Review and Evaluation of the Spontaneous Fission Half-lives of ²³⁸Pu, ²⁴⁰Pu, and ²⁴²Pu and the Corresponding Specific Fission Rates

Abstract

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.