Nuclear Data Sheets最新文献

A widely applied technical measure of nuclear safeguards and nonproliferation for the nondestructive verification and mass-assay of items containing separated plutonium is passive neutron multiplicity counting. The primary source of time correlated passive neutrons is the spontaneous fission of Pu, where the ²⁴⁰Pu is usually the dominant isotope with ²³⁸Pu and ²⁴²Pu also contributing to a degree depending on the grade or burnup of the material. Consequently, knowledge of the ²⁴⁰Pu spontaneous fission rate, expressed as fission per gram per second, and the associated uncertainty is key in interpreting measurement data absolutely. The ²⁴⁰Pu spontaneous fission rate derives (and vice versa) from the ²⁴⁰Pu spontaneous fission half-life. In this work we review and reevaluate the available experimental data on the spontaneous fission (SF) half-life of the Pu isotopes ²³⁸Pu, ²⁴⁰Pu and ²⁴²Pu. The SF half-lives are used to compute the corresponding specific SF rates which are the traditional nuclear data parameters needed for analytical applications, especially the nondestructive assay of plutonium for safety, security and safeguards.

From 7 measurements we recommend a SF half-life of (4.745 ± 0.083) × 10¹⁰ y for ²³⁸Pu corresponding to a specific SF rate of (1171 ± 20) fis ⋅ s^-1 ⋅ g^-1.

There are 16 absolute experimental determinations of ²⁴⁰Pu half-life. Based on this evaluation, we find that the weighted mean of the 16 half-life determinations is (1.1608 ± 0.0091) × 10¹¹ y corresponding to a specific SF rate of (474.7 ± 3.7) fis ⋅ s⁻¹ ⋅ g⁻¹, where the relative uncertainty of about 0.78% is the external standard error.

From 8 measurements we recommend a SF half-life of (6.766 ± 0.037) × 10¹⁰ y for ²⁴²Pu corresponding to a specific SF rate of (807.7 ± 4.4) fis ⋅ s⁻¹ ⋅ g⁻¹. Finally, we conclude that there is a need for more experiments on all three plutonium nuclides using new techniques that have emerged in recent years.

Experimental nuclear structure and decay data are evaluated for all of 12 known nuclides of mass 214 (Hg, Tl, Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa). For each nuclide, detailed evaluated spectroscopic information is presented in each reaction and decay, and the best values combining all available data are recommended for level properties, γ and β radiations, and other spectroscopic properties in the Adopted Levels and Gammas. The present evaluation supersedes the earlier one on A=214 by S.-C. Wu (2009Wu02), published in Nuclear Data Sheets 110, 681 (2009).

Evaluated experimental structure and decay data are presented for 12 known nuclides of mass 219 (Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa, U, Np). Recommended values are given for level parameters, γ and α radiations, and other spectroscopic information. No excited states are known in ²¹⁹Bi, ²¹⁹Po, ²¹⁹Pa, ²¹⁹U and ²¹⁹Np. Except for isotopic identification, no information about its half-life or decay characteristics is available for ²¹⁹Pb. For ²¹⁹Po, several γ rays have been reported from the decay of ²¹⁹Bi β⁻ decay, but no level scheme has been proposed. For ²¹⁹At, only four excited states are known from α decay of ²²³Fr. For ²¹⁹Rn and ²¹⁹Fr, only the low-spin states are known from α decays of ²²³Ra and ²²³Ac, respectively. For ²¹⁹Ra, ²¹⁹Ac and ²¹⁹Th, mainly the data for high-spin states are available from in-beam γ-ray studies. For ²¹⁹Ra, high-spin data are available from 2017He15, 1992Wi02 and 1987Co36, but evaluators find significant differences in relative photon branchings between the three studies. Detailed comments are given in the Adopted dataset for this nuclide. Half-lives of excited states are known only for seven levels in ²¹⁹Rn, thus there is a general lack of knowledge about γ-ray transition probabilities. This work supersedes data in the previous evaluations of A=219 published by 2001Br31, 1992Br10 and 1977Ma30.