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226Ra leaching from fossil bones
NUCLEAR INSTRUMENTS
AND METHODS
151 ( 1 9 7 8 )
599-601
; ©
NORTH-HOLLAND
P U B L I S H I N G CO.
ZZ6Ra LEACHING FROM FOSSIL BONES C. PAPASTEFANOU AND STEF. CHARALAMBOUS
Department of Nuclear Physics, University of Thessalonild, Greece Received 11 October 1977 226Ra and 238U concentrations in fossil bones were measured. A marked disequilibrium between radium and its parent uranium was found. This is probably due to the leaching of radium from fossil bones. A special case of equilibrium between 238U-226Ra is described. For the fossils studied a leaching constant with value of about 10 -11 s -1 is deduced.
It. Introduction Fossil bones exhibit high radioactivityl~). In a recent paper (which hereafter we denote by C-P), Charalambous and PapastefanouS), have described the results of extensive studies of the radioactivity of fossil bones. The radionuclides found were only those of the U-series. Assuming that fossilization is completed more or less in the first 10 4 y (ref. 6), it would be expected that 238U should be in secular equilibrium with its daughters. However, C - P found that 226Ra and 238U are in disequilibrium in the fossils. This disequilibrium is attributed to a leaching of radium. In the present work the 226Ra-238U disequilibrium in the fossil bones is studied. 2. Definition of ZZ6Raleaching factor Consider a sample of fossil bone in its environment of fossilization which contains a given amount (M nuclei) of 238U. Without any leaching the production rate of 226Ra, d N / d t , will be described by the well known equation: dN -d-'/- = 4 1 M - 2 2 N , (1) where 21,22 are the decay constants of 238U and :'26Ra respectively. Now consider that Ra is leached from the los,ills. Let us define the leaching constant k as the probability of one Ra atom being leached in unit time. The constant k should be a function of rnany parameters such as the porosity of the fossil bone, the presence of inorganic radicals in the fos,ill environment etc. Let us assume that k has rernained constant during the last 6000 y (say about 4 TR,) for a given system, i.e. fossil bone-environment. With these assumptions, the net accumulation rate of 226Ra will be described by the equal:ion:
dN dt-
21M - 22N - k N
(2)
Eq. (2) gives: 21M° [ e - ' t ! t - - e -O'2+k)t] N -- k+22_21
(3)
assuming N = 0 at time t = 0. Because )~,~22, it implies the following: 41 < ~ 2 2 q - k
and
e-t'~z+k),~e -'~'t
(4)
Therefore, the term e x p [ - ( 2 2 + k ) t ] in eq. (3) becomes negligible as compared to term e x p ( - 2 1 t) after a long time t, practically after 104 y. Then eq. (3) becomes: N
21Mo
e-Z,t,
k_k_ 22 _ 4 1
(5)
21M k+22_2; .
(6)
or
N=
Now, we define the measurable quantity p 21M - 22N P 2 xM
(7)
Substituting for the number N of the nuclei from eq. (6) in eq. (7), we get: (1-p)
21A// := 2~ k.k_t_j.z_21j,
(8)
or
l -p
2, = k + )~-2]
22 ~ k ~ z,--'-v-'
(9)
because of eq. (4). From eq. (9) we get the value of k, the leaching constant, k--
P
1--p
42.
(10)
The value of p could be calculated experimentally by the activities of radium and uranium.
600
C. P A P A S T E F A N O U
AND STEF. C H A R A L A M B O U S
3. Experimental procedure and results The fossil bones examined were from our Laboratory's collection and had been previously studied for their radioactivityS). The selected fossil bones belong to various geological ages from the earlier Holecene period to the older upper Miocene. They are of various kinds of animals, big (elephants) or small (hipparia), from different locations in Northern Greece. The laboratory numbers given in table 1 correspond to the numbers of table 1 of the work C - P . The information concerning the fossils examined are given there. The procedure of preparation of the samples was that described in C - P . The radioactivity of the samples was measured by gamma-ray analysis.The gamma spectroscopy was done using a 4 0 c m 3 Ge(Li) detector and a 4096-channel P.H.A. Here the measurements are mainly concentrated on the activity of the 63 keV and 186 keV lines. Fig. 1 shows a typical gamma spectrum for a representative sample of " h o t " fossil (spectrum I) and the background (spectrum II). The spectra cover the range from 40 to 200 keV. Seen in spec-
TABLE 1
Reduced difference of the activities of 238U and 226Ra measured in fossil bones with a Ge(Li) detector.
Lab. ref. no.
F-la
F-7 F-8 F-9
F-17 F-10 F-19 F-20 F-12 F-16
F-13
Activity of 63 keV line
21M(dps)
Activity of 186 keV line 22 N (dps)
3.70___0.15 3.47_+0.15 3.75_+0.15 2.76_+0.12 3.39_+0.14 1.63_+0.08 3.05-+0.13 1.40_+ 0.09 1.62 -+0.09 3.48_+0.14 3.62_+0.15
1.15_+0.13 1.04+0.11 1.16_+0.12 0.83_+0.09 0.98_+0.11 0.49_+ 0.06 0.88___0.10 0.42 -+0.05 0.49_+ 0.06 1.04_+0.11 1.19_+0.13
Reduced difference p 0.69___0.08 0.70___0.08 0.69_+0.08 0.70_+0.08 0.71 _+0.08 0.70_+0.09 0.71 -+0.08 0.70-+ 0.09 0.70_+ 0.09 0.70_+0.08 0.67_+0.08
trum I are the important 63 keV gamma transition line of 234Th, first daughter of 238U, and the 186 keV line of ~26Ra. These lines are absent in spectrum II of the background. From the intensity of each of these gammas, their decay characteris-
o K
I
3-
t
.J:
,v I==
2-
®~
-~
0
A •... 1.,
- ~
""" ~.,.,.~A""-"
j
~.--"
i ' , . .
• ..So'-'~_. ,¢ o......~ .. ~ .~..=..,o....,........[-~.
,, .,,
~
}I~ I
:.-.... ,,
,.
-%.
..~....J
|
Fig. 1. Typical gamma spectrum of a hot fossil bone (I) and background (I1) from about 40 to 200 keV.
oAo
226Ra L E A C H I N G
tics, the efficiency curve of the detector etc., it is possible to calculate the activities of 23sU and n6Ra. The determined activities of these nuclides in the fossil bones examined are presented in table 1. Shown in column 4 are the values of the reduced difference p of the activities of the two nuclides. The experimental results indicate that p shows small variations and range from 0.67_+0.08 to 0.71 ___0.09. From these values the constant k is deduced. It is found that k ranges from 2.8 to 3.4× 10 -it s -1 . The value of k found implies that the probability of leaching of radium is very very small and may be considered about the same for all the fossil bones examined. This suggests that: 1) The bones had about the same content before fossilization. 2) The immediate environment of the fossil bones examined did not suffer strong variations in their concentrations of exchange carriers, at least during the last ten thousand years.
601
The leaching of radium from fossil bones as confirmed by the above measurements leads to the hypothesis of leaching of other elements also found in fossil bones. This might happen either to the original constituents of the fossil or the elements adsorbed to the fossils during their histories. This leaching will obviously complicate a radiodating calculation in the cases where the leaching element is in the chain of the radioactive series used. References 1) E. B. Jaffe and A. M. Sherwood, U.S. Geol. Survey Rept. TEM 149 (1951) p. 19. 2) S. H. U. Bowie and D. Atkin, Nature 177 (1956) 487. 3) W. R. Diggle and J. Saxon, Nature 208 (1965) 400. 4) O. Otgonsuren, V. P. Perelygin and D. Chultem, At. Energy 20 (1970) 301. 5) S. Charalambous and C. Papastefanou, Nucl. Instr. and Meth. 142 (1977) 581. 6) K. K. Turekian, D. P. Kharkar, J. Funkhouser and O. A. Schaeffer, Earth Planet. Sci. Lett. 7 (1970) 420. 7) C. L. Lindeken, D. G. Coles and J. W. Meadows, Report UCRL 75659, University of California (1974).
AND METHODS
151 ( 1 9 7 8 )
599-601
; ©
NORTH-HOLLAND
P U B L I S H I N G CO.
ZZ6Ra LEACHING FROM FOSSIL BONES C. PAPASTEFANOU AND STEF. CHARALAMBOUS
Department of Nuclear Physics, University of Thessalonild, Greece Received 11 October 1977 226Ra and 238U concentrations in fossil bones were measured. A marked disequilibrium between radium and its parent uranium was found. This is probably due to the leaching of radium from fossil bones. A special case of equilibrium between 238U-226Ra is described. For the fossils studied a leaching constant with value of about 10 -11 s -1 is deduced.
It. Introduction Fossil bones exhibit high radioactivityl~). In a recent paper (which hereafter we denote by C-P), Charalambous and PapastefanouS), have described the results of extensive studies of the radioactivity of fossil bones. The radionuclides found were only those of the U-series. Assuming that fossilization is completed more or less in the first 10 4 y (ref. 6), it would be expected that 238U should be in secular equilibrium with its daughters. However, C - P found that 226Ra and 238U are in disequilibrium in the fossils. This disequilibrium is attributed to a leaching of radium. In the present work the 226Ra-238U disequilibrium in the fossil bones is studied. 2. Definition of ZZ6Raleaching factor Consider a sample of fossil bone in its environment of fossilization which contains a given amount (M nuclei) of 238U. Without any leaching the production rate of 226Ra, d N / d t , will be described by the well known equation: dN -d-'/- = 4 1 M - 2 2 N , (1) where 21,22 are the decay constants of 238U and :'26Ra respectively. Now consider that Ra is leached from the los,ills. Let us define the leaching constant k as the probability of one Ra atom being leached in unit time. The constant k should be a function of rnany parameters such as the porosity of the fossil bone, the presence of inorganic radicals in the fos,ill environment etc. Let us assume that k has rernained constant during the last 6000 y (say about 4 TR,) for a given system, i.e. fossil bone-environment. With these assumptions, the net accumulation rate of 226Ra will be described by the equal:ion:
dN dt-
21M - 22N - k N
(2)
Eq. (2) gives: 21M° [ e - ' t ! t - - e -O'2+k)t] N -- k+22_21
(3)
assuming N = 0 at time t = 0. Because )~,~22, it implies the following: 41 < ~ 2 2 q - k
and
e-t'~z+k),~e -'~'t
(4)
Therefore, the term e x p [ - ( 2 2 + k ) t ] in eq. (3) becomes negligible as compared to term e x p ( - 2 1 t) after a long time t, practically after 104 y. Then eq. (3) becomes: N
21Mo
e-Z,t,
k_k_ 22 _ 4 1
(5)
21M k+22_2; .
(6)
or
N=
Now, we define the measurable quantity p 21M - 22N P 2 xM
(7)
Substituting for the number N of the nuclei from eq. (6) in eq. (7), we get: (1-p)
21A// := 2~ k.k_t_j.z_21j,
(8)
or
l -p
2, = k + )~-2]
22 ~ k ~ z,--'-v-'
(9)
because of eq. (4). From eq. (9) we get the value of k, the leaching constant, k--
P
1--p
42.
(10)
The value of p could be calculated experimentally by the activities of radium and uranium.
600
C. P A P A S T E F A N O U
AND STEF. C H A R A L A M B O U S
3. Experimental procedure and results The fossil bones examined were from our Laboratory's collection and had been previously studied for their radioactivityS). The selected fossil bones belong to various geological ages from the earlier Holecene period to the older upper Miocene. They are of various kinds of animals, big (elephants) or small (hipparia), from different locations in Northern Greece. The laboratory numbers given in table 1 correspond to the numbers of table 1 of the work C - P . The information concerning the fossils examined are given there. The procedure of preparation of the samples was that described in C - P . The radioactivity of the samples was measured by gamma-ray analysis.The gamma spectroscopy was done using a 4 0 c m 3 Ge(Li) detector and a 4096-channel P.H.A. Here the measurements are mainly concentrated on the activity of the 63 keV and 186 keV lines. Fig. 1 shows a typical gamma spectrum for a representative sample of " h o t " fossil (spectrum I) and the background (spectrum II). The spectra cover the range from 40 to 200 keV. Seen in spec-
TABLE 1
Reduced difference of the activities of 238U and 226Ra measured in fossil bones with a Ge(Li) detector.
Lab. ref. no.
F-la
F-7 F-8 F-9
F-17 F-10 F-19 F-20 F-12 F-16
F-13
Activity of 63 keV line
21M(dps)
Activity of 186 keV line 22 N (dps)
3.70___0.15 3.47_+0.15 3.75_+0.15 2.76_+0.12 3.39_+0.14 1.63_+0.08 3.05-+0.13 1.40_+ 0.09 1.62 -+0.09 3.48_+0.14 3.62_+0.15
1.15_+0.13 1.04+0.11 1.16_+0.12 0.83_+0.09 0.98_+0.11 0.49_+ 0.06 0.88___0.10 0.42 -+0.05 0.49_+ 0.06 1.04_+0.11 1.19_+0.13
Reduced difference p 0.69___0.08 0.70___0.08 0.69_+0.08 0.70_+0.08 0.71 _+0.08 0.70_+0.09 0.71 -+0.08 0.70-+ 0.09 0.70_+ 0.09 0.70_+0.08 0.67_+0.08
trum I are the important 63 keV gamma transition line of 234Th, first daughter of 238U, and the 186 keV line of ~26Ra. These lines are absent in spectrum II of the background. From the intensity of each of these gammas, their decay characteris-
o K
I
3-
t
.J:
,v I==
2-
®~
-~
0
A •... 1.,
- ~
""" ~.,.,.~A""-"
j
~.--"
i ' , . .
• ..So'-'~_. ,¢ o......~ .. ~ .~..=..,o....,........[-~.
,, .,,
~
}I~ I
:.-.... ,,
,.
-%.
..~....J
|
Fig. 1. Typical gamma spectrum of a hot fossil bone (I) and background (I1) from about 40 to 200 keV.
oAo
226Ra L E A C H I N G
tics, the efficiency curve of the detector etc., it is possible to calculate the activities of 23sU and n6Ra. The determined activities of these nuclides in the fossil bones examined are presented in table 1. Shown in column 4 are the values of the reduced difference p of the activities of the two nuclides. The experimental results indicate that p shows small variations and range from 0.67_+0.08 to 0.71 ___0.09. From these values the constant k is deduced. It is found that k ranges from 2.8 to 3.4× 10 -it s -1 . The value of k found implies that the probability of leaching of radium is very very small and may be considered about the same for all the fossil bones examined. This suggests that: 1) The bones had about the same content before fossilization. 2) The immediate environment of the fossil bones examined did not suffer strong variations in their concentrations of exchange carriers, at least during the last ten thousand years.
601
The leaching of radium from fossil bones as confirmed by the above measurements leads to the hypothesis of leaching of other elements also found in fossil bones. This might happen either to the original constituents of the fossil or the elements adsorbed to the fossils during their histories. This leaching will obviously complicate a radiodating calculation in the cases where the leaching element is in the chain of the radioactive series used. References 1) E. B. Jaffe and A. M. Sherwood, U.S. Geol. Survey Rept. TEM 149 (1951) p. 19. 2) S. H. U. Bowie and D. Atkin, Nature 177 (1956) 487. 3) W. R. Diggle and J. Saxon, Nature 208 (1965) 400. 4) O. Otgonsuren, V. P. Perelygin and D. Chultem, At. Energy 20 (1970) 301. 5) S. Charalambous and C. Papastefanou, Nucl. Instr. and Meth. 142 (1977) 581. 6) K. K. Turekian, D. P. Kharkar, J. Funkhouser and O. A. Schaeffer, Earth Planet. Sci. Lett. 7 (1970) 420. 7) C. L. Lindeken, D. G. Coles and J. W. Meadows, Report UCRL 75659, University of California (1974).