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 41mmH 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

100(%)

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geometry source in the generated events of # Total

crystal Ge in the counts ofNumber 100

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

CPLNETEC

• Compare two simulation results

Almost same

• Simulation tests

• EGS4 and GEANT3 test with mixed source-within 7% agreeable

• Relative efficiency check

NaI Crystal HPGe100@CPL HPGe100%@NETEC

Absolute 0.1258% 0.1482% 0.1471%

Relative 100% 117.8% 116.9%

CPL and NETEC simulation result 18% more larger than NaI crystal

Efficiency(%)

top side

Simulation 0.2780 0.7335

Experiment 0.206 0.5441

Ratio(S/E) 1.35 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

Side128mm

• Testing with 152Eu and 137Cs 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 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

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

top side

Simulation 0.2336 0.5179

Experiment 0.206 0.5441

Ratio 1.035 0.952

137Cs source

Relative efficiency

NaI Crystal HPGe100@CPL

Absolute 0.1258% 0.1333%

Relative 100% 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.