- Email: [email protected]
Deep underground measurements of 60Co in steel exposed to the Hiroshima atomic bomb explosion
ARTICLE IN PRESS
Applied Radiation and Isotopes 61 (2004) 173–177
Deep underground measurements of 60Co in steel exposed to the Hiroshima atomic bomb explosion Mikael Hulta,*, Jo.el Gasparroa, Roberto Vassellia, Kiyoshi Shizumab, Masaharu Hoshic, Dirk Arnoldd, Stefan Neumaierd a
Institute for Reference Materials and Measurements, EC-JRC-IRMM, Retieseweg, Geel B-2440, Belgium Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739, Japan c Research Institute for Nuclear Medicine and Biology, Hiroshima University, 1-2-3 Kasumi Minami-ku, Hiroshima 734, Japan d Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig 38116, Germany b
Abstract When using gamma-ray spectrometry performed deep underground, it is possible to measure 60Co activities down to 0.1 mBq in steel samples of some 100 g without any pre-concentration. It is thus still possible to measure 60Co induced by neutrons from the atomic bomb explosion in Hiroshima in pieces of steel collected at distances up to about 1200 m slant range. The results of non-destructive measurements of eight steel samples are compared with the 1986 Dose ReEvaluation (DS86) model calculations. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ultra low-level gamma-ray spectrometry; Underground laboratory;
1. Introduction Man’s knowledge of the risk of ionising radiation to human populations is to a large extent based on followups of Hiroshima and Nagasaki victims. In the US– Japan joint reassessment of the atomic bomb radiation doses, the so-called DS86 (RERF, 1987; Shizuma et al., 1999) and more recently DS02 (RERF, 2003) reports, the effective doses at various locations have been determined retrospectively with computer model calculations. These model calculations have been compared with measurements of activation products from various locations. A significant improvement in the correlation between model calculations and measurements has been achieved in recent years. This is due to both better computers as well as improved measurements. When a new technique, e.g. accelerator mass spectrometry *Corresponding author. Tel.: +32-14-571-269; fax: +32-14584-273. E-mail address: [email protected] (M. Hult).
60
Co; Hiroshima
(AMS) was brought into use, significant progress was made. This paper describes measurements of eight steel samples from Hiroshima using another relatively new technique, ultra low-level gamma-ray spectrometry (ULGS) in underground laboratories. The technique is useful because: (a) activation products have very low activities in Hiroshima as a result of the long time that passed since the explosion, (b) of interest in better estimates of the doses far away from the epicentre that might be obtained, (c) only small samples are available in some cases, (d) ULGS is non-destructive. No chemistry for pre-concentration of the analyte is needed. The advantage of performing gamma-ray spectrometry underground is that the background induced by cosmic rays is significantly reduced (Heusser, 1996). ULGS was originally developed for fundamental physics and experiments studying double beta decay and construction of neutrino detectors, but the facilities are now also being applied to reference measurements and environmental monitoring (Hult et al., 2003a).
0969-8043/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2004.03.040
ARTICLE IN PRESS M. Hult et al. / Applied Radiation and Isotopes 61 (2004) 173–177
174
2. Materials and methods
2.2. HPGe-detector systems
2.1. Samples and sampling
Two laboratories measured the samples, Institute for Reference Materials and Measurements (IRMM) and Physikalisch-Technische Bundesanstalt (PTB). IRMM did the measurements in the 225 m deep underground laboratory HADES, located at the premises of the Belgian nuclear centre SCKdCEN in Mol (Hult et al., 2003b). PTB performed their measurements in the 925 m deep underground laboratory UDO, located in the salt mine Asse (Neumaier et al., 2000). An overview of the characteristics of the detectors employed in the measurements is given in Table 2. All detectors were shielded with 15–20 cm of lowbackground lead that was lined with 5–15 cm of electrolytic copper. The sample volumes were flushed with nitrogen gas boiling off the liquid nitrogen dewars.
Eight steel samples were analysed in this study. They were cut out from various steel constructions in Hiroshima (Table 1). Samples from the same steel constructions have previously been measured for 60Co above ground using low-level gamma-ray spectrometry of pre-concentrated samples (Shizuma et al., 1998). In this study, the gamma-ray spectrometry was performed without any pre-concentration. The samples had roughly the shape of small rectangular pieces and were oxidised to different degrees. The surface rust was removed by first brushing the samples and later submersion in hydrochloric acid (25%) for 30–60 min, rinsing in water, submersion in nitric acid (1 mol/l) for 5 min, rinsing in water and finally cleansing in isopropanol. A small piece (B1 g) was cut from each sample for the elemental analysis.
Table 1 Description of the quasi-rectangular steel samples in this study Massa (g)
Sample no.
Description
S1 S2 S3 S4 S5
A-Bomb Dome 8.6 Kirin Beer Hall 51.3 Kodokan Building 33.5 City Hall 61.72 Red Cross Hospital 48.37 Pipe Red Cross Hospital 69.97 Ladder Hiroshima Bank of 157.7 Credit Army Food Storehouse 135.21
S6 S7 S8
Thickness (mm) 1.0 6.3 2.8 3.8 3.7 5.1 8.5 9 1.3=11.7
a
After cleaning and after removing a small piece for elemental analysis.
2.3. Data analysis Acquisition times ranged from 12 days (sample S1) to 143 days (sample S7) although most samples were measured for approximately 1 month. Generally spectra were collected daily. Before adding the spectra together, all spectra were carefully investigated for non-conformities. Regular quality controls of the detector systems were performed using standardised reference sources. The measurements of the two laboratories were compared by measuring a steel reference sample weighing . 62 g and with a 60Co activity of 21 mBq (Kohler et al., 2004). The sum spectra were not analysed using automatic peak-fitting routines. Instead, the number of counts in a region-of-interest (RoI) encompassing each peak was determined. A linear continuous background was determined by studying one RoI to the right and one RoI to the left of each peak. These measurements demanded good control of the background counting rate of the two 60Co peaks at 1173 and 1332 keV. Background data have been collected regularly and for long periods of time. The relative
Table 2 Details of the four HPGe detectors employed in this study
Bkg. cra at 1173 keV (day1) Bkg. cra at 1332 keV (day1) Relative efficiency Volume (cm3) Crystal configuration Diameter (cm) Depth of laboratory (m) Overburden m w.e. a
Ge-3 (HADES)
Ge-4 (HADES)
Ge-5 (HADES)
PTB-ULB (UDO/ASSE)
0.33(7) 0.35(9) 60% 251 Coaxial 6.4 225 500
0.79(9) 0.90(10) 106% 412 Coaxial 8.0 225 500
0.72(16) 1.12(26) 50% 150 Planar 8.0 225 500
0.68(15) 0.61(14) 95% 357 Coaxial 7.7 925 2100
Bkg. cr.=Counting rate of the peak in question in the background spectrum.
ARTICLE IN PRESS M. Hult et al. / Applied Radiation and Isotopes 61 (2004) 173–177
uncertainty of the background peak counting rate was in the order of 15% for the different detectors. The activity was calculated by applying the following formula to each peak: A¼
lCeltd ; Eð1 eltm Þ
ð1Þ
where A is the activity at the reference date, l the decay constant, C the net number of counts in the peak in question, td the decay time, e the absolute efficiency per decay and tm the measurement time. In case a signal above decision threshold was determined for both peaks, the weighted mean of the activity from the two peaks was calculated. The absolute detection efficiency per decay was determined using the following formula:
175
2.4. Elemental analysis The elemental analysis of the stable cobalt was performed using inductively coupled plasma optical emission spectroscopy (ICP-OES). Pieces of about 1 g of steel samples as well as a blank and a reference sample were weighed and placed into clean 50 ml plastic, volumetric flasks. First the surface was cleaned and rust removed. In order to dissolve the steel, hydrochloric, nitric and hydrofluoric acids were added. Thereafter followed a dilution with Milli-Qs water to a volume of 50 ml before the measurement was conducted using a Perkin-Elmer Optima 3300DV ICP-OES instrument. Cobalt was measured at 228.616 nm in radial view. A standard series comprising 6 standards (which were 100
Ge- 4
S1
ð2Þ 1
The superscript ‘‘MC’’ indicates that the value was calculated using a Monte Carlo code and ‘‘Exp’’ indicates that the value was determined from the measurement. The subscript ‘‘Sample’’ indicates that the value refers to the sample in question, while the subscript ‘‘Ref’’ indicates that the value refers to a reference sample. The Monte Carlo code employed at IRMM was EGS4 (Nelson et al., 1985) and at PTB it was GESPECOR (Sima and Arnold, 2000). The nuclear decay data of 60Co that was used in the Monte Carlo calculations came from the Table of Radionuclides (B!e et al., 1999). The two gamma rays at 1173 and 1332 keV following the 60Co decay are emitted in cascade. They are weakly correlated (which was neglected), but there is a strong coincidence summing effect in gamma-ray spectrometry in close geometry configurations (i.e. when the sample is very near to the detector). The ratio of the efficiencies calculated with and without coincidence summing for the 1332 keV peak was typically 0.75.
Countrate (d-1 keV-1)
E¼
10
Exp EMC Sample ERef : EMC Ref
0.1 10 S2
PTB-ULB
1 0.1 0.01 1 S3
PTB-ULB
0.1
0.01 1150
1200
1250 1300 Energy (keV)
1350
Fig. 1. Part of the gamma-ray spectrum covering the peaks from 60Co at 1173 and 1332 keV for the three samples nearest to the ground zero (solid line). The name of the detector employed for the measurement is given in each graph, as well as the background spectrum (circles) of that detector.
Table 3 Results of the activity measurements and the elemental analyses Sample no.
Description
Cobalt conc. (mg kg1)
60 Co activity at the start of measurement (mBq)
This study 60Co/cobalt at Aug 6, 1945 (Bq/mg)
S1 S2 S3 S4 S5 S6 S7 S8
A-Bomb Dome Kirin Beer Hall Kodokan Building City Hall Red Cross Hospital Pipe Red Cross Hospital Ladder Hiroshima Bank of Credit Army Food Storehouse
250(7) 139(3) 109(3) 163(3) 122(4) 91(2) 120(2) 81(2)
12.1(18) 1.85(26) o0.24 0.25(11) o0.042 o0.19 o0.051 o0.24
10.3(16) 0.41(7) o0.12 0.047(21) o0.014 o0.06 o0.005 o0.04
ARTICLE IN PRESS M. Hult et al. / Applied Radiation and Isotopes 61 (2004) 173–177
176
1.2 1.0 0.8 0.6 0.4
made up in the same acid matrix as the samples) was used to establish a calibration curve.
Ge-4
S4
3. Results
0.2 0 S5
Ge-3
S6
Ge-5
S7
Ge-4
Table 3 gives the results of the activity measurements as well as for the elemental analysis. The uncertainties are expressed as combined standard uncertainties, following the Guide to the Expression of Uncertainty in Measurement (ISO/IEC/OIML/BIPM, 1995). They are given in brackets after the relevant number and their numerical value refers to the corresponding last digit(s) of the quoted result. The main contribution to the uncertainty for the activity measurements is the counting statistics. The second most important contribution is from the full-energy peak efficiency, although this is generally lesser than 5%. The half-life of 60Co is known with a high accuracy (0.02%); so although a decay correction of 11 half-lives was necessary it contributed only 0.13% to the combined standard uncertainty. All the results were compared to the decision thresholds using a significance level of 0.05 (ISO, 2000). Fig. 1 shows the gamma-ray peaks of interest for the three samples closest to the epicentre and Fig. 2 shows the same as Fig. 1 for samples S4–S7. The background spectrum is given as comparison in each plot.
Countrate (d-1 keV-1)
0.3 0.2 0.1 0 0.6 0.4 0.2 0 0.4 0.2 0 1150
1200
1250 Energy (keV)
1300
1350
Fig. 2. Part of the gamma-ray spectrum covering the peaks from 60Co at 1173 and 1332 keV for samples S4–S7 (solid line). The name of the detector employed for the measurement is given in each graph, as well as the background spectrum (circles) of that detector.
4. Discussion Fig. 3 shows the present data, together with a curve representing the DS86 model calculations. Although some of the values in this study are only given as
Activity of 60Co/mass of Co (Bq/mg)
100 Value with combined standard uncertainty
S1, A-dome
10
decision threshold
1 From Red Cross Hospital Separated in x-position in order to be better displayed
S2
0.1 S6, ladder
S3 S4
S5, pipe
0.01 S7
0.001 500
700
900
1100
1300
1500
1700
1900
Slant range (m) Fig. 3. The graph shows the decay corrected 60Co activity (reference date: August 6, 1945) divided by the stable cobalt mass. The solid line represents the DS86 model calculation.
ARTICLE IN PRESS M. Hult et al. / Applied Radiation and Isotopes 61 (2004) 173–177
decision thresholds, they are still very useful because they are very low and even lower than some previously measured values (Shizuma et al., 1998). Preliminary data from the DS02 report indicate only minor refinements of model calculations compared to the DS86 report. The height of the Hiroshima explosion was, for example, increased from 580 to 600 m and moved 15 m to the west. This should result in a slightly better agreement between the model calculations and the measured data for S1 and S2. The main consequence of the DS02 study is the improved confidence in dose estimates, since most of the discrepancies between measurements and model calculations have been resolved. The measurement result of sample S3 (Kodokan building) deviates from the pattern of the other results. There is currently no explanation for this, but further studies are planned. The method of underground ultra low-level gammaray spectrometry as presented here is very robust, since it involves a minimum of sample preparation. The main source of uncertainty is from counting statistics. It is possible to achieve superior results (by lowering decision thresholds or uncertainties) by measuring the samples for longer periods of time and by using more massive samples.
Acknowledgements The HADES crew of SCKdCEN in Mol, Belgium, is gratefully acknowledged for their work. The input and support of Prof. Kazuhisa Komura (Kanazawa University) and Dr. Saturo Endo, (Hiroshima University) is gratefully acknowledged. We are thankful to Leigh Holmes, Geert van Britsom, Michel Bickel (IRMM) for the ICP-OES measurements, and finally we thank Prof. Peter N. Johnston (Royal Melbourne Institute of Technology) for many fruitful discussions.
References B!e, M.M., Lam!e, J., Piton, L.F., Coursol, N., Legrand, J., Duchemin, B., Morillon, C., Browne, E., Chechev, V., . Helmer, R., Schonfeld, E., 1999. Table of Radionuclides on CD-Rom, Version 1-98, CEA/DIMRI, 91191 Gif-surYvette, France.
177
Heusser, G., 1996. Low radioactivity background techniques. Ann. Rev. Nucl. Part. Sci. 45, 543–590. . Hult, M., Gasparro, J., Johnston, P.N., Kohler, M., 2003a. Underground gamma-ray spectrometry of environmental radioactivity: European examples. In: Kamata, N. (Ed.), Proceedings from International Symposium of Kanazawa University’s COE Program. Kanazawa University, Japan, pp. 16–23. Hult, M., Gasparro, J., Johansson, L., Johnston, P.N., Vasselli, R., 2003b. Ultra Sensitive Measurements of Gamma-ray Emitting Radionuclides Using HPGe-detectors in the Underground Laboratory HADES. Environmental Radiochemical Analysis II, Royal Society of Chemistry, Cambridge, UK, pp. 375–382. ISO/IEC/OIML/BIPM, 1995. Guide to the Expression of Uncertainty in Measurement (1st corrected edition). International Standard Organisation, Geneva, Switzerland, 1995. ISO, 2000. Determination of the Detection Limit and Decision Threshold for Ionising Radiation Measurements—Part 3, 1st Edition. International Standards Organisation, Geneva, Switzerland. . Kohler, M., Hult, M., Arnold, D., Laubenstein, M., Reyss, J.-L., 2004. Reference measurements of 60Co in steel. Appl. Radiat. Isot., this issue, doi:10.1016/j.apradiso.2004.03.047. Nelson, W.R., Hirayama, H., Rogers, D.W.O., 1985. The EGS4 code system. SLAC Report 265. . Neumaier, S., Arnold, D., Bohm, J., Funck, E., 2000. The PTB underground laboratory for dosimetry and spectrometry. Appl. Radiat. Isot. 53, 173–178. Radiation Effects Research Foundation (RERF), 1987. In: Roesch, W.C. (Ed.), US–Japan Joint Reassessment of Atomic Bomb Radiation Dosimetry in Hiroshima and Nagasaki, Vols. 1 and 2. DS86, Dosimetry System 1986. Radiation Effects Research Foundation, Hiroshima, Japan. Radiation Effects Research Foundation (RERF), 2003. US– Japan Joint Reassessment of Atomic Bomb Radiation Dosimetry in Hiroshima and Nagasaki, DS02, Dosimetry System 2002. Radiation Effects Research Foundation, Hiroshima, Japan. Shizuma, K., Iwatani, K., Hasai, H., Oka, T., Endo, S., Takada, J., Hoshi, M., Fujita, S., Watanabe, T., Imanaka, T., 1998. Residual 60Co activity in steel samples exposed to the Hiroshima atomic-bomb neutrons. Health Phys. 75 (3), 278–284. Shizuma, K., Hoshi, M., Hasai, H., 1999. Uncertainties of DS86 and prospects for residual radioactivity measurements. J. Radiat. Res. 40, 138–144. Sima, O., Arnold, D., 2000. Accurate computation of coincidence summing corrections in low level gamma-ray spectrometry. Appl. Radiat. Isot. 53 (1–2), 51–56.
Applied Radiation and Isotopes 61 (2004) 173–177
Deep underground measurements of 60Co in steel exposed to the Hiroshima atomic bomb explosion Mikael Hulta,*, Jo.el Gasparroa, Roberto Vassellia, Kiyoshi Shizumab, Masaharu Hoshic, Dirk Arnoldd, Stefan Neumaierd a
Institute for Reference Materials and Measurements, EC-JRC-IRMM, Retieseweg, Geel B-2440, Belgium Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739, Japan c Research Institute for Nuclear Medicine and Biology, Hiroshima University, 1-2-3 Kasumi Minami-ku, Hiroshima 734, Japan d Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig 38116, Germany b
Abstract When using gamma-ray spectrometry performed deep underground, it is possible to measure 60Co activities down to 0.1 mBq in steel samples of some 100 g without any pre-concentration. It is thus still possible to measure 60Co induced by neutrons from the atomic bomb explosion in Hiroshima in pieces of steel collected at distances up to about 1200 m slant range. The results of non-destructive measurements of eight steel samples are compared with the 1986 Dose ReEvaluation (DS86) model calculations. r 2004 Elsevier Ltd. All rights reserved. Keywords: Ultra low-level gamma-ray spectrometry; Underground laboratory;
1. Introduction Man’s knowledge of the risk of ionising radiation to human populations is to a large extent based on followups of Hiroshima and Nagasaki victims. In the US– Japan joint reassessment of the atomic bomb radiation doses, the so-called DS86 (RERF, 1987; Shizuma et al., 1999) and more recently DS02 (RERF, 2003) reports, the effective doses at various locations have been determined retrospectively with computer model calculations. These model calculations have been compared with measurements of activation products from various locations. A significant improvement in the correlation between model calculations and measurements has been achieved in recent years. This is due to both better computers as well as improved measurements. When a new technique, e.g. accelerator mass spectrometry *Corresponding author. Tel.: +32-14-571-269; fax: +32-14584-273. E-mail address: [email protected] (M. Hult).
60
Co; Hiroshima
(AMS) was brought into use, significant progress was made. This paper describes measurements of eight steel samples from Hiroshima using another relatively new technique, ultra low-level gamma-ray spectrometry (ULGS) in underground laboratories. The technique is useful because: (a) activation products have very low activities in Hiroshima as a result of the long time that passed since the explosion, (b) of interest in better estimates of the doses far away from the epicentre that might be obtained, (c) only small samples are available in some cases, (d) ULGS is non-destructive. No chemistry for pre-concentration of the analyte is needed. The advantage of performing gamma-ray spectrometry underground is that the background induced by cosmic rays is significantly reduced (Heusser, 1996). ULGS was originally developed for fundamental physics and experiments studying double beta decay and construction of neutrino detectors, but the facilities are now also being applied to reference measurements and environmental monitoring (Hult et al., 2003a).
0969-8043/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2004.03.040
ARTICLE IN PRESS M. Hult et al. / Applied Radiation and Isotopes 61 (2004) 173–177
174
2. Materials and methods
2.2. HPGe-detector systems
2.1. Samples and sampling
Two laboratories measured the samples, Institute for Reference Materials and Measurements (IRMM) and Physikalisch-Technische Bundesanstalt (PTB). IRMM did the measurements in the 225 m deep underground laboratory HADES, located at the premises of the Belgian nuclear centre SCKdCEN in Mol (Hult et al., 2003b). PTB performed their measurements in the 925 m deep underground laboratory UDO, located in the salt mine Asse (Neumaier et al., 2000). An overview of the characteristics of the detectors employed in the measurements is given in Table 2. All detectors were shielded with 15–20 cm of lowbackground lead that was lined with 5–15 cm of electrolytic copper. The sample volumes were flushed with nitrogen gas boiling off the liquid nitrogen dewars.
Eight steel samples were analysed in this study. They were cut out from various steel constructions in Hiroshima (Table 1). Samples from the same steel constructions have previously been measured for 60Co above ground using low-level gamma-ray spectrometry of pre-concentrated samples (Shizuma et al., 1998). In this study, the gamma-ray spectrometry was performed without any pre-concentration. The samples had roughly the shape of small rectangular pieces and were oxidised to different degrees. The surface rust was removed by first brushing the samples and later submersion in hydrochloric acid (25%) for 30–60 min, rinsing in water, submersion in nitric acid (1 mol/l) for 5 min, rinsing in water and finally cleansing in isopropanol. A small piece (B1 g) was cut from each sample for the elemental analysis.
Table 1 Description of the quasi-rectangular steel samples in this study Massa (g)
Sample no.
Description
S1 S2 S3 S4 S5
A-Bomb Dome 8.6 Kirin Beer Hall 51.3 Kodokan Building 33.5 City Hall 61.72 Red Cross Hospital 48.37 Pipe Red Cross Hospital 69.97 Ladder Hiroshima Bank of 157.7 Credit Army Food Storehouse 135.21
S6 S7 S8
Thickness (mm) 1.0 6.3 2.8 3.8 3.7 5.1 8.5 9 1.3=11.7
a
After cleaning and after removing a small piece for elemental analysis.
2.3. Data analysis Acquisition times ranged from 12 days (sample S1) to 143 days (sample S7) although most samples were measured for approximately 1 month. Generally spectra were collected daily. Before adding the spectra together, all spectra were carefully investigated for non-conformities. Regular quality controls of the detector systems were performed using standardised reference sources. The measurements of the two laboratories were compared by measuring a steel reference sample weighing . 62 g and with a 60Co activity of 21 mBq (Kohler et al., 2004). The sum spectra were not analysed using automatic peak-fitting routines. Instead, the number of counts in a region-of-interest (RoI) encompassing each peak was determined. A linear continuous background was determined by studying one RoI to the right and one RoI to the left of each peak. These measurements demanded good control of the background counting rate of the two 60Co peaks at 1173 and 1332 keV. Background data have been collected regularly and for long periods of time. The relative
Table 2 Details of the four HPGe detectors employed in this study
Bkg. cra at 1173 keV (day1) Bkg. cra at 1332 keV (day1) Relative efficiency Volume (cm3) Crystal configuration Diameter (cm) Depth of laboratory (m) Overburden m w.e. a
Ge-3 (HADES)
Ge-4 (HADES)
Ge-5 (HADES)
PTB-ULB (UDO/ASSE)
0.33(7) 0.35(9) 60% 251 Coaxial 6.4 225 500
0.79(9) 0.90(10) 106% 412 Coaxial 8.0 225 500
0.72(16) 1.12(26) 50% 150 Planar 8.0 225 500
0.68(15) 0.61(14) 95% 357 Coaxial 7.7 925 2100
Bkg. cr.=Counting rate of the peak in question in the background spectrum.
ARTICLE IN PRESS M. Hult et al. / Applied Radiation and Isotopes 61 (2004) 173–177
uncertainty of the background peak counting rate was in the order of 15% for the different detectors. The activity was calculated by applying the following formula to each peak: A¼
lCeltd ; Eð1 eltm Þ
ð1Þ
where A is the activity at the reference date, l the decay constant, C the net number of counts in the peak in question, td the decay time, e the absolute efficiency per decay and tm the measurement time. In case a signal above decision threshold was determined for both peaks, the weighted mean of the activity from the two peaks was calculated. The absolute detection efficiency per decay was determined using the following formula:
175
2.4. Elemental analysis The elemental analysis of the stable cobalt was performed using inductively coupled plasma optical emission spectroscopy (ICP-OES). Pieces of about 1 g of steel samples as well as a blank and a reference sample were weighed and placed into clean 50 ml plastic, volumetric flasks. First the surface was cleaned and rust removed. In order to dissolve the steel, hydrochloric, nitric and hydrofluoric acids were added. Thereafter followed a dilution with Milli-Qs water to a volume of 50 ml before the measurement was conducted using a Perkin-Elmer Optima 3300DV ICP-OES instrument. Cobalt was measured at 228.616 nm in radial view. A standard series comprising 6 standards (which were 100
Ge- 4
S1
ð2Þ 1
The superscript ‘‘MC’’ indicates that the value was calculated using a Monte Carlo code and ‘‘Exp’’ indicates that the value was determined from the measurement. The subscript ‘‘Sample’’ indicates that the value refers to the sample in question, while the subscript ‘‘Ref’’ indicates that the value refers to a reference sample. The Monte Carlo code employed at IRMM was EGS4 (Nelson et al., 1985) and at PTB it was GESPECOR (Sima and Arnold, 2000). The nuclear decay data of 60Co that was used in the Monte Carlo calculations came from the Table of Radionuclides (B!e et al., 1999). The two gamma rays at 1173 and 1332 keV following the 60Co decay are emitted in cascade. They are weakly correlated (which was neglected), but there is a strong coincidence summing effect in gamma-ray spectrometry in close geometry configurations (i.e. when the sample is very near to the detector). The ratio of the efficiencies calculated with and without coincidence summing for the 1332 keV peak was typically 0.75.
Countrate (d-1 keV-1)
E¼
10
Exp EMC Sample ERef : EMC Ref
0.1 10 S2
PTB-ULB
1 0.1 0.01 1 S3
PTB-ULB
0.1
0.01 1150
1200
1250 1300 Energy (keV)
1350
Fig. 1. Part of the gamma-ray spectrum covering the peaks from 60Co at 1173 and 1332 keV for the three samples nearest to the ground zero (solid line). The name of the detector employed for the measurement is given in each graph, as well as the background spectrum (circles) of that detector.
Table 3 Results of the activity measurements and the elemental analyses Sample no.
Description
Cobalt conc. (mg kg1)
60 Co activity at the start of measurement (mBq)
This study 60Co/cobalt at Aug 6, 1945 (Bq/mg)
S1 S2 S3 S4 S5 S6 S7 S8
A-Bomb Dome Kirin Beer Hall Kodokan Building City Hall Red Cross Hospital Pipe Red Cross Hospital Ladder Hiroshima Bank of Credit Army Food Storehouse
250(7) 139(3) 109(3) 163(3) 122(4) 91(2) 120(2) 81(2)
12.1(18) 1.85(26) o0.24 0.25(11) o0.042 o0.19 o0.051 o0.24
10.3(16) 0.41(7) o0.12 0.047(21) o0.014 o0.06 o0.005 o0.04
ARTICLE IN PRESS M. Hult et al. / Applied Radiation and Isotopes 61 (2004) 173–177
176
1.2 1.0 0.8 0.6 0.4
made up in the same acid matrix as the samples) was used to establish a calibration curve.
Ge-4
S4
3. Results
0.2 0 S5
Ge-3
S6
Ge-5
S7
Ge-4
Table 3 gives the results of the activity measurements as well as for the elemental analysis. The uncertainties are expressed as combined standard uncertainties, following the Guide to the Expression of Uncertainty in Measurement (ISO/IEC/OIML/BIPM, 1995). They are given in brackets after the relevant number and their numerical value refers to the corresponding last digit(s) of the quoted result. The main contribution to the uncertainty for the activity measurements is the counting statistics. The second most important contribution is from the full-energy peak efficiency, although this is generally lesser than 5%. The half-life of 60Co is known with a high accuracy (0.02%); so although a decay correction of 11 half-lives was necessary it contributed only 0.13% to the combined standard uncertainty. All the results were compared to the decision thresholds using a significance level of 0.05 (ISO, 2000). Fig. 1 shows the gamma-ray peaks of interest for the three samples closest to the epicentre and Fig. 2 shows the same as Fig. 1 for samples S4–S7. The background spectrum is given as comparison in each plot.
Countrate (d-1 keV-1)
0.3 0.2 0.1 0 0.6 0.4 0.2 0 0.4 0.2 0 1150
1200
1250 Energy (keV)
1300
1350
Fig. 2. Part of the gamma-ray spectrum covering the peaks from 60Co at 1173 and 1332 keV for samples S4–S7 (solid line). The name of the detector employed for the measurement is given in each graph, as well as the background spectrum (circles) of that detector.
4. Discussion Fig. 3 shows the present data, together with a curve representing the DS86 model calculations. Although some of the values in this study are only given as
Activity of 60Co/mass of Co (Bq/mg)
100 Value with combined standard uncertainty
S1, A-dome
10
decision threshold
1 From Red Cross Hospital Separated in x-position in order to be better displayed
S2
0.1 S6, ladder
S3 S4
S5, pipe
0.01 S7
0.001 500
700
900
1100
1300
1500
1700
1900
Slant range (m) Fig. 3. The graph shows the decay corrected 60Co activity (reference date: August 6, 1945) divided by the stable cobalt mass. The solid line represents the DS86 model calculation.
ARTICLE IN PRESS M. Hult et al. / Applied Radiation and Isotopes 61 (2004) 173–177
decision thresholds, they are still very useful because they are very low and even lower than some previously measured values (Shizuma et al., 1998). Preliminary data from the DS02 report indicate only minor refinements of model calculations compared to the DS86 report. The height of the Hiroshima explosion was, for example, increased from 580 to 600 m and moved 15 m to the west. This should result in a slightly better agreement between the model calculations and the measured data for S1 and S2. The main consequence of the DS02 study is the improved confidence in dose estimates, since most of the discrepancies between measurements and model calculations have been resolved. The measurement result of sample S3 (Kodokan building) deviates from the pattern of the other results. There is currently no explanation for this, but further studies are planned. The method of underground ultra low-level gammaray spectrometry as presented here is very robust, since it involves a minimum of sample preparation. The main source of uncertainty is from counting statistics. It is possible to achieve superior results (by lowering decision thresholds or uncertainties) by measuring the samples for longer periods of time and by using more massive samples.
Acknowledgements The HADES crew of SCKdCEN in Mol, Belgium, is gratefully acknowledged for their work. The input and support of Prof. Kazuhisa Komura (Kanazawa University) and Dr. Saturo Endo, (Hiroshima University) is gratefully acknowledged. We are thankful to Leigh Holmes, Geert van Britsom, Michel Bickel (IRMM) for the ICP-OES measurements, and finally we thank Prof. Peter N. Johnston (Royal Melbourne Institute of Technology) for many fruitful discussions.
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