Direct determination of kerma for a d(48.5)+Be therapy beam

An experimental determination of the neutron kerma ratio between muscle tis sue and A-150 plastic was performed at the newly commissioned d(48.5) + Be therapy facility in Detroit. Low-pressure proportional counters with separa te walls made from A-150 plastic, graphite, zirconium oxide and zirconium s erved to measure ionization yield spectra. The absorbed dose in the wall of each counter was determined and rendered the A-150 and carbon kerma direct ly, whilst that for oxygen was deduced from differences between the matched metal oxide and metal pair. This enabled the evaluation of an effective ke rma ratio as a function of radiation field size and hydrogenous filtration. Although filtration was observed to harden the beam, the application of a single kerma ratio for the various irradiation conditions investigated was found to he appropriate. A neutron kerma ratio of 0.90 +/- 0.03 was assessed for the Detroit facilit y, which is lower at the 1 sigma level than the 0.95 currently recommended in the dosimetry protocol for high-energy neutron beams.