R. Stephenson

A positron camera for industrial application

Abstract A positron camera for application to flow tracing and measurement in mechanical subjects is described. It is based on two 300 × 600 mm 2 hybrid multiwire detectors; the cathodes are in the form of lead strips plated onto printed-circuit board, and delay lines are used to determine the location of photon interactions. Measurements of the positron detection efficiency (30 Hz μCi −1 for a centred unshielded source), the maximum data logging rate (3 kHz) and the spatial resolving power (point source response = 5.7 mmfwhm) are presented and discussed, and results from initial demonstration experiments are shown.

The Rutherford Appleton laboratory's mark I multiwire proportional counter positron camera

Abstract A small (30 cm × 30 cm) model of a proposed large aperture positron camera has been developed at Rutherford Appleton Laboratory. Based on multiwire proportional counter technology it uses lead foil cathodes which function simultaneously as converters for the 511 keV gamma rays and readout electrodes for a delay line readout system. The detectors have been built up into a portable imaging system complete with a dedicated computer for data taking, processing and display. This has permitted evaluation of this type of positron imaging system in the clinical environment using both cyclotron generated isotopes ( 15 O, 11 C, 18 F, 124 I) and available isotopic generator systems ( 82 Rb, 68 Ga). At RAL we provided a complete hardware system and sufficient software to permit our hospital based colleagues to generate useful images with the minimum of effort. A complete description of the system is given with performance figures and some of the images obtained in three hospital visits are presented. Some detailed studies of the imaging performance of the positron camera are reported which have bearing on the design of future, improved systems.

Two digital X-ray imaging systems for applications in X-ray diffraction

Abstract Two digital X-ray imaging systems developed at the Rutherford Appleton Laboratory are described: the Mark I and the Mark II. Both use a bidimensionally sensitive multiwire proportional counter (MWPC) as the basic X-ray image transducer coupled, in the case of the Mark I to a Digital LSI 11–23 microcomputer system via CAMAC, and in the case of the Mark II to a Digital LSI 11–73 microcomputer system via custom-built data acquisition hardware mounted directly on the Q-bus of the microcomputer. The Mark I system provides the advantages of high speed, high sensitivity digital imaging directly into the computer with the potential for software control of the sample orientation and environment. The Mark II system adds the novel features of signal averaging and multiframe exposures. The dedicated digital memories have a resolution of 512×512 pixels of 16 bits, matching well to the spatial resolution of the xenon-filled MWPC (0.5 mm fwhm over an aperture of 200 mm×200 mm). A 512×512×4 bit video graphics system displays the images in grey scales or colour.

High speed quantitative digital beta autoradiography using a multistep avalanche detector and an Apple II microcomputer

Abstract The development of an electronic, digital beta autoradiography system is described. Using a multistep avalanche/multiwire proportional counter (MSA/MWPC) detector system fitted with delay line readout, high speed digital imaging is demonstrated with submillimeter spatial resolution. In the case of autoradiography with a tritium label, image acquisition requires about one hour compared with several weeks for conventional film techniques. Good proportionality of observed counting rate relative to the known tritium activity is demonstrated. The application of the system to autoradiography in immunoelectrophoresis, histopathology and DNA sequencing is described (using 125I, 14C and 35S labels in addition to 3H).

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.

The development of gas microstrip detectors for high energy physics applications

Abstract Results are presented from the on-going programme of work at Rutherford Appleton Laboratory aimed at the development of high spatial resolution, high rate tracking counters for the next generation of collider experiments.

PETRRA: preliminary experimental results from the first full size detector and dead time simulation of the count rate performance of a unique whole body PET camera

The design and construction of the first of two BaF/sub 2/-TMAE large area detectors to be incorporated in a unique double headed whole body PET system (PETRRA) has been completed. Preliminary experimental work carried out with the first large area detector shows an absolute detection efficiency of 20.5% and a time resolution of 3.5 ns (FWHM) and 8.0 ns (FWTM), using 8 mm thick scintillation crystals. An analytical model of the electronic read-out system has been developed to investigate the rate capability of the new scanner. This model predicts that under typical imaging conditions, with 40 MBq of activity in the field of view, the proposed scanner will acquire coincidence data at 20-25 kcps. >

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.

Studies of the gain properties of gas microstrip detectors relevant to their application as X-ray and electron detectors

The microstrip gas counter (MSGC) makes an excellent planar (position-sensitive) amplifier of incident electron clouds because both the anodic and cathodic gain-defining elements are produced lithographically on the same rigid substrate. We have studied the dependence of the gas gain and pulse-height resolution of the plate as a function of various geometric and gas parameters. The results show that an MSGC can be made very insensitive to. the shape of the drift electrode, allowing it to be used in a wide variety of applications. An example of an electron-yield XAFS study is given. The aim of the work reported in this paper is to produce a well-defined technology platform from which to build detectors that meet the requirements of high-flux synchrotron radiation and neutron facilities, both of which are key CLRC facilities.

Studies of Lead Tungstate Crystal Matrices in High-Energy Beams for the CMS Electromagnetic Calorimeter at the LHC

Using matrices of lead tungstate crystals, energy resolutions better than 0.6% at 100 GeV have been achieved in the test beam in 1995. It has been demonstrated that a lead tungstate electromagnetic calorimeter read out by avalanche photodiodes can consistently achieve the excellent energy resolutions necessary to justify its construction in the CMS detector. The performance achieved has been understood in terms of the properties of the crystals and photodetectors.