T. Carter

The optimisation of small CsI(Tl) gamma-ray detectors

As part of the INTEGRAL (International Gamma-Ray Astrophysical Laboratory) project, CsI(Tl) scintillation elements with a cross-sectional area of 1 cm/sup 2/ and between 1 and 6 cm in length have been developed to form the basis of the imager detector plane. The crystal preparation and wrapping, crystal geometry, optical coupling, and matching to the photodiode have all been optimized in order to maximize the light output from the crystal, and hence produce the lowest energy threshold and best spectral resolution for any given readout electronics. Energy resolutions of 22% of 122 keV and 7% at 662 keV have been obtained using a 1 cm/sup 3/ crystal on a 10*10 mm crystal. With standard laboratory electronics, a low energy threshold of approximately 40 keV has been obtained. >

The non-invasive inspection of baggage using coherent X-ray scattering

An inspection technique based on angular dispersive X-ray diffraction (ADXRD) has been developed to chemically specific detection of explosives for screening applications. The technique identifies explosives from benign materials using the characteristic Bragg features seen in coherently scattered X-rays. To implement this technique in a prototype screening system, a detector has been designed which, when coupled with the appropriate analysis algorithm, provides chemically specific material identification. Profile analysis has been performed using the Singular Value Decomposition (SVD) maximum likelihood technique. The detector system combines a custom designed collimator with an X-ray image intensifier tube and is capable of acquiring diffraction profiles from several independent volume elements (voxels) within the region of interest. The acquisition of spatially and energy resolved diffraction profiles for both benign and explosive materials using a separate cooled germanium detector, has allowed the data analysis algorithm to be optimised. Results from the prototype detector system show that explosives such as Semtex, RDX, and PETN have sufficient order to produce unique diffraction profiles, which may then be differentiated from benign materials.

The optimisation of CsI(Tl)—PIN photodiode detectors

Abstract A detailed study of small, discrete, CsI(Tl)—PIN photodiode detectors has been carried out for the INTEGRAL project. Optical Monte Carlo simulations and noise calculations show that the maximum signal-to-noise ratio may be obtained by using photodiodes whose sensitive area covers approximately 50–60% of the scintillator output surface area. The measured performance for a range of photodiode sizes coupled to a 1 cm 2 scintillator has been compared with the results of the simulations.

Pulse shape analysis of signals from a CsI(T1)/photodiode detector

Abstract Use of 1 cm 2 CsI(T1) scintillators with photodiode readout in the hard X-ray band is currently limited to above ∼ 40 keV by the lack of intrinsic gain in the photodiode and the electronic noise in the readout system. However, below this energy the photodiode itself becomes a practical X-ray detector. We report on the use of a pulse-shape analysis technique in an attempt to separate the signals from direct X-ray interactions in the photodiode and from scintillation events in the CsI(T1). We have demonstrated that separation of the two types of event is certainly possible, and the use of an appropriate spectral reconstruction algorithm enables us to normalise the signals from the two detectors. In this way we have constructed a hybrid detector capable of operating in the energy range 10 keV to 1 MeV. We discuss, using both simulations and laboratory measurements, the optimisation which can be carried out by changing the thickness of the silicon photodiode. We present details of the pulse-shape analysis system, demonstrate the identification and separation of the two types of signal, and present energy spectra for a range of energies.

A hexagonal multi-element CsI(Tl)-photodiode module for gamma-ray imaging

The technique of producing images of gamma-ray sources by means of coded masks, wherein the shadow of an aperture is cast onto a position-sensitive detector plane is already well-known and has been employed for some time in the fields of nuclear physics, medical diagnostics and high energy astronomy. In order to evaluate several design and scientific aspects of a proposed astronomical gamma-ray images, a prototype module has been constructed consisting of 37 hexagonal CsI(Tl) elements, each of 1 cm/sup 2/ cross-section and 3 cm length. Each bar is viewed by a photodiode for read-out thus constituting a two dimensional position-sensitive array which can operate from 0.1 to 6 MeV. In this paper we describe the experimental arrangement, the readout electronics and some of the experimental results.

An imager prototype for gamma-ray astronomy

A two layer hexagonal pixellated imaging gamma-ray detector has been constructed to evaluate both the design and scientific aspects of the INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) Imager. This prototype imager consists of two layers each comprising of 37 elements. Where each element is a discrete hexagonal 11 mm A/F/spl times/30 mm CsI(Tl) scintillator coupled to a p-i-n photodiode detector. In this paper we review the scientific and engineering aspects of the prototype and present details of the instrument calibration.<<ETX>>

PRODUCTION OF RADIONUCLIDES BY 1.7 GEV PROTON-INDUCED REACTIONS ON CDTE CRYSTALS

Abstract High levels of background radiation are one of the major problems associated with satellite-borne telescopes operating in the gamma-ray domain. Although there are many contributions to this background, cosmic-proton induced radioactivity is perhaps the most difficult to remove because of the delayed gamma-emission. As a part of the INTEGRAL project a series of studies have been performed on a range of materials to determine the production cross sections for unstable isotopes produced by energetic protons. In this paper we present results for the irradiation of small (5×5×2 mm) cadmium telluride (CdTe) crystal with 1.7 GeV protons. Data are given for experimentally determined production cross sections which are compared to simulation predictions by the GEANT/GCALOR code.

High energy proton-induced radioactivity in HgI2 crystals

Abstract Mercuric iodide (HgI2) semiconductor crystals are generating a lot of interest as room temperature solid state detectors for hard X-ray astronomy observations. For these applications one of the most important background sources is the cosmic proton induced radioactivity in the detector material. In order to study this background noise contribution a 1×1×1 cm HgI2 crystal was irradiated with high energy protons. The resulting long-lived unstable isotopes and their production rates have been identified and compared with Monte Carlo simulations.

Imager for gamma-ray astronomy: balloon prototype

A novel low energy astronomical gamma-ray detector is being developed for future satellite missions. Recent advances in the technology of photodiodes and small, low noise amplifier circuits have meant that more compact detectors can be assembled in a complex array in order to give a 3-D position reconstruction capability. In a mask-detector telescope this capability is potentially very useful since it allows the reconstruction of the path of the incident gamma rays making it valuable both for imaging and background rejection. A small prototype of a 3-D detector has been realized for test in a balloon mission. The detector is based on a 12 X 8 array of position sensitive CsI(T1) bars, typically 15 cm long with 1.3 X 1.3 cm cross section, viewed at each end by photodiodes. The detector includes four 1.3 X 1.3 X 2.5 cm CsI(T1) scintillators located above the main array in order to evaluate the low energy response of the imager. The detector constitutes an active block of 2400 cm3 of scintillator that can operate in the 0.2 - 10 MeV energy range. The energy resolution is 13% at 662 keV and the positional resolution is of the order of 1.5 cm in each dimension. An active shield of CSI(T1) and plastic scintillators surrounds the bar detector. The overall experiment is briefly described in general and preliminary results of laboratory tests are presented.

Imager prototype for gamma-ray astronomy: functional performance

A prototype module for the imager on INTEGRAL (International Gamma Ray Astrophysics Laboratory) has been constructed for laboratory and space tests by means of a balloon launch. The main detector is designed to operate in the energy range from 0.25 to 10 MeV and consists of 96 CsI(Tl) bars, each 15 cm in length. At 662 keV, the typical performance is 1.5 cm full width half maximum (FWHM) spatial resolution (1 sigma ) along the bar, and 15% energy resolution. The work carried out during instrument construction and calibration is described, with emphasis on the simple method which has been implemented to normalize the nonuniformity of the performance of the bars. The measurements of energy threshold and energy and position resolution are presented and discussed. >