GEANT4 Simulations of TIGRESS

1. GEANT4 Simulations of TIGRESS TRIUMF-ISAC Gamma-Ray Escape-Suppressed Spectrometer M.A. Schumaker University of Guelph, Ontario, Canada For the TIGRESS Collaboration GEANT4 User’s Workshop, CERN, Geneva November 11 – 15, 2002

2. What is TIGRESS? • TIGRESS is a Gamma-Ray Spectrometer • Assembled over 6 years, ending in 2008 • Will be one of the most advanced, most efficient gamma-ray spectrometer in the world • High gamma detection efficiency and large photopeak-to-background ratio • Will perform nuclear astrophysics, nuclear structure, nuclear reactions experiments, and beyond

3. TIGRESS Location • TIGRESS will be an experimental facility for the ISAC-II radioactive ion beam accelerator at the TRIUMF Laboratory in Vancouver, Canada

4. ISAC • Isotope Separator and Accelerator • Advanced radioactive ion beam accelerator facility • For ISAC, radioactive nuclei are produced by bombarding targets with up to 100 uA of 500 MeV protons from the TRIUMF main cyclotron • Can then be separated and accelerated for a variety of experiments • Accelerated masses up to 30 amu • Energies up to 1.5 MeV per nucleon

5. ISAC-II Upgrade • Scheduled for completion in 2007 • The ISAC-II upgrade to ISAC will increase the mass and energy limits • Accelerated nuclei up to 150 amu • Energies up to 6.5 MeV per nucleon • Up to 15 MeV for light nuclei • Allows Coulomb Excitation and Fusion experiments due to higher energies • TIGRESS will take maximum advantage of the new capabilities ISAC-II provides

6. ISAC II 2008 ISAC II 0.15 – 6.5 MeV/A ISAC II High b SCRF SC LINAC ISAC I 0.15 – 1.5 MeV/A Med b SCRF Tigress Low b SCRF DTL1 Thick/Hot Target high-energy proton beam DTL2 Ion Source RFQ Production Accelerator TRIUMF 500 MeV Cyclotron 100 mA Ion Beam A/q < 30 Low-Energy Experiments Isotope Separator CSB

7. TIGRESS Design • Composed of sixteen separate detectors • Modular design is versatile • Detectors can be replaced with other detection devices • Detectors can be used with other experiments • Allows the full array to be assembled over time

8. Geometry Forward Configuration Provides Maximum Efficiency Back Configuration Provides Maximum Peak-to-Background Ratio

9. TIGRESS Simulation Using GEANT4 • Goal was to create a detailed simulation to determine the expected efficiency and peak-to-background response of the detectors, and to aid in design optimization studies

10. TIGRESS: Reality vs. Simulation • Simulation (Current Implementation) • Particle Gun set at centre of the array • Emits gamma rays of a certain energy • Random directions • Germanium crystals and Suppression shields set as Sensitive Detectors • Energy deposition in each is recorded and analyzed • EM Physics List • Only processes for photons, electrons and positrons needed • Reality • Beam of radioactive nuclei passes through detector, collides with target in the centre of the array • Low Energy • Beam particles become embedded in target material, and decay • High Energy • Coulomb excitation or fusion with target particles, creating exotic nuclei • Gamma rays emitted in random directions • Detected by Ge Crystals • Compton Suppression Shield detect scattered gamma rays

11. TIGRESS HPGe Crystals • TIGRESS uses High-Purity Germanium Crystals for gamma detection • HPGe provides excellent energy resolution • Crystals are designed to maximize the solid angle coverage inside the detector • Leads to high gamma-ray detection efficiency

12. BGO Compton Escape Suppression Compton Scattered Gamma Ray Compton Suppression • Compton Suppression shields detect events which scattered in the germanium crystal • When an event is detected in both the germanium crystal and the suppression shield, the event is not included in the data • Detecting scattered events decreases the magnitude of the spectral background, so it is important to optimize the shield design

13. These come together to form… A TIGRESS Detector

14. Comparison of GEANT4 Simulation to Previous Monte Carlo Program Counts (total=1 million) Energy (keV) Testing the Simulation

15. TIGRESS Efficiency

16. Geometry • Detector geometry is determined by a small number of variables set in a configuration file • Configuration file is read at run time • Geometry was designed to be extremely malleable • From run to run, the detector measurements can be changed very easily • All components, but most notably the Compton suppression shield measurements can be changed for different runs, and the results compared

17. Suppression Shield Optimization • The suppressor shields should detect as many events as possible, though cost is an issue • The thicknesses of the sides, back, and front extensions were examined • Runs were performed to investigate the optimal design • Performed in both the forward and backward configuration

18. Suppression Shield Optimization

19. Suppression Shield Optimization

20. Suppression Shield Optimization

21. TIGRESS

22. Summary • A simulation of TIGRESS was created to determine the expected efficiency at various energies, and the behaviour of the Compton suppression shield • This was used to maximize the peak-to-background ratio by simulating different Compton suppressor designs • We have made significant progress with GEANT4, but there is much more to do

23. Future Goals • Make the simulation closer to reality • Simulate electrode dead layers in Germanium crystals • Aluminum shell around the suppression shield, beamline vacuum chamber, etc. • Suppression shield scintillation? • Compare the simulation to the first TIGRESS detector prototype • HPGe detector expected November 2002 • Suppressor expected spring 2003

24. Acknowledgements • University of Guelph (Guelph, Ontario, Canada) • Carl Svensson • Paul Finlay • Geoff Grinyer • TRIUMF (Vancouver, British Columbia, Canada) • Helen Scraggs and Kelly Cheung • Greg Hackman and Gordon Ball • McMaster University (Hamilton, Ontario, Canada) • Jim Waddington • The Full TIGRESS Collaboration