INFLUENCE OF CAVITY SIZE ON THE RESPONSE OF CAVITY CHAMBERS TO 25-MEVAND 45-MEV NEUTRONS

We calculated the response of gas-cavity dosimeters to 25- and 45-MeV neutrons in order to estimate the portion of the response that origina tes from neutron interactions within the gas cavity. This affords insi ght into the validity of the Bragg-Gray theory for dosimeters with fin ite cavities and also provides a basis for modifying gas-to-wall absor bed dose conversion factors deduced from the Bragg-Gray theory for use with small-cavity dosimeters. For atypical ionization chamber, e.g., with a 1-cm(3) propane-based tissue-equivalent gas at standard tempera ture and pressure, the fraction of the total absorbed dose to the gas cavity from neutron interactions within the gas is 0.09% at 25-MeV and 0.006% at 45-MeV neutron energies. For microdosimetric detectors, the neutron interactions within the gas are negligible above 25 MeV. (C) 1997 American Association of Physicists in Medicine.