Search

Thallium bromide (TlBr) has been under development for room temperature gamma ray spectroscopy. As a result of improvements in material processing and detector fabrication over the last several years, the electron and hole mobility-lifetime products of TlBr are close to those of CdTe. This has allowed us to fabricate 10 mm thick arrays. Without any correction, 4.1% FWHM energy resolution was measured from one pixel of 3×3 array at 662 keV at room temperature. To minimize the polarization effect, lowering the operation temperature was applied to 13 mm thick array and the stable operation beyond 8 months could be seen without any significant degradation of detector performance. Much improved energy resolution of 2.2% or less at 662 keV has been also obtained from one pixel with depth correction. In this paper improved charge collection with Cr contacts and our recent progress made in long term stability of TlBr detectors at room temperature by applying post annealing are presented.

In recent years, progress in processing and crystal growth methods have led to a significant increase in the mobility-lifetime product of electrons in thallium bromide (TlBr). This has enabled single carrier collection devices with thickness greater than 1-cm to be fabricated. In this paper we report on our latest results from pixellated devices with depth correction as well as our initial results with Frisch collar devices. After applying depth corrections, energy resolution of approximately 2% (FWHM at 662 keV) was obtained from a 13-mm thick TlBr array operated at -18°C and under continuous bias and irradiation for more than one month. Energy resolution of 2.4% was obtained at room temperature with an 8.4-mm thick TlBr Frisch collar device.

Thallium bromide (TlBr) and related ternary compounds, TlBrI and TlBrCl, have been under development for room temperature gamma-ray spectroscopy due to high density, high Z and wide bandgap of the material. Low melting point and cubic crystal structure of selected compositions of these compounds facilitate crystal growth by melt techniques. Recent advances in material purification, crystal growth, and device processing have led to mobility–lifetime products of electrons in the mid 10 −3 cm 2 /V range enabling working detectors of greater than 15 mm thickness to be fabricated. In this paper we report on our recent progress on TlBr detector development and first results from TlBr x Cl 1− x devices. Pulse height spectra will be presented from TlBr arrays as thick as 18 mm. Depth corrected spectra will also be presented. For a 5 mm thick TlBr array, energy resolution of less than 1% (FWHM at 662 keV) was obtained after depth correction.

TlBr and TlBrxI1−x are wide bandgap semiconductor materials being investigated for applications in γ-ray spectroscopy. They have a good combination of density and atomic numbers, promising to make them very efficient detectors. Their low melting points and simple cubic and orthorhombic crystal structures are favorable for bulk crystal growth. However, these semiconductors need to be extremely pure to become useful as radiation detectors. Impurities can lead to charge trapping and scattering, reducing the charge transit lengths and limiting the detector thickness to

Radiation Monitoring Devices previously reported to have fabricated, using a planar processed, deep diffused silicon avalanche photodiodes (APDs) and position sensitive APDs (PSAPDs) that can be used for direct or scintillation-based spectroscopic and imaging applications. We have developed high gain (∼1000), high quantum efficiency (40–70% in the 200–900 nm region) at unity gain, relatively low noise, and magnetically insensitive APDs up to 45 cm 2 in area and PSAPDs up to 2.8×2.8 cm 2 in area. These detectors have begun to be implemented in applications such as positron emission tomography (PET) and single photon emission computerized tomography (SPECT) for medical imaging, high-energy physics experiments as water Cherenkov detectors and liquefied noble gas calorimeters, and receivers for long-range optical communication at near infrared (IR) wavelengths (1064 nm). Also, our PSAPDs have been combined with photocathode structures, similar to a photomultiplier tube (PMT), to fabricate hybrid devices. Here, we present a small review and a sample of results showing various applications utilizing our planar processed APDs and PSAPDs.

Vapor deposited films of thallium bromide are evaluated as potential photoconductive layers in new large-area radiographic detectors. The attractiveness of the material lies in its inherent high effective atomic number and high density. Polycrystalline films up to 200 /spl mu/m have been grown and show a columnar structure with grains reaching 100 /spl mu/m in diameter. Current-voltage (IV) tests indicate a bulk resistivity of 10/sup 9/-10/sup 10/ /spl Omega//spl middot/cm, limited by ionic conduction. The instability of current with time is also observed, but it can be minimized with cooling. The films demonstrate high gain at relatively low field strengths as compared to other photoconductive layers. Benefits and drawbacks of TIBr are compared to other materials, and possible solutions are discussed.

Semiconductors photodetectors have long seemed an attractive alternative for scintillation detection, but only recently have semiconductor photodiodes been proven suitable for some room temperature applications. There are many applications, however for which the performance of standard silicon p-i-n photodiodes is not satisfactory. This article reviews recent progress in two different families of novel semiconductor photodetectors: (1) wide bandgap compound semiconductors and (2) silicon photodetectors with enhanced signal-to-noise ratio. The compounds discussed and compared in this paper are HgI 2 , PbI 2 , InI, TlBr, TlBr 1− x I x and HgBr 1− x I x . The paper will also examine unity gain silicon drift diodes and avalanche photodiodes with maximum room temperature gain greater than 10 000.

Recent progress in the development of room-temperature lead iodide (Phi,) X-ray detectors is reported. Progress has been made in the areas of detector fabrication and preamplifier electronics, and this has resulted in improved detection performance. An energy resolution of 415 eV (FWHM) has been reported for 5.9 keV X-rays (“Fe source) with 1 mm’ detector at room temperature. A better estimation of the Fano factor in PbIz has been carried out and the upper limit of the Fano factor is calculated to be 0.19. Larger lead iodide detectors (up to 2.5 mm*) have been fabricated and their spectroscopic performance has been evaluated. The timing characteristics of lead iodide detectors have been investigated. A compact, portable lead iodide probe assembly has been designed and built for X-ray spectroscopic applications. Finally, optical and charge particle detection properties of lead iodide detectors have also been characterized.

Silicon drift photodiodes have been constructed in a two-dimensional array format for the purpose of position sensitive detection. The arrays are dimensioned 4/spl times/4, with pixel size 3 mm /spl times/3 mm, and are intended to be used with discrete-channel readout electronics. This aspect of the design limits the overall array dimension in future prototype versions to approximately 10/spl times/10, but allows for higher total count rates than multiplexed or serial-read arrays. The photodiode arrays use the floating cathode ring design described by others. The design reported here, however, differs in that these devices have been specifically processed to maximize the detection efficiency of visible light. This feature makes these detectors attractive for use in position-sensitive scintillator readout. Measurements are presented using these photodiodes coupled to common scintillator crystals to detect gamma-rays. Measurements are also shown using these photodiodes to directly detect low energy X-rays. Typical room temperature noise (ENC) measured in the prototype arrays is 60 electrons rms.

Lead iodide (PbI2) is a wide bandgap semiconductor (Eg = 2.32 eV) and has been studied over the past several years as a semiconductor material for use in solid state X-ray and gamma-ray detectors. Small lead iodide detectors have been found to operate with low noise and good energy resolution (500 eV FWHM for 5.9 keV 55Fe X-rays). In the interest of reducing the electronic noise in lead iodide detection systems we have characterized the measured noise in the lead iodide detectors, and have compared the measurements with known noise models. It has been assumed that the noise sources in the lead iodide detectors would take form of a combination of series thermal noise, detector shot noise, and 1ƒ noise. Detectors with differing areas and thicknesses were analyzed by measuring their noise as a function of the amplifier integration time. A computer fitting program was used to obtain the magnitude of each noise source. Based on this analysis, 1ƒ noise was found to be the dominant noise source in most detectors at larger integration times of 4 μs to 12 μs, in which range the detectors are normally operated. The 1ƒ noise was also found to be proportional to the input capacitance, indicating that it is dominated by the series1ƒ noise. The 1ƒ noise magnitude appears to be dependent on the detector fabrication procedures, and may be reduced in future detectors by using more suitable fabrication procedures.

Over the last few years, thallium bromide (TlBr) has been investigated for use as a semiconductor radiation detection material. This article reviews the history of TlBr detector development and describes the fabrication of state-of-the-art TlBr γ-ray detectors. Studies of TlBr detectors of different chemical purity indicate that their performance is no longer limited by chemical purity, as was true previously. This is a result of our purification methods, which employ multipass zone-refining. We also report on the performance of vacuum-deposited (by sublimation, ≅ 100 μm thick) TlBr films as single photon detectors. They have surprisingly high μτ for sublimed films, and X-ray detectors made from these films may find use in some X-ray imaging applications, including xeroradiography.

We have characterized photodetection properties of a new ternary semiconductor, HgBrxI2−x. High quality crystals of HgBrxI2−x are produced using purified starting materials. These crystals are characterized by measuring their optical, electrical and crystallographic properties. Photodetectors are fabricated from these ternary crystals and these photodetectors are evaluated by measuring their quantum efficiency as a function of the wavelength of incident light and their performance as scintillation spectrometers. The dominant factors limiting the performance of these photodetectors are identified and the direction of the future research in order to overcome these limitations is also presented.

Recent improvements have been made in growing large, oriented lead iodide (PbI 2 ) single crystals for room temperature X-ray detector applications. The effect of ampoule design on the growth of crystals by the Bridgman-Stockbarger technique was studied. Crystals, optimal for detector labrication, were obtained using a necked ampoule with a 60° bend. This ampoule produced single crystals with the c -axis parallel to the axis of the ampoule.

Thallium bromide (TlBr) is a dense, high-Z, wide bandgap semiconductor that has potential as an efficient, compact, room temperature nuclear radiation detector. In this paper we report on our recent progress in TlBr nuclear detector development. In particular, improvements in material purification have led to an order of magnitude increase in the mobility-lifetime product of electrons, (mutau)e, to as high as 5 times 10-3 cm2/V. This has enabled much thicker detectors with good charge collection to be fabricated. We fabricated and tested small pixel TlBr arrays up to 10 mm thick. The energy resolution ~2% FWHM at 662 keV was recorded with 5-10 mm thick devices without 3-D spectral correction. We also investigated the long-term detector stability and were able to constantly operate a thin (0.5 mm) detector for five months at -18degC, under an electric field and with irradiation.

TlBr is a promising semiconductor for gamma-ray detection at room temperature, but it has to be extremely pure to become useful. We investigated the purification and crystal growth of TlBr to improve the mobility and lifetime of charge carriers, and produce TlBr detectors for radioisotopic detection. Custom equipment was built for purification and crystal growth of TlBr. The zone refining and crystal growth were done in a horizontal configuration. The process parameters were optimized and detector grade material with an electron mobility-lifetime product of up to 3x10-3 cm2/V has been produced. The material analysis and detector characterization results are included.

The development of a model which enables the calculation of charge collection efficiencies in a planar semiconductor detector with a nonconstant electric field is described. The validity of the model is verified analytically by comparing the results predicted by it with known solutions for the limiting case of a constant field. The correctness of the model is also established by comparing its predictions with experimental results. The utility of this model is shown by computing an electric field distribution which would lead to improved energy resolution in a semiconductor detector. >

The performance of semiconductor radiation detectors is strongly influenced by the trapping and detrapping of electrons and holes in the semiconductor crystals used in their fabrication. Unfortunately, these effects are very difficult to predict analytically, even when assuming simple conditions such as constant electric field. We report on a technique which computes the induced current pulse as a function of time in semiconductor detectors using Monte Carlo methods. The charge collection efficiency may then be computed from the current pulse by integration. This technique uses a computer program which simulates the trapping and detrapping of individual charge carriers within a semiconductor detector under a broad range of conditions. For boundary conditions where analytical solutions exist, excellent agreement is obtained with the results of our simulation.

In this paper, we discuss recent advances in the high gain, very large area silicon avalanche photodiode technology. Very large area APDs (square and octagonal design, up to 45 cm/sup 2/ area) have been built using the planar process. These APDs have been successfully operated as scintillation spectrometers. When cooled to -40 /spl deg/C, the energy resolution of 5.9 keV X-rays (/sup 55/Fe source) upon direct interaction with the 45 cm/sup 2/ APD was measured to be 2.4 keV (FWHM). The APD was coupled to a CsI(Tl) scintillator and irradiated with 662 keV gamma rays from /sup 137/Cs source and the energy resolution was measured to be /spl sim/10% (FWHM) at the temperature of -40 /spl deg/C. A rise time of < 2 ns was measured with the 40 cm/sup 2/ APD upon irradiation with 5.5 MeV alpha particles. A new packaging design has been developed to allow cooling of these large APDs to liquid nitrogen temperature. Evaluation of the 45 cm/sup 2/ APD has been conducted at LN/sub 2/ temperatures. Its gain was found to be as high as 10/sup 4/, while the noise was measured to be /spl sim/0.8 electrons (rms). Detection of low intensity optical signal (

We have investigated a new position sensitive avalanche photodiode (PSAPD) for indirect and direct radiation imaging. This PSAPD exhibits minimal image distortion and still has all the attractive characteristics of our normal high gain APDs. The arc-PSAPD incorporates a resistive arc between the corner contacts which eliminates the 'pincushion' or 'barrel' effect commonly seen with four corner contact devices. Simulations have been performed to model the position distortion of such a device. Position and energy resolution have also been measured with these devices. Gamma ray imaging with various scintillator arrays and direct charged particle and low energy X energy resolution-ray images have been acquired.

The use of junction field effect transistors (JFETs) as radiation detectors is described. Measurements of the energy resolution of n-channel JFETs operated as X-ray spectrometers are presented. An energy resolution of 210 eV FWHM at 5.9 keV was obtained at room temperature using a commercially available JFET as a detector. An analysis of the behavior of JFETs as X-ray spectrometers is presented together with suggestions on designing future devices specifically for use as radiation detectors. It is concluded that detectors approaching the performance of cooled Si(Li) detectors but capable of operating at room temperature could be made from JFETs. >

A method of charge-sensitive amplifier feedback is described, in which the input FET drain current is controlled by the preamplifier output voltage producing stable operation. The circuit design has the benefit of eliminating the feedback resistor, thereby eliminating its parallel white and 1/f noise components. The circuit is simple, requiring no specialized FET designs. The FETs may remain in their packaged state because they do not have to be light sensitive for purposes of reset. Measurements were made to compare this feedback mode to that using resistor feedback. The noise level of the drain current feedback amplifier was observed to be substantially less than the resistor feedback at all shaping times used. >

This paper describes the research performed in developing low noise, high quantum efficiency lead iodide photodetectors for use with scintillators. These photodetectors operate with very low leakage current and show high quantum efficiency (>60% in 350 to 500 mm region). Successful scintillation studies have been performed at room temperature as well as elevated temperatures (100/spl deg/C) using PbI/sub 2/ photodetectors with LSO and CsI(Na) scintillators. Detailed analysis of noise has also been performed and potential applications are discussed.

Junction field-effect transistors (JFETs) were fabricated directly onto a high resistivity silicon wafer and tested as readout devices for radiation detectors. The JFETs were fabricated using an n-channel epitaxial process. We report on the design and electrical characteristics of these devices as well as the noise performance and energy resolution obtained with these devices when operated as X-ray spectrometers. >

This paper reports on the evaluation of photodetectors fabricated from a ternary semiconductor, TlBr/sub x/I/sub 1-x/ for application in scintillation spectroscopy. These photodetectors are characterized in terms of their resistivity, charge transport parameters, quantum efficiency as a function of wavelength, and finally their performance as scintillation spectrometers. The details about TlBr/sub x/I/sub 1-x/ purification, crystal growth and device fabrication are also addressed. >

Lead iodide shows great promise as an X-ray spectrometer material for use at room temperature and above. The pioneering work on lead iodide detectors was promising but resulted in detectors with poor energy resolution. Later, we discovered that by carefully preparing the starting materials, lead iodide detectors with good energy resolution (< 1 keV FWHM at 5.9 keV) could be fabricated. We have continued to develop these detectors with significant progress being made in the areas of purification, crystal growth and device fabrication.

We report on the performance of T1Br nuclear detectors fabricated on crystals grown from highly purified material. The detectors are characterized in terms of their energy and timing resolution. In addition, parameters important to detector performance are presented. Finally, the effect of material processing on device performance is addressed.

The authors report on measurements of the two most important transport parameters, the mobility mu and the mean trapping time tau for electrons and holes, in TlBr crystals prepared in the laboratory. The results using the transient charge technique are presented along with the data obtained by the pulse height spectrum analysis. The values of ( mu tau )/sub e/ and ( mu tau )/sub h/ measured for TlBr are still lower than those reported for more established detector materials such as CdTe and HgI/sub 2/. It is noted, however, that purer TlBr exhibited somewhat improved transport characteristics, implying that mu tau may still be impurity-limited. For this reason, future improvements in purification will probably yield detectors having improved charge carrier transport. >

Large area (≈1 cm 2 ) silicon avalanche photodiodes (SiAPDs) have been fabricated and their performance as optical detectors for use with scintillating crystals has been measured. Light sensitivity is measured for hexagonal SiAPDs of 1.57 cm 2 total package area, and the energy and timing resolution is measured for these devices coupled to CsI(Tl) scintillators operating as gamma spectrometers.

This chapter reviews the properties of lead iodide and radiation detectors fabricated from it. The chapter briefly reviews the physical properties of lead iodide crystals followed by a description of methods to produce crystals of this material for detector use. It discusses the electrical properties of crystals that have been produced in the laboratory, followed by a discussion of the properties of detectors fabricated from these crystals. The chapter outlines the potential applications of lead iodide detectors and speculates on the future of this promising material. Lead iodide is an interesting and potentially useful material for the fabrication of radiation detectors. In general, lead iodide crystals and detectors are very similar to those produced by the more well-known detector material mercuric iodide but lead iodide is currently in a more primitive state of development because fewer researchers have investigated it.

We describe the theory of operation, design, and estimated performance of an n-channel JFET designed to be operated as a detector in an X-ray spectrometer system. We estimate that a room temperature (300 K) JFET detector can be built with performance comparable to a small area, cryogenically cooled Si(Li) detector.

Traditionally, researchers developing improved gamma ray detectors have used analytical techniques or, rarely, computer simulations to predict the performance of new detectors. However, with the advent of inexpensive personal computers, it is now possible for virtually all detector researchers to perform some form of numerical computation to predict detector performance. Although general purpose code systems for semiconductor detector performance do not yet exist, it is possible to perform many useful calculations using commercially available, general purpose numerical software packages (such as `spreadsheet' programs intended for business use). With a knowledge of the rudimentary mechanics of detector simulation most researchers, including those with no programming skills, can effectively use numerical simulation methods to predict gamma ray detector performance. In this paper we discuss the details of the numerical simulation of gamma ray detectors with the hope of communicating the simplicity and effectiveness of these methods. In particular, we discuss the steps involved in simulating the pulse height spectrum produced by a semiconductor detector.

Photoconductive detectors have been developed from a new wide bandgap (E g equals 2.01 eV) semiconductor, InI, and these photodetectors are intended for use in scintillation spectroscopy of nuclear radiation. InI single crystals were produced using the Bridgman process and these crystals were characterized by measuring their optical transmission spectrum, micro-hardness, electrical resistivity, and charge transport properties. Photodetectors were fabricated from InI crystal slices by evaporating thin ( 60%) in the 300 nm to 600 nm wavelength region and their uniform photo-response over the active detector area. Finally, these photodetectors were coupled to CsI(Tl) scintillator and were successfully tested as spectrometers at room temperature by irradiating the scintillator with 5.5 MeV (alpha) particles ( 241 Am source) and 662 keV (gamma) rays ( 137 Cs source).

The requirements of a semiconductor material intended to operate in a gamma-ray detector at room temperatures are discussed, and the status of the search for alternative materials is reviewed. The important material characteristics of a semiconductor gamma-ray detector material are high average atomic number, material's uniformity, resistivity, and electron and holes transport properties. Materials under investigation include GaAs, InP, TlBr, and PbI2. Theoretically, it is considered to be feasible to built a large volume semiconductor gamma-ray detector capable of good energy resolution at room temperature. But it is very unlikely that a semiconductor detector with germanium-like performance will be available in the next five years.

Silicon drift photodiodes have been developed over the last five years and are a derivative of silicon drift chambers. These devices, while lacking the position sensitivity of the silicon drift chamber, retain the qualities of low capacitance and large area. These properties make them attractive for use in applications requiring low noise high efficiency photodetection, such as for scintillation light detectors in nuclear spectroscopy. These devices might also find other uses in photonics; as replacements for silicon p-i-n photodiodes in other applications demanding low noise operation. We report on our progress in fabricating silicon drift photodiodes for use as scintillator photodetectors, specifically optimized for detecting the 550 nm emission from CsI(Tl). The design we chose to build was a square photodiode 1 cm 2 in area, having the general features of that described by Avset et al. Although some technical problems have temporarily delayed us from producing working drift photodiodes, we have made some diagnostic measurements on our devices and have made observations that may be of general interest.

Monte Carlo techniques have been used to simulate gamma-ray pulse-height spectra from rectangularly shaped CdTe detectors in the energy range of a few keV to approximately 1 MeV. The simulation results were found to agree well with measured CdTe gamma-ray pulse-height spectra. The various physical events occurring within the detector are simulated in a naturally sequential manner. Specifically, the algorithm simulates such effects as exponentially absorbed radiation, multiple Compton scattering within the detector, charge-carrier trapping, and electronic noise (Gaussian). Advantages of this simulation method are the compactness of the code (allowing the use of microcomputers) and speed of operation (approximately 15 events per second). >

Semiconductor radiation detectors have been constructed from indium phosphide to investigate the feasibility of using them in a solar neutrino detector. The 115 In (76% by mass) in an InP detector is expected to undergo the reverse beta decay reaction induced by the solar neutrino flux. This reaction produces energetic photons and electrons which might be used to identify and measure the energy of low energy neutrinos. InP detectors up to 1.0 cm 3 in volume capable of detecting individual electrons and photons have been constructed.

Crystals of highly purified thallium bromide were used to fabricate radiation detectors. These detectors were rested with isotopic photon sources and successfully detected gamma and X-rays in the 0.01- to 1 MeV energy range. The detectors were tested at and below room temperature, and their performance approaches that of other semiconductor detector materials which have been under development. The performance of TlBr detectors is due to the similarity in the transport characteristics of electrons and holes in TlBr and the relatively large number of charge carriers produced per unit of energy of detected particle. The performance of these detectors is currently limited by shot noise induced by the leakage current in the TlBr. >

The authors fabricated particle detectors made from InP:Fe, InP:Zn, and InP:Cu and evaluated the performance of these devices. The InP detectors were suggested for gamma-ray detection with varying degrees of success. Gamma-ray pulse-height spectra were acquired and stored for detectors of each type. It was found that the InP:Fe detector performance is currently limited by the trapping of charge carriers by the iron dopant in the material InP:Zn detectors are limited by the low mobilities ( approximately=3 cm/sup 2/ V/sup -1/ s/sup -1/) existing in the material at the operating temperature of 14 K. The InP:Cu detectors fabricated are limited by defects induced in the material by the thermal cycling of the copper diffusion. >

We report on radiation detectors fabricated from boron phosphide (BP) layers. These devices were fabricated by growing 1 to 10 μm thick layers of BP by chemical vapor deposition (CVD) on (100) oriented n-type silicon substrates. Ohmic contacts were applied to the Si (Au-Sb). Schottky barrier contacts (also Au-Sb) were applied to the BP layer. The devices were tested as radiation detectors and were found to be capable of detecting individual 5.5 MeV alpha particles. With some improvements we hope to fabricate neutron detectors from these devices, making use of the very high cross-section of boron for thermal neutrons.