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Strontium90 for determination of time since death
Forensic Science International 99 (1999) 47–51
Strontium90 for determination of time since death a, b b b c P. Neis *, R. Hille , M. Paschke , G. Pilwat , A. Schnabel , C. Niess c , H. Bratzke c a
Department of Forensic Pathology, University of Mainz, Am. Pulverturm 3, 55131 Mainz, Germany b ¨ ¨ , Julich , Germany Department for Safety and Radiation Protection, Forschungszentrum Julich c Department of Forensic Pathology, University of Frankfurt, Frankfurt, Germany Received 22 June 1998; received in revised form 20 September 1998; accepted 5 October 1998
Abstract Strontium90 (Sr90) is an artificial nuclear fission product of the atmospheric a-bomb testing between 1945 and 1979. It was spread throughout the atmosphere in the following years. Sr90 is an analogue to calcium and therefore enriched in human bones. Several studies especially in the 1960s and 1970s were undertaken to investigate the Sr90 burden and the resulting incorporated radiation in humans, but present studies are missing. In this study nine bone samples, three from 1931 / 32 and six from 1989 to 1994 were examined by measuring the Sr90 radiation. The samples from 1931 / 32 did not show any Sr90 activity. All the samples from 1989 and later showed a Sr90 activity, but the intensity was very variable. Subsequent investigations should be done to determine the cut-off year for measurable Sr90 activity. Furthermore the determination of a specific time since death depending on Sr90 activity should be possible, due to the ranging Sr90 pollution between 1950 and 1980 and different uptake in adolescents and adults. 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Time since death; Strontium90; Bones; Human remains
1. Introduction The determination of time since death is a well-studied subject in forensic sciences. In particular the determination concerning the first hours and days after death are well investigated, but for a longer time there are no reliable parameters known. Some physicochemical parameters like bone staining with ninhydrin [1,2] or estimating the *Corresponding author. 0379-0738 / 99 / $ – see front matter 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0379-0738( 98 )00175-3
48
P. Neis et al. / Forensic Science International 99 (1999) 47 – 51
degree of decomposition of the bone structure as well as the intensity of UVfluorescence were established. The results of these investigations are not very reliable since they are influenced by various factors such as temperature, humidity, soil composition and others. So the results of these investigations however are more or less accurate. In fact, none of these methods are adequate to distinguish in cases where the individual has died several to 100 years before. Radioactive disintegration is a typical process that is independent of all environmental factors. After a defined time only half of a radioactive substance is left, due to the emission of radiation and changing into other elements. Therefore, it would be a perfect marker for the determination of time since death. Another important factor is the enrichment and storage of the radioactive material in the body during the lifetime, as well as the duration of the half-life of the substance. Strontium90, a fallout product of the atmospheric a-bomb testing, for example, is very interesting due to its chemical and physical properties. With an amount of 5% it is one of the most frequent radioactive products after a-bomb explosions [3]. The atmospheric currents have spread Sr90 and the other fallout products all over the world. In the northern hemisphere, there is a three to five times higher Sr90 burden than in the southern hemisphere [4]. Sr90, as an earth alkaline metal, has similar chemical properties to calcium, which is the main earth alkaline metal in bone and many other tissues. Many investigations were undertaken to measure the Sr90 burden in bones [5–8]. As Sr90 is a calcium analogue substance it is accumulated in the bone tissue for a long time, the biological half-life is between 7.5 and 18 years, the physical half-life 28.1 years [9–11]. Sr90 is also one of the most investigated artificial radioactive elements due to the dangerous beta-radiation which was the reason for many investigations between the 1950s and 1970s [12]. After natural and medical radiation as leading radiation sources, Sr90 was in the 1960s at third place. First investigations of Sr90 activity in bones have revealed a significant difference between skeletal remains of the last century and remains of individuals that have died after the a-bomb testing [13].
2. Materials and methods Nine skull bone samples were analyzed for Sr90 activity. The samples were taken from the occipital area of the skull. Samples no. 1 to 3 were from the years 1931 / 32 and samples no. 4 to 9 from the years 1989 to 1994. All samples are remains of people who died in these years and were not buried. This information was obtained from autopsy protocols performed at the department of forensic pathology of the University of Frankfurt. The age of sample no. 2 was 7 years old at the time of death, no. 5 26 years, no. 6 was 16 and no. 9 was 54 years of age at the time of death. The ages of the other samples are not known since the remains of the individuals could not be identified. The analytical procedure for the analysis of Sr90 is mainly carried out using the nitric acid method first described by Harley [14] and is based on the different solubility of the nitrates of calcium and strontium in concentrated nitric acid. Briefly, after dry ashing at 7008C the bone ash is dissolved in nitric acid. Strontium carrier solution is added and the nitrates are precipitated by adding fuming nitric acid. Strontium is then separated from
P. Neis et al. / Forensic Science International 99 (1999) 47 – 51
49
calcium by repeated precipitations with fuming nitric acid. Coprecipitated elements like barium, radium and lead are separated as chromates. Yttrium90 (Y90), the daughter isotope of Sr90 is removed as hydroxide after adding a Y-carrier solution. Date and time are recorded. Strontium is precipitated as carbonate and the weight is determined for Sr recovery. After 2 weeks to establish the equilibrium between Sr90 and Y90, yttrium carrier is added and yttrium is precipitated as hydroxide. For counting, yttrium is then prepared as oxalate. The activity is recorded in a low-level-beta-counter. The yttrium yield measurement is performed by photometric determination of the arsenazo-IIIcomplex.
3. Results and discussion The net count rates of samples 1–3 were 0.0192–0.0763 counts per min. The count rates for samples 4–9 were 0.2370–8.354 counts per min (Table 1). In order to decide that the procedure will be sufficiently precise to yield a satisfactory quantitative estimate the detection limit for the Sr90 activity for each sample was calculated according to DIN 25482-1 ( [15], German Industry Norm) assuming a first degree error of 0.14% and a second degree error of 5%. The activity was corrected for the decay since the year of death (except values below the detection limit). An inspection of the values in Table 2 shows that they can be divided into four groups. In group one (1931–32) the specific activity of samples 1–3 was below the detection limit of the procedure. This is in agreement with the fact that the year of death was before the a-bomb testing. The activity of sample 9 (second group) with a specific activity of 1.2 mBq / g Ca can be explained that the growth of the skeleton was already completed before the fall-out. This is supported by the known date of birth. For the next group (samples 4 and 8) with specific activities of 17.2 and 17.7 mBq / g Ca, respectively, no age or other facts are known, due to unknown identity of these samples. A possible explanation for the lower values of the activities in contrast to the value of the last group is that the growth of the skeleton was not already completed before the fall-out. Table 1 Dry masses and net count rates of samples 1 to 9 Sample no.
Dry mass (g)
1 2 3 4 5 6 7 8 9
36.2 45.5 38.4 68.2 88.1 64.1 55.9 22.3 51.5
Live age 7
26 16
54
Net counts per min 0.0192 0.0763 0.0306 3.425 8.354 6.813 6.622 1.3655 0.237
50
P. Neis et al. / Forensic Science International 99 (1999) 47 – 51
Table 2 Specific activities for Sr90 in mBq / g Ca. The samples are assorted with increasing Sr90 activity Sample no.
Year of death
1 2 3 9 4 8 5 6 7
1931 1931 1932 1994 1989 1992 1990 1990 1991
Age 7 54
26 16
Sr90 activity per g Ca ,0.6 ,0.5 ,0.6 1.2 17.2 17.7 31.3 44.2 41.6
The last group (samples 5–7) shows the highest activities with values between 31.3 and 44.2 mBq / g Ca. High radiation rate was expected because the dates of birth are during the years of the a-bomb testing. Sr90 as an artificial radioactive isotope of strontium is appropriate to determine the time since death of a certain individual. The three samples from the years 1931 and 1932 show no significant Sr90 activity. One can argue that the complete absence of Sr90 activity is a result of the storage inside a building (Department of Forensic Pathology, Frankfurt, Germany), the samples were never buried and therefore never exposed to Sr90-contaminated soil. But this proposal is rather unlikely for the following reasons. Firstly the presence of Sr90 in the bones is the result of an active metabolism that binds the radioactive nuclide in the bone structure. Sr90 behaves similarly to calcium and undergoes an equivalent metabolism. Secondly the depth in which human remains are usually buried is between 1.5 and 2 m below the surface. The penetration depth of Sr90 however is only about 1 cm per year [16] so that Sr90 activity can never reach further below the surface than 80 cm. Finally it has been shown that skeletons exhumed on Ninoshima Island 10 and 26 years after the a-bomb explosion in Hiroshima showed different Sr90 activities for different parts of the skeleton (i.e. femur, tibia, humerus and vertebra). The ground and soil was extremely contaminated with radioactive reaction products like Sr90. One would expect that if there is a considerable influence of Sr90 in the soil on the bones, the activity of the different parts should be in the same range [17]. A problem is to determine a cut-off year for Sr90 determination in skeletal remains. The Sr90 activity rose continuously especially during the first 2 years following a bigger atomic bomb test series. Such as in the years 1960 and 1964 / 65 as well as in 1958 and 1963 (with |90 a-bomb tests and a total of 417 atmospheric tests in the years between 1945 and 1980). The first expanding series (about 20 explosions) were done in 1951 which means that the first measurable Sr90 activity should be detected in remains of individuals who have died in 1953 or later. Additional investigations of Sr90 contents in bones are definitely necessary to verify this proposal and determine the exact cut-off year. It would be then possible to identify every bone piece without Sr90 activity as being older than the cut-off year. This thesis was confirmed by the samples that were from the years of 1931 / 32, which showed no significant radioactivity. A British study on archaeological bone samples revealed higher levels of Sr90 activity than our current
P. Neis et al. / Forensic Science International 99 (1999) 47 – 51
51
investigations but it must be considered that the exact death dates of the archaeological as well as the newer material from this study were not known [13], nor was the age at death of these individuals. The activities of the recent samples in that study ranged from 2.18 to 3.15 mBq / g Ca and were, except for our sample of the 54-year-old individual, 10 to 20 times lower than ours. Our investigations show that the Sr90 method is very simple and reliable for the question of whether an individual died before or after the 50th year of our century. Further investigations however need to be done to estimate the exact cut-off year and to find a possible correlation between the Sr90 burden and the year of death. Right now these experiments are being performed on 35 samples for 1997 and another 140 from the years of 1950 to 1990.
References [1] H.G. Koslowsky, Untersuchungen uber die Altersbestimmungen von Skeletteilen, Thesis, Erlangen University, Germany, 1953. [2] T. Spiegel, Ninhydrin-Reaktion zur Altersbestimmung des Knochens, Thesis, Erlangen University, Germany, 1954. ¨ und Umwelt, Die zivilisatorisch bedingte Strahlenbelas[3] H.H. Henk, 2. Beitrag zum Thema Radioaktivitat tung, 1978, pp. 14–17. ¨ [4] G. Cavelius, Vergleichende Studie uber den Stoffwechsel von Calcium, Strontium, Barium und Radium ¨ unter besonderer Berucksichtigung des Calcium-Bedarfs des Menschen, Thesis, Homburg, Germany, 1989. ¨ [5] O. Kuntzen, Experimentelle Untersuchungen uber die Wirkung des stabilen Strontium in Knochen, ¨ Germany, 1966. Thesis, Koln, [6] F.A. Mettler, W.K. Sinclair, L. Anspaugh, et al., The 1986 and 1988 UNSCEAR Reports: findings and implications, Health Phys. 58(3) (1990) 241–250. [7] F.W. Spiers, Dose to bone from strontium-90: implications for the setting of the maximum permissible body burden, Radiat. Res. 28 (1966) 624–642. [8] F.W. Spiers, Radionuclides and bone — from 226Ra to 90Sr, Br. J. Radiol. 47 (1974) 833–844. [9] W. Fletcher, J.F. Loutit, D.G. Papworth, Interpretation of levels of strontium-90 in human bone, Br. Med. J. 2 (1966) 1225–1230. [10] J.F. Loutit, The metabolism of strontium 90 in bone. Chem. Ind. (1962) 1228–1229. [11] J.F. Loutit, What is the turnover of bone mineral?, Calc. Tissue Res. 2 (1968) 111–114. [12] D.G. Papworth, J. Vennart, The uptake and turnover of Sr90 in the human skeleton, Phys. Med. Biol. 29(9) (1984) 1045–1061. [13] S.M. Maclaughlin-Black, R.J.M. Herd, K. Willson, M. Myers, I.E. West, Strontium-90 as an indicator of time since death: a pilot investigation, Forensic Sci. Int. 57 (1992) 51–66. [14] D.H. Harley (Ed.), HASL Procedure Manual US Atomic Energy Commission, New York, 1972, 1976. [15] DIN 25482-1 Limit of Detection and Decision for Nuclear Radiation Measurements; Counting measurements, Neglecting the Influence of Sample Treatment, Beuth Verlag, Berlin, 1989. [16] I. Gans, J. Arndt, Die Verteilung der langlebigen Spaltprodukte Strontium90 und Caesium 137 des Kernwaffen-Fallouts im Boden, Institute for Water, Soil and Air Hygiene, German Health agency, 1987. [17] H. Kawamura, et al., Strontium-90 activity in bones of the a-bomb exposed in Hiroshima and exhumed on Ninoshima island, J. Radiat. Res. 28 (1987) 109–116.
Strontium90 for determination of time since death a, b b b c P. Neis *, R. Hille , M. Paschke , G. Pilwat , A. Schnabel , C. Niess c , H. Bratzke c a
Department of Forensic Pathology, University of Mainz, Am. Pulverturm 3, 55131 Mainz, Germany b ¨ ¨ , Julich , Germany Department for Safety and Radiation Protection, Forschungszentrum Julich c Department of Forensic Pathology, University of Frankfurt, Frankfurt, Germany Received 22 June 1998; received in revised form 20 September 1998; accepted 5 October 1998
Abstract Strontium90 (Sr90) is an artificial nuclear fission product of the atmospheric a-bomb testing between 1945 and 1979. It was spread throughout the atmosphere in the following years. Sr90 is an analogue to calcium and therefore enriched in human bones. Several studies especially in the 1960s and 1970s were undertaken to investigate the Sr90 burden and the resulting incorporated radiation in humans, but present studies are missing. In this study nine bone samples, three from 1931 / 32 and six from 1989 to 1994 were examined by measuring the Sr90 radiation. The samples from 1931 / 32 did not show any Sr90 activity. All the samples from 1989 and later showed a Sr90 activity, but the intensity was very variable. Subsequent investigations should be done to determine the cut-off year for measurable Sr90 activity. Furthermore the determination of a specific time since death depending on Sr90 activity should be possible, due to the ranging Sr90 pollution between 1950 and 1980 and different uptake in adolescents and adults. 1999 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Time since death; Strontium90; Bones; Human remains
1. Introduction The determination of time since death is a well-studied subject in forensic sciences. In particular the determination concerning the first hours and days after death are well investigated, but for a longer time there are no reliable parameters known. Some physicochemical parameters like bone staining with ninhydrin [1,2] or estimating the *Corresponding author. 0379-0738 / 99 / $ – see front matter 1999 Elsevier Science Ireland Ltd. All rights reserved. PII: S0379-0738( 98 )00175-3
48
P. Neis et al. / Forensic Science International 99 (1999) 47 – 51
degree of decomposition of the bone structure as well as the intensity of UVfluorescence were established. The results of these investigations are not very reliable since they are influenced by various factors such as temperature, humidity, soil composition and others. So the results of these investigations however are more or less accurate. In fact, none of these methods are adequate to distinguish in cases where the individual has died several to 100 years before. Radioactive disintegration is a typical process that is independent of all environmental factors. After a defined time only half of a radioactive substance is left, due to the emission of radiation and changing into other elements. Therefore, it would be a perfect marker for the determination of time since death. Another important factor is the enrichment and storage of the radioactive material in the body during the lifetime, as well as the duration of the half-life of the substance. Strontium90, a fallout product of the atmospheric a-bomb testing, for example, is very interesting due to its chemical and physical properties. With an amount of 5% it is one of the most frequent radioactive products after a-bomb explosions [3]. The atmospheric currents have spread Sr90 and the other fallout products all over the world. In the northern hemisphere, there is a three to five times higher Sr90 burden than in the southern hemisphere [4]. Sr90, as an earth alkaline metal, has similar chemical properties to calcium, which is the main earth alkaline metal in bone and many other tissues. Many investigations were undertaken to measure the Sr90 burden in bones [5–8]. As Sr90 is a calcium analogue substance it is accumulated in the bone tissue for a long time, the biological half-life is between 7.5 and 18 years, the physical half-life 28.1 years [9–11]. Sr90 is also one of the most investigated artificial radioactive elements due to the dangerous beta-radiation which was the reason for many investigations between the 1950s and 1970s [12]. After natural and medical radiation as leading radiation sources, Sr90 was in the 1960s at third place. First investigations of Sr90 activity in bones have revealed a significant difference between skeletal remains of the last century and remains of individuals that have died after the a-bomb testing [13].
2. Materials and methods Nine skull bone samples were analyzed for Sr90 activity. The samples were taken from the occipital area of the skull. Samples no. 1 to 3 were from the years 1931 / 32 and samples no. 4 to 9 from the years 1989 to 1994. All samples are remains of people who died in these years and were not buried. This information was obtained from autopsy protocols performed at the department of forensic pathology of the University of Frankfurt. The age of sample no. 2 was 7 years old at the time of death, no. 5 26 years, no. 6 was 16 and no. 9 was 54 years of age at the time of death. The ages of the other samples are not known since the remains of the individuals could not be identified. The analytical procedure for the analysis of Sr90 is mainly carried out using the nitric acid method first described by Harley [14] and is based on the different solubility of the nitrates of calcium and strontium in concentrated nitric acid. Briefly, after dry ashing at 7008C the bone ash is dissolved in nitric acid. Strontium carrier solution is added and the nitrates are precipitated by adding fuming nitric acid. Strontium is then separated from
P. Neis et al. / Forensic Science International 99 (1999) 47 – 51
49
calcium by repeated precipitations with fuming nitric acid. Coprecipitated elements like barium, radium and lead are separated as chromates. Yttrium90 (Y90), the daughter isotope of Sr90 is removed as hydroxide after adding a Y-carrier solution. Date and time are recorded. Strontium is precipitated as carbonate and the weight is determined for Sr recovery. After 2 weeks to establish the equilibrium between Sr90 and Y90, yttrium carrier is added and yttrium is precipitated as hydroxide. For counting, yttrium is then prepared as oxalate. The activity is recorded in a low-level-beta-counter. The yttrium yield measurement is performed by photometric determination of the arsenazo-IIIcomplex.
3. Results and discussion The net count rates of samples 1–3 were 0.0192–0.0763 counts per min. The count rates for samples 4–9 were 0.2370–8.354 counts per min (Table 1). In order to decide that the procedure will be sufficiently precise to yield a satisfactory quantitative estimate the detection limit for the Sr90 activity for each sample was calculated according to DIN 25482-1 ( [15], German Industry Norm) assuming a first degree error of 0.14% and a second degree error of 5%. The activity was corrected for the decay since the year of death (except values below the detection limit). An inspection of the values in Table 2 shows that they can be divided into four groups. In group one (1931–32) the specific activity of samples 1–3 was below the detection limit of the procedure. This is in agreement with the fact that the year of death was before the a-bomb testing. The activity of sample 9 (second group) with a specific activity of 1.2 mBq / g Ca can be explained that the growth of the skeleton was already completed before the fall-out. This is supported by the known date of birth. For the next group (samples 4 and 8) with specific activities of 17.2 and 17.7 mBq / g Ca, respectively, no age or other facts are known, due to unknown identity of these samples. A possible explanation for the lower values of the activities in contrast to the value of the last group is that the growth of the skeleton was not already completed before the fall-out. Table 1 Dry masses and net count rates of samples 1 to 9 Sample no.
Dry mass (g)
1 2 3 4 5 6 7 8 9
36.2 45.5 38.4 68.2 88.1 64.1 55.9 22.3 51.5
Live age 7
26 16
54
Net counts per min 0.0192 0.0763 0.0306 3.425 8.354 6.813 6.622 1.3655 0.237
50
P. Neis et al. / Forensic Science International 99 (1999) 47 – 51
Table 2 Specific activities for Sr90 in mBq / g Ca. The samples are assorted with increasing Sr90 activity Sample no.
Year of death
1 2 3 9 4 8 5 6 7
1931 1931 1932 1994 1989 1992 1990 1990 1991
Age 7 54
26 16
Sr90 activity per g Ca ,0.6 ,0.5 ,0.6 1.2 17.2 17.7 31.3 44.2 41.6
The last group (samples 5–7) shows the highest activities with values between 31.3 and 44.2 mBq / g Ca. High radiation rate was expected because the dates of birth are during the years of the a-bomb testing. Sr90 as an artificial radioactive isotope of strontium is appropriate to determine the time since death of a certain individual. The three samples from the years 1931 and 1932 show no significant Sr90 activity. One can argue that the complete absence of Sr90 activity is a result of the storage inside a building (Department of Forensic Pathology, Frankfurt, Germany), the samples were never buried and therefore never exposed to Sr90-contaminated soil. But this proposal is rather unlikely for the following reasons. Firstly the presence of Sr90 in the bones is the result of an active metabolism that binds the radioactive nuclide in the bone structure. Sr90 behaves similarly to calcium and undergoes an equivalent metabolism. Secondly the depth in which human remains are usually buried is between 1.5 and 2 m below the surface. The penetration depth of Sr90 however is only about 1 cm per year [16] so that Sr90 activity can never reach further below the surface than 80 cm. Finally it has been shown that skeletons exhumed on Ninoshima Island 10 and 26 years after the a-bomb explosion in Hiroshima showed different Sr90 activities for different parts of the skeleton (i.e. femur, tibia, humerus and vertebra). The ground and soil was extremely contaminated with radioactive reaction products like Sr90. One would expect that if there is a considerable influence of Sr90 in the soil on the bones, the activity of the different parts should be in the same range [17]. A problem is to determine a cut-off year for Sr90 determination in skeletal remains. The Sr90 activity rose continuously especially during the first 2 years following a bigger atomic bomb test series. Such as in the years 1960 and 1964 / 65 as well as in 1958 and 1963 (with |90 a-bomb tests and a total of 417 atmospheric tests in the years between 1945 and 1980). The first expanding series (about 20 explosions) were done in 1951 which means that the first measurable Sr90 activity should be detected in remains of individuals who have died in 1953 or later. Additional investigations of Sr90 contents in bones are definitely necessary to verify this proposal and determine the exact cut-off year. It would be then possible to identify every bone piece without Sr90 activity as being older than the cut-off year. This thesis was confirmed by the samples that were from the years of 1931 / 32, which showed no significant radioactivity. A British study on archaeological bone samples revealed higher levels of Sr90 activity than our current
P. Neis et al. / Forensic Science International 99 (1999) 47 – 51
51
investigations but it must be considered that the exact death dates of the archaeological as well as the newer material from this study were not known [13], nor was the age at death of these individuals. The activities of the recent samples in that study ranged from 2.18 to 3.15 mBq / g Ca and were, except for our sample of the 54-year-old individual, 10 to 20 times lower than ours. Our investigations show that the Sr90 method is very simple and reliable for the question of whether an individual died before or after the 50th year of our century. Further investigations however need to be done to estimate the exact cut-off year and to find a possible correlation between the Sr90 burden and the year of death. Right now these experiments are being performed on 35 samples for 1997 and another 140 from the years of 1950 to 1990.
References [1] H.G. Koslowsky, Untersuchungen uber die Altersbestimmungen von Skeletteilen, Thesis, Erlangen University, Germany, 1953. [2] T. Spiegel, Ninhydrin-Reaktion zur Altersbestimmung des Knochens, Thesis, Erlangen University, Germany, 1954. ¨ und Umwelt, Die zivilisatorisch bedingte Strahlenbelas[3] H.H. Henk, 2. Beitrag zum Thema Radioaktivitat tung, 1978, pp. 14–17. ¨ [4] G. Cavelius, Vergleichende Studie uber den Stoffwechsel von Calcium, Strontium, Barium und Radium ¨ unter besonderer Berucksichtigung des Calcium-Bedarfs des Menschen, Thesis, Homburg, Germany, 1989. ¨ [5] O. Kuntzen, Experimentelle Untersuchungen uber die Wirkung des stabilen Strontium in Knochen, ¨ Germany, 1966. Thesis, Koln, [6] F.A. Mettler, W.K. Sinclair, L. Anspaugh, et al., The 1986 and 1988 UNSCEAR Reports: findings and implications, Health Phys. 58(3) (1990) 241–250. [7] F.W. Spiers, Dose to bone from strontium-90: implications for the setting of the maximum permissible body burden, Radiat. Res. 28 (1966) 624–642. [8] F.W. Spiers, Radionuclides and bone — from 226Ra to 90Sr, Br. J. Radiol. 47 (1974) 833–844. [9] W. Fletcher, J.F. Loutit, D.G. Papworth, Interpretation of levels of strontium-90 in human bone, Br. Med. J. 2 (1966) 1225–1230. [10] J.F. Loutit, The metabolism of strontium 90 in bone. Chem. Ind. (1962) 1228–1229. [11] J.F. Loutit, What is the turnover of bone mineral?, Calc. Tissue Res. 2 (1968) 111–114. [12] D.G. Papworth, J. Vennart, The uptake and turnover of Sr90 in the human skeleton, Phys. Med. Biol. 29(9) (1984) 1045–1061. [13] S.M. Maclaughlin-Black, R.J.M. Herd, K. Willson, M. Myers, I.E. West, Strontium-90 as an indicator of time since death: a pilot investigation, Forensic Sci. Int. 57 (1992) 51–66. [14] D.H. Harley (Ed.), HASL Procedure Manual US Atomic Energy Commission, New York, 1972, 1976. [15] DIN 25482-1 Limit of Detection and Decision for Nuclear Radiation Measurements; Counting measurements, Neglecting the Influence of Sample Treatment, Beuth Verlag, Berlin, 1989. [16] I. Gans, J. Arndt, Die Verteilung der langlebigen Spaltprodukte Strontium90 und Caesium 137 des Kernwaffen-Fallouts im Boden, Institute for Water, Soil and Air Hygiene, German Health agency, 1987. [17] H. Kawamura, et al., Strontium-90 activity in bones of the a-bomb exposed in Hiroshima and exhumed on Ninoshima island, J. Radiat. Res. 28 (1987) 109–116.