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