Simulation of HPGe detector efficiency Eunkyung Lee Ewha Womans University

1. Introduction It is important to obtain efficiency by simulation in order to calculate accurate amount of radioactive isotopes. Compare experimental efficiency with simulation one with a mixed source in Marinelli Beaker. - Efficiency discrepancy was 40~50%. We assumed several reason of this efficiency discrepancy then check it various ways. Test with point sources placed top and side of HPGe detector Compare gamma peaks and Compton edge part Check relative efficiency : compare HPGe to NaI using a Co-60 source with simulation results Consider inactive Ge layer – active Ge volume Using this geometry we will calculate amount of radioactive isotopes in CsI crystal, CsI powder and PMT background.

R 41mm H 86.3mm Geometry of 100% HPGe detector at CPL (provided by manufacturer)

2. Experiment 2.1 Mixed source experiment

Efficiency Calculation ⅰ ) Calculation of measured efficiency ⅱ ) Calculation of simulated efficiency

Experiment data * 1.4 Experiment data Simulation result 40% more larger than experiment result Compare experimental data to simulation of HPGe detector at CPL Efficiency discrepancy was shown in CPL HPGe detector. We assumed three possibilities. 1. due to incorrect detector geometry 2. due to problems in GEANT4 program 3. due to incorrect source information

Experiment data * 1.4 Experiment data 2.2 Efficiency difference Compare experimental data to simulation of HPGe at NETEC Simulation result 40% more larger than experiment result

CPL NETEC Compare two simulation results Almost same

Simulation tests EGS4 and GEANT3 test with mixed source -within 7% agreeable Relative efficiency check NaI Absolute0.1258%0.1482%0.1471% Relative100%117.8%116.9% CPL and NETEC simulation result 18% more larger than NaI crystal

Efficiency(%)topside Simulation Experiment Ratio(S/E)1.35 Simulation efficiency larger than experiment result but Compton Edge shape is almost same 137Cs source at top and side position Compare efficiency and Compton edge shape

HPGe 204mm Top Side 128m m Testing with 152 Eu and 137 Cs sources Two Eu-152 (KRISS, NIST) at top position

3 Ge dead layer Dead layer correction check-up mentioned on other papers  NIMA vol.496 p390 dead layer correction HPGe 28% dead layer 0.65mm Active volume / (2.65% reduced)  NIMA vol.498 p340 charge collection time correction  NIMA vol.487 p477 dead layer correction HPGe 40% dead layer 0.6mm Al-Ge distance increased by 8mm

 The CPL HPGe detector geometry correction  Outer Ge dead layer : 1.12mm  Active volume : cm3 (7.2% reduced)

topside Simulation Experiment Ratio  137Cs source  Relative efficiency NaI Absolute0.1258%0.1333% Relative100%105.96%

4. Measurement of radioisotopes activities

6. Conclusion With manufacturer geometry, efficiency differences was occurred between experimental data and simulation data. We assumed three possibilities-a. detector geometry, b. simulation program, c. source information, then tested each possibility. Our tests indicate that there are unknown parameters like inactive layers in HPGe detector. Adding inactive Ge layer in simulation code, efficiency difference reduced. Using this HPGe geometry included inactive Ge layer, calculation of amount radioactive isotopes in CsI crystal and PMT can be obtained.