Experimental Determination of the Uranium-239 Neutron Capture Reaction by Measurement of Uranium-240 by Accelerator Mass Spectrometry

Scott Tumey | 18-FS-023


The measurement of the 239 U(n,g) 240 U reaction cross section is very challenging because a short half-life (23.45 minutes) makes producing a suitable target for irradiation nearly impossible. Only two experimental measurements of the cross section have been made, and they resulted in poor agreement and large uncertainty. Furthermore, the experiments were conducted at relatively low neutron energy rather than higher energies relevant to mission applications.

Our study demonstrated the feasibility of experimental measurement of the 239 U(n,g) 240 U reaction cross section using accelerator mass spectrometric (AMS) measurements of irradiated depleted uranium targets. AMS offers the advantage of being insensitive to other radioactive isotopes and molecular isobars allowing for greater sensitivity than other methods. Initial measurements were performed using a thermalized neutron spectrum so the results could be validated to previous measurements. At the same time, this work lays the groundwork for future cross section measurements at higher neutron energies for which no experimental data exist.

Impact on Mission

As a product of the detonation of a nuclear weapon, 240 U is an important signature in both stockpile radiochemistry and nuclear forensics. This project established the feasibility of a new capability for providing improved nuclear data to enhance the fidelity and robustness of radiochemical assessments of historical nuclear tests and the attribution of illicit use of nuclear weapons. These outcomes are well aligned with Lawrence Livermore National Laboratory's nuclear threat reduction and nuclear weapons science mission focus areas, and they advance the chemical and isotopic signatures core competency of the Laboratory.