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
TIGRESS is a sophisticated gamma ray spectrometer scheduled to begin assembly next year. The complete array will be assembled by 2008.
When it is complete, TIGRESS will be the most advanced, most efficient gamma-ray spectrometer in the world. This is due to a very high gamma ray detection efficiency

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
ISAC exists, and runs. Make a point of this. But don’t say too much more

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
Nuclear evaporation experiments (clarification if someone asks)

6. Production Accelerator
ISAC II
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
CSB is charge state booster (knocks more electrons off ion) when they come out of the target they are 1+
RFQ is radio frequency quadrupole
DTL drift tube linac
SC linac is superconducting (niobium)
Highest current used is 40 uA from TRIUMF (melt target)
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
This should really discuss the flexibility of design, not really the flexibility of TIGRESS

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
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
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

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
Good HPGe gives 2 keV FWHM at 1.33 MeV (Co-60 second photopeak)
Best for energy above 100 keV (silicon also works… but lots of compton scattering)
11 cm across somehow point this out

12. 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
Compton
Escape
Suppression
BGO
Compton
Scattered
Gamma Ray
Say that we’re interested in optimizing this feature of the detector

13. These come together to form…
60 cm long
A TIGRESS Detector

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

15. TIGRESS Efficiency
Slide after geometry – put in spectrum and explain that we wanted to test the output of the code (and explain gamma ray spectra)
And scanning test
Say what was in the previous slide

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
Nothing is hardcoded. That’s a better way to say it

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
Go with 10 mm
Show the other results

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