We perform an investigation into the scintillation processes and performance of elpasolites Cs 2 LiLaBr 6-x Cl x :Ce (CLLBC) and Cs 2 LiYBr 6 :Ce (CLYB) using a thermal cycle over a range of -20 to +50° C. At 10° increments, we acquire data with both a waveform digitizer and charge-integrating electronics. We identify decay components and evaluate decay times, thermal neutron gamma-equivalent energy (GEE), and pulse shape discrimination (PSD) performance. Results are compared to common Cs 2 LiYCl 6 :Ce (CLYC).

A new class of elpasolite scintillators has garnered recent attention due to the ability to perform as simultaneous gamma spectrometers and thermal neutron detectors. Such a dual-mode capability is made possible by pulse-shape discrimination (PSD), whereby the emission waveform profiles of gamma and neutron events are fundamentally unique. To take full advantage of these materials, we have developed the Compact Advanced Readout Electronics for Elpasolites (CAREE). This handheld instrument employs a multi-channel PSD-capable ASIC, custom micro-processor board, front-end electronics, power supplies, and a 2 in. photomultiplier tube for readout of the scintillator. The unit is highly configurable to allow for performance optimization amongst a wide sample of elpasolites which provide PSD in fundamentally different ways. We herein provide an introduction to elpasolites, then describe the motivation for the work, mechanical and electronic design, and preliminary performance results.

Cs2LiYCl6 : Ce3+ (CLYC) is a promising new inorganic scintillator for gamma-ray spectroscopy and thermal neutron detection with the capability for pulse-shape discrimination (PSD). We verify the scintillation mechanisms responsible for optical emission under gamma-and neutron-induced excitation by fitting decay functions to the waveform structures. Under gamma-excitation, we observe the ultrafast, fast, intermediate, and slow scintillation mechanisms reported in the literature. Thermal neutron waveforms, however, show no evidence of ultrafast decay. We then investigate the thermal dependence of the waveforms in a range from -20 to +50 degrees C. Despite some thermally-variant emission components, we conclude that PSD is feasible at the range of temperatures investigated.

The coupling of Cs2LiYCl6:Ce (CLYC) scintillator to silicon photon converters has been evaluated with the goal of investigating replacements for the traditional photomultiplier tube (PMT) in small handheld spectrometers. Energy spectra produced under irradiation by a range of gamma-ray and neutron sources were collected with CLYC mounted to several avalanche photodiodes, PIN photodiodes, and silicon photomultipliers. The performance for both gamma rays and neutrons was compared to that obtained by coupling CLYC to PMTs. None of the silicon devices evaluated provide comparable performance to that of a PMT with CLYC. This is attributed to the photon-detection efficiency of the silicon detectors over the wavelength range of CLYC emissions, as well as the noise characteristics of the devices.

Cs2LiYCl6:Ce3+ (CLYC) is a promising new inorganic scintillator for gamma-ray spectroscopy and thermal neutron detection with the capability for pulse-shape discrimination (PSD). We verify the scintillation mechanisms responsible for optical emission under gamma- and neutron-induced excitation by fitting decay functions to the waveform structures. Under gamma-excitation, we observe the ultrafast, ...

We have utilized CS 2 LiYCl 6 :Ce 3+ (CLYC) scintillators in a hand-held instrument for radioisotope identification, known as the Advanced Radiation Monitoring Device (ARMD). The CLYC crystals in ARMD are each read out by a PMT and custom electronics designed to exploit CLYC's pulse-shape discrimination (PSD) capabilities. ARMD is designed to function in temperatures ranging from -20 to +500°C. CLYC scintillation emission light yield and pulse shapes are a function of temperature, due to the thermal dependence of the responsible scintillation mechanisms. Additionally, PMT gain and electronics readout also exhibit temperature dependence. Gain stabilization and compensation for varying waveform profiles are therefore necessary for robust isotope identification and PSD. We present the results of a complete thermal cycle over the specified range on an ARMD core detector module and describe our method of gain stabilization and PSD compensation to account for thermallydependent waveform profiles.

Cs2LiYCI6:Ce3+ (CLYC) is a new inorganic scintillator that has recently garnered attention for its ability to detect and discriminate between gammas and thermal neutrons. While scintilla tors are typically coupled to traditional photomultiplier tubes for data acquisition, this setup may not be feasible in all applications. Solid state photomultipliers offer potential advantages including smaller size, added robustness, no high voltage requirement, and imperviousness to magnetic fields. We investigate the waveform structure of CLYC emission when coupled to a SensL silicon photomultiplier and a Hamamatsu multi-pixel photon counter. We report on the ability to provide pulse-shape discrimination for gamma and thermal neutron separation.

Rotating modulation is a technique for indirect imaging in the hard X-ray and soft gamma-ray energy bands, which may offer an advantage over coded aperture imaging at high energies. A rotating modulator (RM) consists of a single mask of co-planar parallel slats typically spaced equidistance apart, suspended above an array of circular non-imaging detectors. The mask rotates, temporally modulating the transmitted image of the object scene. The measured count rate profiles of each detector are folded modulo the mask rotational period, and the object scene is reconstructed using pre-determined characteristic modulation profiles. The use of Monte Carlo simulation to derive the characteristic count rate profiles is accurate but computationally expensive; an analytic approach is preferred for its speed of computation. We present both the standard and a new advanced characteristic formula describing the modulation pattern of the RM; the latter is a more robust description of the instrument response developed as part of the design of a wide-field high-resolution telescope for gamma-ray astronomy. We examine an approximation to the advanced formula to simplify reconstruction software and increase computational speed, and comment on both the inherent limitations and usefulness of the approach. Finally, we show comparisons to the standard formula and demonstrate image reconstructions from Monte Carlo simulations.

A rotating modulator (RM) is capable of imaging hard x-rays and gamma rays by the temporal modulation of incident photons. It combines a single mask of equally-wide slats and slits, rotating above an array of detectors with diameter equal to the slats. Since the RM works in the temporal domain, appropriate analysis of the measured data enables super-resolution (resolution better than the geometric resolution), which consequently allows for larger detector elements, better efficiency at higher energies, and a simplified detector design. We describe the RM concept and discuss briefly a novel image reconstruction technique to achieve super-resolution and suppress fluctuations arising from noise. We present experimental results from a laboratory prototype and the concept and expected results for a high-altitude balloon flight of an RM. The High-Altitude Rotating Modulator for Energetic Radiation Imaging (HARMEnI) is a prototype balloon-borne instrument that offers a 20° field-of-view and 1.9° intrinsic resolution with a sensitivity in the range of 30-700 keV. At its intrinsic resolution, HARMEnI will obtain a 20σ observation of the Crab Nebula and Cygnus X-1 in approximately one hour. For a 1-day flight from Ft. Sumner, New Mexico, exposure times for these sources will exceed 3 hours, enabling enhancement of image resolution.

A Rotating Modulator (RM) is one of a class of techniques for indirect imaging of an object scene by modulation and detection of incident photons. Comparison of the RM to more common imaging techniques, the Rotating Modulation Collimator and the coded aperture, reveals trade-offs in instrument weight and complexity, sensitivity, angular resolution, and image fidelity. In the case of a high-energy (hundreds of keV to MeV), wide field-of-view, satellite or balloon-borne astrophysical survey mission, the RM is shown to be an attractive option when coupled with a reconstruction algorithm that can simultaneously achieve super-resolution and suppress fluctuations arising from statistical noise. We describe the Noise-Compensating Algebraic Reconstruction (NCAR) algorithm, which is shown to perform better than traditional deconvolution techniques for most object scene distributions. Results from Monte Carlo simulations demonstrate that NCAR achieves super-resolution, can resolve multiple point sources and complex distributions, and manifests noise as fuzzy sidelobes about the true source location, rather than spurious peaks elsewhere in the image as seen with other techniques.

A Rotational Modulator (RM) gamma ray imager, consisting of a single grid of lead slats rotating above an array of detectors with diameter equal to the slat spacing, has the capability of providing angular resolution significantly better than the geometric resolution (i.e., the ratio of detector diameter to mask/detector separation). The sensitivity, weight, and angular resolution are comparable to that of a coded aperture device, but with significantly less complexity. As the grid rotates, the transmission from a source is modulated on each detector between 0 and 100%. The count profile is cross-correlated with precalculated modulation profiles to produce an approximate source image. Deconvolution of this image with the known imager response can accurately resolve point sources and complex emissions. The appropriate deconvolution technique can achieve angular resolution better than the basic geometrical resolution of the instrument. A prototype RM developed at Louisiana State University features high sensitivity and energy resolution, functional angular resolution of 15, and a simple readout system. The detector array consists of 19 1.5 × 1 thick cerium-doped lanthanum bromide (LaBr3:Ce) crystals. LaBr3 produces significantly more light than other common scintillators, offering < 3% FWHM energy resolution at 662 keV. A grid spaced ~1.2 m from the detection plane with slat width 1.5 offers a 13.8° field of view. We present our reconstruction technique, deconvolution algorithms, and simulated and experimental imaging results.

A time modulation imaging device uses a periodic structure which translates or rotates above one or few position-insensitive detectors. One common design, a Rotational Modulation Collimator (RMC) uses a bi-grid collimator which rotates above a single detector, and is able to attain very good angular resolution. The two grids cost sensitivity and weight, however, making the RMC unattractive for certain applications. A Rotational Modulator (RM) consists of a single grid of transparent and opaque slats of width a, above an array of several detectors with diameter d, subject to the constraint a = d. The sensitivity, weight, and angular resolution can be comparable to that of a coded aperture device. As the grid rotates, the transmission from a source is modulated on each detector between 0 and 100%. This count profile is cross-correlated with pre-calculated modulation profiles to produce an initial source image. Further processing of the image with a “cleaning” technique that incorporates information from the point-spread function can accurately resolve point sources. In an RMC imager recently constructed at LSU, LaBr3:Ce detectors are used, which produce significantly more light than other common scintillators, offering ≪ 3% FWHM energy resolution at 662 keV. The instrument features high sensitivity and energy resolution, angular resolution of 0.8° (1σ), and a simple readout system. The detector array consists of 19 1:5″×1″ thick LaBr3:Ce detectors in a concentric circular layout. A grid spaced ∼1.2 m from the detection plane with slat width 1:5″ offers a field of view radius of 6:9°. We present our reconstruction technique, cleaning algorithms, and imaging results for the RM prototype.

A rotational modulator (RM) gamma-ray imager is capable of obtaining significantly better angular resolution than the fundamental geometric resolution defined by the ratio of detector diameter to mask-detector separation. An RM imager consisting of a single grid of absorbing slats rotating ahead of an array of a small number of position-insensitive detectors has the advantage of fewer detector elements (i.e., detector plane pixels) than required by a coded aperture imaging system with comparable angular resolution. The RM therefore offers the possibility of a major reduction in instrument complexity, cost, and power. A novel image reconstruction technique makes it possible to deconvolve the raw images, remove sidelobes, reduce the effects of noise, and provide resolving power a factor of 6 - 8 times better than the geometric resolution. A 19-channel prototype RM developed in our laboratory at Louisiana State University features 13.8 deg full-angle field of view, 1.9 deg geometric angular resolution, and the capability of resolving sources to within 35' separation. We describe the technique, demonstrate the measured performance of the prototype instrument, and describe the prospects for applying the technique to either a high-sensitivity standoff gamma-ray imaging detector or a satellite- or balloon-borne gamma-ray astronomy telescope.

Cerium-doped lanthanum bromide is an attractive scintillator for use in hard x-ray/low energy gamma ray astronomy instruments. Its high light output and correspondingly excellent energy resolution, fast decay time, and relatively high density offer advantages, but the relatively high internal background due to the presence of the naturally occurring radioactive isotope 138 La is a disadvantage. It is desirable to expose LaBr 3 to a near-space environment in order to investigate the effects of space exposure on the internal background of the crystal. We have flown an unshielded 1 inch diameter cylindrical LaBr 3 and an identical NaI(Tl) scintillator on the ATIC long duration balloon flight from Antarctica for a total of 16 days exposure. We present results from this test, including the background rates at float altitude and changes in the measured spectra over time. No indication is seen of any increase in rate due to activation. The results indicate that LaBr 3 appears to be satisfactory as a balloon- or satellite-borne x-ray/gamma ray detector.

A gamma-ray imager based on rotational modulation is capable of obtaining significantly better angular resolution than the fundamental geometric resolution defined by the ratio of detector diameter to mask-detector separation. Compared to a standard coded aperture or Compton telescope, a Rotational Modulator (RM) requires a detection plane with only modest spatial resolution, and is consequently less complex. A prototype of this concept developed at LSU features high sensitivity and energy resolution, and a simple readout system. The instrument consists of a single grid of 1.5" wide lead slats separated by 1.5" and spaced 1.2 m in front of an array of 19 1.5" × 1" thick LaBr 3 :Ce scintillators in a concentric circular layout. As the grid rotates, the transmission from a source is modulated on each detector between 0 and 100%. This count profile is cross-correlated with pre-calculated modulation profiles to produce an initial source image. A novel reconstruction technique deconvolves this image with the point-spread function to reduce the effects of noise and resolve sources to within 20', or about 6x better than the geometric resolution of the instrument. We describe this reconstruction technique, and present imaging and spectral results for the RM prototype.

Astrophysical X-ray/gamma-ray telescopes and standoff detectors for national security applications may require detector areas on the order of a meter squared or more. A fine grained meter squared scale scintillator with mm-scale resolution may require ~106 pixels and electronics channels. In many applications, this can be prohibitive. Since LaBr3:Ce produces significantly more scintillation light (63 photons/keV) than other materials, it offers the possibility of a crossed optical fiber readout approach needing only a few thousand channels. A layer of 2 mm square, double clad waveshifting fibers can be laid in the x-direction across the top of a LaBr3 detection plane (separated from the LaBr3 by a thin glass seal that provides a moisture barrier), with a second layer of fibers in the y-direction laid across the bottom. With an absorption peak matched to the emission of LaBr3, the fibers will absorb the scintillation light and re-emit it isotropically, a portion of which will be trapped in the fiber and propagated down the fiber axis, where the fiber ends are viewed by 64-channel MAPMTs. The crossed fiber layers are intended to measure x- and y-position only. Since only a small fraction of the light is trapped in the fibers, the energy is measured in nine large "energy measuring" PMTs viewing the scintillator through the bottom fiber layer. A lab-scale crossed-fiber LaBr3:Ce imager has been constructed as a protype for this concept. Results of measurements of energy, position, and angular resolution are given. The use of pre-processed Detector Pixel Source Image (DPSI) files, which provide a significant speedup in the computationally intensive reconstruction process, is explained in detail.