Fundamental low energy nuclear science research for nuclear forensics: Producing americium-240

Paul Ellison, University of California, Berkeley

Due to its role in nuclear forensics and stockpile stewardship science, it is desirable to experimentally measure the neutron-induced fission cross section of americium-240. The smallest amount of material that such a measurement can be performed upon is 20-100 nanograms. However, because of americium-240’s 50.8 hour half-life, it is difficult to produce in even these quantities. This poster summarizes my efforts to investigate a new production reaction for americium-240 that involves the bombarding of plutonium-242 with protons at an energy that maximizes the evaporation of three neutrons.

The targets used for the investigation of this nuclear reaction consist of plutonium-242 electrodeposited onto thin titanium foils. The foils are then wrapped in protective catcher foils and clamped to a water-cooled metal block. This block is then attached to an electrically-isolated irradiation apparatus at the 88-Inch Cyclotron at Lawrence Berkeley National Laboratory. The proton beam intensity is measured through the use of nickel and titanium monitor foils. After irradiation, the foils are separated and measured individually using high-purity germanium gamma ray spectrometers. By taking many short measurements, the decay curves of the produced radioisotopes can be monitored and the end-of-bombardment activity levels for each measured.

Over the past year, improvements have been made to the experimental apparatus and procedures for studying this nuclear reaction. Improvements to the irradiation apparatus allow for more safe and reliable cross section measurements. Also, work has been performed developing computer programs to sum the gamma spectra, fit peaks, extract peak areas, and minimize the error in the back-extrapolated end-of-bombardment activity levels. Several measurements were performed in late 2009 and have been analyzed in early 2010. The results from these experiments are useful in assessing the feasibility of using this nuclear reaction for the production of americium-240 on the scale of several hundred nanograms.

Abstract Author(s): P.A. Ellison, J.P. McLaughlin, H. Nitsche