Radiation quality investigations around a neutron therapy beam, using a high-pressure ionization chamber

The mean quality factor (Q̄) and the dose mean linear energy transfer [LET (L̄(d))] a few centimeters outside a neutron therapy beam have been measured using a high-pressure ionization chamber. Measurements have been carried out in air, 5 and 100 cm outside the treatment field edge on the patient plane for p(46)-Be neutrons and are compared with the results obtained at the same points with a low-pressure proportional counter (Rossi counter) and a Bonner spheres system. General agreement is found between these independent methods. The dose equivalent at 5 and 100 cm distances outside the field edge is 2% and 0.4% of the dose equivalent at the isocenter. Measurements were also performed at the same points as a function of proton energy on the Be target. Proton energies of 25-46 MeV on Be were applied. The results indicate increase in the mean quality factor at proton energies below 30 MeV.