Efficiency Calibration of NaI Detectors for a Measurement of the 12C(n, 2n)11C Cross Section
One possible inertial confinement fusion diagnostic involves tertiary neutron activation of a carbon disk via the 12C(n, 2n)11C reaction. A recent experiment to measure this reaction cross section involved coincidence counting the annihilation gamma rays produced by the positron decay of 11C. This requires an accurate value for the full-peak coincidence efficiency of the NaI detector system. The GEANT 4 toolkit was used to develop a Monte-Carlo simulation of the detector system, which was used to calculate the required efficiencies. For validation, simulation predictions have been compared with the results of two types of experiments. In the first, full-peak coincidence positron annihilation efficiencies were measured for 22Na decay positrons that annihilate in a small plastic scintillator. In the second, a NIST-calibrated 68Ge source was used. A comparison of calculated with measured efficiencies, and the resulting 12C(n, 2n)11C cross sections, are presented.
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