Studies on spectrum shapes of low-energy photon sources

The spectrum shapes of Low-Energy Photon (LEP) sources undergo interesting changes in the presence of scattering media of low atomic numbers. Two readily observed changes in the spectrum shape are (a) downward shift of the peak energy and (b) apparent deterioration of the energy resolution of the detector. A study of these changes is useful in vivo assessment of LEP-emitters such as 241Am. A computer program, based on Monte Carlo principles, has been evolved to compute the magnitudes of these changes in pulse-height distribution. Calculations were performed for point sources of low-energy photons located on the axis of the detector and for different thickness of the scattering medium interposed between the source and the detector. The magnitudes of both the above mentioned changes in the pulse-height spectra of LEP sources embedded under media of low effective atomic number, such as water (Z = 7.42), tissue (Z = 7.35) and perspex (Z = 6.47), are found to be dependent to a great extent on the energy resolution of the detector employed, apart from the electron density and effective atomic number of the scattering medium. The theoretical results are presented and compared, wherever possible, with experimental values obtained with NaI(Tl) and Ge(Li) detectors. (author)