Experimental Progress Report--Modernizing the Fission Basis

In 2010 a proposal (Modernizing the Fission Basis) was prepared to 'resolve long standing differences between LANL and LLNL associated with the correct fission basis for analysis of nuclear test data'. Collaboration between LANL/LLNL/TUNL has been formed to implement this program by performing high precision measurements of neutron induced fission product yields as a function of incident neutron energy. This new program benefits from successful previous efforts utilizing mono-energetic neutrons undertaken by this collaboration. The first preliminary experiment in this new program was performed between July 24-31, 2011 at TUNL and had 2 main objectives: (1) demonstrating the capability to measure characteristic γ-rays from specific fission products; (2) studying background effects from room scattered neutrons. In addition, a new dual fission ionization chamber has been designed and manufactured. The production design of the chamber is shown in the picture below. The first feasibility experiment to test this chamber is scheduled at the TUNL Tandem Laboratory from September 19-25, 2011. The dual fission chamber design will allow simultaneous exposure of absolute fission fragment emission rate detectors and the thick fission activation foils, positioned between the two chambers. This document formalizes the earlier experimental report demonstrating the experimental capability to make accurate (< 2 %) precision gamma-ray spectroscopic measurements of the excitation function of high fission product yields of the 239Pu(n,f) reaction (induced by quasimonoenergetic neutrons). A second experiment (9/2011) introduced an compact double-sided fission chamber into the experimental arrangement, and so the relative number of incident neutrons striking the sample foil at each bombarding energy is limited only by statistics. (The number of incident neutrons often limits the experimental accuracy.) Fission chamber operation was so exceptional that 2 more chambers have been fabricated; thus fission foils of different isotopes may be left in place with sample changes. The scope of the measurements is both greatly expanded and the results become vetted. Experiment 2 is not reported here. A continuing experiment has been proposed for February 2012.