DETERMINATION OF PHOTON BACKSCATTER FROM SEVERAL CALIBRATION PHANTOMS

The photon backscatter factor for several irradiation phantoms was det ermined by calculational and experimental methods and compared with a phantom constructed of International Commission on Radiation Units and Measurements (ICRU) four-element tissue. Calculations of the photon b ackscatter factor over the energy range from 10 to 2000 keV were perfo rmed using the MCNP 4A code for each of the phantoms. Measurements of the backscatter factor were carried out using a thin-walled ionisation chamber that was exposed to X-ray beams with narrow energy spectra. T he calculations and measurements agreed to within uncertainties and de monstrated that the backscatter factors over this energy range for a w ater-filled phantom recommended by the International Organization for Standardization (ISO) and a phantom constructed of tissue-equivalent ( RS-I) plastic are nearly the same as that of the ICRU tissue reference phantom. Hd Never, the backscatter from a polymethylmethacrylate (PMM A) phantom was up to about 8% higher. In addition, it was found that a composite phantom of polytetrafluoroethylene (PTFE) and PMMA also pro duced a backscatter factor quite similar to that of the ICRU tissue re ference phantom.