geant4 simulations of tigress triumf-isac gamma-ray escape-suppressed spectrometer m.a. schumaker...
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GEANT4 Simulations of TIGRESSTRIUMF-ISAC Gamma-Ray
Escape-Suppressed Spectrometer
M.A. SchumakerUniversity of Guelph, Ontario, Canada
For the TIGRESS Collaboration
GEANT4 User’s Workshop, CERN, GenevaNovember 11 – 15, 2002
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 Location• TIGRESS will be an
experimental facility for the ISAC-II radioactive ion beam accelerator at the TRIUMF Laboratory in Vancouver, Canada
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-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
ISAC II 2008
ISAC II
Production Accelerator
TRIUMF500 MeV
Cyclotron100 A
high-energy proton beam
Thick/Hot Target
Ion Source
Isotope Separator
IonBeam
Low-EnergyExperiments
Low-EnergyExperiments
DTL1
RFQ
CSB
ISAC II0.15 – 6.5 MeV/A
ISAC II0.15 – 6.5 MeV/A
A/q < 30
DTL2
High SCRF
Med SCRF
LowSCRF
SC
LINAC
ISAC I0.15 – 1.5 MeV/A
ISAC I0.15 – 1.5 MeV/A
Tigress
• 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
TIGRESS Design
GeometryForward Configuration
Provides Maximum Efficiency
Back ConfigurationProvides Maximum Peak-
to-Background Ratio
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
TIGRESS: Reality vs. Simulation• 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
• 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
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
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
ComptonEscapeSuppression
BGO
ComptonScatteredGamma Ray
A TIGRESS Detector
These come together to form…
Testing the Simulation
Energy (keV)
Cou
nts
(to
tal=
1 m
illion
)Comparison of GEANT4 Simulation to Previous Monte
Carlo Program
TIGRESS Efficiency
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
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
Suppression Shield Optimization
Suppression Shield Optimization
Suppression Shield Optimization
TIGRESS
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
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
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