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@clermont.in2p3.fr MGS meeting

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 MGS meeting

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 MGS meeting

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 detector’s well MGS meeting

5. Experimental measurements • 137Cs E = 32 keV 137Cs E = 662 keV MGS meeting

6. Experimental measurements • 60Co E ~1250 keV MGS meeting

7. Model • Efficiency simulated with geometry given by Canberra manufacturer • 137Cs E = 32 keV 137Cs E = 662 keV MGS meeting

8. Model • 60Co E ~ 1250 keV MGS meeting

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) MGS meeting

10. Results • 137Cs E = 32 keV 137Cs E = 662 keV MGS meeting

11. Results • 60Co E ~ 1250 keV MGS meeting

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

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) MGS meeting

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 MGS meeting

15. Tools • Gate : user interface over Geant4 • Advantages : • Language (script) which doesn’t 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 MGS meeting