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.
• Geometry of 100% HPGe detector at CPL (provided by manufacturer)
R 41mm
H 86.3mm
2. Experiment
2.1 Mixed source experiment
• Efficiency Calculation
ⅰ) Calculation of measured efficiency
N0 
A0

N  N 0 ( e
 t i
e
 t f
)
Rnet
 (%) 
 100
N  I
ⅱ) Calculation of simulated efficiency
Number of counts in the Ge crystal
  100
Total # of events generated in the source geometry
• 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
2.2 Efficiency difference
• Compare experimental data to simulation of HPGe at NETEC
Experiment data * 1.4
Experiment data
Simulation result 40% more larger than experiment result
• Compare two simulation results
CPL
NETEC
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
137Cs source at top and side position
Compare efficiency and Compton edge shape
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
• Testing with 152Eu and 137Cs sources
• Two Eu-152 (KRISS, NIST) at top position
Top
204mm
HPGe
Side
128m
m
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)


137Cs source
top
side
Simulation
0.2336
0.5179
Experiment
0.206
0.5441
Ratio
1.035
0.952
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.