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Thorium and uranium content determination using double gamma irradiation
Pergamon
Radiation Measurements, Vol. 28, Nos I-6, pp. 393-396, 1997 © 1997 ElsevierScience Ltd Printed in Great Britain. All rights reserved 1350-4487/97 $17.00 + 0.00 PII: S 1350-4487(97)00106-6
THORIUM AND URANIUM CONTENT DETERMINATION USING DOUBLE GAMMA IRRADIATION
L. ENKHJIN AND V.P. PERELYGIN Joint Institute for Nuclear Research, 141980, Dubna, Russian Federation ABSTRACT A new technique for the determination of Th and U content in solids was proposed. The ratio of U and Th fission reaction yields for "infinite thick" metall layers was measured in the y-ray energy range from 7 MeV to 23.5 MeV. For this purpose metallic sheets of U and Th placed next to polyethyleneterephtalate track detectors were irradiated at the Microtron MT-25. The method is based on the fact that the ratio Nu/Nrh differs sufficiently from 1.7 to 3.3 depending on the )'-ray energy. Irradiating an Uranium ethalon and samples with y-rays of (20-23) MeV (No/Nrh = 2.2) and (15-17) MeV (Nu/Nrh = 3.3) one can determine simultaneously both U and Th content in samples. The comparison of the results of measuring of U and Th content in different samples obtained by the proposed and the known methods is presented. KEYWORDS Thorium; Uranium; SSNTD; polyethyleneterephtalate; photonuclear fission cross section of U and Th; photonuclear fission yield of U and Th INTRODUCTION We have previously reported the improved track technique for Th content determination in solids using neutrons and y-rays (Otgonsuren et al., 1995). There are some other track detector methods for Th analysis, such as, methods using irradiations with neutrons and protons, neutrons and alpha particles, neutrons and alpharadiography (Hair et al., 1971; Bimbot et al.; 1967, Flerov, 1979) As a rule, Thbearing geological samples contain U, too. Because both 23SU and 232Th nuclei undergo fission by y-rays, alpha particles, protons, fast neutrons, each of the above mentioned techniques requires thermal neutrons irradiation for the determination of U contents in samples. In this paper, we present a track detector method for simultaneous determination of U and Th using double y-irradiation. M E T H O D AND E X P E R I M E N T A L Ratio Nt, /NTh The proposed technique of Th and U content determination is based on the variation of the ratio of fission yields of reactions U(7, f) and Th(y, f) depending on the energy of y-rays in the range from 7 MeV to 23 MeV. The ratio of fission yields of these reactions were measured with the aid of layers "infinitely thick" to fission fragments i.e metallic sheets of U and Th placed next to polyethyleneterephtalate (lavsan) track detectors. This target-detector assembly was irradiated with '/-rays at beam intensity of 10 IxA for (5-30) s at the Microtron MT-25, Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research. Before the irradiation the metallic sheets were polished for surface oxide layers to be removed. The behavior of the ratio of fission fragments track density for U to that for Th depending on y-rays energy is shown in Fig 1.
393
394
PROCEEDINGS OF TIlE 18TH INTERNATIONAL CONFERENCE
4.0 3.5
{
3.0
!
2.5
f
2.0 1.5 1.0
,
5
I
10
i
I
,
15
I
i
20
25
E, MeV Fig. 1. Dependence of the ratio of fission fragment tracks for U and Th "thick" layers on T-rays energy One can see from Fig. 1 that the ratio Nu/Nrh ranges from 1.7 at (7-8) MeV to 3.3 at (15-17) MeV. However, at "/-ray energy of (7-8) MeV, fission fragment yield per BA.h of exposure is less than that at higher energies by I0-15 times.Therefore we preferred to use in our experiments `/-rays of energy (1517) MeV and (20-23) MeV.
Formulaefor Cv and Crh Track densities in detectors (polyethyleneterephtalate) irradiated in contact with a sample and an uranium standard (calibrated thin layer) with "/-rays are equal to [the superscripts 1 and 2 correspond to the energy ranges of?-rays (20-23) MeV and (15-17) MeV, respectively)]:
NIs = @l ¢ (Cu O'Iu RefU A/238 +Crh o'lTh RcfTh A/232)
())
N2s = @2e (Cu O2u Rcfu A/238 +CTh O2rhRefTh A/232)
N'~t = @t olug Cst A/238 (2) N2st = ~)2 0.2 u g Cst A/238 where @ is `/-rays fluence, cu and arh are fssion cross section of 238U and 232 Th nuclei, e is registration efficiency of the detector, ILfu and l ~ t ~ are thickness of effective matter layer of U and Th fission fragments in mg.cm"2, A is Avogadro's number, Cu and C~ are U and Th content in the sample, Ca is U content of the standard sample in mg.cm"~. Because the values au and a n are difficult to measure the ratios alu/a~rh and O2u/cJrh were replaced by the ratios NIu/Nlrh and N2u/N2wh while solving the equation systems (I), (2). Thus, assuming that l~fu *, RefTh and -- Rcf , we Call obtain formulae for U and Th content in samples:
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
--
Cu
395
I 1 2 2 N,/N, - N,/N, N /Nb -NX/N
c.
= Cst
1
N2s
Ref Ns~
I
(3) 2
2
N s//N st x N st//Ns - I I NT /NoI
(4)
--i
Samples investigation In order to check the proposed method of U and Th content determination, some samples of coal ash, cigarette ash and soil samples were investigated by both the present method and the one described by (Otgonsuren et al, 1995). A comparison of the results is given in Table 1. Errors are due to counting statistics and errors by calculating effective matter layer R,f. Table 1. Results of measuring U and Th content in samples obtained by two methods Sample
Cth (ppm)
Cth (ppm) n/y20-method
Y2o/yi6-method
n/y2o-method
y2o/y16-method
Coal ash 1
115.4±13.8
117.8±14.1
64.4±7.7
73.3±8.8
2
114.5±13.7
109.3±13.1
78.9±9.4
86.8±10.4
3
71.1±8.5
65.5±7.9
18.3±2.2
18.4±2.2
1
0.32±0.05
0.39±0.06
0.15±0.02
0.17±0.03
2
0.20±0.03
0.24±0.04
0.42±0.06
0.39±0.06
3
0.11±0.01
0.15±0.02
0.24±0.04
0.24±0.04
1
2.4±0.4
2.1±0.3
2.4±0.4
2.8±0.5
2
1.6±0.2
1.8±0.3
3.9±0.6
4.4±0.6
3
2.2±0.3
2.0±0.3
6.6±1.0
7.2±i.1
Cigarette ash
Soil
CONCLUSION As one can see from Table 1, the results of Th and U content measurements obtained by the methods of double irradiations with thermal neutrons and y - rays and with y - rays at the energies E y ~ 20 MeV and ~ 15 MeV are in good agreement within error bars. The sensitivity of the method reaches 10"Tg/gof Th, U, per gram of solid sample. The proposed method is valid with the condition that U content does not exceed Th content in a sample by a factor of 2. The advantage of the method of Th and U content determination using double y-irradiation is that it takes 1-2 h at beam intensity of 10 gA for exposure with y-rays of each energy range while irradiation with thermal neutrons at a microtron requires more than 10 h.
396
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
The proposed method of determination of Th and U content in samples was applied successfully in experiments for the analysis of man-made Pu (Perelygin et al., 1995). The matter is microquantities of Pu in samples are determined with the aid of irradiation with thermal neutrons; possible minor admixture of U and Th in Pu fractions aRer chemical extraction are measured by means of double yirradiation. Acknowledgment-We are grateful to A.N.Belov for carrying out a large number of irradiations at the Microtron
MT-25, FLNR, JINR.
REFERENCES Bimbot R., Maurette M., Pellas P. (1967) A New Method for Measuring the Ratio of the Atomic Concentrations of Thorium and Uranium in Minerals and Natural Glasses, Preliminary Application to Tektites. Geochim. Cosmochim. Acta 31,263-274. Flerov G.N., Berzina I.G. (1979) Radiography of Minerals, Rocks and Ores, Atomizdat, Moscow. Hair M.W., Kauthold J., Maurette M., Walker R.M. (1971) Th Microanalysis Using Fission Tracks. Rad. Effects 7, 285-287. Otgonsuren O., Enkhjin L., Gerbish Sh., Perelygin V.P., Petrova R.I. (1995) On Determination of Th Concentration in Specimens by means of Gamma Rays and Alpha Particles. Radiation Measurements 24, 83-86. Perelygin V.P., Chuburkov Yu.T., Zwara I., Khritonov Yu.P., Enkhjin L., Drobina T.P., Belov A.G. (1995) On Determination of Man-Made Plutonium Content in Naturel Samples, Plants and Living Species. Paper presented at 3rd Meeting "Nuclear Physics for Protection of the Environment". 23-28 May 1995, Dubna.
Radiation Measurements, Vol. 28, Nos I-6, pp. 393-396, 1997 © 1997 ElsevierScience Ltd Printed in Great Britain. All rights reserved 1350-4487/97 $17.00 + 0.00 PII: S 1350-4487(97)00106-6
THORIUM AND URANIUM CONTENT DETERMINATION USING DOUBLE GAMMA IRRADIATION
L. ENKHJIN AND V.P. PERELYGIN Joint Institute for Nuclear Research, 141980, Dubna, Russian Federation ABSTRACT A new technique for the determination of Th and U content in solids was proposed. The ratio of U and Th fission reaction yields for "infinite thick" metall layers was measured in the y-ray energy range from 7 MeV to 23.5 MeV. For this purpose metallic sheets of U and Th placed next to polyethyleneterephtalate track detectors were irradiated at the Microtron MT-25. The method is based on the fact that the ratio Nu/Nrh differs sufficiently from 1.7 to 3.3 depending on the )'-ray energy. Irradiating an Uranium ethalon and samples with y-rays of (20-23) MeV (No/Nrh = 2.2) and (15-17) MeV (Nu/Nrh = 3.3) one can determine simultaneously both U and Th content in samples. The comparison of the results of measuring of U and Th content in different samples obtained by the proposed and the known methods is presented. KEYWORDS Thorium; Uranium; SSNTD; polyethyleneterephtalate; photonuclear fission cross section of U and Th; photonuclear fission yield of U and Th INTRODUCTION We have previously reported the improved track technique for Th content determination in solids using neutrons and y-rays (Otgonsuren et al., 1995). There are some other track detector methods for Th analysis, such as, methods using irradiations with neutrons and protons, neutrons and alpha particles, neutrons and alpharadiography (Hair et al., 1971; Bimbot et al.; 1967, Flerov, 1979) As a rule, Thbearing geological samples contain U, too. Because both 23SU and 232Th nuclei undergo fission by y-rays, alpha particles, protons, fast neutrons, each of the above mentioned techniques requires thermal neutrons irradiation for the determination of U contents in samples. In this paper, we present a track detector method for simultaneous determination of U and Th using double y-irradiation. M E T H O D AND E X P E R I M E N T A L Ratio Nt, /NTh The proposed technique of Th and U content determination is based on the variation of the ratio of fission yields of reactions U(7, f) and Th(y, f) depending on the energy of y-rays in the range from 7 MeV to 23 MeV. The ratio of fission yields of these reactions were measured with the aid of layers "infinitely thick" to fission fragments i.e metallic sheets of U and Th placed next to polyethyleneterephtalate (lavsan) track detectors. This target-detector assembly was irradiated with '/-rays at beam intensity of 10 IxA for (5-30) s at the Microtron MT-25, Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research. Before the irradiation the metallic sheets were polished for surface oxide layers to be removed. The behavior of the ratio of fission fragments track density for U to that for Th depending on y-rays energy is shown in Fig 1.
393
394
PROCEEDINGS OF TIlE 18TH INTERNATIONAL CONFERENCE
4.0 3.5
{
3.0
!
2.5
f
2.0 1.5 1.0
,
5
I
10
i
I
,
15
I
i
20
25
E, MeV Fig. 1. Dependence of the ratio of fission fragment tracks for U and Th "thick" layers on T-rays energy One can see from Fig. 1 that the ratio Nu/Nrh ranges from 1.7 at (7-8) MeV to 3.3 at (15-17) MeV. However, at "/-ray energy of (7-8) MeV, fission fragment yield per BA.h of exposure is less than that at higher energies by I0-15 times.Therefore we preferred to use in our experiments `/-rays of energy (1517) MeV and (20-23) MeV.
Formulaefor Cv and Crh Track densities in detectors (polyethyleneterephtalate) irradiated in contact with a sample and an uranium standard (calibrated thin layer) with "/-rays are equal to [the superscripts 1 and 2 correspond to the energy ranges of?-rays (20-23) MeV and (15-17) MeV, respectively)]:
NIs = @l ¢ (Cu O'Iu RefU A/238 +Crh o'lTh RcfTh A/232)
())
N2s = @2e (Cu O2u Rcfu A/238 +CTh O2rhRefTh A/232)
N'~t = @t olug Cst A/238 (2) N2st = ~)2 0.2 u g Cst A/238 where @ is `/-rays fluence, cu and arh are fssion cross section of 238U and 232 Th nuclei, e is registration efficiency of the detector, ILfu and l ~ t ~ are thickness of effective matter layer of U and Th fission fragments in mg.cm"2, A is Avogadro's number, Cu and C~ are U and Th content in the sample, Ca is U content of the standard sample in mg.cm"~. Because the values au and a n are difficult to measure the ratios alu/a~rh and O2u/cJrh were replaced by the ratios NIu/Nlrh and N2u/N2wh while solving the equation systems (I), (2). Thus, assuming that l~fu *, RefTh and -- Rcf , we Call obtain formulae for U and Th content in samples:
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
--
Cu
395
I 1 2 2 N,/N, - N,/N, N /Nb -NX/N
c.
= Cst
1
N2s
Ref Ns~
I
(3) 2
2
N s//N st x N st//Ns - I I NT /NoI
(4)
--i
Samples investigation In order to check the proposed method of U and Th content determination, some samples of coal ash, cigarette ash and soil samples were investigated by both the present method and the one described by (Otgonsuren et al, 1995). A comparison of the results is given in Table 1. Errors are due to counting statistics and errors by calculating effective matter layer R,f. Table 1. Results of measuring U and Th content in samples obtained by two methods Sample
Cth (ppm)
Cth (ppm) n/y20-method
Y2o/yi6-method
n/y2o-method
y2o/y16-method
Coal ash 1
115.4±13.8
117.8±14.1
64.4±7.7
73.3±8.8
2
114.5±13.7
109.3±13.1
78.9±9.4
86.8±10.4
3
71.1±8.5
65.5±7.9
18.3±2.2
18.4±2.2
1
0.32±0.05
0.39±0.06
0.15±0.02
0.17±0.03
2
0.20±0.03
0.24±0.04
0.42±0.06
0.39±0.06
3
0.11±0.01
0.15±0.02
0.24±0.04
0.24±0.04
1
2.4±0.4
2.1±0.3
2.4±0.4
2.8±0.5
2
1.6±0.2
1.8±0.3
3.9±0.6
4.4±0.6
3
2.2±0.3
2.0±0.3
6.6±1.0
7.2±i.1
Cigarette ash
Soil
CONCLUSION As one can see from Table 1, the results of Th and U content measurements obtained by the methods of double irradiations with thermal neutrons and y - rays and with y - rays at the energies E y ~ 20 MeV and ~ 15 MeV are in good agreement within error bars. The sensitivity of the method reaches 10"Tg/gof Th, U, per gram of solid sample. The proposed method is valid with the condition that U content does not exceed Th content in a sample by a factor of 2. The advantage of the method of Th and U content determination using double y-irradiation is that it takes 1-2 h at beam intensity of 10 gA for exposure with y-rays of each energy range while irradiation with thermal neutrons at a microtron requires more than 10 h.
396
PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE
The proposed method of determination of Th and U content in samples was applied successfully in experiments for the analysis of man-made Pu (Perelygin et al., 1995). The matter is microquantities of Pu in samples are determined with the aid of irradiation with thermal neutrons; possible minor admixture of U and Th in Pu fractions aRer chemical extraction are measured by means of double yirradiation. Acknowledgment-We are grateful to A.N.Belov for carrying out a large number of irradiations at the Microtron
MT-25, FLNR, JINR.
REFERENCES Bimbot R., Maurette M., Pellas P. (1967) A New Method for Measuring the Ratio of the Atomic Concentrations of Thorium and Uranium in Minerals and Natural Glasses, Preliminary Application to Tektites. Geochim. Cosmochim. Acta 31,263-274. Flerov G.N., Berzina I.G. (1979) Radiography of Minerals, Rocks and Ores, Atomizdat, Moscow. Hair M.W., Kauthold J., Maurette M., Walker R.M. (1971) Th Microanalysis Using Fission Tracks. Rad. Effects 7, 285-287. Otgonsuren O., Enkhjin L., Gerbish Sh., Perelygin V.P., Petrova R.I. (1995) On Determination of Th Concentration in Specimens by means of Gamma Rays and Alpha Particles. Radiation Measurements 24, 83-86. Perelygin V.P., Chuburkov Yu.T., Zwara I., Khritonov Yu.P., Enkhjin L., Drobina T.P., Belov A.G. (1995) On Determination of Man-Made Plutonium Content in Naturel Samples, Plants and Living Species. Paper presented at 3rd Meeting "Nuclear Physics for Protection of the Environment". 23-28 May 1995, Dubna.