D.M. Duxbury

Wish: The New Powder and Single Crystal Magnetic Diffractometer on the Second Target Station

Powder diffractometers are naturally suited for short-pulse spallation sources, as they optimally exploit the brilliance from the sharp neutron pulses, with a relative time-resolution constant over a broad wavelength range. Their design has considerably progressed over the last three decades, in particular due to advances in detector technology. In the 80s, the first instruments such as HRPD at Argonne and HRPD at ISIS employed backscattering geometry, where the instruments are naturally focussed, and have long primary flight-paths in order to acquire high-resolution data. To improve count rates, large solid angles could be covered but detectors were positioned on a locus that allowed geometrical focusing, where time-of-flight (TOF) histograms produced by individual detector elements have equal resolution and can be directly grouped into a single diffraction pattern.

Preliminary results from the new large-area PETRRA positron camera

The PETRRA positron camera is based on the use of BaF/sub 2/ scintillators interfaced to large area multiwire proportional chambers filled with a photo-sensitive vapour (TMAE). The camera consists of two 60 cm/spl times/40 cm annihilation photon detectors mounted on a rotating gantry. Initial measurements with the camera show that the spatial resolution is /spl sim/6.5/spl plusmn/1 mm FWHM all through the field-of-view, and the timing resolution is between 7 ns and 10 ns FWHM. Detection efficiency for annihilation photons is /spl sim/30% per detector. The count rates obtained, by using a 20 cm diameter by 11 cm long water filled phantom containing /sup 18/F, were /spl sim/1.35/spl times/10/sup 6/ singles and /spl sim/1.2/spl times/10/sup 5/ cps raw coincidences at which point data-rates are limited by the dead-time in the readout system. The randoms rate varies between 5 and 50% with activity in the field of view of 10-100 MBq (0.27-2.7 mCi) with a timing gate of 20 ns. Initial results show that the randoms corrected sensitivity is >3-4 kcps/kBq/ml (120-150 kcps//spl mu/Ci/ml) for activities up to 30 MBq (0.81 mCi) in a 20 cm diameter water-filled phantom. However, these values include a high (60%) scatter fraction due to detector support structures. The camera has not yet been fully optimised and it is expected that the performance will substantially improve by further tuning and a reduction in the scattering structures. With a 40 cm axial FoV the camera is ideally suited to whole-body imaging in oncology.

A gas microstrip wide angle X-ray detector for application in synchrotron radiation experiments

Abstract The Gas Microstrip Detector has counting rate capabilities several orders of magnitude higher than conventional wire proportional counters while providing the same (or better) energy resolution for X-rays. In addition the geometric flexibility provided by the lithographic process combined with the self-supporting properties of the substrate offers many exciting possibilities for X-ray detectors, particularly for the demanding experiments carried out on Synchrotron Radiation Sources. Using experience obtained in designing detectors for Particle Physics we have developed a detector for Wide Angle X-ray Scattering studies. The detector has a fan geometry which makes possible a gas detector with high detection efficiency, sub-millimetre spatial resolution and good energy resolution over a wide range of X-ray energy. The detector is described together with results of experiments carried out at the Daresbury Laboratory Synchrotron Radiation Source.

Energy resolution in X-ray detecting micro-strip gas counters

Abstract Systematic measurements of the energy resolution available from a Micro-Strip Gas Counter (MSGC) are presented. The effect of factors such as bias potential, gas filling and strip geometry on the energy resolution are examined in detail and related to a simple model. The geometry of the MSGC is adapted to permit “wall-less” detection of X-rays and this results in useful improvements in the pulse height spectra.

A 2-D MSGC-based imaging detector for neutrons

The development and testing of a two-dimensional (2-D) prototype detector based on a microstrip gas chamber (MSGC) is reported using a gas mixture of 2.5 bar /sup 3/He and 2.5 bar CF/sub 4/. The second coordinate is obtained by utilising a plane of wires as pick up electrodes. The detector is operated with the wire plane at such a potential so as not to induce any gain around the wires. This means that the high tolerances normally associated with wire planes in multi wire proportional counters are not mandatory. The detector comprises of 48 individually instrumented channels in both X (MSGC strips) and Y (orthogonal wire plane). A specially designed encoding module has been constructed which feeds digital addresses for each event to the ISIS Data Acquisition Electronics (DAE) system. An intrinsic detector resolution of /spl sim/1 mm full-width at half-maximum has been measured for both dimensions (in experimental exposures on the ROTAX beamline at ISIS) which is degraded slightly by the digital resolution for the overall system. This readout method is shown to be very tolerant of a poor signal to noise ratio in the readout channels (unlike traditional analogue wire chamber readout systems) and permits the operation of the MSGC at low avalanche gains (/spl sim/10) which helps to maximize the rate and lifetime performance of the detector as well as permitting data capture rates in the megahertz range. The event timing resolution is comfortably submicrosecond and is, therefore, suitable for applications on spallation neutron sources.

Singles transmission scanning for rotating positron cameras with large-area detectors

This paper describes the novel geometry and initial results of a singles transmission system for PETRRA, a whole-body PET camera with 400/spl times/600 mm BaF/sub 2/-TMAE detectors. Special frames mounted on to the front face of each detector support a collimated Cs-137 source in various positions. Cone-beam transmission data are acquired during 360/spl deg/ gantry rotation and images are reconstructed using the Feldkamp algorithm. The radial field of view (FOV) can be increased by using two source positions displaced either side of the central position. The truncated data acquired from each source position are combined to form a complete dataset. The axial FOV depends on the amount of collimation in the axial direction and could be increased by horizontal couch motion. Preliminary singles transmission images of both real and simulated thorax phantoms have been obtained. The effects of varying the linear sampling interval, the number of projections and the axial FOV have been studied. The singles transmission scanning system, described here and developed for PETRRA, could be applied to other PET cameras with large planar detectors.

SPRINTER: A New Detector System for the INTER Neutron Reflectometer

The development and testing of the SuPerior Rate for INTER (SPRINTER) detector is described. Based on a microstrip gas chamber (MSGC), the aim of the project was to produce a matched pair of high-counting-rate detectors capable of replacing the existing 3He tubes currently used on the INTER reflectometer of the ISIS spallation neutron source. The detector system is described, and the results from the first neutron beam tests carried out on an ISIS neutron reflectometer are also shown.

THE PIN PIXEL DETECTOR - NEUTRON IMAGING

The development and testing of a neutron gas pixel detector intended for application in neutron diffraction studies is reported. Using standard electrical connector pins as point anodes, the detector is based on a commercial 100 pin connector block. A prototype detector of aperture 25.4mm x 25.4mm has been fabricated, giving a pixel size of 2.54mm which matches well to the spatial resolution typically required in a neutron diffractometer. A 2-dimensional resistive divide readout system has been adapted to permit the imaging properties of the detector to be explored in advance of true pixel readout electronics. The timing properties of the device match well to the requirements of the ISIS pulsed neutron source.

Performance Characteristics of the OSMOND Neutron Detector

The performance characteristics of the Off Specular MicrOstrip Neutron Detector (OSMOND) are described. Based on microstrip gas chamber (MSGC) detectors, OSMOND is a high counting rate detector, with sub-millimeter position resolution, capable of replacing the existing rate limited scintillator detectors currently in use on the suite of reflectometer instruments of the ISIS spallation neutron source. The detector system is described together with results of neutron beam tests carried out on the CRISP and POLREF reflectometers.

A 2-dimensional MSGC-based imaging detector for X-rays

The development and testing of a two-dimensional (2-D) prototype detector based on a microstrip gas chamber (MSGC) are reported. The second coordinate is obtained by utilising a plane of wires as pick up electrodes. The detector is operated with the wire plane at such a potential so as not to induce any gain around the wires. This means that the high tolerances normally associated with wire planes in multiwire proportional counters are not necessary, making the manufacture and repair of such a device relatively easier. The detector comprises of 48 individually instrumented channels in both X (MSGC strips) and Y (transverse wire plane). A specially designed encoding module has been constructed which feeds digital addresses for each event to the ISIS data acquisition electronics (DAE) system. (ISIS is the spallation neutron source at the Rutherford Appleton Laboratory.) An intrinsic detector resolution of /spl sim/0.5 mm full-width at half-maximum (FWHM) has been measured for both dimensions which is degraded slightly by the digital resolution for the overall system. This readout method is shown to be very tolerant of a poor signal-to-noise ratio in the readout channels (unlike traditional analogue wire chamber readout systems) and permits the operation of the MSGC at moderate avalanche gains (/spl sim/1000) which helps to maximize the rate and lifetime performance of the detector as well as permitting data capture rates in the MHz range.