The Lunar Vulkan Imaging and Spectroscopy Explorer (Lunar-VISE) mission consists of a suite of instruments to explore the Gruithuisen Domes located in the northern lunar hemisphere. One of instruments to be mounted on a rover is a gamma ray and neutron spectrometer. The spectrometer uses a Li-6 enriched Cs26LiYCl6 (CLYC) scintillator to detect both gamma rays and neutrons. The design of the instrument balances mass, power and performance to obtain high-quality gamma ray spectroscopy with a 6.4-cm diameter, CLYC crystal encased in a gadolinium shell to veto thermal neutrons. To obtain a measure of the thermal neutron albedo to assess the concentration of local neutron moderating materials, a 5.0-cm diameter CLYC without a gadolinium shell is used to measure the full spectral, neutron flux. To minimize power, the system design utilizes a multi-channel configuration to provide pulse shape analysis and spectroscopy using a single, readout channel.
We present a preliminary design for a neutron detection system that is compact, lightweight, and low power consuming, utilizing the CubeSat platform making it suitable for space-based applications. This is made possible using the scintillating crystal lithium indium diselenide (LiInSe26), the first crystal to include Li6 in the crystalline structure, and a silicon avalanche photodiode. The schematics of this instrument are presented as well as the response of the instrument to initial testing under alpha radiation. A principal aim of this work is to demonstrate the feasibility of such a neutron detection system within a CubeSat platform. The entire end-to-end system presented here is 10×10×15 cm3, weighs 670 g, and requires 5 V direct current at 3 W.
Gamma rays produced passively by cosmic ray interactions and by the decay of radioelements convey information about the elemental makeup of planetary surfaces and atmospheres. Orbital missions mapped the composition of the Moon, Mars, Mercury, Vesta, and now Ceres. Active neutron interrogation will enable and/or enhance in situ measurements (rovers, landers, and sondes). Elemental measurements support planetary science objectives as well as resource utilization and planetary defense initiatives. Strontium iodide, an ultra-bright scintillator with low nonproportionality, offers significantly better energy resolution than most previously flown scintillators, enabling improved accuracy for identification and quantification of key elements. Lanthanum bromide achieves similar resolution; however, radiolanthanum emissions obscure planetary gamma rays from radioelements K, Th, and U. The response of silicon-based optical sensors optimally overlaps the emission spectrum of strontium iodide, enabling the development of compact, low-power sensors required for space applications, including burgeoning microsatellite programs. While crystals of the size needed for planetary measurements (>100 cm3) are on the way, pulse-shape corrections to account for variations in absorption/re-emission of light are needed to achieve maximum resolution. Additional challenges for implementation of large-volume detectors include optimization of light collection using silicon-based sensors and assessment of radiation damage effects and energetic-particle induced backgrounds. Using laboratory experiments, archived planetary data, and modeling, we evaluate the performance of strontium iodide for future missions to small bodies (asteroids and comets) and surfaces of the Moon and Venus. We report progress on instrument design and preliminary assessment of radiation damage effects in comparison to technology with flight heritage.
The detection of illicit nuclear sources and SNM requires an ubiquitous network of sensors. While 3He proportional
counters are excellent neutron detectors, there is an insufficient global supply of 3He to create the required number of
detectors. Alternatives to 3He must be efficient, insensitive to gamma radiation, easily manufactured, rugged, and
inexpensive to enable the procurement of a large numbers of sensors. The use of a high sensitivity solid-state optical
detector coupled to scintillation materials, loaded with a neutron absorber such as 6Li or 10B, can fulfill these design
constraints. In this work, we compare the properties of neutron-sensitive scintillation materials utilizing Monte Carlo
simulations and experiments. Cs2LiLaBr6:Ce is compared to commercially available boron-loaded plastic scintillators
and 3He tubes. The scintillators are compared for neutron detection efficiency, limitations on size, gamma-rejection
ratio, neutron detection limits, manufacturing cost, and availability for mass-production.
CMOS solid-state photomultipliers (CMOS-SSPM) are new, potentially very inexpensive, photodetectors that have
the promise of supplanting photomultiplier tubes and standard photodiodes for many nuclear radiation detection
measurements using scintillator crystals. The compact size and very high gain make SSPMs attractive for use in
applications where photomultiplier tubes cannot be used and standard photodiodes have insufficient sensitivity. In this
effort, the use of SSPMs was investigated for the detection of neutrons with the goal of designing a detector for portable
systems that has the capability of discriminating neutrons from gamma rays.
The neutron scintillation signatures were measured using boron-loaded plastic scintillators. Our detector concept
design incorporates a dual-scintillator design with both a neutrons sensitive organic scintillator (a boron-loaded gel) and
a gamma ray sensitive inorganic scintillator (LYSO). Using this design, the gamma ray signal is suppressed and the
neutron events are clearly resolved. The design was modeled to optimize the detection efficiency for both thermal and
energetic neutrons. In addition, the detection of thermal neutrons in the presence of gamma rays was examined using the
SSPM coupled to Cs2LiYCl6:Ce scintillator (CLYC).
Thomas Prettyman, William Feldman, Bruce Barraclough, Maria Teresa Capria, Angioletta Coradini, D. Enemark, Kenneth Fuller, David Lawrence, D. Patrick, Carol Raymond, S. Storms, R. Williford
Dawn is a NASA discovery mission that will explore the main belt asteroids (1) Ceres and (4) Vesta. Ceres and Vesta are among the oldest bodies in the solar system and represent very different evolutionary paths. By studying these ancient, complementary asteroids, we will answer fundamental questions about the early solar system and planetary formation processes. The Dawn payload consists of a Framing Camera (FC), a visual and infrared mapping spectrometer (VIR), and a Gamma Ray and Neutron Detector (GRaND). The instruments provide data needed to investigate the structure, geology, mineralogy, and geochemistry of the asteroids. GRaND provides the data for the geochemistry investigation, including maps of most major elements and selected radioactive and trace elements. An updated description of the GRaND instrument is given along with the expected performance of GRaND at Vesta and Ceres. Approaches to combine data from FC, VIR and GRaND are discussed.
Galactic cosmic rays (GCR) constantly impinge all planetary bodies and produce characteristic gamma-ray lines and leakage neutrons as reaction products. Together with gamma-ray lines produced by radioactive decay, these nuclear emissions provide a powerful tool for remotely measuring the chemical composition of planetary surfaces having little or no atmospheres. While lunar gamma-ray spectroscopy was first demonstrated with Apollo Gamma-Ray measurements, the full value of combined gamma-ray and neutron spectroscopy was shown for the first time with the Lunar Prospector Gamma-Ray (LP-GRS) and Neutron Spectrometers (LP-NS). Any new planetary mission will likely have the requirement that instrument mass and power be kept to a minimum. To satisfy such requirements, we have been designing a GR/NS instrument which combines all the functionality of the LP-GRS and LP-NS for a fraction of the mass and power. Specifically, our design uses a BGO scintillator crystal to measure gamma-rays from 0.5 - 10 MeV. A borated plastic scintillator and a lithium glass scintillator are used to separately measure thermal, epithermal, and fast neutrons as well as serve as an anticoincidence shield for the BGO. All three scintillators are packaged together in a compact phoswich design. Modifications to this design could include a CdZnTe gamma-ray detector for enhanced energy resolution at low energies (0.5 - 3 MeV). While care needs to be taken to ensure that an adequate count rate and background suppression is achieved for specific mission designs, previous mission successes demonstrate that a combined GR/NS provides essential information about planetary surfaces.
Surface processing plays a major role in manufacturing CdZnTe semiconductor devices used for radiation detection. We are conducting a thorough, systematic study of surfaces and contacts and their effect on charge transport and signal formation in CdZnTe devices. We are investigating wet chemical processing techniques as well as treatment of surfaces with energetic neutral atoms. Our goal is to develop and implement improved surface treatment methods and device manufacturing techniques for large-volume CdZnTe detectors. In addition, we will determine how surfaces and electrical contacts affect the performance of CdZnTe devices used for radiation detection. In this paper, we will show how surface electronic properties influence carrier transport and signal formation in devices designed to simulate coplanar grid detectors. By altering the surface using a wet chemical process, we will show that charge collection is significantly effected by the conductivity of the surface.
KEYWORDS: Sensors, Gamma radiation, Electronics, Detector arrays, Spectroscopy, Data acquisition, Energy efficiency, Manufacturing, Monte Carlo methods, Fiber amplifiers
We present results of experiments to characterize two large- volume, multi-element CdZnTe detectors for gamma-ray spectroscopy at high energy. The first detector consisted of four 1.5cm x 1.5cm x 0.75cm coplanar grid detectors. The measurements for the four-element design were performed with various configurations. The second detector consisted of eight 1 cm x 1 cm x 0.5 cm coplanar grid detectors arranged in a 2x2x3 array. The high-energy gamma-ray sources included 60Co(1332), 228Th(2614), 244Cm/13C(6129), and Fe(n,(gamma) )(7645). The front end-electronics consisted of eight spectroscopy-grade preamplifiers/shapes/pulse stretchers, built on circuit boards close to the arrays. For the four-element measurements the shapers/pulse stretchers were replaced with commercial amplifiers. An eight-channel data acquisition system with list mode output was used to record gamma-ray events for each detector element in each array. The list mode data were analyzed to produce coincidence and single spectra and efficiencies for the various sources. The Compton continuum and the escape peaks are suppressed in the coincidence spectra relative to the singles spectra. We compare these spectra and efficiencies at high energy to results at lower energies and to Monte Carlo predictions.
In this paper, we present results of experiments to characterize a large-volume (4 cm3), multi-element CdZnTe detector for high-efficiency, gamma-ray spectroscopy. The module includes an array of eight 0.5 cm3 coplanar grid detectors manufactured by eV Products. An eight-channel data acquisition system with list mode output is used to record gamma-ray events for each detector in the array. The list mode data are analyzed to determine the efficiency for coincidence events and to demonstrate different modes of operation (e.g., Compton suppression). The total efficiency of the array is found to match Monte Carlo calculations to within a few percent; however, the full-energy (photopeak) efficiency is significantly lower than predicted by Monte Carlo. The observed difference is probably caused by a combination of electrode design and charge transport properties. Approaches to improve full-energy efficiency are proposed.
Preliminary results of experiments to investigate charge collection in CdZnTe detectors are presented. The experiments support the development of semiconductor- modeling tool for device engineering that will be used to design large volume CdZnTe detectors for gamma ray spectroscopy. Improved diagnostic methods are described, including an automated alpha particle scanner for charge pulse mapping. Semiconductor modeling techniques are presented along with methods to visualize charge transport. Experimental results are compared to a physical model that has been used routinely in research on room temperature devices for gamma ray detection.
Single-element CdZnTe detectors are limited in size, and therefore efficiency, by the poor hole transport, even with a coplanar grid. We are investigating the possibility of a 27-element array using 15 mm X 15 mm X 15 mm elements for gamma-ray energies to 10 MeV for NASA planetary missions. We present experimental results for combinations of various size coplanar grid detectors using NIM electronics and energies to 6.1 MeV. Summation of the signals after linear gating and requiring coincidence produces only a small increase in the energy resolution. Our results indicate that good efficiency and spectrum not complicated by a large Compton continuum can be achieved by simply summing the spectra from 15 X 15 X 15 mm3 detectors for gamma-ray energies below about 2 MeV. Above 2 MeV, 2-fold coincidence might be required, depending on the spectrum, to suppress the Compton continuum and escape peaks. We use a Monte Carlo calculation to predict the performance of the 27-elements array for a lunar highlands spectrum. Such ambient-temperature, high-efficiency, good- resolution arrays will facilitate new NASA mission to determine elemental composition of planetary bodies and terrestrial applications requiring high-efficiency, good- resolution portable instruments.
This paper describes the development of cylindrical coplanar grid CdZnTe detectors for gamma ray spectroscopy. Cylindrical detector offer a number of advantages over established designs. For example, grid structures for cylindrical detectors are simpler than those for rectangular designs. The goal of our work is to design a cylindrical coplanar grid detector with excellent resolution at low- and high-energy. Information on detector design and manufacturing is presented. Six detectors are characterized. The pulse height resolution of the best detector is 13.5 keV full width at half maximum at 662 keV and 5.5 keV FWHM at 122 keV.
A prototype Compton camera using ambient-temperature semiconductor detectors is developed for gamma ray spectroscopic imaging. Two camera configurations are evaluated, one using an intrinsic silicon detector for the front plane detector and the other using a CdZnTe detector for the front plane. Both configurations use a large-volume coplanar grid CdZnTe detector for the back plane. The effect of detector noise, energy resolution, and timing resolution on camera performance is described. Technical issues underlying the development of Compton cameras for spectroscopic imaging are presented and imaging of radioactive sources is demonstrated.
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