DOE OSTI2019
The primary motivation for the fissionTPC experiment is to provide precise, accurate data for evaluation of the 239 Pu(n,f) cross section. Neutron induced fission cross sections are typically measured as ratios, with a well known standard cross section being in the denominator. Measuring a ratio removes the need to quantify the neutron beam flux which is difficult to determine accurately. Any cross section ratio measurement is ultimately limited by the accuracy of the standard used. While the 235 U(n,f) standard is well measured some light particle reactions are known as well or better. Light particle reactions also remove some of the shared systematic uncertainties that are present in an actinide-only ratio measurement. To this end the NIFFTE collaboration originally proposed to measure the 239 Pu(n,f)/ 1 H(n,el) cross section ratio. The 1 H(n,el) reaction is well measured, smoothly varying as a function of energy and has good theoretical support, making it an ideal candidate for a cross section standard. Through simulation-based study and initial data collection with the fissionTPC of a 235 U(n,f)/ 1 H(n,el) cross section ratio it was determined that the measurement uncertainties could not be reduced sufficently to take full advantage of the high accuracy of the 1 H(n,el) reaction. The primary difficulties faced were the low energy of the scattered protons which do not provide enough signal to measure the neutron time-of-flight (nToF) with the fissionTPC fast cathode amplifier, and the the background (n,p) signals induced by the high energy component of the WNR beam. It should be noted that these challenges would be greatly reduced if we were to make such a measurement at a monoenergetic neutron facility. In light of these challenges other reactions were investigated. The 6 Li(n,t)a reaction was identified as a good alternative. The 4.8 MeV Q-value of the reaction provides a relatively large signal for nToF determination and the two charged particles (t & a) released in the reaction provide a signal well suited for background suppression in the fissionTPC. The 6 Li(n,t) reaction is considered a standard up to 1 MeV and is currently evaluated up to 3 MeV incident neutron energy. While this is a narrower energy range than the 1 H(n,el) reaction it overlaps well with the fission neutron spectrum. A mock evaluation was performed and it was estimated that a measurement of 239 Pu(n,f)/ 6 Li(n,t) would have an impact on the 239 Pu(n,f) evaluation comparable to a measurement using a the 1 H(n,el) reaction. While the 6 Li(n,t) reaction is not as well measured as the 1 H(n,el) reaction, the lack of previous measurements in the GMA database means that a new measurement of 239 Pu(n,f)/ 6 Li(n,t) will have a greater weighting and therefore a large impact on a future evaluation. In addition to providing a reduction in the 239 Pu(n,f) cross section evaluation uncertainties, a measurement of 6 Li(n,t) in the fissionTPC will provide data to resolve a discrepancy in previous measurements of the reaction above 1 MeV. Furthermore, the 6 Li data collected in the fissionTPC can also be analyzed to measure the 6 Li(n,nd) reaction for which there is limited discrepant data in the GMA database. This report will focus on: the motivation for making a 239 Pu(n,f)/ 6 Li(n,t) cross section ratio measurement; a preliminary look at 235 U(n,f)/ 6 Li(n,t) data collected which will emphasize the capability of the fissionTPC to make an nToF measurement with the fast cathode amplifier; and finally will provide a brief update on the 239 Pu(n,f)/ 235 U(n,f) cross section analysis.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗