R.J. Cooper

Compton imaging using the SmartPET detectors

Image reconstruction from Compton camera data is a complex problem requiring investigation. Generally reconstruction is conducted using iterative reconstruction methods such as Maximum Likelihood - Expectation Maximization (MLEM). However, iterative reconstruction into volumetric grids is a computational burden. Analytic methods of image reconstruction have been proposed which relieve the computational expense. However, such methods usually have unrealistic sampling assumptions or are not directly extendable to situations where the scattering detector is extended when compared to the detector-source distance. Starting from a standard inversion technique, a generic method of filtering inversion co-efficients is developed. By dynamically assigning the strength of co-efficients on an event-by-event basis, artifacts arising from the difference between assumed and actual forward transform may be reduced. Dynamic assignment allows inversion of single cone-surfaces, so that the generic technique may be implemented in volumetric reconstruction. Results are compared to direct-back-projection in a limited-angle tomography context. The ability to apply event-by-event analytic image reconstruction provides many advantages when compared to standard iterative techniques.

Status and Performance of an AGATA asymmetric detector

High‐resolution gamma‐ray detectors based on high‐purity germanium crystals (HPGe) are one of the key workhorses of experimental nuclear science. The technical development of such detector technology has been dramatic in recent years. Large volume, high‐granularity, electrically segmented HPGe detectors have been realised and a methodology to improve position sensitivity using pulse‐shape analysis coupled with the novel technique of gamma‐ray tracking has been developed. Collaborations have been established in Europe (AGATA) [1] and the USA (GRETA/GRETINA) [2] to build gamma‐ray tracking spectrometers. This paper discusses the performance of the first AGATA (Advanced GAmma Tracking Array) asymmetric detector that has been tested at the University of Liverpool. The use of a fully digital data acquisition system has allowed detector charge pulse shapes from a selection of well defined photon interaction positions to be analysed, yielding important information on the position sensitivity of the detector.

γ‐ray Spectroscopy of Proton Drip‐Line Nuclei in the A∼130 Region using SPIRAL beams

A fusion‐evaporation experiment has been performed with a SPIRAL 76Kr radioactive beam in order to study the deformation of rare‐earth nuclei near the proton drip‐line. The experimental setup consisted in the EXOGAM γ‐array, coupled to the light‐charged particles (LCP) DIAMANT detector and to the VAMOS heavy‐ion spectrometer. The difficulties inherent to such measurements are enlightened. The coupling between EXOGAM and DIAMANT has been used to decrease the huge background caused by the radioactivity of the beam. It further permits assigning new γ‐ray transitions to specific residual nuclei. A γ‐ray belonging to the 130Pm level scheme has thus been observed for the first time.

Orthogonal Strip HPGe Planar SmartPET Detectors in Compton Configuration

The evolution of germanium detectors over the last decade has lead to the possibility that they can be used in medical imaging and security scanning. The potential of increased sensitivity and energy resolution that germanium affords takes away the necessity of mechanical collimators that would be required in a gamma camera. Without mechanical collimation the resulting increase in statistics leads to the possibility of decreased patient dose or increased system throughput. In terms of security imaging segmented germanium provides directionality and excellent spectroscopic information for nuclide identification.

{gamma}-Spectroscopy and Radioactive Beams: How To Perform Channel Selection ?

An experiment has been performed using a SPIRAL 76Kr radioactive beam at GANIL to investigate rare‐earth nuclei near the proton drip‐line. The EXOGAM gamma array was coupled with the DIAMANT light charged‐particle detector and the VAMOS spectrometer. We report here on the powerful of this setup to extract fusion‐evaporation γ‐rays from a large beam contamination.

First Results with TIGRESS and Accelerated Radioactive Ion Beams from ISAC: Coulomb Excitation of 20,21,29Na

The TRIUMF‐ISAC Gamma‐Ray Escape Suppressed Spectrometer (TIGRESS) is a state‐of‐the‐art γ‐ray spectrometer being constructed at the ISAC‐II radioactive ion beam facility at TRIUMF. TIGRESS will be comprised of twelve 32‐fold segmented high‐purity germanium (HPGe) clover‐type γ‐ray detectors, with BGO/CsI(Tl) Compton‐suppression shields, and is currently operational at ISAC‐II in an early‐implementation configuration of six detectors. Results have been obtained for the first experiments performed using TIGRESS, which examined the Au2009=u200920, 21, and 29 isotopes of Na by Coulomb excitation.

Effects of Incorrect Interaction Identification on Image Resolution in HPGe Compton Cameras

The performance of Compton imaging systems is limited by the angular uncertainty arising from the detector geometry and spatial resolution [Ordonez et al., 1997 amd 1999], When closely spaced multiple interactions are incorrectly recorded as a single event [Solomon and Ott, 1988], termed interaction packing, the system response has considerable angular and directional uncertainty. In this situation, we test two methods for assigning the event location; binning the combined interaction to a central location, and sub-sampling to an energy weighted centroid. We considered a dual layer camera geometry containing two SmartPET detectors [Hall et al., 2003], and employed Geant4 [Agostinelli et al., 2003] to simulate the response to 662 keV photons. A cone-intersection algorithm [Wilderman et al., 2000] is utilised to reconstruct the activity distribution. The major findings are as follows. Interaction packing in the scatter detector leads to an increase in reconstructed background levels. Interaction packing in the absorption detector leads to a broader point spread function, but this is only observed using the energy centroid approximation. While both of these effects are small for the SmartPET based geometry, they may lead to image degradation when using different detection geometries, such as more closely spaced detection volumes.