Report on 240Am(n,x) surrogate cross section test measurement

The main goal of the test measurement was to determine the feasibility of the 243Am(p,t) reaction as a surrogate for 240Am(n,f). No data cross section data exists for neutron induced reactions on 240Am; the half-life of this isotope is only 2.1 days making direct measurements difficult, if not impossible. The 48-hour experiment was conducted using the STARS/LIBERACE experimental facility located at the 88 Inch Cyclotron at Lawrence Berkeley National Laboratory in August 2011. A description of the experiment and results is given. The beam energy was initially chosen to be 39 MeV in order to measure an equivalent neutron energy range from 0 to 20 MeV. However, the proton beam was not stopped in the farady cup and the beam was deposited in the surrounding shielding material. The shielding material was not conductive, and a beam current, needed for proper tuning of the beam as well as experimental monitoring, could not be read. If the 240Am(n,f) surrogate experiment is to be run at LBNL, simple modifications to the beam collection site will need to be made. The beam energy was reduced to 29 MeV, which was within an energy regime of prior experiments and tuning conditions at STARS/LIBERACE. At this energy, the beam current was successfully tuned and measured. At 29 MeV, data was collected with both the 243Am and 238U targets. An example particle identification plot is shown in Fig. 1. The triton-fission coincidence rate for the 243Am target and 238U target were measured. Coincidence rates of 0.0233(1) cps and 0.150(6) cps were observed for the 243Am and 238U targets, respectively. The difference in count rate is largely attributed to the available target material - the 238U target contains approximately 7 times more atoms than the 243Am. A proton beam current of ∼0.7 nA was used for measurements on both targets. Assuming a full experimental run under similar conditions, an estimate for the run time needed was made. Figure 2 shows the number of days needed as a function of acceptable uncertainty for a measurement of 1-20 MeV equivalent neutron energy, binned into 200 keV increments. A 5% measurement will take 3 days for U, but 20 days for Am. It may be difficult to be the sole user of the LBNL cyclotron, or another facility, for such an extended period. However, a 10% measurement will take 19 hours for U, and 5 days for Am. Such a run period is more reasonable and will allow for the first ever measurement of the 240Am(n,f) cross section. We also anticipate 40% more beam time being available at Texas A and M Cyclotron Institute compared to LBNL in FY2012. The increased amount of beam time will allow us to accumulate better statistics then what would have been available at LBNL.