J.R. Cresswell

The GREAT triggerless total data readout method

Recoil decay tagging (RDT) is a very powerful method for the spectroscopy of exotic nuclei. RDT is a delayed coincidence technique between detectors usually at the target position and at the focal plane of a spectrometer. Such measurements are often limited by dead time. This paper describes a novel triggerless data acquisition method, which is being developed for the Gamma Recoil Electron Alpha Tagging (GREAT) spectrometer, that overcomes this limitation by virtually eliminating dead time. Our solution is a total data readout (TDR) method where all channels run independently and are associated in software to reconstruct events. The TDR method allows all the data from both target position and focal plane to be collected with practically no dead-time losses. Each data word is associated with a timestamp generated from a global 100-MHz clock. Events are then reconstructed in real time in the event builder using temporal and spatial associations defined by the physics of the experiment.

The GRT4 VME pulse processing card for segmented germanium detectors

A four channel VME card with 14 bit, 80 MHz digitizers and powerful on-board processing has been designed, built and used in tests of digital pulse processing techniques for gamma-ray tracking. This paper explains the background (rationale for the project), describes the VME card (known as the GRT4) and presents a 64 channel GRT4 digitizing system which was used to instrument two segmented Germanium detectors during in-beam tests. Results obtained using the GRT4 card are presented as well as some applications.

Validation of Pulse Shape Simulations for an AGATA Prototype Detector

An AGATA symmetric, coaxial, high-purity germanium (HPGe) detector has been scanned in coincidence mode. Charge pulse shapes from the 36-fold segmented outer contacts and center contact were stored for events at more than 2000 precisely determined 3-D interaction positions spread over ten depths (z). A database (basis) of the 37 average experimental pulse shapes at each position was generated. The electric field simulation code Multi Geometry Simulation (MGS) was used to generate the pulse shapes for the geometry on a 1.0-mm cubic grid. A minimization between the experimental pulse shapes at each position and the MGS basis yielded mean displacements of between 1.5 and 3.0 mm in the x- y plane. The vectors of these displacements were biased in the direction of the center of the detector. This effect is attributed to cross-talk. The maximum level of derivative cross-talk was measured and shown to be 534%ns. However due to the lack of a global clock in the acquisition system, it could not be accounted for throughout the basis.

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.

Digital gamma-ray tracking algorithms in segmented germanium detectors

A gamma-ray tracking algorithm has been implemented and tested, using simulated data, for gamma rays with energies between 0.1 and 2 MeV, and its performance evaluated for a 90-mm-long, 60-mm-diameter, cylindrical, 36 (6 /spl times/ 6) segment detector. The performance of the algorithm in two areas was determined: Compton suppression and Doppler shift correction. It was found that for gamma rays of energies around 1 MeV, a ratio of photopeak counts to total counts of 2:3 could be obtained using the tracking algorithm, with only a 2% reduction in detection efficiency, compared to the untracked data. Approximately 80% of first interaction points could be correctly identified, enabling a good Doppler shift correction. A detector of the type simulated has recently been delivered, together with a compactPCI digital data acquisition system comprising 36 12-bit, 40-MHz flash ADCs, and 6200-MHz DSPs. Some initial data has been recorded using this system, and the performance of the tracking algorithm on this real data is comparable to its performance on simulated data.

Towards combining in‐beam γ‐ray and conversion electron spectroscopy

The SAGE spectrometer will combine a segmented Si‐detector with a Ge‐detector array aiming to take the simultaneous in‐beam γ‐ray and conversion electron spectroscopy to the next level. It will be coupled with the GREAT focal plane spectrometer and the RITU gas‐filled recoil separator at the accelerator laboratory of the University of Jyvaskyla, Finland. Its high efficiency and resolution will open the door to a new era of complete spectroscopy directed, amongst others, at the study of superheavy nuclei aiming to investigate the properties of the next spherical proton shell above Z = 82.

The SAGE spectrometer: A tool for combined in-beam γ-ray and conversion electron spectroscopy

The SAGE spectrometer allows simultaneous in-beam γ-ray and internal conversion electron measurements, by combining a germanium detector array with a highly segmented silicon detector and an electron transport system. SAGE is coupled with the ritu gas-filled recoil separator and the great focal-plane spectrometer for recoil-decay tagging studies. Digital electronics are used both for the γ ray and the electron parts of the spectrometer. SAGE was commissioned in the Accelerator Laboratory of the University of Jyvaskyla in the beginning of 2010.

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.

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.