H. C. Boston

Characterisation Results From an AGATA Prototype Detector

An Advanced GAmma Tracking Array (AGATA) symmetric prototype high purity Germanium (HPGe) detector has been tested. The detector was illuminated with a 1 mm collimated beam of 137Cs (662 keV) gamma rays. The beam was raster scanned across the front and sides of the detector and the charge sensitive preamplifier output pulse shapes from all 37 channels (36 segments plus the centre contact) were digitised and stored for off-line analysis. Rise time and image charge asymmetry magnitudes were measured as a function of interaction position to study the charge transport properties through the crystal volume. These parameters were then utilised as a calibrated look up table with which in-beam data was analysed and Doppler corrected. An average position resolution of approximately 9 mm (FWHM) was achieved with a crude analysis.

Quadrupole moments of collective structures up to spin similar to 65(h)over-bar in Er-157 and Er-158: A challenge for understanding triaxiality in nuclei

The transition quadrupole moments. Q(t), of four weakly populated collective bands up to spin similar to 65h in Er-157,Er-158 have been measured to be similar to II eb demonstrating that these sequences are associated with large deformations. However, the data are inconsistent with calculated values from cranked Nilsson-Strutinsky calculations that predict the lowest energy triaxial shape to be associated with rotation about the short principal axis. The data appear to favor either a stable triaxial shape rotating about the intermediate axis or, alternatively, a triaxial shape with larger deformation rotating about the short axis. These new results challenge the present understanding of triaxiality in nuclei

Two-neutron transfer reaction mechanisms in 12 C(6 He, 4 He) 14 C using a realistic three-body 6 He model

The reaction mechanisms of the two-neutron transfer reaction 12C(6He,4He) have been studied at Elab=30 MeV at the TRIUMF ISAC-II facility using the Silicon Highly-segmented Array for Reactions and Coulex (SHARC) charged-particle detector array. Optical potential parameters have been extracted from the analysis of the elastic scattering angular distribution. The new potential has been applied to the study of the transfer angular distribution to the 2+2 8.32 MeV state in 14C, using a realistic three-body 6He model and advanced shell-model calculations for the carbon structure, allowing to calculate the relative contributions of the simultaneous and sequential two-neutron transfer. The reaction model provides a good description of the 30-MeV data set and shows that the simultaneous process is the dominant transfer mechanism. Sensitivity tests of optical potential parameters show that the final results can be considerably affected by the choice of optical potentials. A reanalysis of data measured previously at Elab=18 MeV, however, is not as well described by the same reaction model, suggesting that one needs to include higher-order effects in the reaction mechanism.

Development of the ProSPECTus semiconductor Compton camera for medical imaging

The ProSPECTus project is the development of a prototype semiconductor Compton camera for use in nuclear medical imaging applications. The proposed system has the potential to improve the sensitivity of conventional mechanically col-limated Single Photon Emission Computed Tomography (SPECT) systems through the use of electronic collimation techniques. In addition, the use of compatible semiconductor technology within a Magnetic Resonance Imaging (MRI) system could potentially lead to simultanous SPECT/MRI data acquisition. This paper outlines the consideration of key design features for the new system. Such design factors include the geometrical setup, suitable energy and position resolution values for the detectors and the ability of the system to function in a magnetic field. The ProSPECTus protoype imaging system will now be built according to optimised specifications.

High-spin structure in K-40

High-spin states of K-40 have been populated in the fusion-evaporation reaction C-12(Si-30,np)K-40 and studied by means of gamma-ray spectroscopy techniques using one triple-cluster detector of the ...

Semiconductor detectors for Compton imaging in nuclear medicine

An investigation is underway at the University of Liverpool to assess the suitability of two position sensitive semiconductor detectors as components of a Compton camera for nuclear medical imaging. The ProSPECTus project aims to improve image quality, provide shorter data acquisition times and lower patient doses by replacing conventional Single Photon Emission Computed Tomography (SPECT) systems. These mechanically collimated systems are employed to locate a radioactive tracer that has been administered to a patient to study specifically targeted physiological processes. The ProSPECTus system will be composed of a Si(Li) detector and a High Purity Germanium (HPGe) detector, a configuration deemed optimum using a validated Geant4 simulation package. Characterising the response of the detectors to gamma irradiation is essential in maximising the sensitivity and image resolution of the system. To this end, the performance of the HPGe ProSPECTus detector and a suitable Si(Li) detector has been assessed at the University of Liverpool. The energy resolution of the detectors has been measured and a surface scan of the Si(Li) detector has been performed using a finely collimated 241Am gamma ray source. Results from the investigation will be presented.

Characterisation of a Broad Energy Germanium (BEGe) detector. Simulation and experimental results

Characterisation of High Purity Germanium (HPGe) detectors in order to predict the response of the detector to different gamma ray interactions is one of the current goals in the Nuclear physics community. This purpose includes a theoretical study of the detector from the simulation point of view and an experimental stage to validate the goodness of the considerations performed. In this work, the detector under study is a Broad Energy Germanium detector. The simulation has been performed with the Multi Geometry Simulation (MGS) program, that provides the predicted electric field and the charge pulse shapes expected at the contacts for a given detector geometry. The experimental setup included two type of scans with different collimated sources across its front and bottom faces, storing the data with a fully digital acquisition system. Subsequent data analysis and the use of Pulse Shape Analysis (PSA) techniques has allowed the knowledge of internal characteristics of the detector such as the contact limits or orientation of crystallographic axes; as well as the comparison between experimental and simulated pulse shapes.

Prospectus: Development of a Compton Camera for Medical Imaging

Single Photon Emission Computed Tomography (SPECT) is an established method of studying physiological functions. However, novel gamma-ray Compton camera systems which provide electronic collimation have the potential to greatly improve the sensitivity of this technique. Compton cameras have been employed in high energy applications but have not yet been fully implemented for clinical applications at low energies. This paper describes the optimization of imaging efficiency for the ProSPECTus medical imaging Compton camera system with 99mTc. Experimental factors which degrade the image quality will also be assessed and quantified.

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.

Collective structures up to spin ~ 65hslash in the N = 90 isotones 158Er and 157Ho

A new collective band with high dynamic moment of inertia in 158Er at spins beyond band termination has been found in addition to the two previously reported ones. The measured transition quadrupole moments (Qt) of these three bands are very similar. These three bands have been suggested to possess a triaxial strongly deformed shape, based on comparisons with calculations using the cranked Nilsson-Strutinsky model and with tilted axis cranking calculations using the Skyrme-Hartree-Fock model. In addition, three collective bands with similar high dynamic moments of inertia, tentatively assigned to 157Ho, have been observed. Thus, it is suggested that all these structures share a common underlying character and that they are most likely associated with triaxial strongly deformed minima which are predicted to be close to the yrast line at spin 50 – 70.