The 12C(n, 2n)11C cross section was measured from just below threshold to 26.5 MeV using the Pelletron accelerator at Ohio University. Monoenergetic neutrons, produced via the 3H(d,n)4He reaction, were allowed to strike targets of polyethylene and graphite. Activation of both targets was measured by counting positron annihilations resulting from the β+ decay of 11C. Annihilation gamma rays were detected, both in coincidence and singly, using back-to-back NaI detectors. The incident neutron flux was determined indirectly via 1H(n,p) protons elastically scattered from the polyethylene target. Previous measurements fall into upper and lower bands; the results of the present measurement are consistent with the upper band.

One possible diagnostic technique for characterizing inertial confinement fusion reactions involves tertiary neutron activation of C via the C(n,2n)C reaction. Because the cross section for this reaction is not well measured in the energy range of interest, a new measurement was recently made at Ohio University. Part of this experiment involves counting the positron annihilation 511 keV gamma rays from the C decay using two sodium iodide detectors in coincidence. A new technique has been developed to measure the coincidence efficiency by detecting the positron prior to its annihilation, and requiring that the 1275 keV gamma ray also emitted by the 22Na be in the full-peak in another NaI(Tl) detector. Measurements and simulation results for the absolute coincidence full-peak efficiencies are presented.

Alexander Lipnicki, Joshua Troyer and Mark Yuly. Department of Physics, Houghton College, One Willard Avenue, Houghton, New York 14744. The 200 keV electrostatic electron accelerator at Houghton College is being upgraded. The previous design used a makeshift electron gun that did not allow the beam to be easily controlled, and did not produce a welldefined beam spot. A new electron gun, taken from a RCA 3RP1 cathode ray tube, is being installed which will allow the beam intensity, focus and initial accelerating voltage to be remotely controlled. Using a 2000 V test system, a beam current of 0.1 μA was obtained into a beam spot of less than 1 mm diameter. The electron gun is to be remotely operated via an Ethernet-GPIB-RS232-fibre optic link to a BASIC Stamp-2 microcontroller inside the high voltage terminal of the accelerator. The microcontroller will set the voltages on the grids of the electron gun with a 12-Bit four channel DAC7624 digital-to-analog converter feeding four 4-transistor power amplification circuits which energize four EMCO G20 DC to HV DC converters.

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.