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The Improved Landmine Detector System, a militarily fielded, teleoperated vehicle-mounted multi-sensor landmine detector, uses a thermal neutron analysis (TNA) detector to confirm the presence of a mine by detecting the bulk nitrogen in its explosives. To improve the nitrogen sensitivity or measurement times of the TNA detector, higher gamma ray rates will be required. The chief bottleneck to achieving the maximum possible performance from the present TNA or future versions is the relatively slow fluorescent decay time of the NaI(TI) scintillators which are currently used. An experimental investigation was undertaken to compare a number of modern, fast inorganic scintillators to NaI(TI) with respect to parameters relevant to TNA landmine detection, including efficiency, energy resolution, linearity, available size and cost. This paper presents results in the context of the high-rate, high-gamma-energy environments expected in such a TNA application. Large (7.62 cm x 7.62 cm) La[Br.sub.3]:Ce scintillators, and to a lesser degree La[Cl.sub.3]:Ce, were found to stand-out as as the principal candidates for the detector upgrade to the TNA confirmation system. Their properties also make them ideal candidates for fast neutron analysis and associated particle imaging bulk explosives detectors. Index Terms--Gamma ray detection, gamma ray spectroscopy, inorganic scintillators, security applications, scintillation detectors, thermal neutron capture gamma ray analysis.

Compton imaging is a gamma ray imaging technique that has many possible applications including homeland security and medical imaging. Using the Compton scattering formula the origin of a scattered [lambda]-ray can be localized to a point on the surface of a cone using a minimum of two position and energy measurements. List-mode Maximum likelihood expectation maximization (MLEM) is an iterative statistical algorithm that makes successive approximations to the most probable source distribution that would have led to the observed data. Conventional MLEM is often performed in two dimensions, which still requires knowledge of the source-to-detector distance. We propose iteration over all three spatial dimensions in order to maximize the probability in three-dimensional space. This paper reports on the development of a three-dimensional MLEM (3DMLEM) algorithm to reconstruct images from a Compton imager in three dimensions for single, multiple and line sources. Several techniques for reducing convergence times are also discussed. Index Terms--Algorithms, compton scattering, nuclear imaging.

The Nuclear Compton Telescope (NCT) is a balloon-borne telescope designed to study astrophysical sources of nuclear line emission and polarization at soft gamma-ray (0.2-10 MeV) energies. NCT uses high-purity germanium strip detectors for 3D tracking of photon interactions. Compton imaging enables effective background rejection, resulting in a compact but highly efficient instrument. The NCT prototype completed a successful flight from Fort Sumner, New Mexico in 2005. We have since integrated additional detectors, updated the readout electronics, and improved other flight systems. Two flights of the full instrument are upcoming: a conventional flight in New Mexico and a long duration flight from Australia. We give an overview of the instrument and its status prior to the planned balloon flights. Index Terms--Compton imaging, gamma-ray astronomy detectors, gamma-ray imaging, germanium radiation detectors.

This study compares the light yield as a function of gamma-ray energy for several Lu-and Y-based silicate and aluminate scintillators. These comparisons illustrate the effects of crystal matrix, growth atmosphere, activator concentration, and co-doping on proportionality. The radioisotopic sources used in this study produce gamma-rays and X-rays from 22 keV to 1.3 MeV. The samples investigated are 10 x 10 x 10 mm in size and have the following single-phase compositions: LSO:Ce; LSO:Ce, Ca; LPS:Ce; LuAG:Pr; YSO:Ce; and YSO:Ce, Ca. The light yield vs. gamma-ray energy response is similar for scintillators with similar crystal structures, despite significant variations in growth atmosphere and activator concentration. Additionally, sample-to-sample variation is significantly reduced in the Ce-doped oxyorthosilicates when calcium is added as a co-dopant. Overall, the silicates measured in this study exhibit similar non-proportional responses, and in general are less proportional in the lower energy range compared to LuAG and other aluminates. Index Terms--Non-proportionality, scintillation, scintillators.

Thallium bromide (TlBr) is a high atomic number (81, 35), dense (7.56 g/[cm.sup.3]) wide band gap (2.68 eV) semiconductor. In addition, TlBr has a cubic crystal structure and melts congruently at a relatively low temperature (~460 C). Recently, mobility-lifetime product of electrons in TlBr has been reported to be greater than 0.001 [cm.sup.2]/V [5]. These properties make TlBr a promising material for room temperature gamma radiation detection. Employing device designs such as small pixel arrays that depend primarily on the motion of a single carrier type allows fabrication of thicker devices with better energy resolution than planar devices of the same thickness. We report on our recent progress in developing larger TlBr detectors. Over the past several months we have increased the electron mobility-lifetime product of our TlBr by more than one order of magnitude. Electron mobility-lifetime values as high as 3.0 x [10.sup.-3] [cm.sup.2]/V have been measured. Devices with small pixel design have been built with 3, 5, and 10 nun thickness and pixel pitch of 1 mm, 1.5, and 2.0 mm respectively. Pulse height spectra have been recorded over a range of energies from 60 keV to 662 keV. Energy resolution (FWHM) as high as approximately 5% at 122 keV and 1.7% at 662 keV has been obtained without any 3-D corrections. Such arrays are well suited for 3-D correction techniques similar to those applied to CZT devices, indicating that further improvement in energy resolution should be achievable. These latest results demonstrate promise for TlBr as a room temperature semiconductor gamma ray detector. Index Terms--Detector arrays, energy resolution, gamma-ray spectroscopy, semiconductor detectors.

Over 25 1-[cm.sup.3] CdZnTe crystals produced using the Traveling Heater Method at Redlen Technologies have been characterized. The charge carrier mobility and lifetime, and charge carrier transport uniformity of each crystal were measured using alpha particles. Some of the crystals were made into coplanar-grid detectors and their performance characterized using 662 keV gamma rays. The average electron mobility-lifetime product for these crystals was found to be a factor of about five times greater than that measured from crystals obtained over the last decade from two other commercial crystal growers. The coplanar-grid detectors produced from the material typically achieved an energy resolution at 662 keV near 2% FWHM when operated at room temperature. This is comparable to the best coplanar-grid detectors commercially produced today. Index Terms--CdZnTe detectors, gamma-ray spectroscopy detectors, charge carrier lifetime, charge carrier mobility.

A semiconductor Compton camera that combines silicon (Si) and Cadmium Telluride (CdTe) detectors was developed, and its imaging capability was examined with various kinds of gamma-ray targets such as a point source, arranged point sources and an extended source. The camera consists of one double-sided Si strip detector and four layers of CdTe pad detectors, and was designed to minimize the distance between a scatterer and the target. This is because the spatial resolution with Compton imaging improves as the target approaches the scatterer. This new camera realizes a minimum distance of 25 mm. By placing the target at a distance of 30 mm from the detector, resolving power better than 3 mm was demonstrated experimentally for a 364 keV ([sup.131]I) gamma-ray. Positional determination with accuracy of 1 mm was also demonstrated. As a deconvolution method, we selected the iteration algorithm (called List-Mode Expectation-Maximizing Maximum Likelihood), and applied it to several kinds of experimental data. The Compton back projection images of the arranged point sources and an extended object were successfully deconvolved. Index Terms--CdTe detector, Compton camera, gamma-ray imaging, silicon strip detector.

In any gamma-ray detector, each event produces electrical signals on one or more circuit elements. From these signals, we may wish to determine the presence of an interaction; whether multiple interactions occurred; the spatial coordinates in two or three dimensions of at least the primary interaction; or the total energy deposited in that interaction. We may also want to compute listmode probabilities for tomographic reconstruction. Maximum-likelihood methods provide a rigorous and in some senses optimal approach to extracting this information, and the associated Fisher information matrix provides a way of quantifying and optimizing the information conveyed by the detector. This paper will review the principles of likelihood methods as applied to gamma-ray detectors and illustrate their power with recent results from the Center for Gamma-ray Imaging. Index Terms--Depth of interaction, gamma-ray detectors, maximum-likelihood estimation, scintillation cameras, semiconductor arrays.

Analytic formulas and Monte Carlo simulation are used to calculate and compare the sensitivity of circular and polygonal orbits at different points in the Field of View (FoV) for both pinhole and slit-slat collimation. Results show that for a given slit-slat collimator an N-sided polygonal orbit tangent to the FoV generally provides average sensitivity lower than the tightest circular orbit consistent with the same aperture angle, but with better spatial resolution that can be traded for sensitivity for a constant-resolution comparison. This generally results in a slight advantage for the polygonal orbit. However, this advantage depends on the clearance that must be allowed between the orbit and the FoV and decreases quickly, vanishing when even a few millimeters of space are left, which in practice is necessary to accommodate mechanical constraints. For a pinhole collimator the advantage for the tangent polygonal orbit is more consistent, but similar conclusions are reached again when clearance is considered. A direct comparison at constant resolution between slit-slat and pinhole collimation in a single transverse plane is shown to be possible with parameters typical of small-animal imaging applications only for detectors with excellent intrinsic resolution; in this case, pinhole collimation is shown to be more sensitive in magnifying geometries, but reduced axial FoV and increased axial blurring should also be considered for a more complete comparison. Index Terms--Gamma-ray cameras, medical imaging, SPECT.

Polymer insulating materials installed in electrical and electronic devices are widely used in spacecraft and nuclear power stations where the polymers are exposed to energetic radioactive rays. The chemical structure can be changed by radiation induced reactions that possibly gives rise to the alteration of characteristics of surface charge, which can enhance the local electrical field and cause surface discharge that damages the insulation. For the safe use of the material, it's necessary to gain a good understanding of radiation effect on surface charge accumulation. In this paper, polyethylene was employed as test sample which was previously irradiated in air up to 100 kGy and then up to 1000 kGy with dose rate of 10 kGy / h by using a [sup.60]Co gamma-source. The sample surface was corona charged by applying a series of dc stresses on needle electrode that was set 6 mm above the surface. An electrostatic probe was designed to measure the surface charge density immediately after the charging process. Obtained results show the dependence of surface charge distribution upon the radiation dose varying as a function of the polarity of charging particles. It is suggested that surface charge accumulation depends upon the behavior of localized surface states, which is altered by radiation-induced degradation and cross-linking reactions of the chemical structure. Index Terms--polyethylene, gamma-ray irradiation, surface charge, localized surface states, cross-linking, degradation.

We present a new standoff imaging technique able to provide 3-dimensional (3D) images of gamma-ray sources distributed in the environment. Unlike standard 3D tomographic methods, this technique does not require the radioactive sources to be bounded within a predefined physical space. In the present implementation, the gamma-ray imaging system is based on two large planar HPGe double sided segmented detectors, which are used in a Compton camera configuration. A LIDAR system is used in conjunction with the gamma-ray imaging system to confine the gamma-ray image space to the interior of physical objects situated within the detection range of the gamma-ray imager. This approach results in superior image contrast and efficient image reconstruction. Results demonstrating the operating principle are reported. Index Terms--Compton scatter imaging, gamma-radiation, radiation imaging, semiconductor detectors.

This paper measures the sample to sample variation in the fight yield proportionality of NaI(TI), and so explores whether this is an invariant characteristic of the material or whether it depends on the chemical and physical properties of the tested samples. We report on the electron response of nine crystals of NaI(TI), differing in shape, volume, age, manufacturer and quality. The proportionality has been measured at the SLYNCI facility in the energy range between 3.5 to 460 keV. We observe that while samples produced by the same manufacturer at approximately the same time have virtually identical electron response curves, there are significant sample to sample variations among crystals produced by different manufacturers or at different times. In an effort to correlate changes in the electron response with details of the scintillation mechanism, we characterized other scintillation properties, including the gamma response and the x-ray excited emission spectra and decay times, for the nine crystals. While sample to sample differences in these crystals were observed, we have been unable to identify the underlying fundamental mechanisms that are responsible for these differences. Index Terms--Nonproportionality, radiation detector, scintillator.

In this paper we present the studies performed on a set of [Lu.sub.3] [Al.sub.5][O.sub.12] :Pr (LuAG:Pr) crystals with praseodymium concentration between 1.5 and 10%, grown by the micro-pulling-down ([micro]PD) technique. The research comprises the measurements of X-ray excited emission spectra and [sup.137]Cs gamma-ray pulse height spectra in a range from 78 to 600 K, and thermoluminescence glow curves. Based on experimental data we discuss the dependence of scintillation properties of [Lu.sub.3][Al.sub.5][O.sub.12]:Pr on praseodymium content and temperature. The main attention is focused on a distinct increase of scintillation yield with temperature, which we attribute to existence of shallow electron traps and their temperature-dependent contribution to scintillation of LuAG:Pr. An active role of traps is demonstrated by a novel experiment combining X-ray and laser excitation. Index Terms--LuAG:Pr, light yield, scintillation mechanism, trap.

The Gamma Ray Energy Tracking In-Beam Nuclear Array (GRETINA), capable of determining the energy and position (within 2 mm) of each gamma-ray interaction point and tracking multiple gamma-ray interactions, has been designed. GRETINA will be composed of seven detector modules, each with four highly pure germanium crystals. Each crystal has 36 segments and one central contact instrumented by charge sensitive amplifiers. Two custom designed modules, the Digitizer/Digital Signal Processing (DSP) and the Trigger Timing and Control, compose the electronics of this system. The Digitizer/DSP converts the analog information with 14-bit analog to digital converters (operating at 100 MS/s, and digitally processes the data to determine the energy and timing information of the gamma interactions with the crystal. Each Digitizer/DSP is controlled by and sends trigger information to the Trigger Timing & Control system through a bidirectional Gbit link. Presently four different trigger algorithms are planned for the trigger system and can be selected for trigger decision. In this paper the details of the electronics and algorithms of the GRETINA data acquisition and trigger system are presented and the performance is reviewed. Index Terms--Data acquisition systems, digital signal processing, gamma-ray spectrometer, trigger systems.

High-energy (> 100 keV) photon detectors are often made thick relative to their lateral resolution in order to improve their photon-detection efficiency. To avoid issues of parallax and increased signal variance that result from random interaction depth, we must determine the 3D interaction position in the imaging detector. With this goal in mind, we examine a method of calibrating response statistics of a thick-detector gamma camera to produce a maximum-likelihood estimate of 3D interaction position. We parameterize the mean detector response as a function of 3D position, and we estimate these parameters by maximizing their likelihood given prior knowledge of the pathlength distribution and a complete list of camera signals for an ensemble of gamma-ray interactions. Furthermore, we describe an iterative method for removing multiple-interaction events from our calibration data and for refining our calibration of the mean detector response to single interactions. We demonstrate this calibration method with simulated gamma-camera data. We then show that the resulting calibration is accurate and can be used to produce unbiased estimates of 3D interaction position. Index Terms--3D interaction position, depth of interaction, gamma-ray imaging, mean detector response calibration, maximum likelihood estimation, multiple-hit event filtering.

A computed tomography (CT) system using high-energy monochromatic photons is one of the best choices for non-destructive tests, to detect flaws or defects in manufactured items for the purpose of improving quality control, and quality assurance. We have developed such a CT system based on laser-Compton X-rays, which were generated with an electron accelerator and a laser beam. Using 10 MeV laser-Compton X-rays, an experimental study for non-destructive tests of concrete specimens with computed tomography were performed. We successfully obtained CT images. Index Terms--Computed tomography, electron accelerator, gamma-ray, laser-Compton, non-destructive test.

We have constructed a unique instrument for use in the search for orphan sources. The system uses gamma-ray imaging to 'see through' the natural background variations that effectively limit the sensitivity of current devices. The imager is mounted in a 4.9-m-long trailer and can be towed by a large personal vehicle. Source locations are determined both in range and along the direction of travel as the vehicle moves. A fully inertial platform coupled to a global positioning system receiver is used to map the gamma-ray images onto overhead geospatial imagery. The resulting images provide precise source locations, allowing rapid follow-up work. The instrument simultaneously searches both sides of the street to a distance of 50 m (100-m swath) for millicurie-class sources with excellent performance as determined using source injection studies and Receiver-Operator-Characteristic techniques. Index Terms--Coded-aperture imager, gamma-ray detectors, gamma-ray imagers, nuclear imaging, orphan source search.

Experiments using dose rates of 40 and 1270 rads/min from Co-60 and dual optical wavelengths, 850 and 1310 nm, are conducted to estimate fiber optic attenuation at other dose rates using low order kinetics models. The irradiation-induced attenuation is fit to power and saturating exponential models. The 1310 nm data exhibit a linearly increasing term with the saturating exponential whereas the 850 nm results do not. Representation of the long-term low-dose-rate induced attenuation is an impetus for developing a difference equation fitting routine that better characterizes the signal loss and has the capability of predicting the attenuation at saturation. Index Terms--Gamma-ray effects, optical fiber cables, optical fiber radiation effects, photon radiation effects.

Frisch-ring CdZnTe detectors have demonstrated both good energy resolution, < 1% FWHM at 662 keV, and good efficiency in detecting gamma rays, highlighting the strong potential of CdZnTe materials for such applications. We are designing a hand-held gamma-ray spectrometer based on Frisch-ring detectors at Brookhaven National Laboratory. It employs an 8 x 8 CdZnTe detector array to achieve a high volume of 19.2 [cm.sup.3], so greatly improving detection efficiency. By using the front-end application-specific integrated circuits (ASICs) developed at BNL, this spectrometer has a small profile and high energy-resolution. It includes a signal processing circuit, digitization and storage circuits, a high-voltage module, and a universal serial bus (USB) interface. In this paper, we detail the system's structure and report the results of our tests with it. Index Terms--CdZnTe, CZT detector array, Frisch-ring detector, gamma-ray spectrometer.

In this work, we present results of measurements of spectra of [alpha]-particles carried out on semi-insulating (SI) InP detectors. The detectors were fabricated from Ti and Zn co-doped SI InP crystals grown by Czochralski technique. Tests of detectors performance with s-particles emitted from [sup.241]Am (5.48 MeV) radioactive source are reported. Excellent detectors performance has been evaluated by values of charge collection efficiency 99% and energy resolution ~ 1% obtained from the spectra measured at 230 K. Such high performance can be explained by InP doping with suitable Ti atoms and co-doping with low concentration of Zn acceptors sufficient for the full compensation of shallow donors, to reach SI properties. The electron mobility of reported SI InP detectors co-doped with Ti and Zn has been found to be equal to 2510 [cm.sup.2][V.sup.-1][s.sup.-1] at room temperature. This value is much higher than that obtained previously with our SI InP co-doped with Ti and Mn or with InP crystals converted to SI state by temperature annealing. Excellent detector performance and high electron mobility makes the reported InP material promising to be used for the of X-rays and [gamma]-radiation. Index Terms--Gamma ray, InP, particle detector, radiation detector, semi-insulating.

In this paper, a new pulse shape discrimination algorithm for a triple-layer phoswich detector is implemented and tested. The three-layer detector which was used in this experiment was originally designed for simultaneous detection of beta particles and gamma rays in a mixed radiation field. The first two layers are specifically designed for beta particles and the third layer is designed for gamma-ray interaction. The pulse output of the photomultiplier is digitized and sent to the host computer for further processing. A de-noising algorithm based on the Wavelet Transform (WT) is implemented to reduce the effect of noise introduced by the noisy analog channel and by the photomultiplier tube. Three new timing features were extracted and given as input to a fuzzy interface system (FIS). The FIS output indicates to which scenario the input signal belongs. Compared to the non-fuzzy method which was initially implemented for this detector, the fuzzy algorithm shows a better performance in separating beta and gamma spectra, especially at high energies. Also, fewer pulses are rejected, due to classification as unknown pulses, and interaction in multiple layers is detected more efficiently. Index Terms--Digital pulse shape discrimination, fuzzy logic, nuclear spectroscopy, phoswich detectors, wavelet de-noising.

Image quality in nuclear medicine is a trade-off between image contrast and resolution, since there are conflicting requirements of crystal thickness and collimator-hole dimensions. The sensitivity, or efficiency, which determines the contrast-to-noise ratio, however, is constrained primarily by pulse-mode operation, in which gamma ray interactions are recorded once every deadtime period. Nuclear images also suffer from image blurring due to gamma rays penetrating the collimator septa and due to misplaced pileup events arising from the coincidence detection of Compton-scattered gamma rays. Multi-zone detector architecture (MZDA) and the use of pileup prevention circuitry (PPC) lead to reduction of the detector's effective deadtime, and hence to significant extension of the count-rate range of the camera. This in turn allows the source activity to be increased to make up for sensitivity that will be lost on changing the detector and collimator-hole dimensions to improve spatial resolution. Misplaced pileup suppression (MPS) circuitry, also described in this paper, enables image blurring from Compton-scattered radiation to be reduced to 1/P of its level in a single zone detector, P being the PMT array size. The paper first develops the theoretical relationships relating to the trade-offs between efficiency and resolution, the distribution of scintillation light from interaction points in the crystal, the share of the light exposure of individual PMTs and of clusters of PMTs, and the cross-talk between clusters of PMTs. The paper then presents an outline of the implementation of an S-zone camera with PPC of order 1, based on hexagonal and rectangular arrays of PMTs. A hexagonal array of 331 1.27-cm PMTs, which will have S [approximately equal to] 35 detector-zones on average, can, for instance, use a 1.7-mm thick and 29-cm diameter NaI(TI) crystal to design a camera with twice the sensitivity of a single-zone camera of the same array size, which will also provide a spatial resolution of 2.1 mm at the collimator face and 3.4 mm at 10 cm from the collimator face. Index Terms--Contrast-to-noise ratio, depth of interaction (DOI), geometric efficiency, intrinsic spatial resolution, misplaced pileup suppression (MPS), multi-zone detector architecture (MZDA), pileup prevention circuitry (PPC).

98 SPEAR sensors from eV Products have been evaluated for their use in a portable Compton camera. The sensors have a 5 mm x 5 mm x 5 mm CdZnTe crystal and are provided together with a preamplifier. The energy resolution was studied in detail for all sensors and was found to be 6% on average at 59.5 keV and 3% on average at 662 keV. The standard deviations of the corresponding energy resolution distributions are remarkably small (0.6% at 59.5 keV, 0.7% at 662 keV) and reflect the uniformity of the sensor characteristics. For a possible outside use the temperature dependence of the sensor performances was investigated for temperatures between 15 and 45 deg Celsius. A linear shift in calibration with temperature was observed. The energy resolution at low energies (81 keV) was found to deteriorate exponentially with temperature, while it stayed constant at higher energies (356 keV). A Compton camera built of these sensors was simulated. To obtain realistic energy spectra a suitable detector response function was implemented. To investigate the angular resolution of the camera a [sup.137]Cs point source was simulated. Reconstructed images of the point source were compared for perfect and realistic energy and position resolutions. The angular resolution of the camera was found to be better than 10 deg. Index Terms--CdZnTe, Compton camera, CZT, energy resolution, gamma-ray, imaging, simulation.

We expand upon the idea of using [gamma]-rays for nuclear photofission of [sup.238]U at the giant dipole resonance to generate rare neutron-reach nuclei. The SPIRAL II project proposes the employment of 10-20-MeV Bremsstrahlung [gamma]-rays generated by a 45-MeV electron beam/http://ganinfo.in2p3.fr/research/ developments/spira12/index.html/. In this paper, we explore the possibility of using a Compton [gamma]-ray source for such a process. The Collider Accelerator Department at Brookhaven National Laboratory is developing high-current (up to 1 A), high-brightness (down to l-mm x mrad normalized emittance), and high-energy energy-recovery linacs (up to 20-GeV electron beam energy for eRHIC). These electron beams are perfectly suited for generating photon beams with tremendous average power, approaching the megawatt level. The range of photon's energy extends from subelectronvolts from free-electron lasers to 10 GeV from the Compton process. In this paper, we focus on a [gamma]-ray source for producing rare isotopes. Index Terms--Energy recovery linac, free electron laser, gamma rays, isotopes far away from the island of stability, laser Compton backscattering, laser-generated [gamma]-ray beam, nuclear resonance, nucleosynthesis in supernova explosions, photodisintegration, photofission, polarization, polarized positron source, rare isotopes, transmutation of used nuclear fuel.

Pixellated thallium bromide (TlBr) gamma-ray spectrometers were fabricated and evaluated with the depth sensitive single polarity charge sensing technique. The detectors exhibited energy resolutions of 2.22%, 1.22% and 1.11% for 356 keV, 662 keV and 1.33 MeV, respectively, at room temperature. Direct measurements of electron mobility-lifetime products in TlBr detectors were performed. An electron mobility-lifetime product of 4.1 x [10.sup.-3] [cm.sup.2]/V was obtained from a TlBr detector. Index Terms--Position-sensitive detector, semiconductor detector, thallium bromide.

The present work describes the procedure developed for the standardization of [sup.18]F by means of a coincidence system using a plastic scintillator detector in 4[pi] geometry, named 4[pi](PS)[beta]--[gamma] which was developed at the Nuclear Metrology Laboratory in IPEN, Sao Paulo, Brazil. The main advantage of this detector system is the ability to perform primary standardizations without the need for a coating of a metal layer on the radioactive source film for rendering it conductive, as usually necessary for proportional counter measurements. The measurements were also performed with a conventional 4[pi](PC)[beta]--[gamma] coincidence system, which makes use of a 4[pi] proportional counter for charged particles or X-ray detection coupled to a pair of NaI(T1) scintillation counters. The gamma-ray window was set to measure the 511 keV gamma-rays produced by positron annihilation. The detector efficiency was changed by moving the electronic discriminator threshold. The observed activity values were extrapolated to 100 % efficiency and the results showed good agreement between the two detector systems. Index Terms--Coincidence, fluorine-18, plastic scintillator, standardization.

The spectral response of several crystals grown by the Traveling Heater Method (THM) were investigated. An energy resolution of 0.98% for a Pseudo Frisch-Grid of 4 x 4 x 9 [mm.sup.3] and 2.1% FWHM for a coplanar-grid of size 11 x 11 x 5 [mm.sup.3] were measured using [sup.137]Cs-662 keV. In addition a 4% FWHM at 122 keV has also been measured on 20 x 20 x 5 [mm.sup.3] monolithic pixellated devices. The material shows great potential toward producing large-volume detectors with spectral performance that meets the requirement for high-resolution gamma-ray spectroscopy. Index Terms--Coplanar-grid, CZT, depth correction, gamma-ray, THM.

Lanthanum halide (LA[X.sub.3] : Ce) scintillators are currently being assessed by ESA for use as remote sensing gamma-ray spectrometers on future planetary missions. Such missions place a raft of constraints and requirements on sensor technology, which invariably stem from environmental and resource issues. In the past, NaI(T1) and CsI(T1) based spectrometers offered great advantages in term of device simplicity, reliability and detection efficiency but with poor energy resolution. Lanthanum halides combine these attributes with much higher energy resolution and their extensive deployment in future missions seems assured. In fact, La[Br.sub.3] spectrometers are currently baselined for most missions in planning. In this paper, we describe the needs of remote gamma-ray sensing and efforts by the European Space Agency to develop and qualify the next generation of spectrometers based on La[Br.sub.3] for interplanetary space missions. We contrast their performance to large volume Ge detectors and show that in the context of minimally resourced spacecraft, they represent a more attractive solution. Index Terms--Extraterrestrial measurements, Gamma-ray spectroscopy detectors, remote sensing, scintillation detectors.

Ce[Br.sub.3] crystals have been studied to assess their utility as potential gamma ray spectrometers for future ESA planetary missions. Pulse height spectra, scintillation time profiles, X-ray excited emission spectra, and photoinminescence spectra have been recorded as a function of temperature between 78 and 600 K. In addition, the influence of exposing Ce[Br.sub.3] to various doses of gamma rays from a strong [sup.60]Co source on its scintillation performance has been investigated. Index Terms--Ce[Br.sub.3], energy resolution, La[Br.sub.3]:Ce, radiation damage, scintillation yield, scintillator, self-absorption.

Options for optimizing the energy and spatial resolution of gamma-ray imaging detectors based on thick, monolithic crystals shaped like fiat-topped pyramids were studied. Monte Carlo simulations were made of the scintillation light transport for evaluating the effect of four parameters on the energy resolution, the spatial resolutions, and the depth of interaction (DOI) resolution of the gamma-ray imaging detector. These four parameters are: the reflectivity of the surface coating; the scatter mean free path; the absorption mean free path of the scintillation light; and the angle that defines the inclination of the sides of the pyramidal frustum. In real detectors, the values for the mean free paths for optical photons are normally not known. We estimated these by comparing MC simulations of detector resolutions to measurements for three gamma-ray imaging detectors with LYSO and LSO from different suppliers and with different surface coatings and geometries. The gamma-ray imaging detector measures the energy, centroids, and depth of interaction of the gamma-ray. DOI enhanced charge dividing readouts were used to measure the depth of interaction. Index Terms--Crystal geometry, gamma ray imaging, interaction depth, monolithic scintillator, surface coating.

We have developed a computational model of energy transfer processes in scintillators using the kinetic Monte Carlo (KMC) approach. In this publication, we focus on the alkali halide compound CsI both pure and doped with a range of thallium concentrations. The KMC model makes use of an explicit atomistic representation of the crystal lattice, activator sites, defect sites, and individual electron-hole pairs. The probability of individual diffusion, recombination, and scintillation events is calculated from rate equations parameterized with data published in the literature. Scintillation decay curves, relative intensities of emission peaks, and light yields are computed and found to be in good agreement with experimental data for a range of temperatures and thallium concentrations. This demonstrates that the KMC scintillation model is capable of reproducing both the kinetics and the efficiency of the scintillation process in CsI. In addition, novel predictions emerge from our simulations such as the diffusion distance distributions of serf-trapped holes and excitous. Finally, the KMC scintillation model provides a framework for probing possible physical processes responsible for the nonlinear relationship between scintillation light yield and incident gamma-ray energy. Index Terms--Alkali halides, cesium iodide, emission spectra, gamma-ray radiation, kinetic Monte Carlo, light yield, nonlinearity, scintillation decay curves, scintillation mechanisms, thallium.

This paper presents new development methods for property-screening design rules, using structure-property relationships for two fundamental properties of activated scintillating based gamma radiation detection-luminosity and stopping power. The first and most evident goal in developing screening models of luminosity and stopping power, as indicated by the weight and electron densities, is to obtain new candidate cerium scintillating materials. However, a second and more strategic goal is to extract design rules, which define the structural limitations on materials consistent with desirable detector properties. These design rules are based on our capability to predict the luminescence and stopping power of a material from a set of structural descriptors. Predictive models are generated using statistical multiple linear regression over a set of 24 descriptors. We find that within a set of ten cerium-doped scintillator materials we can quantitatively predict luminosity with a correlation coefficient of ~ 0.94 based on 4 of the 24 descriptors, improving to ~ 0.99 with 6 descriptors; and electron density to ~ 0.99 with 3 descriptors. Furthermore, we show in this circumstance that the luminosity and stopping power are only nominally related. In particular, luminosity depends largely on matrix valence electron properties and their coupling to activator sites-properties that do not require high atomic masses or atomic numbers per se, requirements for high stopping power. Index Terms--Density, efficiency, informatics, luminescence, scintillation detectors.

Effect of Ga substitution in the [(Lu, Pr).sub.3][Al.sub.5][O.sub.12](Pr : LuAG) scintillator was examined at the crystals grown by the micropulling down ([mu]-PD) method. Host luminescence due to an exciton localized around the Lu-Al antisite defect was suppressed by Ga admixture. Concentration depenendence was investigated and suppression was observed even at the lowest Ga concentration of 5 mol%. Faster decay time as well as less intense slower components were obtained upon increasing the amount of Ga. In the thermostimulated luminescence measurement, the glow curve peaks are gradually shifted to lower temperatures with the gallium admixture, and noticeable intensity decrease was observed only for the [(Lu, Pr).sub.3][([Ga.sub.0.2][Al.sub.0.8]).sub.5][O.sub.12]. The undesired quenching effect of Ga on the [Pr.sup.3+] luminescence seems to be reasonably low up to [(Lu, Pr).sub.3][(Ga.sub.0.2][Al.sub.0.8]).sub.5][O.sub.12] sample. It is estimated to introduce nonradiative losses of less than 10% with respect to Ga-free Pr:LuAG. Bulk crystals were also grown by the Czochralski (Cz) method and these effects were confirmed. Index Terms--Gamma-ray detectors, garnets, nuclear imaging, praseodymium compounds, radioluminescence.

By means of a photomultiplier tube, a Si-photodiode, and a Si-avalanche photodiode, the absolute scintillation yield of recently developed La[Br.sub.3]:Ce, La[Cl.sub.3]:Ce, and [(Lu, Y).sub.2]Si[O.sub.5]:Ce scintillators and traditional [Lu.sub.2]Si[O.sub.5]:Ce, [Bi.sub.4][Ge.sub.3][O.sub.12], NaI:Tl CsI:Tl, and CsI:Na scintillators were determined. These are all well known scintillators that cover emission wavelengths from 250 nm to 750 nm. By comparing the scintillation yield independently measured with the three different photon detectors reliable yield values are obtained. Index Terms--Absolute scintillation yield, gamma detectors, photodiode readout, scintillators.

The authors constructed a prototype Time-Encoded Signature (TES) system, complete with automated detection algorithms that can be used to detect point-like gamma-ray sources in search applications where detectors observe large variability in background count rates beyond statistical (Poisson) noise. The person-carried system consists of two cesium iodide scintillators placed on opposite sides of a lead shield. This geometry mitigates systematic background variation and induces a unique signature upon encountering point-like sources. This manuscript focuses on the development of detection algorithms that identify point-source signatures while remaining computationally simple. The latter constraint derives from the instrument's mobile (and thus low power) operation. The authors evaluated algorithms using both simulated and field data. The results of this analysis demonstrate the capability to detect sources at a wide range of source-detector distances using computationally simple algorithms. Index Terms--Detection statistic, gamma ray detection, handheld detector, matched filter, point source detection, standoff distance.

The assessment of the durability of candidate space components and materials requires the use of ground laboratory testing facilities when no reliable information exists in the published literature. Accelerated tests simulating the radiation/temperature/vacuum environment need to be performed to investigate possible synergistic degradation effects. An irradiation chamber combining high vacuum and thermal cycling with a large range of gamma dose rates has been designed and constructed for this purpose at SCK.CEN, the Belgian Nuclear Research Center. The irradiation chamber module with actively-controlled vacuum is designed to fit in our gamma irradiation facilities. The vacuum irradiation module operates in the high-vacuum regime even in the presence of moderate outgassing from polymer materials. A pressure of [10.sup.-5] mbar can be routinely achieved but lower pressure can be obtained in the case of inert and mineral materials. In addition to the facility description, our paper discusses preliminary results of radiation-assisted outgassing of common polymers obtained using this new vacuum irradiation module. Index Terms--Gamma radiation, hardness assurance, irradiation facility, radiation assessment, vacuum.

We have investigated the variation of the optical absorption induced by pulsed (dose rate > [10.sup.8] rad/s) and continuous ( Index Terms--Color centers, optical fibers, pulsed X-rays, radiation-induced effects, [gamma]-rays.

Cerium-doped gadolinium pyrosilicate single crystals Ce : [Gd.sub.2][Si.sub.2][O.sub.7] (Ce:GPS) with various Ce concentrations of 2.5-30 mol% with respect to the total rare-earth sites were prepared by the floating zone method. Their scintillation performances were investigated under irradiation of gamma rays of [sup.137]Cs. The Ce concentration dependence of scintillation characteristics and high-performance scintillation characteristics were obtained: 3-6 times greater light output than that of BGO single crystals, rapid decay, and good energy resolutions of 5.1-8.4%. Furthermore, X-ray diffraction patterns of Ce:GPS revealed Ce concentration dependence of the crystal structure of Ce:GPS. Index Terms--Crystal growth, gadolinium compounds, [Gd.sub.2][Si.sub.2][O.sub.7], neutron detectors, radiation detectors, scintillators.