Simulation of detection of gamma radiation by germanium detector

1
Simulation of detection of gamma radiation by
germanium detector

• Courtine Fabien
• Equipe Thermoluminescence
• Laboratoire de Physique Corpusculaire
• Clermont-Ferrand
• courtine_at_clermont.in2p3.fr

2
Introduction

• Measurement of gamma activity of solid or liquid
  samples in an energy range from 20 keV to 3 MeV?
  dosimetry
• 2 geometry well and marinelli

3
Problematical

• Efficiency calibration of a germanium detector
• Efficiency nb gamma photopeak/nb gamma emitted
• No available calibrated source to cover all the
  energy range
• Correction of self-attenuation
• Cascade effect
• Calibration fully experimental impossible

4
Method

• Monte Carlo calculations need full knowledge of
  geometry, which is not the case
• Calibration needs to be a combination of
  experimental measurements and Monte Carlo
  calculations unknown dimensions are calculated
  by comparing simulated and experimental
  efficiency
• Experimental measurements done with two
  point-like sources (137Cs et 60Co) displaced
  inside the detectors well

5
Experimental measurements

• 137Cs E 32 keV 137Cs E 662 keV

6
Experimental measurements

• 60Co E 1250 keV

7
Model

• Efficiency simulated with geometry given by
  Canberra manufacturer
• 137Cs E 32 keV 137Cs E 662
  keV

8
Model

• 60Co E 1250 keV

9
Model

• Introduction of two inactive layers in model
• Size of these layers is determined by successive
  adjustment between experimental and silmulated
  efficiency
• Internal inactive layer thickness calculated with
  low energy gamma (32 keV)
• External inactive layer thickness calculated with
  higher energy gamma (662 keV)

10
Results

• 137Cs E 32 keV 137Cs E 662 keV

11
Results

• 60Co E 1250 keV

12
Interpretation

• Physical meaning of inactive layers ?
• 2 effects
• Electrical field effect ? Mgs (cortesy of C.
  santos, P. Medina, C. Parisel), passive area

13
Interpretation

• Lithium diffusion on external face and bore
  implantation on interne face? dead layer
• Good agreement simulation-experience at low and
  mean energy but disagreement at high energy
• E 32 keV R (0.992/-0.006)
• E 662 keV R (0.997/-0.002)
• E 1250 keV R (0.925/-0.001)

14
Tools

• Geant4 simulation of the passage of particles
  throught matter
• Advantages
• Cross sections validited at low energy
• Complex geometry like boolean operation or shapes
  with hole.
• Geant4 is written in C
• Possibility of use many package for analyse and
  graphical interface (Root, OpenScientist, Aidda)
• Disadvantages
• Need to know C
• No graphical interface already built
• No package for analysis

15
Tools

• Gate user interface over Geant4
• Advantages
• Language (script) which doesnt need the
  knowledge of C
• Easy to use
• Disadvantages
• No possibility to make geometry as complex as in
  Geant4
• No graphical interface
• No package for analysis