Application of Unfolding Technique to HPGe Detector Using Response Functions Calculated with the EGS4 Monte Carlo Code

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The EGS4 Monte Carlo simulation technique was used to obtain the energy spectra of photons arriving at a detector from the pulse height distributions measured by the same detector. First, the measured pulse height distribution for incident photons from several radiation sources such as 60Co, 137Cs, 152Eu and 207Bi with a collimator are compared with those calculated using the EGS4 code to investigate the feasibility of the simulation. The comparison showed good agreement of 98.7% for 60Co, 92.5% for 207Bi on the total counts. Second, the pulse height distributions were measured in the open space and then unfolded. The measurement of the distributions was done with changing the source to detector distance (SDD) from 10 cm to 100 cm for 60Co and 137Cs respectively. In the unfolding process, response functions of a high purity Ge (HPGe) detector were calculated using the EGS4 code. The calculated pulse height distributions were then normalized to the measured ones at the peaks of the incident photon energies. The ratio of the sum of counts of the main peaks to the total count in the unfolded spectra for 60Co varied from 5.4 to 5.7 times greater than those in the measured pulse height distributions, while from 2.5 to 2.9 times for 137Cs. Electron contribution to the unfolded spectra for 137Cs decreased as the source to detector distance increased, becoming negligible above 50 cm. The pulse height distributions at the center of the reference plane at 100 cm from the 60Co and 137Cs dummy sources located inside each irradiator were also measured and unfolded to obtain the real pulse height distribution. In the unfolded spectra, the photons scattered from the surrounding materials were reduced to approximately one fourth of those measured in the open space due to the small size of apertures of the irradiators. The ratio of the sum of counts for the main peaks to the total count was larger than those in the measured pulse height distributions by the factor of 5.0 for 60Co and 3.4 for 137Cs. The uncertainties estimated in the unfolding processes were around 0.1% for 60Co and 0.07% for 137Cs.