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The LargE Area burst Polarimeter (LEAP) is one of two NASA Missions of Opportunity proposals that are currently in a Phase A Concept Study, with a final selection due later this year. It is a wide Field of View (FoV) Compton polarimeter designed to study Gamma-Ray Burst (GRB) polarization over the energy range from 50- 500 keV and to measure GRB spectra in the range from 20 keV - 5 MeV. During the Phase A Concept Study, lab measurements were conducted with a small-scale (5x5) prototype polarimeter module. This included both spectral and polarization measurements with laboratory calibration sources. Here the prototype measurements and the comparisons made with simulations of the prototype detector are described. These results demonstrate the basic functionality of the LEAP design.

Observing cosmic sources in the medium-energy gamma-ray regime (~0.4 - 10 MeV) requires highly efficient instruments with high angular resolution and robust background rejection. Artificial single-crystal diamond detectors (SCDDs) are comparable to traditional silicon solid-state detectors (SSDs) in terms of energy range, energy resolution, and threshold levels. However, they exceed SSD performance with faster rise times, improved radiation hardness, and insensitivity to light and temperature. CeBr3 scintillator is a high density, high Z material with fast rise times and good energy resolution ( 4% FWHM at 662 keV) make it a promising gammaray calorimeter. Here, we outline ongoing work by Southwest Research Institute (SwRI) to develop readout and data acquisition electronics to characterize SCDDs. Additional work is ongoing at Los Alamos National Laboratory to characterize CeBr3 scintillator detectors that are read out with silicon photomultipliers (SiPMs). Currently, an off the shelf ASIC system from PETsys Electronics (TOFPET2 ASIC),1 developed for time-of-flight (ToF) positron emission tomography (PET), is used to record the CeBr3 data. After characterization of the CeBr3 and SCDDs, we plan to bring them together to form a prototype Compton telescope. Performance of the prototype will benchmark simulations of a functional Compton Telescope to predict the sensitivity of an optimized instrument for a satellite platform.

The LargE Area Burst Polarimeter (LEAP) will radically improve our understanding of some of the most energetic phenomena in our Universe by exposing the underlying physics that governs astrophysical jets and the extreme environment surrounding newborn compact objects. LEAP will do this by making the highest fidelity polarization measurements to date of the prompt gamma-ray emission from a large sample of Gamma-Ray Bursts (GRBs). The science objectives are met with a single instrument deployed as an external payload on the ISS – a wide FOV Compton polarimeter that measures GRB polarization from 50–500 keV and GRB spectra from ~10 keV to 5 MeV. LEAP measures polarization using seven independent polarimeter modules, each with a 12x12 array of optically isolated high-Z and low-Z scintillation detectors readout by individual PMTs. LEAP is one of two NASA Missions of Opportunity proposals that are currently in a Phase A Concept Study, with a final selection due later this year.

The Advanced Scintillator Compton Telescope (ASCOT) is a medium-energy gamma-ray Compton telescope flown on NASA’s high-altitude scientific balloon from Palestine, TX on 5th July 2018. It uses commercially available highperformance scintillators like Cerium Bromide (CeBr3) and p-terphenyl along with compact readout devices - silicon photomultipliers (SiPMs) - for an improved instrument response. ASCOT was built to address the existing need for observations in the gamma-ray energy range of 0.4 - 20 MeV. Operating stably throughout the mission, it reached an altitude of 120,000 ft and observed the Crab Nebula at MeV energies for ~5 hours. Built on the legacy of COMPTEL (onboard CGRO), along with the hardware advancement ASCOT also makes use of the Time-of-Flight (ToF) background rejection technique for effective imaging. Presented here is the Energy and ToF calibrated flight data with optimal data cuts (Earth Horizon Cut, Pulse Shape Discrimination Cut). The growth curves generated using this data from 5 to 100 g/cm2 of residual atmosphere in conjunction with the Monte Carlo simulations of the instrument response have been used to obtain the Cosmic Diffuse Gamma-ray (CDG) flux value of (1.28±0.37)×10-5 photons/cm2 /s/sr/keV for 0.4 – 0.7 MeV energy range. The 3σ upper limit for CDG flux is 1.8×10-5 photons/cm2 /s/sr/keV for 0.7-1.5 MeV and 2×10-6 photons/cm2 /s/sr/keV for 1.5-2.5 MeV. The analysis of the Crab Nebula from flight observation is underway.

MeV astronomy provides unique and direct diagnostics of cosmic explosions and chemical evolution. Gammaray lines provide detailed information on nuclear physics processes throughout the universe. New instruments sensitive to MeV gamma-ray lines are needed. A new instrument concept with high energy and angular resolution is proposed to study nuclear astrophysics in the medium gamma-ray energy range. The system consists of a 12U CubeSat carrying onboard CdZnTe detectors that will be placed in orbit around the moon. The Lunar Occultation Technique will be used to analyze the changes in counting rate during occultations of the moon to pinpoint the location of gamma-ray line flux at precise locations in the sky. The lack of lunar atmosphere, stable background, and low-mass spacecraft provides the perfect environment for high sensitivity gamma-ray spectroscopy and good angular resolution. Simulation studies using MCNP6 were used to determine the expected instrument eciency and lunar gamma-ray background environment. An initial simulation study indicates that the proposed instrument can achieve sensitivity to gamma-ray lines comparable to much larger proposed Compton Telescopes.

Observing cosmic sources in the medium-energy gamma-ray regime (∼0.4 - 10 MeV) will require a highly efficient instrument with good angular resolution and background rejection. Artificial single-crystal diamond detectors (SCDDs) have comparable energy ranges, energy resolution, and threshold levels as traditional silicon solidstate detectors (SSDs), but with faster rise times, improved radiation hardness, and insensitivity to light and temperature. CeBr3 scintillator is a high density, high Z material with fast rise times and good energy resolution make it a promising gamma-ray calorimeter. This work outlines ongoing work at Southwest Research Institute (SwRI) to develop readout and data acquisition electronics to characterize SCDDs. Additionally, work is ongoing at Los Alamos National Laboratory to characterize CeBr3 scintillator detectors that are read out with silicon photomultipliers (SiPMs) and recorded via an off the shelf ASIC system (TOFPET ASIC), developed for timeof-flight (ToF) positron emission tomography. The ultimate goal of the project is to individually characterize SCDDs and CeBr3 and bring them together to form a prototype Compton telescope, which will benchmark simulations of a functional diamond Compton telescope and predict the sensitivity of an optimized instrument for a satellite platform.

The origin of the cosmic diffuse gamma-ray (CDG) background in the 0.3 – 30 MeV energy range is a mystery that has persisted for over 40 years. The Mini Astrophysical MeV Background Observatory (MAMBO) is a CubeSat mission concept motivated by the fact that, since the MeV CDG is relatively bright, only a small detector is required to make highquality measurements of it. Indeed, the sensitivity of space-based gamma-ray instruments to the CDG is limited not by size, but by the locally generated instrumental background produced by interactions of energetic particles in spacecraft materials. Comparatively tiny CubeSat platforms provide a uniquely quiet environment relative to previous gamma-ray science missions. The MAMBO mission will provide the best measurements ever made of the MeV CDG spectrum and angular distribution, utilizing two key innovations: 1) low instrumental background on a 12U CubeSat platform; and 2) an innovative shielded spectrometer design that simultaneously measures signal and background. We describe the MAMBO instrument and mission concept in detail, including simulations and laboratory measurements demonstrating the key measurement concept.

With the help of a new observing technique, we have detected diurnal variations in both lunar albedo protons and incident galactic cosmic rays (GCRs) at the Moon. Using the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on the Lunar Reconnaissance Orbiter (LRO), we have combined specific, targeted observations of the lunar horizon with our nominal nadir-viewing data to find a tentative enhancement in the yield of lunar albedo protons at the local sunrise terminator of the Moon relative to the local sunset terminator. A diurnal variation in hydrogenation in the top few cm of regolith is one possible interpretation of this result. We have also measured an unanticipated AM/PM difference in the arriving GCRs at the Moon which can be explained by a known streaming anisotropy. The relatively small data set used here (compared to previous mapping studies of CRaTER data) and heretofore untried horizon observations required us to develop improved data analysis techniques, which we will use in the future to revisit previous data sets and to analyze planned new horizon observations of lunar albedo protons.

A medium-energy gamma-ray Compton telescope called the Advanced Scintillator Compton Telescope (ASCOT) was designed to address the existing need for observations in the gamma-ray energy range of 0.4 - 20 MeV. Built on the legacy of COMPTEL instrument onboard NASA’s CGRO, ASCOT uses commercially available high-performance scintillators, such as Cerium Bromide (CeBr 3 ) and p-terphenyl in conjunction with Silicon Photomultipliers (SiPM) as compact readout devices to improve the instrument response. ASCOT also makes use of the Time-of-Flight background rejection technique along with the hardware advancement, an important tool for effective imaging in this energy range. ASCOT was developed with the goal of imaging the Crab Nebula at MeV energies during a high-altitude balloon flight. The instrument was successfully launched by NASA from Palestine (TX) on 5th July 2018. It operated stably and observed the Crab for ~5 hours from an altitude of 120,000 ft. Based on pre-flight calibrations and simulations results we expect a ~4.5 sigma detection of the Crab in the 0.2 - 2 MeV band. We present here the calibrated flight data along with preliminary results. The findings from ASCOT will demonstrate an improvement in the energy, timing, and position resolution using this advanced technology.

We present our work to demonstrate the suitability of artificial single-crystal diamond detectors (SCDDs) for use as the scattering medium in Compton telescopes for medium-energy gamma-ray astronomy. SCDDs offer the possibility of position and energy resolution comparable to those of silicon solid-state detectors (SSDs), combined with efficiency and timing resolution so-far only achievable using fast scintillators. It has been shown that SCDDs fabricated using chemical vapor deposition (CVD) show good energy resolution (~7 keV FWHM), comparable to SSDs, with much faster time response (~ns rise time) due to higher electron/hole mobilities. They are also temperature- and light-insensitive, and radiation hard. In addition, diamond is lowZ, composed entirely of carbon, but relatively high-density (3.5 g cm-3) compared to silicon or organic scintillator. SCDDs are therefore an intriguing possibility for a new Compton scattering element: if patterned with ~mm-sized readout electrodes and combined with a calorimeter composed of fast inorganic scintillator, such as CeBr 3 , read out by silicon photomultipliers (SiPMs), SCDDs will enable a compact and efficient Compton telescope using time-of-flight (ToF) discrimination to achieve low background. Such an instrument offers the exciting potential for unprecedented sensitivity, especially at energies < 1-2 MeV, on a small-scale mission utilizing recently available SmallSat buses (payload mass < 100 kg). We present the status of our laboratory development effort to design, fabricate, and test a small prototype Compton telescope.

The origin of Extragalactic Background (EGB) light in the 300 keV - 30 MeV energy range is an unsolved question that has persisted for more than 40 years. MAMBO (Mini Astrophysical MeV Background Observatory) is a CubeSat mission concept motivated by the realization that an instrument purpose-built for measuring the MeV EGB from a CubeSat can provide quality measurements that exceed the best that are currently available. There are two key reasons for this: (1) the EGB is so bright at MeV energies that it only requires an instrument with a small effective area and (2) the best existing measurements are compromised by instrumental background, which scales with the mass of the spacecraft/instrument. Here, small is beautiful. We present a design for a gamma-ray spectrometer capable of both providing quality measurements of the spectrum and angular distribution of the MeV EGB and meeting the tight Size, Weight, and Power (SWaP) constraints imposed by deployment on a 6U CubeSat. During a two year baseline mission, MAMBO will be able to collect measurements of the MeV EGB background that are superior to the best currently available and place important constraints on MeV EGB origin.

The mergers of compact binaries with at least one neutron star component are the potential leading sites of the production and ejection of $r$-process elements. Discoveries of galactic binary pulsars, short gamma-ray bursts, and gravitational-wave detections have all been constraining the rate of these events, while the gravitational wave plus broadband electromagnetic coverage of binary neutron star merger (GW170817) has also placed constraints on the properties (mass and composition) of the merger ejecta. But uncertainties and ambiguities in modeling the optical and infrared emission make it difficult to definitively measure the distribution of heavy isotopes in these mergers. In contrast, gamma rays emitted in the decay of these neutron-rich ejecta may provide a more direct measurement of the yields. We calculate the gamma production in remnants of neutron star mergers, considering two epochs: a kilonova epoch, lasting about two weeks, and a much later epoch of tens and hundreds of thousands of years after the merger. For the kilonova epoch, when the expanding ejecta is still only partially transparent to gamma radiation, we use 3D radiative transport simulations to produce the spectra. We show that the gamma-ray spectra associated with beta- and alpha-decay provide a fingerprint of the ejecta properties and, for a sufficiently nearby remnant, may be detectable, even for old remnants. We compare our gamma spectra with the potential detection limits of next generation detectors, including the Lunar Occultation Explorer (LOX), the All-sky Medium Energy Gamma-ray Observatory (AMEGO), and the Compton Spectrometer and Imager (COSI). We show that fission models can be discriminated via the presence of short-lived fission fragments in the remnant spectra., Comment: 14 pages, 11 figures

We present the results of the first high-altitude balloon flight test of a concept for an advanced Compton telescope making use of modern scintillator materials with silicon photomultiplier (SiPM) readouts. There is a need in the fields of high-energy astronomy and solar physics for new medium-energy gamma-ray (~0.4 - 10 MeV) detectors capable of making sensitive observations. A fast scintillator- based Compton telescope with SiPM readouts is a promising solution to this instrumentation challenge, since the fast response of the scintillators permits the rejection of background via time-of-flight (ToF) discrimination. The Solar Compton Telescope (SolCompT) prototype was designed to demonstrate stable performance of this technology under balloon-flight conditions. The SolCompT instrument was a simple two-element Compton telescope, consisting of an approximately one-inch cylindrical stilbene crystal for a scattering detector and a one-inch cubic LaBr3:Ce crystal for a calorimeter detector. Both scintillator detectors were read out by 2 x 2 arrays of Hamamatsu S11828-3344 MPPC devices. Custom front-end electronics provided optimum signal rise time and linearity, and custom power supplies automatically adjusted the SiPM bias voltage to compensate for temperature-induced gain variations. A tagged calibration source, consisting of ~240 nCi of Co-60 embedded in plastic scintillator, was placed in the field of view and provided a known source of gamma rays to measure in flight. The SolCompT balloon payload was launched on 24 August 2014 from Fort Sumner, NM, and spent ~3.75 hours at a float altitude of ~123,000 feet. The instrument performed well throughout the flight. After correcting for small (~10%) residual gain variations, we measured an in-flight ToF resolution of ~760 ps (FWHM). Advanced scintillators with SiPM readouts continue to show great promise for future gamma-ray instruments., Comment: 39 pages, 25 figures, to appear in NIM-A

The soft gamma-ray concentrator is a telescope mission concept utilizing a suitable arrangement of bent multilayer structures of alternating low- and high-density materials. This lens is able to channel gamma-ray photons via total external reflection and concentrate the incident radiation to a point. The channeling technique offers the potential for concentrating gamma rays with focal lengths 100 keV, beyond the reach of current grazing-incidence hard x-ray mirrors. For the performance estimation of such an instrument, we have developed a flexible set of computer modeling tools to compute the optical properties of multilayer structures, predict the channeling efficiency for a given multilayer configuration, and aid in the optimization of potential gamma-ray concentrator-based telescope designs. This modeling includes the multilayer optical properties calculated by the IMD software, the ray tracing using an IDL code, and the focal plane detector simulation by MEGAlib. We illustrate the potential of this approach by presenting simulated astronomical observations from a balloon-borne platform. The final result, including simulated effective area, instrument sensitivity, and polarization performance, shows that the gamma-ray concentrator will provide greatly increased sensitivity for next-generation soft gamma-ray missions with modest cost and complexity.

The Gamma Ray Polarimeter Experiment (GRAPE) is designed to investigate gamma-ray bursts (GRB) in the important energy range of 50-500 keV. Our eventual goal is to fly GRAPE on a long duration balloon (LDB) platform to collect data on a significant sample of GRBs. Our experience with two balloon flights (in 2011 and 2014), coupled with further design efforts focused on orbital payloads, has led to an improved polarimeter concept that represents a natural evolution of the current design. The new concept employs a large number of small (2 cm3 ), optically-isolated scintillator cubes, each of which is read out by its own silicon photomultiplier (SiPM). These cubes are stacked in an arrangement that allows the determination of event interaction locations in three dimensions. The resulting three-dimensional location data provides a moderate level of Compton imaging capability (1σ angular resolution of 10-15). This level of imaging can be used to significantly reduce the instrumental background by limiting the impact of the cosmic diffuse flux, dramatically improving the polarization sensitivity. Here we shall describe this concept, some results from initial laboratory studies, and the expected performance parameters. We are currently working to optimize this design in preparation for a prototype balloon flight in the summer of 2020. Our long-term goal (pending acquisition of continued funding) is to fly a prototype balloon payload in the summer of 2020 and to be prepared for a first long duration balloon (LDB) flight at the end of 2021.

We have investigated the use of multilayer thin film structures for channeling and concentrating soft gamma rays with energies greater than 100 keV, beyond the reach of current grazing-incidence hard X-ray mirrors. A suitable arrangement of bent multilayer structures of alternating low and high-density materials will channel soft gamma-ray photons via total external reflection and then concentrate the incident radiation to a point. We describe the properties of W/Si multilayer structure produced by magnetron sputter technique with the required thicknesses and smoothness. We also have developed a flexible set of computer modeling tools to compute the optical properties of multilayer structures, predict the channeling efficiency for a given multilayer configuration and aid in the optimization of potential gamma-ray concentrator-based telescope designs. This modeling includes multilayer optical properties calculated by the IMD software, IDL gamma ray tracing code and a focal plane detector simulation by MEGAlib. This technology offers the potential for soft gamma-ray telescopes with focal lengths of less than 10 m, removing the need of formation flying spacecraft and providing greatly increased sensitivity for modest cost and complexity and opening the field up to balloon-borne instruments.

We describe our ongoing work to develop a new medium-energy gamma-ray Compton telescope using advanced scintillator materials combined with silicon photomultiplier readouts and fly it on a scientific balloon. There is a need in high-energy astronomy for a medium-energy gamma-ray mission covering the energy range from approximately 0.4 - 20 MeV to follow the success of the COMPTEL instrument on CGRO. We believe that directly building on the legacy of COMPTEL, using relatively robust, low-cost, off-the-shelf technologies, is the most promising path for such a mission. Fortunately, high-performance scintillators, such as Cerium Bromide (CeBr3) and p-terphenyl, and compact readout devices, such as silicon photomultipliers (SiPMs), are already commercially available and capable of meeting this need. We are now constructing an Advanced Scintillator Compton Telescope (ASCOT) with SiPM readout, with the goal of imaging the Crab Nebula at MeV energies from a high-altitude balloon flight. We expect a ~4-sigma detection at ~1 MeV in a single transit. We present calibration results of the detector modules, and updated simulations of the balloon instrument sensitivity. If successful, this project will demonstrate that the energy, timing, and position resolution of this technology are sufficient to achieve an order of magnitude improvement in sensitivity in the medium-energy gamma-ray band, were it to be applied to a ~1 cubic meter instrument on a long-duration balloon or Explorer platform.

We are reporting the investigation result of using multilayer thin film structures for channeling and concentrating soft gamma rays with energies greater than 100 keV, beyond the reach of current grazing-incidence hard X-ray mirrors. This will enable future telescopes for higher energies with same mission parameters already proven by NuSTAR. A suitable arrangement of bent multilayer structures of alternating low and high-density materials will channel soft gamma-ray photons via total external reflection and then concentrate the incident radiation to a point. We present the latest results of producing Ir/Si and W/Si multilayers with the required thicknesses and smoothness by using magnetron sputter technique. In addition to experimental works, we have been working on gamma-ray tracking model of the concentrator by IDL, making use of optical properties calculated by the IMD software. This modeling allows us to calculate efficiency and track photon for different energy bands and materials and compare them with experimental result. Also, we describe combine concentrator modeling result and detector simulation by MEGAlib to archive a complete package of gamma-ray telescope simulation. This technology offers the potential for soft gamma-ray telescopes with focal lengths of less than 10 m, removing the need for formation flying spacecraft and providing greatly increased sensitivity for modest cost and complexity and opening the field up to balloon-borne instruments.

Space-based gamma-ray detectors for high-energy astronomy and solar physics face severe constraints on mass, volume, and power, and must endure harsh launch conditions and operating environments. Historically, such instruments have usually been based on scintillator materials due to their relatively low cost, inherent ruggedness, high stopping power, and radiation hardness. New scintillator materials, such as LaBr 3 :Ce, feature improved energy and timing performance, making them attractive for future astronomy and solar physics space missions in an era of tightly constrained budgets. Despite this promise, the use of scintillators in space remains constrained by the volume, mass, power, and fragility of the associated light readout device, typically a vacuum photomultiplier tube (PMT). In recent years, silicon photomultipliers (SiPMs) have emerged as promising alternative light readout devices that offer gains and quantum efficiencies similar to those of PMTs, but with greatly reduced mass and volume, high ruggedness, low voltage requirements, and no sensitivity to magnetic fields. In order for SiPMs to replace PMTs in space-based instruments, however, it must be shown that they can provide comparable performance, and that their inherent temperature sensitivity can be corrected for. To this end, we have performed extensive testing and modeling of a small gamma-ray spectrometer composed of a 6 mm×6 mm SiPM coupled to a 6 mm×6 mm ×10 mm LaBr 3 :Ce crystal. A custom readout board monitors the temperature and adjusts the bias voltage to compensate for gain variations. We record an energy resolution of 5.7% (FWHM) at 662 keV at room temperature. We have also performed simulations of the scintillation process and optical light collection using Geant4, and of the SiPM response using the GosSiP package. The simulated energy resolution is in good agreement with the data from 22 keV to 662 keV. Above ~1 MeV, however, the measured energy resolution is systematically worse than the simulations. This discrepancy is likely due to the high input impedance of the readout board front-end electronics, which introduces a non-linear saturation effect in the SiPM for large light pulses. Analysis of the simulations indicates several additional steps that must be taken to optimize the energy resolution of SiPM-based scintillator detectors.

[1] Dose spectra from energetic particles and neutrons (DoSEN) are an early-stage space technology research project that combines two advanced complementary radiation detection concepts with fundamental advantages over traditional dosimetry. DoSEN measures not only the energy but also the charge distribution (including neutrons) of energetic particles that affect human (and robotic) health in a way not presently possible with current dosimeters. For heavy ions and protons, DoSEN provides a direct measurement of the lineal energy transfer (LET) spectra behind shielding material. For LET measurements, DoSEN contains stacks of thin-thick Si detectors similar in design to those used for the Cosmic Ray Telescope for the Effects of Radiation. With LET spectra, we can now directly break down the observed spectrum of radiation into its constituent heavy-ion components and through biologically based quality factors that provide not only doses and dose rates but also dose equivalents, associated rates, and even organ doses. DoSEN also measures neutrons from 10 to 100 MeV, which requires enough sensitive mass to fully absorb recoil particles that the neutrons produce. DoSEN develops the new concept of combining these independent measurements and using the coincidence of LET measurements and neutron detection to significantly reduce backgrounds in each measurement. The background suppression through the use of coincidence allows for significant reductions in size, mass, and power needed to provide measurements of dose, neutron dose, dose equivalents, LET spectra, and organ doses. Thus, we introduce the DoSEN concept: a promising low-mass instrument that detects the full spectrum of energetic particles, heavy ions, and neutrons to determine biological impact of radiation in space.

High school students launch their own high-altitude payloads and learn from their successes and failures through a science research training program led by the University of New Hampshire.

To elucidate the radiation mechanism of gamma ray bursts, we are developing the polarimeter which consists of segmented plastic scintillators and GAGG(Ce) scintillators attached with multi-anode photomultipliers and avalanche photodiodes, respectively. Constructing the bread board model with 36 pieces of plastic scintillator and 24 pieces of GAGG(Ce) scintillator, we have carried out the beam experiments at the KEK Photon Factory in Japan. At the same time, we carried out also computer simulation and we compared the experimental data with results of simulations. As the results, it was recognized that the modulation factor by the experiments is consistent with that by the simulations. On the other hand, though the detection efficiency obtained by the experiments is in rough agreement with that by the simulation, the value of the detection efficiency can not be well explained reasonably. This discrepancy is likely due to the incomplete energy calibration of the plastic scintillators.

Theoretical models show that a more complete understanding of the inner structure of -ray bursts (GRBs), including the geometry and physical processes close to the central engine, requires the exploitation of -ray polarimetry. Over the past several years, we have developed the Gamma Ray Polarization Experiment (GRAPE) to measure the polarization of -rays from GRBs over the energy range of 50 to 500 keV. GRAPE is a large FoV instrument with a sensitive energy range covering the peak energy distribution of GRBs. The design is based on an array of independent modules, each of which consists of an array of (high-Z and low-Z) scintillator elements read out by a multi-anode PMT (MAPMT). Our eventual goal is to y GRAPE on a long duration balloon (LDB) platform to collect data on a significant sample of GRBs. Our experience with two balloon flights (in 2011 and 2014), coupled with further design efforts focused on orbital payloads, has led to an improved polarimeter concept that represents a natural evolution of the current design. The new concept employs a large number of optically-isolated scintillator elements, each of which is designed to provide a depth-of-interaction (DOI) using two (or perhaps more) readout sensors. The resulting three-dimensional location data provides a moderate level of Compton imaging capability (1 angular resolution of ~ 10 - 15°. Even this level of imaging can be used to significantly reduce the instrumental background by limiting the impact of the cosmic diffuse flux, dramatically improving the polarization sensitivity. Here we shall describe this concept and the expected performance for GRB polarization measurements.

We describe a project to develop new medium-energy gamma-ray instrumentation by constructing and flying a balloon-borne Compton telescope using advanced scintillator materials combined with silicon photomultiplier readouts. There is a need in high-energy astronomy for a medium-energy gamma-ray mission covering the energy range from approximately 0.4 - 20 MeV to follow the success of the COMPTEL instrument on CGRO. We believe that directly building on the legacy of COMPTEL, using relatively robust, low-cost, off-the-shelf technologies, is the most promising path for such a mission. Fortunately, high-performance scintillators, such as Lanthanum Bromide (LaBr3), Cerium Bromide (CeBr3), and p-terphenyl, and compact readout devices, such as silicon photomultipliers (SiPMs), are already commercially available and capable of meeting this need. We have conducted two balloon flights of prototype instruments to test these technologies. The first, in 2011, demonstrated that a Compton telescope consisting of an liquid organic scintillator scattering layer and a LaBr3 calorimeter effectively rejects background under balloon-flight conditions, using time-of-flight (ToF) discrimination. The second, in 2014, showed that a telescope using an organic stilbene crystal scattering element and a LaBr3 calorimeter with SiPM readouts can achieve similar ToF performance. We are now constructing a much larger balloon instrument, an Advanced Scintillator Compton Telescope (ASCOT) with SiPM readout, with the goal of imaging the Crab Nebula at MeV energies in a one-day flight. We expect a ~4σ detection up to ~1 MeV in a single transit. We present calibration results of the first detector modules, and updated simulations of the balloon instrument sensitivity. If successful, this project will demonstrate that the energy, timing, and position resolution of this technology are sufficient to achieve an order of magnitude improvement in sensitivity in the mediumenergy gamma-ray band, were it to be applied to a ~1 cubic meter instrument on a long-duration balloon or Explorer platform.

We are investigating the use of thin-film, multilayer structures to form optics capable of concentrating soft gamma rays with energies greater than 100 keV, beyond the reach of current grazing-incidence hard X-ray mirrors. Alternating layers of low- and high-density materials (e.g., polymers and metals) will channel soft gamma-ray photons via total external reflection. A suitable arrangement of bent structures will then concentrate the incident radiation to a point. Gamma-ray optics made in this way offer the potential for soft gamma-ray telescopes with focal lengths of less than 10 m, removing the need for formation flying spacecraft and opening the field up to balloon-borne instruments. Following initial investigations conducted at Los Alamos National Laboratory, we have constructed and tested a prototype structure using spin coating combined with magnetron sputtering. We are now investigating whether it is possible to grow such flexible multi-layer structures with the required thicknesses and smoothness more quickly by using magnetron sputter and pulsed laser deposition techniques. We present the latest results of our fabrication and gamma-ray channeling tests, and describe our modeling of the sensitivity of potential concentrator-based telescope designs. If successful, this technology offers the potential for transformational increases in sensitivity while dramatically improving the system-level performance of future high-energy astronomy missions through reduced mass and complexity.

The detection of fast neutrons has important applications in a wide variety of fields including geospace, solar and planetary physics, and applications within Defense and Security programs. Scintillator-based technologies have a proven record for detecting and measuring fast neutrons. They have high stopping power, good energy resolution, and fast timing properties. Modern organic scintillators such as stilbene and p-terphenyl, provide improved light output and pulse shape discrimination the ability to distinguish gamma-from neutron-induced signals. Furthermore, modern readout devices such as silicon photomultipliers offer an ideal alternative to photomultiplier tubes given their inherently compact size and low operating voltages. The combination of modern scintillators and silicon photomultipliers enables new instrument designs for small satellite platforms such as CubeSats. We present the performance of a double-scatter neutron spectrometer based on p-terphenyl equipped with silicon photomultipliers. We also present preliminary results for pulse-shape discrimination using advanced waveform digitization techniques.

We describe a project to develop new medium-energy gamma-ray instrumentation by constructing and flying a balloon-borne Compton telescope using advanced scintillator materials combined with silicon photomultiplier readouts. There is a need in high-energy astronomy for a medium-energy gamma-ray mission covering the energy range from approximately 0.4–20 MeV to follow the success of the COMPTEL instrument on CGRO. We believe that directly building on the legacy of COMPTEL, using relatively robust, low-cost, off-the-shelf technologies, is the most promising path for such a mission. Fortunately, high-performance scintillators, such as Lanthanum Bromide (LaBr 3 ), Cerium Bromide (CeBr 3 ), and p-terphenyl, and compact readout devices, such as silicon photomultipliers (SiPMs), are already commercially available and capable of meeting this need. We have conducted two balloon flights of prototype instruments to test these technologies. We have now begun work on a much larger balloon instrument, an Advanced Scintillator Compton Telescope (ASCOT) with SiPM readout, with the goal of imaging the Crab Nebula at MeV energies in a one-day flight. We expect a ∼4σ detection at ∼1 MeV in a single transit. If successful, this will demonstrate that the energy, timing, and position resolution of this technology are sufficient to achieve an order of magnitude improvement in sensitivity in the medium-energy gamma-ray band, were it to be applied to a ∼1 cubic meter instrument on a long-duration balloon or Explorer platform.

We have begun to investigate the use of thin-film, multilayer structures to form optics capable of concentrating soft gamma rays with energies greater than 100 keV, beyond the reach of current grazing-incidence hard X-ray mirrors. Alternating layers of low- and high-density materials (e.g., polymers and metals) will channel soft gamma-ray photons via total external reflection. A suitable arrangement of bent structures will then concentrate the incident radiation to a point. Gamma-ray optics made in this way offer the potential for soft gamma-ray telescopes with focal lengths of less than 10 m, removing the need for formation flying spacecraft and opening the field up to balloon-borne instruments. Building on initial investigations at Los Alamos National Laboratory, we are investigating whether it is possible to grow such flexible multi-layer structures with the required thicknesses and smoothness using magnetron sputter and pulsed laser deposition techniques. We present the initial results of tests aimed at fabricating such structures by combining magnetron sputtering with either spin coating or pulsed laser deposition, and demonstrating gamma-ray channeling of 122 keV photons in the laboratory. If successful, this technology offers the potential for transformational increases in sensitivity while dramatically improving the system-level performance of future high-energy astronomy missions through reduced mass and complexity.

The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4–50 MeV) and bridge the huge sensitivity gap between the COMPTEL and OSSE experiments on the Compton Gamma Ray Observatory and the visionary Advanced Compton Telescope (ACT) mission. The scientific goals include compiling a much larger catalog of sources in this energy range, performing far deeper searches for supernovae, better measuring the galactic continuum and line emissions, and identifying the components of the cosmic diffuse gamma-ray emission. MEGA records and images gamma rays by completely tracking Compton and pair creation events in a stack of double-sided Si strip detectors surrounded by a pixellated CsI calorimeter. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results from the prototype and describe the proposed satellite mission.

The Medium Energy Gamma-ray Astronomy (MEGA) telescope concept will soon be proposed as a MIDEX mission. This mission would enable a sensitive all-sky survey of the medium-energy gamma-ray sky (0.4–50 MeV) and bridge the huge sensitivity gap left after the demise of the COMPTEL and OSSE experiments on the Compton Gamma-Ray Observatory. The scientific goals include compiling a much larger catalog of sources in this energy range, performing far deeper searches for long-lived nuclear lines from supernovae, novae, and supernova remnants, studying prompt decay lines from solar flares and the interstellar medium, better measuring the diffuse galactic continuum and line emissions, identifying the components of the cosmic diffuse gamma-ray emission, searching for nuclear resonance absorption features in bright continuum spectra, and studying the medium-energy properties of black holes, pulsars, and gamma-ray bursts. MEGA detects and images gamma rays by completely tracking Compton and pair creation interactions in a stack of double-sided silicon strip track detectors surrounded by a pixellated CsI calorimeter. A prototype instrument has been developed and calibrated in the laboratory and at a gamma-ray beam facility. We present calibration results and describe future plans for the prototype, and describe the proposed satellite mission.

The primary scientific mission of the Black Hole Finder Probe (BHFP), part of the NASA Beyond Einstein program, is to survey the local Universe for black holes over a wide range of mass and accretion rate. One approach to such a survey is a hard X-ray coded-aperture imaging mission operating in the 10–600 keV energy band, a spectral range that is considered to be especially useful in the detection of black hole sources. The development of new inorganic scintillator materials provides improved performance (for example, with regards to energy resolution and timing) that is well suited to the BHFP science requirements. Detection planes formed with these materials coupled with a new generation of readout devices represent a major advancement in the performance capabilities of scintillator-based gamma cameras. Here, we discuss the Coded Aperture Survey Telescope for Energetic Radiation (CASTER), a concept that represents a BHFP based on the use of the latest scintillator technology.

The Advanced Compton Telescope (ACT) has been selected by NASA for a one-year “vision mission” study. The study’s main goal is to determine feasible instrument configurations to achieve ACT’s sensitivity requirements, and to give recommendations for technology development. Space-based instruments operating in the energy range of nuclear lines are subject to complex backgrounds generated by cosmic rays, earth albedo radiations, trapped particles, and diffuse gamma rays; typically measurements are significantly background-dominated. Therefore accurate, detailed simulations of the background induced in different ACT configurations, and exploration of event selection and reconstruction techniques for reducing these backgrounds, are crucial to determining the capabilities of a given instrument configuration. The ACT simulation team has assembled a complete suite of tools that allows the generation of particle backgrounds for a given orbit, their propagation through any instrument and spacecraft geometry – including delayed photon emission from instrument activation – as well as the selection and reconstruction of Compton events in the given detectors. We describe here the scope of the ACT simulation effort and the suite of tools used.

A new telescope for Medium Energy Gamma-Ray Astronomy, MEGA, is being developed for the energy band 0.4–50 MeV as a successor to COMPTEL and EGRET. MEGA records gamma rays by detecting and tracking Compton as well as pair creation events in a stack of double-sided Si-strip detectors and stopping them in a surrounding pixelated CsI calorimeter. Its goal is to improve sensitivity by at least an order of magnitude over that of COMPTEL. The extensive simulation tools compiled for the ACT Vision Mission Concept Study are applied to estimate the performance of a potential MEGA space mission. The tools allow the detailed simulation of the different background components expected for the satellite’s desired low-earth orbit. We present the expected narrow-line sensitivities of a MIDEX-sized telescope and show that a sensitivity 10 times better than that of COMPTEL is achievable.

MEGA, short for Medium Energy Gamma-ray Astronomy, is the development of a new technology telescope in the energy band 0.4–50 MeV. The wide energy range of MEGA, which spans nuclear γ-ray lines and energetic continuum spectra, the large field of view, and the capacity for polarimetry enables unique investigations into cosmic nucleosynthesis, particle accelerators around compact objects, and explosive high-energy events. We describe the development and tests of a prototype detector. Results from laboratory tests using radioactive sources and from a beam test calibration are presented and an outlook of a potential space mission is sketched.

We describe the development and tests of the prototype for a new telescope for Medium Energy Gamma-ray Astronomy (MEGA) in the energy band 0.4–50 MeV. As a successor to COMPTEL and EGRET (at low energies), MEGA aims to improve the sensitivity for astronomical sources by at least an order of magnitude. It could thus fill the severe sensitivity gap between scheduled or operating hard-X-ray and high-energy gamma-ray missions and open the way for a future Advanced Compton Telescope. MEGA records and images γ-rays by completely tracking Compton and Pair creation events in a stack of double-sided Si-strip track detectors surrounded by a pixelated CsI calorimeter. A scaled down prototype has been built and we describe technical details of its design and properties. Results from calibrations using radioactive sources and from measurements with an accelerator generated, fully polarized, γ-ray beam are presented and an outlook to future plans with MEGA is given.

We present results from the second-generation Energetic X-ray Imaging Telescope (EXITE2) observations of the black hole X-ray binary Cyg X-1 during the experiment's 1997 and 2001 flights. The EXITE2 phoswich [NaI(Tl)/CsI(Na)] detector is designed to image cosmic X-ray sources in the hard X-ray band by using the coded-aperture imaging technique from a high-altitude scientific balloon. The sky image reconstruction methodology used for EXITE2 is also discussed in detail. Background reduction (PSD rejection), subtractive flat-fielding, pixel shuffling, and image functions are introduced. During the observations from the EXITE2 1997 and 2001 flights, Cyg X-1 is easily detected in the 37-237 keV energy range. During the 1997 observations, the spectrum is well fitted by a Comptonization model. The spectral signatures and the observed 100 keV flux, together with the RXTE ASM light curve, indicate that the source was in the typical low state during this observation. Evidence is seen for hard X-ray variability on timescales on the order of 10 minutes. During the 2001 flight the spectrum is best fit by an extended power law with no observable cutoff. This is possible evidence of a transition to the high state, which is indeed seen in the RXTE ASM light curve shortly after our observation.

We describe the calibration measurements of the MEGA prototype, a tracking Compton and pair creation telescope. The measurements were performed at the high intensity gamma-ray source (HIγS) facility from April 21 to May 06, 2003. The main goal of this calibration was directed at higher energies, above those available from radioactive lab sources, and at polarization. A total of 15.5×10 6 triggered events at 10 energies in the range of 0.7–49 MeV and at six angles of incidence (0°–180°) were recorded.

We present a concept for an Advanced Compton Telescope (ACT) based on the use of pixelized gas micro-well detectors to form a three-dimensional electron track imager. A micro-well detector consists of an array of individual micro-patterned proportional counters opposite a planar drift electrode. When combined with thin film transistor array readouts, large gas volumes may be imaged with very good spatial and energy resolution at reasonable cost. The third dimension is determined from the drift time of the ionization electrons. The primary advantage of this approach is the excellent tracking of the Compton recoil electron that is possible in a gas volume. Such good electron tracking allows us to reduce the point spread function of a single incident photon dramatically, greatly improving the imaging capability and sensitivity. The polarization sensitivity, which relies on events with large Compton scattering angles, is particularly enhanced. We describe a possible ACT implementation of this technique, in which the gas tracking volume is surrounded by a CsI calorimeter, and present our plans to build and test a small prototype over the next three years., Comment: 7 pages, 6 figures, to appear in New Astronomy Reviews (proceedings of the Seeon Conference "Astronomy with Radioactivities IV and Filling the Sensitivity Gap in MeV Astronomy")

We describe the development of a new telescope for Medium Energy Gamma-Ray Astronomy (MEGA) for the energy band 0.4–50 MeV. As a successor to COMPTEL and EGRET (low energies), MEGA aims to improve the sensitivity for astronomical sources by at least an order of magnitude. It could thus fill the severe sensitivity gap between scheduled or operating hard-X-ray and high-energy γ-ray missions and open the way for a future Advanced Compton Telescope. MEGA records and images γ-rays by completely tracking Compton and Pair creation events in a stack of double sided Si-strip track detectors surrounded by a pixelated CsI calorimeter. A scaled down prototype has been built and calibrations using radioactive sources and exposures to an accelerator generated γ-ray beam were performed in 2003. A balloon flight is planned for 2004.

We report on the long term monitoring of X-ray dips from the ultracompact low-mass X-ray binary (LMXB) XB 1916-053. Roughly one-month interval observations were carried out with the Rossi X-ray Timing Explorer (RXTE) during 1996, during which the source varied between dim, hard states and more luminous, soft states. The dip spectra and dip lightcurves were compared against both the broadband luminosity and the derived mass accretion rate Mdot. The dips spectra could be fitted by an absorbed blackbody plus cut-off power law non-dip spectral model, with additional absorption ranging from 0 to >100 x 10^22 cm^-2. The amount of additional blackbody absorption was found to vary with the source luminosity. Our results are consistent with an obscuration of the inner disk region by a partially ionized outer disk. The size of the corona, derived from the dip ingress times, was found to be ~10^9 cm. The corona size did not correlate with the coronal temperature, but seemed to increase when \Mdot also increased. We discuss our findings in the context of an evaporated accretion disk corona model and an ADAF-type model., Accepted for publication in ApJ, 15 pages

We present the spectral analysis system for the second-generation Energetic X-ray Imaging Telescope Experiment (EXITE2) balloon payload. EXITE2 is an imaging hard X-ray telescope using a coded-aperture mask and a NaI/CsI phoswich detector operating in the energy range 20--600 keV. The instrument was flown on a high-altitude scientific balloon from Ft. Sumner, NM on 1997 May 7-8. We describe the details of the EXITE2 spectral analysis system, with emphasis on those aspects peculiar to coded-aperture instruments. In particular, we have made our analysis compatible with the standard X-ray spectral fitting package XSPEC by generating a response matrix in the appropriate format including all the effects of a coded-aperture system. The use of XSPEC, which may be a first for coded-aperture data, permits great flexibility in the fitting of spectral models. The additional effects of our phoswich system, or any other detector-specific considerations, may be easily included as well. We test our spectral analysis using observations of the Crab Nebula, and find that the EXITE2 Crab spectrum is consistent with those recorded by previous instruments operating in this energy range., Comment: 17 pages LaTeX, 6 figures, accepted to Astroparticle Physics

The ability to detect the presence of radioactive sources and Special Nuclear Materials (SNM) is of extreme importance for homeland security efforts. Today, we face terrorist threats in the form of explosive nuclear devices and Radiation Dispersal Devices (RDDs). The ability to remotely locate and identify the materials used to create these devices is imperative in the interdiction of terrorist threats. The Neutron SPECTrometer (NSPECT) developed by Michigan Aerospace Corporation and the University of New Hampshire is a dual-species instrument that detects both neutrons and gamma rays emitted from SNM, and gamma rays emitted from radioactive material. NSPECT provides spectroscopic information on the radioactive and fissile sources present while simultaneously imaging the sources within the instrumental field of view for enhanced detection capabilities. Creation and application of a detailed instrument response function through numerical simulations, analytical descriptions, and real NSPECT data provides the most detailed information on the detected neutrons and gamma rays, thereby allowing for the most sensitive and reliable detection of nuclear and radioactive threats.

The Gamma RAy Polarimeter Experiment (GRAPE) was first own on a 26-hour balloon flight in the fall of 2011. GRAPE consists of an array of Compton polarimeter modules (based on traditional scintillation technologies) designed to operate in the energy range from 50 keV up to 500 keV. The ultimate goal of our program is to operate GRAPE in a wide FoV configuration for the study of gamma-ray bursts. For the first balloon flight, GRAPE was configured in a collimated mode to facilitate observations of known point sources. The Crab nebula/pulsar, the active Sun, and Cygnus X{1 were the primary targets for the first flight. Polarization results from this flight are summarized. Plans for the next GRAPE balloon flight, which is scheduled to take place in the fall of 2014 from Ft. Sumner, NM, will also be presented. These plans involve modifications designed to improve the polarization sensitivity, including an expansion of the array of polarimeter modules from 16 to 24 and improvements to the instrument shielding. These improvements to the instrument will significantly improve the polarization sensitivity, enabling a measurement of the Crab Nebula polarization to be made during the 2014 balloon flight.

The primary science goal of the Polarimeters for Energetic Transients (POET) mission is to measure the polarization of gamma-ray bursts over a wide energy range, from X rays to soft gamma rays. The higher-energy portion of this band (50 - 500 keV) will be covered by the High Energy Polarimeter (HEP) instrument, a non-imaging, wide field of view Compton polarimeter. Incident high-energy photons will Compton scatter in low-Z, plastic scintillator detector elements and be subsequently absorbed in high-Z, CsI(Tl) scintillator elements; polarization is detected by measuring an asymmetry in the azimuthal scatter angle distribution. The HEP design is based on our considerable experience with the development and flight of the Gamma-Ray Polarimeter Experiment (GRAPE) balloon payload. We present the design of the POET HEP instrument, which incorporates lessons learned from the GRAPE balloon design and previous work on Explorer proposal efforts, and its expected performance on a two-year SMEX mission.

A small pixellated CdZnTe array is tested for suitability in a prototype SPECT system designed to acquire both emission and transmission data. Determining the optimum contact design and obtaining performance estimates of single photon acquisition are the primary focus. Flood field and collimated 57 Co sources irradiated the 16 pixel array (5 mm thick and 1.5 mm pixels) to determine photopeak efficiencies and detector response with different event collection techniques. Intrinsic full energy peak efficiency averaged 72% for an 18 keV acceptance window. A small irradiation spot scanned an array region, revealing detector response from nearby pixels. Post processing spectra compare coincident and anti-coincident acquisition. Additionally, current mode tests compare linearity with a CdWO 4 /Si p-i-n detector.