This subproject is to develop a series of low-energy neutron beams (lowest dose-weighted mean energy <30 keV) with minimal dose contributions by higher energy neutrons and ( rays, and which have dose rates usable for cell radiobiology. Protons with energies from 1.95-2.08 MeV are used to produce neutron spectra with energies from nearly 0 to 30-100 keV using the 7Li(p,n) reaction. The MCNP4 transport computer code has been used to calculate the spectral characteristics of the source and to optimize the design of the target, to minimize the contribution of scattered neutrons with energies above the desired maximum. Many hundreds of simulations have been performed to optimize the geometry. A 1/2""""""""-diameter spherical hydrogen recoil proportional counter has been purchased and is being used to measure the actual neutron spectra. Depleted uranium has been obtained and machined as part of the multi-component filter designed to reduce the dose contribution of 0.477 MeV gamma rays produced by the competing 7Li(p,p')7Li* reaction to acceptable levels while minimizing the effect on the neutron dose rate. Construction of the rotator assembly is complete, and construction of the vacuum chamber is underway. To reduce energy fluctuations in the proton beam which result in variations in the neutron energies, we have purchased a new voltage stabilization system for the Van de Graaff accelerator. This system, combined with beam slit feedback, reduces the energy fluctuations of the proton beam to 11 keV.
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