Energy Dependent Neutron Absorption Coefficients in Polyethylene

In recent years, the Suffolk University Physics Department and the Massachusetts General Hospital (MGH) Radiation Oncology Department in Boston have collaborated in a series of experiments to determine the neutron energy distribution of an Americium-Beryllium (AmBe) source located at the MGH Proton Center. (The current and previous work has been supported by research grants from the Society of Physics Students.) The research reported builds on that work and makes use of the known energy distribution of the MGH AmBe source to determine the linear absorption coefficient for neutrons in different energy windows passing through polyethylene. Using an array of commercially available energy threshold bubble detectors and a series of polyethylene blocks of different thicknesses, we have determined neutron absorption coefficients in energy windows ranging from 0 to 20 MeV. An experimental design was used to mitigate backscattering that can cause artificially high bubble counts. The bubble detectors have energy dependent thresholds and known non-uniform cross sections in the different energy regions. The AmBe source activity previously determined in these six energy regions and the known bubble detector neutron cross sections was used to unfold the absorption coefficients of polyethylene. The results can be of use in neutron shielding studies.