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Simulation of HPGe detector efficiency

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.

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Simulation of HPGe detector efficiency

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  1. Simulation of HPGe detector efficiency Eunkyung Lee Ewha Womans University

  2. 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.

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

  4. 2. Experiment 2.1 Mixed source experiment

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

  6. Compare experimental data to simulation of HPGe detector at CPL Experiment data * 1.4 Experiment data • 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 Simulation result 40% more larger than experiment result

  7. 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

  8. Compare two simulation results CPL NETEC Almost same

  9. Simulation tests • EGS4 and GEANT3 test with mixed source-within 7% agreeable • Relative efficiency check CPL and NETEC simulation result 18% more larger than NaI crystal

  10. 137Cs source at top and side position • Compare efficiency and Compton edge shape Simulation efficiency larger than experiment result but Compton Edge shape is almost same

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

  12. 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 121.574 /124.879 (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

  13. The CPL HPGe detector geometry correction • Outer Ge dead layer : 1.12mm • Active volume : 374.15cm3 (7.2% reduced)

  14. 137Cs source • Relative efficiency

  15. 4. Measurement of radioisotopes activities

  16. 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.

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