NAC protocol for neutron beam calibration

The instrumentation used for neutron beam calibration consists of several tissue equivalent ionization chambers flushed with methane-based tissue equivalent gas (64.4% methane, 32.4% carbon dioxide and 3.2% nitrogen) which are connected through a low noise cable to an electrometer. The identical instrumentation including interconnecting cable is used for calibrating the ionization chambers in a 60Co beam. Exradin type T-2 0.5 cm3 tissue equivalent ionization chambers are the primary standards although other types of chambers are available for special applications and for comparison purposes. The procedures used to calibrate the monitor ionization chambers in the neutron therapy beam in terms of absolute total dose (neutron plus gamma) are specified. The procedures basically involve calibration of the ionization chambers in air in a 60Co beam, calibration of the neutron beam monitor chambers relative to the dose to tissue measured at a reference point in a water phantom and routine constancy checks. 8 refs