Coincidence Efficiency Measurement Using 11B(p,n)11c

An attempt to measure the 12C(n,2n)11C cross section for high energy neutrons in the range of 20-30 MeV was conducted using Ohio University’s accelerator facility as a fast neutron source. The neutrons were incident on a graphite target and the β+ decay of the activated carbon-11 nuclei were observed in an on-axis gamma ray detector pair. To predetermine the efficiency of this gamma ray detector system, a boron-11 activation experiment was performed. Using SUNY Geneseo’s 1.7 MV tandem pelletron accelerator, 3.1 MeV protons were incident upon the 11B foil inducing the 11B(p,n)11C reaction to occur at a high rate of activation. The 11C decays via β+ emission, then upon annihilation with an electron creates characteristic 511-511 keV photon pairs which were counted using coincidence methods. Since the 11B(p,n) cross section is well defined, a calculation was performed to determine the expected number of activations and later compared to the total number of decays observed in the counting system. Funded in part by a grant from the DOE through the Laboratory for Laser Energetics.