The feasibility of using compact, medium-energy accelerators for production of neutron radiotherapy beams has been examined in recent reports. The use of hydrogeneous filters in the neutron beams to harden the beam may allow smaller accelerators to produce a beam with acceptable penetration. Indeed, neutron radiotherapy units wherein a beryllium target is bombarded by a beam of protons from a cyclotron are currently being marketed; polyethylene filtration of the neutron beam is offered in these machines to harden the beam. In order to understand fully the hardening effected by such filtration and to provide spectral information needed for accurate dosimetry of these beams, the authors have measured the neutron energy spectra produced by filtering p-Be neutron beams with polyethylene filters of 1.84, 3.68 and 5.52 g cm-2thicknesses.
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