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

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 Pu-240 is usually the dominant isotope with Pu-238 and Pu-242 also contributing to a degree depending on the grade or burnup of the material. Consequently, knowledge of the Pu-240 spontaneous fission rate, expressed as fission per gram per second, and the associated uncertainty is key in interpreting measurement data absolutely. The Pu-240 spontaneous fission rate derives (and vice versa) from the Pu-240 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 Pu-238, Pu-240 and Pu-242. 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) x 10(10) y for Pu-238 corresponding to a specific SF rate of (1171 +/- 20) fis.s(-1).g(-1). There are 16 absolute experimental determinations of Pu-240 half-life. Based on this evaluation, we find that the weighted mean of the 16 half-life determinations is (1.1608 +/- 0.0091) x 10(11) 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) x 10(10) y for Pu-242 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.

Automated summary

Passive neutron multiplicity counting is a widely applied technical measure for nuclear safeguards and nonproliferation, especially for the nondestructive verification and mass-assay of items containing separated plutonium. Studies on the spontaneous fission half-lives of Pu isotopes Pu-238, Pu-240, and Pu-242 are crucial for interpreting measurement data, and it is recommended to conduct more experiments for further research and improvement.