The Burst and All Sky Imaging Survey (BASIS) is a proposed mission to provide ∼3 arc second locations of approximately 90 Gamma-Ray Bursts (GRBs) per year. The BASIS coded aperture imaging system requires a segmented detector plane able to detect the interaction position of (10 - 150 keV) photons to less than 100 μm. To develop prototype detector arrays with such fine position resolution we have fabricated many 15 mm × 15 mm × 2 mm 100 μm pitch CdZnTe strip detectors. We have assembled these fine pitch CdZnTe strip detectors into prototype 2 × 2 and 6 × 6 element arrays read out by ASIC electronics. The assembly and electronics readout of the 6 × 6 flight prototype array will be discussed, and preliminary data illustrating the uniformity and efficiency of the array will be presented.

We describe initial results of our program to develop and test Cd-Zn-Te (CZT) detectors with a pixellated array readout. Our primary interest is in the development of relatively thick CZT detectors for use in astrophysical coded aperture telescopes with response extending over the energy range ∼ 10 – 600 keV. The coded aperture imaging configuration requires only relatively large area pixels (1–3 mm), whereas the desired high energy response requires detector thicknesses of at least 3–5 mm. We have developed a prototype detector employing a 10 × 10 × 5 mm CZT substrate and 4 × 4 pixel (1.5 mm each) readout with gold metal contacts for the pixels and continuous gold contact for the bias on the opposite detector face. This MSM contact configuration was fabricated by RMD and tested at Harvard for uniformity, efficiency and spatial as well as spectral resolution. We have developed an ASIC readout (IDE-VA-1) and analysis system and report results, including ∼ 4% (FWHM) energy resolution at 60 keV. A prototype design for a full imaging detector array is discussed.

One of the critical issues in CdZnTe detector fabrication is the surface treatment. This will not only affect the electrical properties, such as leakage current, but also influence the physical properties, such as smoothness and adhesion between the metal and the semiconductor. The latter will determine the wire bonding yield. Historically, there has been a problem in achieving both low leakage current and excellent wire bonding yield. In this paper, we report our new approach to fabricate high performance doubled sided CdZnTe strip detectors. The new surface treatments involve chemical etching and post-annealing. The leakage current, interstrip resistance and energy resolution were studied as a function of different etchants/time and post-annealing temperature. It was found that a chemical etch with bromine in ethylene glycol (Br/EG) is suitable for the double sided strip detector process. Keeping a relatively smooth surface is critical for achieving a high yield of good strips. To improve the adhesion of the metal to CdZnTe for wire bonding, the detectors were annealed from 100 to 175°C for 10 hours. It has been observed that after annealing, not only has the strip leakage current decreased, but the interstrip resistance is increased for a temperature less than 150°C.

One of the main electronic noise sources of a room temperature spectroscopy system is the leakage current of a detector. It can be reduced with a pn-junction type detector structure such as a M-i-n configuration, and with cooling. In this work eight CdZnTe detectors with a M-i-n structure were fabricated by indium diffusion. The junction was characterized by a currentvoltage technique. Detector electrical, charge collection and spectroscopic properties were compared to the ones received with the traditional electroless Au contacts, before the junction formation. As a result of the indium diffusion an improved detector leakage current performance was achieved. However, a corresponding improvement in the detector energy resolution was not always observed due to the CdZnTe charge collection properties and process variables.

Simulated and associated experimental results of a high efficiency CdZnTe (CZT) radiation detector response for gamma-ray imaging applications are presented. The model of a high efficiency semiconductor gamma ray detector takes into account several different physical phenomena involved in the detection and correction processes, namely the geometry of the irradiation, the gamma-ray's interaction with the crystal, the physics of the semiconductor's charge collection, the electric field distribution and the pulse height correction method. A few important decoupling assumptions allow us to use a one dimensional charge collection simulation with a two-dimensional field model and a full three dimensional Monte-Carlo calculation of the gamma ray interactions. The model allows calculation of charge collection and gamma ray spectra for non uniform electric field distribution in either planar, striped or pixellated detector.

The model takes also into account the new CZT fast pulse correction method and its associated noise by considering the pulse height and the rise time of electron signals (Bi-Parametric spectrum) for all gamma ray interactions. Specific simulated and experimental spectra at 122 keV are presented for CZT. First, basic spectral changes are calculated for variations in crystal and detector properties like mobility, trapping lifetime and electric field profiles

Second, new experimental results of the fast pulse correction method applied to different CZT detector grades are presented. This method allows to achieve a high detection efficiency (> 80 %) with a good energy resolution (< 6 % FWHM) at 122 keV for a 4×4×6 mm3 CZT detector. No specific contact geometry is needed and the unusual low applied bias voltage allows to limit the ageing and break voltage effects and also the dark current and its associated noise. This fast correction method is expected to be useful for medical imaging and other applications.

Finally, simulated Bi-Parametric (BP) spectra expected with the fast pulse correction method according to the detector properties (electric field profiles, electron lifetime) are simulated and a qualitative comparison is provided.

We report an approach to double gaussian filtering used in classical works as dual parameter pulse processing. This technique has been implemented by creating a bank of gaussian-like digital filters based on wavelet transforms. A simple method to correct for the charge loss inherent to room temperature semiconductor gamma detectors has been developed. This method is based on multi-resolution signal analysis. Results are reported from tests of these algorithms on commercial CZT detectors and two trapped hole charge correction levels are compared. Finally, the advantages and limitations of this new approach to detector pulse processing are discussed.

We have used an elementary statistical technique to derive a closed-form expression for the hole-tailing line shape produced by photoelectric absorption of monoenergetic radiation in a semiconductor X-ray/γ-ray detector. In the case of compound semiconductors, where the drift length for electrons is much greater than that for holes, the line shape is given by a type of power law, except for a small region very near the photopeak. This analytical result agrees well with Monte Carlo simulations and is used to extract approximate μτ products from a 57Co pulse height spectrum. We also present an expression for the maximum obtainable energy resolution of a semiconductor detector in the presence of leakage current noise and intrinsic statistical fluctuations as a function of material parameters, along with a chart of the optimal band gap as a function of temperature and photon energy. Based on these considerations, the optimal band gap for room-temperature operation is approximately 2.0 eV.

In this paper we develop quantitative models to predict the active volume of cadmium zinc telluride (CZT) detectors operated as gamma-ray pulse height spectrometers. Three cases are considered: a conventional planar detector, a unipolar device, and a detector in which electronic signal processing has been applied to correct for charge trapping effects. We find that existing detectors are very limited in their maximum attainable active volume, but unipolar devices with charge correction show promise for producing large active volume devices.

Compact, efficient, spectroscopic detector arrays which preferably operate without cooling have applications in the fields of x-ray astronomy, nuclear medicine and radioactive materials management. We have fabricated a 16 element CdZnTe detector array that provides a stopping efficiency of greater than 80%, a photopeak fraction of 44% and an energy resolution of 3.2 keV FWHM at 122 keV. The 4 × 4 array uses a pixel size of(1.5 mm)2 with 0.2 mm spacing on a 5 mm thick substrate. The variation in spectroscopic properties amongst pixels has been characterized in terms of energy resolution and detection efficiency. Operation in current (or flux) mode for transmission imaging has also been investigated.

A novel electrode configuration for CZT imaging devices is presented. It is made of focusing, non-collecting anode strips, in one dimension, and collecting anode pixels, interconnected in rows, in the orthogonal dimension. The simulation of such an imaging detector is presented. First, field lines in the detector are computed that show that electrons generated in γ-ray events are collected on the pixels. Charge signals, induced on the pixel and on the strip by drifting electrons, are calculated for several points of interaction inside the detector unit cell. These show that this new detector should retain the spectroscopic and detection efficiency advantages of single carrier (electrons) charge sensing devices such as pixel detectors or spectrometers with controlling electrodes. Furthermore, it retains the main advantage of conventional strip detectors, i.e. an N x N array of imaging pixels realized with only 2N electronic channels. An additional potential advantage is the measurement of the third coordinate, i.e. the depth of interaction in the detector.

A portable Gamma-Ray Spectroscopy system has been designed and tested for identification of nuclear materials in the field. A P-I-N CdZnTe detector was developed to minimize the leakage current and to improve energy resolution. The detector has a sensitive volume of 200 mm3 and has been constructed with novel heterojunction contacts. Thermoelectric cooling is used to reduce leakage currents in the detector element and the input thermal noise to the FET. Risetime discrimination is used to offset the negative effects of incomplete charge collection. The detector probe is connected to a single unit comprised of detector electronics, an MCA and a portable computer, all operated from battery power. Results from testing with laboratory radioisotopic sources are presented.

Several analytical techniques have been used in the study of the homogeneity of Cadmium Zinc Telluride (CZT) single crystals grown by the vertical high pressure Bridgman (VHPB) method. The presence of black inclusions and tubular hollow pipes has been observed by a few methods, such as photoluminescence (PL), infrared (IR) transmission microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The crystals investigated were grown commercially in the USA, at the Institute of Single Crystals in Kharkov, Ukraine, and at the Institute of Solid State Physics, Moscow, Russia. We discuss the homogeneity of the various CZT crystals based on the results from these measurement techniques.

The γ ray (57Co) and α particle (241Am) detector response of Cdl-xZnxTe crystals grown by vertical Bridgman technique was studied under both positive and negative bias conditions. Postgrowth processing was utilized to produce a high-resistivity material with improved chargecollection properties. Samples of various Zn concentrations were investigated by I-V measurements and thermally stimulated spectroscopies to determine the ionization energies of deep levels in the band gap. When the post-processing conditions were optimized the lowenergy tailing of the γ-ray photopeaks was significantly reduced and an energy resolution of under 5% was achieved for the 122 keV γ-photon line in crystals with x=0.2 Zn content at room temperature. A peak to background ratio of 14:1 for the 122 keV photopeak from 57Co was observed on the best sample, using a standard planar detection geometry. The low-energy 14.4 keV X-ray line could also be observed and distinguished from the noise.

We are developing modular arrays of CdZnTe radiation detectors for high-resolution nuclear medicine imaging. Each detector is delineated into a 64×64 array of pixels; the pixel pitch is 380 jim. Each pixel is connected to a corresponding pad on a multiplexer readout circuit. The imaging system is controlled by a personal computer. We obtained images of standard nuclear medicine phantoms in which the spatial resolution of approximately 1.5 mm was limited by the collimator that was used. Significant improvements in spatial resolution should be possible with different collimator designs. These results are promising for high-resolution nuclear medicine imaging.

We present the performance characteristics of CdZnTe radiation detectors with a new P-I-N design and their unique advantages over metal-semiconductor-metal (M-S-M) devices. In M-S-M CdZnTe detectors the bulk resistivity of the substrate largely determines the leakage current. High leakage current is a dominant noise factor for CdZnTe detector arrays, coplanar detectors, and detectors used for low X-ray energy applications. P-I-N devices provide low leakage currents. Early CdZnTe detectors exhibited polarization, were limited to small detection volumes, and some required high deposition temperatures. We have developed a new heterojunction design which can be deposited at low temperatures so that even high-pressure Bridgman CdZnTe can be used. Using the P-I-N design, CdZnTe detectors with high detection volumes (>200 mm3) were fabricated and exhibited low leakage current, good energy resolution, and no polarization. These detectors have significant advantages over M-S-M detectors in three specific areas. First, X-ray fluorescence studies require detectors with low leakage currents to provide less spectral broadening due to electronic noise. Second, less expensive vertical Bridgman CdZnTe material can be used for imaging applications since it normally possesses too low of a bulk resistivity to be useful as a M-S-M detector. Third, leakage currents across the anode grid in large volume coplanar detectors can be significantly reduced

A CdZnTe strip detector large area array (∼ 60 cm2 with 36 detectors) with capabilities for high resolution imaging and spectroscopy has been built as a prototype for a space flight gamma ray burst instrument. The detector array also has applications in nuclear medical imaging. Two dimensional orthogonal strip detectors with 100 μm pitch have been fabricated and tested. Details for the array design, fabrication and evaluation of the detectors will be presented.

Semiconductor multiple-electrode detectors have been developed for the purpose of reducing effects of hole trapping in room-temperature radiation detectors. Some reported geometries maintain a nearly-uniform electric field inside the detector, but others generate an electric field that is very non-uniform and highly-concentrated at the anode. This paper reports the results of mapping such a detector (having a non-uniform electric field) with a finely collimated gamma-ray beam to determine the detector response as a function of position.

The deep levels present in semiconducting CdTe and semi-insulating (SI) CdTe:Cl and Cd0.8Zn0.2 Te have been investigated by means of cathodoluminescence (CL), deep level transient spectroscopy (DLTS), photo-DLTS (PDLTS) and photo induced current transient spectroscopy (PICTS). PICTS and PDLTS can be applied to SI materials and allow to determine whether the observed deep levels are hole or electron traps.Among the observed deep centers, we have focused our attention on those involved in the compensation process such as the so called center A and the deep traps located near midgap. We have identified a deep acceptor, labelled H, and a deep donor, labelled E, the latter is peculiar to CdTe:Cl and can be a good candidate for the deep donor level needed to explain the compensation process in SI CdTe:Cl.

An important region of the electromagnetic spectrum for astrophysics is the hard X- and gamma ray band between 10 keV and a few MeV, where several processes occur in a wide variety of objects and with different spatial distribution and time scales. In order to fulfill the observational requirements in this energy range and taking into account the opportunities given by small/medium size missions (e.g. on the ISS), we have proposed a compact, wide field camera based on a thick (1 cm) position sensitive CdTe detector (PSD). The detector is made of an array of 128×96 CdTe microspectrometers with a pixel size of 2×2 mm2. The basic element of the PSD is the linear module that is an independent detection unit with 32 CdTe crystals and monolithic front-electronics (ASIC) supported by a thin (300 μm) ceramic layer. The expected performance of the PSD over the operative energy range and some of the required ASIC functionality are presented and discussed.

We have designed and tested a new compact, very sensitive (>1 μRad), X-ray quantum sensor for high energy (0.5 -10 MeV) flash imaging. Monte Carlo simulations have been computed for several interesting detector materials (CdZnTe, HgI2-a, BGO, CsI, LSO...). From these calculations, we obtain general design rules and curves which allow one to optimize the pixel size of a sensor to the dose levels. An experimental CdZnTe detector array and its performance is presented.