RU2706642C1 - METHOD OF DETERMINING 230Th (THORIUM) ISOTOPE ACTIVITY IN URANIUM-CONTAINING MINERALS - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: invention relates to a method of determining ²³⁰Th alpha activity. Control of chemical yield of target nuclide is carried out by value of ²³⁴Th isotope activity, contained in the studied mineral and in a state of secular equilibrium with parent isotope ²³⁸U. Method comprises successive determination of the full value of uranium-238 alpha activity in a sample, then determination of coefficient of beta-setting, then determination of chemical yield of thorium-230 and full ²³⁰Th alpha-activity in the sample. It is sufficient to use only one radio path required for controlling the chemical output of uranium-238.

EFFECT: determining chemical yield of thorium fraction and radioactivity of isotope ²³⁰Th, with compliance of results with actual experimental data obtained during radiochemical analysis of polycase – Ti-Ta-niobate with mineral formula AB₂O₆.

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Description

The need to determine the total radioactivity of the ²³⁰ Th isotope in the sample arises when solving a number of problems of geochronology of lithosphere objects or constructing a chronological scale for some geological processes, when studying the effects of disturbed radioactive equilibrium in the uranium-238 series as a result of natural fractionation of isotopes or the migration of free uranium . For the correct determination of the alpha activity of ²³⁰ Th, radio tracers are used: ²³⁴ Th, ²²⁸ Th, ²²⁹ Th.

The use of the thorium isotope ²²⁹ Th as a tracer seems to be the most optimal solution to this problem [1-2]. However, in this case, rather high requirements are imposed on the used alpha spectrometric equipment and on the quality of the manufactured alpha sources, since the alpha lines of the tracer and the target nuclide in the total alpha spectrum are separated by an interval of 126-157 keV. The imposition of low-energy alpha-peak tails can adversely affect the accuracy of the results.

A similar problem does not occur when using ²²⁸ Th radionuclide as a tracer. However, in this case, we encounter difficulties of a different origin: ²²⁸ Th is a daughter product of the decay of thorium-232 and is present in all objects of the lithosphere where the maternal isotope is detected. In [3], this problem was overcome by double analysis of the mineral: without tracers and with the addition of radionuclides ²²⁸ Th and ²³² U (purified from the daughter nuclide ²²⁸ Th). In the absence of other possibilities, the described procedure can be used to determine the content of ²³⁰ Th radionulide in lithosphere samples. But it is obvious that it is more time-consuming, and the results obtained are burdened by an increased error.

Probably the easiest way to consider the use of ²³⁴ Th beta emitter as a tracer [4]. Numerous examples of geochronological studies of the ²³⁰ Th / U-method using ²³² U and ²³⁴ Th tracers are described in detail in the monograph [5]. The only natural restriction for the use of the ²³⁴ Th nuclide may be a high concentration of uranium in the test sample, in equilibrium with which is the daughter isotope ²³⁴ Th.

In view of the above circumstances, we propose a method for determining the value of AR ( ²³⁰ Th / U), characterized in that the chemical yield of the thorium fraction is controlled by beta radiation of the ²³⁴ Th isotope ( ^234m Pa) contained in a mineral whose total radioactivity is mineral is determined using a ²³² U radio ^tracer from the measured alpha activity of the ²³⁸ U isotope.

The method is as follows.

1. The alpha activity of the chemical fractions of uranium and thorium is measured. Using the uranium radio tracer and the known detection coefficient (counting coefficient) of alpha radiation (θ _α ), the total activity of ²³⁸ U in the studied sample A ( ²³⁸ U), Bq is calculated. Recorded alpha activity of ²³⁰ Th is designated as A ( ²³⁰ Th), cpm.

2. The background of the beta setup and the beta activity of the ²³⁰ Th ( ²³⁴ Th) source are measured. To delay ²³⁴ Th soft beta radiation, an absorber with a thickness of about 40 mg / cm ² (including the thickness of the beta counter window) is used. Registration is performed using ^234m Ra beta radiation with a maximum energy of 2300 keV and a yield of 98%. The obtained value is recalculated at the time of separation of the thorium fraction from the test sample, taking into account the half-life of the ²³⁴ Th nuclide, equal to 24.1 days. This activity is designated as A ( ²³⁴ Th) cpm.

3. With the same absorber, the beta activity of a standard uranium preparation with a known total alpha activity (Bq) is measured and the detection efficiency (counting coefficient) of beta particles (θ _β ) in this energy range for the solid angle used is calculated.

4. Taking into account the found counting coefficient θ _{β, the} total beta activity of the ²³⁰ Th source is calculated as the ratio A ( ²³⁴ Th), imp / s / θ _β .

5. Knowing the total alpha activity of the ²³⁸ U isotope in the test sample and, therefore, the corresponding activity of the daughter nuclide ²³⁴ Th, we calculate the chemical yield (P _α ) of the ²³⁰ Th isotope:

6. We find the total alpha activity of ²³⁰ Th in the sample of the mineral, taking into account the found chemical yield of the thorium fraction and the counting coefficient θα. The form of the final formula for the calculation is as follows:

Thus, two advantages can be noted when using the ²³⁴ Th nuclide contained in the test sample as a thorium tracer instead of preparing the ²³⁴ Th tracer. Firstly, the need to produce a new tracer solution with known (at the time of manufacture) specific activity of thorium-234 disappears. Secondly, when calculating the activity of ²³⁰ Th, it is not necessary to constantly introduce the moving value of the fraction of the ²³⁴ Th isotope decayed in the time elapsed since the tracer was manufactured into the corresponding equation.

An example of a specific implementation.

The object of research was a sample of metamict mineral (Ti-Ta-niobate). To perform two parallel experiments, 2 weighed samples (No. 1 and No. 2) of a mineral powder of 7 mg each were selected. To determine the content and isotopic composition of U (Th) in the samples, complete decomposition of minerals is necessary. This was achieved by dissolving the samples in concentrated HF, and then in aqua regia. Fractions of U and Th after acid decomposition of the samples were isolated by anion exchange chromatography. Using the well-known technique [6], alpha sources of the uranium fraction and thorium fraction were prepared. The isotope ²³² U was used as a tracer to determine the output of U.

The proposed method includes the sequential determination of the total alpha activity of uranium-238 in the sample, then determining the counting coefficient of the beta installation, and then determining the chemical yield of thorium-230 and the total alpha activity of 230Th in the sample. Each of these procedures is explained below in this example implementation of the claimed method.

1. Determination of the total alpha activity of uranium-238 in samples 1 and 2

The results of two parallel experiments are as follows: the activity of the ²³⁸ U isotope, isolated from the first sample, measured on an alpha spectrometer, was 0.697 ± 0.003 pulse / s, and from the second, 0.478 ± 0.003 pulse / s. In this case, the chemical yield of uranium determined using the ²³² U radio tracer for the first source was 0.720 ± 0.011, for the second - 0.52. Taking into account the values of the chemical yield and the known alpha-particle counting coefficient (0.365), the value of A ( ²³⁸ U) was 2.652 ± 0.041Bq for a sample No. 1 and 2.518 ± 0.047 Bq for a sample No. 2.

2. Determination of the beta counting coefficient

The total alpha activity of the standard uranium-238 preparation is 6.05 Bq.

The beta activity of this source is 2 imp / s. Thus, the beta particle count coefficient is θ _β = 0.33.

3. Determination of the chemical yield of thorium-230 for attachments 1 and 2

The values of beta activity A ( ²³⁰ Th, imp / s) after subtraction of the background and assignment at the moment of separation of the thorium fraction were: 0.588 ± 0.006 imp / s for sample No. 1 and 0.614 ± 0.005 imp / s for sample No. 2. Therefore, the total beta activity of the first source is 0.588: 0.33 = 1.78 ± 0.018 Bq, of the second: 0.614: 0.33 = 1.861 ± 0.015 Bq. Accordingly, the chemical yield P _{α of the} ²³⁰ Th isotope in the first case was: 1.78: 2.65 = 0.672 ± 0.012, in the second case: 1.86: 2.52 = 0.739 ± 0.015.

4. Determination of total alpha activity of ²³⁰ Th in samples

Sample 1. The measured alpha activity of the ²³⁰ Th isotope was 0.738 ± 0.003 imp / s. Taking into account the values of the chemical yield P _α and the alpha-particle counting coefficient, we calculate the total alpha activity of ²³⁰ Th in the sample:

Sample 2. The measured alpha activity of the ²³⁰ Th isotope was 0.781 imp / s. Total alpha activity of ²³⁰ Th in sample:

5. Determination of the activity ratio of ²³⁰ Th / U

If necessary, it is not difficult to find the ratio of the activity of thorium-230 to the activity of ²³⁸ U. The average activity of uranium-238, contained in 7 mg of the mineral, is 2.585 ± 0.031 Bq. Accordingly, the average activity of thorium-230 was 2.952 ± 0.037 Bq. Thus, for the studied mineral AR ( ²³⁰ Th / ²³⁸ U) = 2.955: 2.585 = 1.142 ± 0.020.

The activity ratio of ²³⁰ Th / ²³⁸ U is greater than unity. This value reflects the violation of secular equilibrium in the series of uranium in the studied sample and allows us to calculate the rate constant of incongruent dissolution of uranium under the influence of natural fluids. In turn, this value gives indirect information about the stability of "silt" forms of actinides in mineral-like matrices of radioactive waste.

The results, illustrated by a specific calculation using real experimental data, indicate that the proposed method provides a correct estimate of the radioactivity of ²³⁰ Th. This is achieved by controlling the chemical yield of the thorium fraction by beta radiation of the ²³⁴ Th isotope ( ^234m Pa) contained in the mineral, the total radioactivity of which is determined in the mineral using a ²³² U radio ^tracer from the measured alpha activity of the ²³⁸ U isotope.

Bibliography

[1] Lavison, P. de., Husband, LJ, Jerome, SM, Keightley, JD, Woodman, AP, Woods, DH, Woods, SA, 2000. The standardization of ²²⁹ Th for an environmental yield tracer. Applied Radiation and Isotopes 53 (1-2), 243-249.

[2] Tuovinen, H., Vesterbacka, D., Pohjolainen, E., Read, D., Solatie, D., Lehto, J., 2015. A comparison of analytical methods for determining uranium and thorium in ores and mill tailings . J. of Geochemical Exploration 148, 174-180.

[3] Bogdanov RV, Batrakov Yu. F., Puchkova EV, and Sergeev AS, (2002). On the Kinetics of Natural Leaching of ²³⁸ U and ²³⁴ U from Rocks and Minerals. Geochemistry International, Vol. 40, No. 11, pp. 1056-1065. (Translated from Geokhimiya, No. 11, pp. 1167-1177.)

[4] Maksimov, F, Arslanov, Kh., Kuznetsov, V., Chernov, S., 2006. ²³⁰ Th / U and ¹⁴ C dating of Upper and Middle Pleistocene Interglacial and Interstadial organic deposits from the East-European Plain and Siberia // Pleistocene Environments in Eurasia Chronology, Paleoclimate and Teleconnection. INTAS Final Workshop. Hannover, Germany, 2-3 November, 34-38 (prototype)

[5] Kuznetsov V.Yu., Maksimov F.E. 2012. Methods of Quaternary geochronometry in palaeogeography and marine geology. Saint-Petersburg, NAUKA, 192 p. (In Russian).

[6] Bogdanov, R.V., Puchkova, E.V., Parnikov, N.G., Sergeev, A.S., 2011. Radiogenic Uranium in Paragenetic Mineral Associations. Radiochemistry 53 (6), 651-661, (Translated from Radiokhimiya 53 (6), 549-558).

Claims (1)

A method for determining the activity of the ²³⁰ Th isotope in uranium-containing minerals, including the isolation and isolation of chemical fractions of uranium and thorium by anion exchange chromatography, the production of alpha sources of uranium and thorium by electrodeposition, the measurement of the alpha activity of ²³⁸ U and ²³⁰ Th isotopes, taking into account the corresponding alpha spectrometric paths, determination of the beta-radiometer ^counting coefficient with respect to beta radiation ²³⁴ Th ( ^234m Pa) using a standard uranium preparation, measurement of beta activity of a ²³⁰ Th source taking into account the beta radiometer counting coefficient, determining the chemical yield of the thorium fraction and the total alpha activity of the ²³⁰ Th isotope, characterized in that the chemical yield of the thorium fraction is controlled by beta radiation of the ²³⁴ Th isotope ( ^234m Pa) contained in the mineral, the total radioactivity of which in the mineral was determined using a ²³² U radio ^tracer from the measured alpha activity of the ²³⁸ U isotope.