Monte Carlo Simulation of Neutron Background Sources in the Measurement of the 12C(α,γ)16O Reaction Rate

The 12C(α, γ)16O reaction rate strongly affects the relative abundances of chemical elements, as well as when core collapse supernovae occur. There have been several attempts to measure the reaction rate, but the Coulomb barrier between the carbon nucleus and the α-particle inhibits direct measurement at stellar energies. In a proposed experiment, a water-filled bubble chamber will be used to measure the reverse reaction rate. This technique will accurately measure the reaction rate closer to stellar energies than previous experiments have accomplished. A potential background source is photoneutrons from the γ-ray beam collimator entering the bubble chamber and generating a false signal. To minimize this effect, a Monte Carlo simulation has been performed to compare the number of photoneutrons created in lead, copper, and aluminum collimators. It was found that 30 cm of copper would be an effective beam collimator by stopping 99.8% of γ-rays and generating no photoneutrons. The simulation also compared the effectiveness of concrete, polyethylene, and water as neutron shields. These simulations show that polyethylene consistently stops the most neutrons at relevant energies. Further simulation will be required to evaluate shielding materials for cosmic ray neutrons, which can also generate false signals.

The Journal of Young Investigators is not affiliated with the US Department of Energy. This paper was written by a student intern with the Department of Energy and does not constitute a declarative position of either the Department of Energy or the Journal of Young Investigators.
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