Microdosimetric measurements for photons in a water phantom
Microdosimetric event distributions were determined in air at three primary photon energies (60, 660 and 1250 keV) and at depths of 2, 5, and 10 cm in a water phantom (30 x 30 x 30cm) from measurements of charge produced in a walled Rossi-type proportional counter. For 660 and 1250 keV photon energies free air sources of /sup 137/Cs and /sup 60/Co, respectively, were used at the Brookhaven National Laboratory (BNL) calibration facility and for 60 keV a 300 mCi /sup 241/Am source was used. Event size spectra were taken at simulated site diameters of 0.5, 1, and 2 ..mu..m. The frequency mean (anti Y/sub F/) and dose mean (anti Y/sub D/) of lineal energy density were determined from these measurements. Our results without a phantom indicate reasonably good agreement for anti Y/sub F/ and anti Y/sub D/ with measurements reported in the literature for walled proportional counters. Ratios anti Y/sub F/(water)/anti Y/sub F/(air) and anti Y/sub D/(water)/anti Y/sub D/(air) were determined for each site diameter and depth in the phantom. For 60 keV photons the maximum increase in these ratios was approximately 8 percent for a site diameter of 2 microns at 10 cm depth in the phantom. For 660 keV photons the maximum increase in these ratios was 24 percent whereas for 1250 keV photons this maximum increase was approximately 15 percent. However, for 60 keV photons only about 8 percent increase was observed; this small increase is consistent with the fact that about half of the scattered photons are removed by photoelectric processes. Since anti Y/sub D/ in the phantom increased only by about 24 percent large changes in biological effectiveness are not expected as a function of depth in typical biological systems.
- Research Organization:
- Brookhaven National Lab., Upton, NY (USA); Columbia Univ., New York (USA). Radiological Research Lab.
- DOE Contract Number:
- AC02-76CH00016
- OSTI ID:
- 5157989
- Report Number(s):
- BNL-28157; CONF-800944-1; TRN: 80-015834
- Resource Relation:
- Conference: 7. symposium on microdosimetry, Oxford, UK, 8 Sep 1980
- Country of Publication:
- United States
- Language:
- English
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