Application of A150 plastic-equivalent gas in A150 plastic ionization chambers for Co-60 gamma rays and 14.8-MeV neutrons.
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Cobalt Radioisotopes↗
Biomedical subjects
D W Pearson
Publications and source records attributed to D W Pearson.
Performance characteristics of A 150 plastic-equivalent gases in A 150 plastic proportional counters for 14.8-MeV neutrons.
Two recently developed A 150 plastic-equivalent gas mixtures have been tested for suitability in proportional counter applications. Methane- and propane-based "tissue-equivalent" gases were also included for comparison purposes. Event-size weighted dose distributions were measured in a 14.8-MeV neutron beam. Resolution was found to be independent of gas mixture. Moreover the gains of the two A 150 mixtures were the same, and comparable to that of the methane-based gas mixture. The ionization yield per event size was also independent of the hydrogenous gas mixture employed. Neutron doses determined with the proportional counter were in reasonable agreement with those obtained from an ionization chamber.
Fast Neutrons↗
Measurement of dose distributions of linear energy transfer in matter irradiated by fast neutrons.
A detector has been developed and used to measure dose distributions versus linear energy transfer to thin gas targets in spherical geometry from fast neutron irradiation of tissue-equivalent plastic and carbon. The detector is a hemispherical proportional counter with a Cs(T1) scintillator at the center of the hemisphere. The coincidence of the proportional counter signals constrain the measurements to charged particles traversing the radius of the hemisphere. The charged particle energy deposition distributions are directly measured for a known pathlength. The A-150 kerma factor was measured at a neutron energy of 14.8 MeV and is in agreement with tabulated values. The carbon kerma factor measurements are less than the tabulated value at 14.8 MeV. The alpha-particle production in carbon was measured for neutron energies from 14.1 to 14.8 MeV and is compared with existing data.
Energy Transfer↗
Calibration of 192Ir high-dose-rate afterloading systems.
A method is described for calibration of 192Ir high-dose-rate (HDR) brachytherapy afterloading systems. Since NIST does not offer calibration of ionization chambers with the gamma-ray spectrum of iridium-192, an interpolation procedure is employed, using calibrations above (137Cs, 662 keV) and below (250 kVcp, 146-keV x rays) the exposure-weighted average 192Ir energy of 397 keV. The same total wall + cap thickness must be used for both calibrations, and for the 192Ir measurements. A wall + cap thickness of 0.3 g/cm2 is recommended to assure charged particle equilibrium and to exclude secondary electrons emitted from the source encapsulation. Procedures are described for determining the corrections for source-chamber distance and room scatter during the source calibration in inverse-square-law geometry. A new well-type ionization chamber has been designed specifically for convenient routine use with the HDR afterloading system. It can be calibrated by means of a previously calibrated 192Ir source, and offers a simple means for verifying the decay rate and for calibrating 192Ir replacement sources.
Brachytherapy↗