Gamma spectrometry and plastic-scintillator inherent background

The y-ray absorption in PS at medium energies is due mainly to Compton scattering. The high-energy part of the Compton-electron spectrum enables one to determine the resolution and light yield [5, 6]. The calibration method for the energy scale is based on the pulse height corresponding to half the maximum of the Compton distribution. Figure I shows the energy dependence of the light yield for PS detectors of various shapes and irradiation geometries. A linear dependence has been found with this calibration method in numerous experiments with cylindrical and rectangular detectors of length up to i m used with point or distributed y sources giving energies of 0.6-4.4 MeV, no matter what the form of reflecto=. These results agree with many other studies such as [7] and confirm that this energy calibration method is correct. Deviations from linearity have been detected only with small detectors when the ratio of the length of the scintillator (in the y-ray beam direction) to the path length for y rays with the minimum energy is n < 0.5. An increase in y-ray energy for n < 0.5 leads to a shift in the light zones providing most of the signal. The transparency varies by a substantial factor for small transmission lengths for the scintillations [81 because of UV absorption. We measured the energy resolution for a linear dependence of light yield on y-ray energy. If there were Several y lines from a source, we used the one with the highest energy to eliminate distortion due to overlap from the Compton background from y rays of higher energy. Table 1 gives the characheristics of the y sources. The total attenuation coefficients have been taken from [9], which were used to calculate the attenuation lengths for materials of density 1.055 g/cm 3, Figure 2 shows the results for a specimen based on polystyrene of diameter and length 150 ~m used with an FEU-49B photomultJplier and voltage divider selected to give uniformity in the quantum sensitivity of the photocathode. Under all reflection conditions, an increase iny-ray energy at first improves the reso