Shape of the backscatter peak

The effect was studied of the scatterer on the shape of peaks of scattered gamma radiation energy spectra obtained using a 60Co point source placed close to the surface of the scatterer. A NaI(Tl) scintillation crystal with 40 mm diameter and 40 mm length was placed at a distance of 20 or 80 mm above the Pb, Sn, C, Al, or Fe scatterer. The measured spectra showed that low Z elements suppressed the low-energy part of the spectrum to a lesser degree than elements with higher atomic number. Also studied was the effect of scatterer dimensions on the peak shape at a detector distance of 80 mm from the scatterer. Measurements were carried out using slabs of ''infinite dimensions'', 2, 5, 10, 20, 100 mm in thickness, cylinders of 100 mm in height and 30, 60, 100, 170 mm in diameter and hemispheres of the same diameter as the cylinders. All specimens were made from duralumin. The experiments showed that the single scattering peak decreased quicker to lower energies than to higher energies. The slower decrease rate to higher energies was due to photons scattered once at an angle smaller than 180deg. Multiple scattering was observed in increasing the thickness or diameter of the scatterer. Multiple scattering was also observed at greater distances from the point source. It was shown that in studying the local changes in the scatterer thickness in the vicinity of the point source, only photons having an energy corresponding to a single scattering at angles approaching 180deg should be evaluated. (J.P.)