual depth distribution of the radionuclides in the ground, probably due to the oversimplifications in the model. In conclusion, the method has its advantages and disadvantages. Time-consuming sampling and sample preparation are avoided, measuring time is small compared to measuring time in the laboratory, where several samples have to be measured in order to determine the relaxation length and the deposit, and finally the problem of the representativeness of soil samples is mini-
61 5
mized. It is estimated that the data from in situ measurements are representative for about 30 mz area . On the other hand the method cannot be applied independently for radionuclides which emit gamma rays at one energy only without some previous knowledge about relaxation lengths. Our experience is that in-situ measurements could be superior to laboratory measurements with respect to the determination of radionuclides to the soil, especially in emergency conditions. References [1] H.L . Beck, J. DeCampo and C.V . Gogolak, HASL-258 (1972) . [21 C. Munth, H V61kle and O. Huber, Nucl. Instr. and Meth . A243 (1986) 549. [31 U. Tveten (ed.), NKA Project AKTU-200, Environmental Consequences of Releases from Nuclear Accidents, Final Report (1990) . [4] M Abramowitz and I.A. Stegun, Handbook of Mathematical Functions (Dover, New York, 1965).