A microdosimetric characterization of a cyclotron-produced therapeutic neutron beam.

The therapeutic neutron beam of the Cyclotron Corporation's CP-42 negative-ion cyclotron is generated by protons of 42 MeV bombarding a thin beryllium target. Microdosimetric measurements were made for this neutron beam in a full-scatter water phantom at nine positions inside and outside the useful beam. The lineal energy distribution and the variations of dose-mean, frequency-mean, and saturated lineal energy are compared for these positions. The dose fraction due to gamma rays is also calculated at each of these positions, based upon previously published techniques. A theoretical relative biologic effectiveness, based upon the dual radiation action model of Kellerer and Rossi [Curr. Top. Radiat. Res. 8, 85 (1972)] is also shown for the positions of measurement.