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) × 1010y 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) × 1011y 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) × 1010y 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.