I. Lazarus

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.

Detector characteristics of a pixellated germanium Compton camera for nuclear medicine

A pixellated germanium Compton camera is currently being developed for imaging 511 keV sources in nuclear medicine. It was built by ORTEC and consists of two planar Ge detectors (the scatter and the absorption detector) housed within the same cryostat. The scatter and absorption detectors have 152 and 25 4/spl times/4 mm/sup 2/ pixels respectively. The readout electronics was developed at Daresbury Laboratory UK and consists of 15 GRT4 VME cards and a PowerPC. The system is controlled by a PC running MIDAS software. This paper reports the current status of camera development. The pixel energy resolution has been measured to give an average of /spl sim/0.5% at 356 keV using a NIM module and /spl sim/2% using a simple digital algorithm. In addition, a centroiding algorithm that takes into consideration the induced charge from the surrounding pixels, is being implemented to improve the intrinsic spatial resolution of the camera, which is restricted by the size of the pixel (4 mm), to a target value of /spl sim/1mm; so far a X-Y position sensitivity of 2 mm has been achieved. The depth of interaction can be provided by analysis of the pulse rise time, theoretical predictions anticipate a value of the order of 0.5 mm. Currently these parameters are determined offline using simple algorithms, but it is the intention to develop the algorithms and then implement them on the FPGAs available on the GRT4 cards for online 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.

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.

Performance of a 6×6 segmented germanium detector for γ-ray tracking

Abstract A 36 fold segmented germanium coaxial detector has been supplied by EURISYS MESURES. The outer contact is segmented both radially and longitudinally. The signals from the fast preamplifiers have been digitised by 12 bit , 40 MHz ADCs. In this article we report preliminary results obtained using this detector and their relevance for future germanium γ-ray tracking arrays.

Conceptual design of a hybrid neutron-gamma detector for study of β-delayed neutrons at the RIB facility of RIKEN

BRIKEN is a complex detection system to be installed at the RIB-facility of the RIKEN Nishina Center. It is aimed at the detection of heavy-ion implants, s-particles, ?-rays and s-delayed neu- trons. The whole detection setup involves the Advanced Implantation Detection Array (AIDA), two HPGe Clover detectors and a large set of 166 counters of 3He embedded in a high-density polyethy- lene matrix. This article reports on a novel methodology developed for the conceptual design and optimisation of the 3He-tubes array, aiming at the best possible performance in terms of neutron detection. The algorithm is based on a geometric representation of two selected parameters of merit, namely, average neutron detection efficiency and efficiency flatness, as a function of a reduced num- ber of geometric variables. The response of the detection system itself, for each configuration, is obtained from a systematic MC-simulation implemented realistically in Geant4. This approach has been found to be particularly useful. On the one hand, due to the different types and large number of 3He-tubes involved and, on the other hand, due to the additional constraints introduced by the ancillary detectors for charged particles and gamma-rays. Empowered by the robustness of the al- gorithm, we have been able to design a versatile detection system, which can be easily re-arranged into a compact mode in order to maximize the neutron detection performance, at the cost of the gamma-ray sensitivity. In summary, we have designed a system which shows, for neutron energies up to 1(5) MeV, a rather flat and high average efficiency of 68.6%(64%) and 75.7%(71%) for the hybrid and compact modes, respectively. The performance of the BRIKEN system has been also quantified realistically by means of MC-simulations made with different neutron energy distributions.

AIDA: A 16-channel amplifier ASIC to read out the Advanced Implantation Detector Array for experiments in nuclear decay spectroscopy

We have designed a read-out ASIC for nuclear decay spectroscopy as part of the AIDA project — the Advanced Implantation Detector Array. AIDA will be installed in experiments at the Facility for Antiproton and Ion Research in GSI, Darmstadt. The AIDA ASIC will measure the signals when unstable nuclei are implanted into the detector, followed by the much smaller signals when the nuclei subsequently decay. Implant energies can be as high as 20GeV; decay products need to be measured down to 25keV within just a few microseconds of the initial implants. The ASIC uses two amplifiers per detector channel, one covering the 20GeV dynamic range, the other selectable over a 20MeV or 1GeV range. The amplifiers are linked together by bypass transistors which are normally switched off. The arrival of a large signal causes saturation of the low-energy amplifier and a fluctuation of the input voltage, which activates the link to the high-energy amplifier. The bypass transistors switch on and the input charge is integrated by the high-energy amplifier. The signal is shaped and stored by a peak-hold, then read out on a multiplexed output. Control logic resets the amplifiers and bypass circuit, allowing the low-energy amplifier to measure the subsequent decay signal. We present simulations and test results, demonstrating the AIDA ASIC operation over a wide range of input signals.

AIDA: A 16-channel amplifier ASIC to read out the advanced implantation detector array for experiments in nuclear decay spectroscopy

We have designed a read-out ASIC for nuclear decay spectroscopy as part of the AIDA project - the Advanced Implantation Detector Array. AIDA will be installed in experiments at the Facility for Antiproton and Ion Research in GSI, Darmstadt. The AIDA ASIC will measure the signals when unstable nuclei are implanted into the detector, followed by the much smaller signals when the nuclei subsequently decay. Implant energies can be as high as 20GeV; decay products need to be measured down to 25keV within just a few microseconds of the initial implants. The ASIC uses two amplifiers per detector channel, one covering the 20GeV dynamic range, the other selectable over a 20MeV or 1GeV range. The amplifiers are linked together by diodes and bypass transistors which are normally switched off. The arrival of a large signal causes saturation of the low-energy amplifier and a fluctuation of the input voltage, which forward biases the link diode to the high-energy amplifier. The bypass transistors switch on and the remainder of the charge is integrated by the high-energy amplifier. The signal is shaped and stored by a peak-hold, then read out on a multiplexed output. Control logic resets the amplifiers and bypass circuit, allowing the low-energy amplifier to measure the subsequent decay signal. We present simulations and test results, demonstrating the AIDA ASIC operation over a wide range of input signals.