Optimisation of a Dual Head Semiconductor Compton Camera using Geant4
L J Harknessa, J Gillamb, D Oxleya, T Beveridgeb, A J Bostona, H C Bostona, R J Coopera, J R Cresswella,
A N Grinta, I Lazarusc, P J Nolana, D P Scraggsa
a)Department of Physics, University of Liverpool, Oliver Lodge Laboratory, Liverpool, L69 7ZE, UK
b)School of Physics and Materials Engineering, Monash University, Melbourne, Australia
c) CCLRC Daresbury Laboratory, Warrington, Cheshire, UK
Introduction
Position sensitive High Purity germanium detectors are the sensor of choice for gamma-ray
detection in the field of nuclear physics due to their excellent intrinsic energy resolution and the
potential for excellent spatial resolution using segmentation and pulse shape analysis techniques [1].
Members of the University of Liverpool imaging group are currently developing a Compton Camera
for medical and security applications, which will exploit the characteristics of high purity germanium
detectors. Previous work done by the group has shown the potential of semiconductor Compton
camera systems but show limitations to the current imaging equipment [2]. Such limitations arise
because SmartPET [3] has been designed and developed for Positron Emission Tomography (PET)
experiments, at a higher energy range than the Compton Camera for Single Positron Emission
Tomography (SPECT).
Previous Experiments
The two ORTEC position sensitive High Purity Germanium segmented SmartPET detectors have been
operated in a dual head Compton Camera configuration to image a 152Eu point source [2]. The
separation between the scatter detector and analyser detector was varied at 3cm, 5cm, 7cm, 9cm
and 11cm whilst the source was rotated from 0° to 15°, 30°, 45° and 60° for each separation,
totalling 25 acquisition positions. Simple reconstruction codes were implemented to obtain images
of the source.
Fig 1: Reconstructed images of
152
Eu point source for 3cm (top)
and 5cm (bottom) separation at
0° (left) and 60° (right)
Fig 2: The SmartPET detectors in
Compton Camera Configuration
with source at 0° [2]
The majority of incident gammas of energy <244keV were absorbed in the 20mm thick scatter
detector, showing that the detectors are not ideal for the energy of medical interest, 141keV. In
addition, experiments have been carried out using a 0.5mm thick Double Sided Strip Silicon Detector
as the scatter detector and a SmartPET detector as the analyser detector [4]. It was observed in this
acquisition that limitations arose from high noise levels on the DSSSD, leading to a reduced number
of events. For medical applications this is not desirable, as dose to the patient should be kept to a
minimum therefore requiring high sensitivity instruments. It is proposed that using germanium as
an alternative to silicon could improve on this, as noise levels could be lower than on the DSSSD.
Geant4
Geant4 [5] has been used to simulate the SmartPET detectors in Compton camera configuration, Fig.
3, and has been successfully validated against experimental data. The simulation is currently being
used to optimise the geometry of a semiconductor Compton camera system, by varying the detector
thicknesses and geometry. The setup will be optimised for 141keV but will be useful across an
energy range to be determined by the simulations. The optimal performance will be maximum
single hit scattering in the front detector and maximum absorption in the back detector. It can be
seen in Fig.4 that the majority of the incident 141keV radiation is absorbed in the thickness of the
SmartPET detector, and that by decreasing the thickness, a higher proportion will scatter out of the
detector.
Fig 3: Geant 4 Simulation output of 2
SmartPet detectors (blue), guard ring (red)
and aluminium can (yellow). Isotropic
source Gamma rays are shown in green.
Fig 4: Simulated interaction position in x across
the scatterer detector for incident energy
141keV
Conclusion
A germanium Compton camera is being designed and optimised using Geant4 simulations. A proofof-principle has been demonstrated with the SmartPET system for energies >344keV range. The aim
is to increase sensitivity of current SPECT systems by implementing a Compton camera system and
removing the need for mechanical collimation. The system will be optimised for 141keV but will be
simulated across a wide energy range. Applications of interest include nuclear medical imaging and
security.
[1]D. Bazzacco, Nuclear Physics A 746 (2004)
[2] H.C. Boston et al., IEEE Nuclear Science Symposium Conference Record (2006)
[3] R.J. Cooper et al., Nuclear Instruments and Methods in Physics Research A 579 (2007)
[4] A. Grint, IOP Nuclear Physics Conference (2008)
[5] S. Agostinelli Nuclear Instruments and Methods in Physics Research A 506 (2003) 250–303