The LargE Area burst Polarimeter (LEAP) is a proposed Compton scattering polarimeter that will, for the first time, measure the level of polarization for a significant number of GRBs with sufficient sensitivity to determine the magnetic field structure, composition, energy dissipation mechanism of GRB jets, and determine the prompt emission mechanism of GRBs. Once approved, LEAP will be deployed as an external payload on the International Space Station (ISS) where it will measure GRB polarization over the energy range from 50–1000 keV, perform GRB spectroscopy from 20 keV to 6 MeV, and self-sufficiently determine the source direction. LEAP is uniquely suited to fill a critical gap in our knowledge regarding GRBs, by exposing the underlying physics that governs astrophysical jets and the extreme environment surrounding newborn compact objects.

After many years of flying in space primarily for educational purposes, CubeSats - tiny satellites with form factors corresponding to arrangements of "1U" units, or cubes, each 10 cm on a side - have come into their own as valuable platforms for technology advancement and scientific investigations. CubeSats offer comparatively rapid, low-cost access to space for payloads that be built, tested, and operated by relatively small teams, with substantial contributions from students and early career researchers. Continuing advances in compact, low-power detectors, readout electronics, and flight computers have now enabled X-ray and gamma-ray sensing payloads that can fit within the constraints of CubeSat missions, permitting in-orbit demonstrations of new techniques and innovative high-energy astronomy observations. Gamma-ray-sensing CubeSats are certain to make an important contribution in the new era of multi-messenger, time-domain astronomy by detecting and localizing bright transients such as gamma-ray bursts, solar flares, and terrestrial gamma-ray flashes; however, other astrophysical science areas requiring long observations in a low-background environment, including gamma-ray polarimetry, studies of nuclear lines, and measurement of diffuse backgrounds, will likely benefit as well. We present the primary benefits of CubeSats for high-energy astronomy, highlight the scientific areas currently or soon to be studied, and review the missions that are currently operating, under development, or proposed. A rich portfolio of CubeSats for gamma-ray astronomy already exists, and the potential for a broad range of creative and scientifically productive missions in the near future is very high.

The origin of the cosmic diffuse gamma-ray (CDG) background in the 0.3 – 10 MeV energy range is a mystery that has persisted for over 40 years. The best existing measurements have large systematic uncertainties and sparse sky coverage. Intriguingly, the latest theoretical models based on emission from active galactic nuclei and supernovae differ significantly from these data below ~1.5 MeV. The Mini Astrophysical MeV Background Observatory (MAMBO) is a new CubeSat mission under development at Los Alamos National Laboratory with the goal of making high-quality measurements of the MeV CDG to help solve this puzzle. Scheduled for launch in early 2025, the concept is motivated by the fact that the sensitivity of space-based MeV 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 measurement ever made of the MeV CDG spectrum and uniformity across the sky utilizing an innovative shielded spectrometer that simultaneously measures signal and background. Primary detector, background monitor, and active shield volumes made of bismuth germanate (BGO) scintillator are read out by custom silicon photomultiplier (SiPM) arrays to enable an efficient, compact instrument suitable for CubeSat deployment. A tagged calibration source maintains constant gain across detector elements despite expected temperature variations. We present detailed calibration measurements of the MAMBO instrument which verify the required energy range, energy resolution, and stable performance over the expected operating temperature range. These measurements also validate a detailed Monte Carlo simulation model, enabling prediction of detector counting rates and scientific performance in orbit.

Scintillators with pulse-shape discrimination (PSD) capability are of great interest to many fields in the scientific community. The ability to discern a gamma ray from a neutron using PSD varies between different types of scintillator materials and dopants. A new generation of organic scintillator materials with PSD capability were studied to determine their radiation hardness to ionizing and non-ionizing radiation. The PSD capability, average pulse shapes, and light output of four types of organic scintillator were characterized before and after neutron and gamma-ray irradiation. The main goal of this investigation is to study the effects of long-term irradiation that may be experienced in space applications on the light output and particle discriminating capabilities of each material. EJ-270, EJ-276, organic glass, and Stilbene were tested. Damage due to non-ionizing (neutron) radiation was not observed in any of the scintillators up to $2.56\times10^{11}$ n/cm$^2$, except for Stilbene which showed a small (12$\%$) decrease in light output. All scintillators presented some light output reduction after ionizing (gamma-ray) irradiation, with reductions of 17$\%$ (EJ-276 and OGS), 32$\%$ (EJ-270), and 42$\%$ (Stilbene) observed immediately after 100 kRad.

In collaboration between the University of Michigan and Los Alamos National Laboratory, a 3D position-sensing CdZnTe (CZT) detector prototype was built and integrated into a high-altitude balloon platform to evaluate its performance in a space-like mixed-radiation environment. The detector prototype, Orion Eagle, was designed to operate in near-vacuum environments without any temperature regulation. Orion Eagle was hand-launched from NASA's Columbia Scientific Balloon Facility (CSBF) at Fort Sumner, NM on September 26, 2021, and successfully operated throughout a 9-hour flight, which reached 38.5 km in altitude. The flight met its objectives, successfully detecting atmospheric gamma rays and galactic cosmic rays, and raising the Technical Readiness Level from 4 to 6 for large-volume 3D CZT detector technology for space applications. Ionization tracks produced by charged particles create spatial signatures in the detector that are distinguishable from discrete gamma-ray interactions. Therefore, the 3D position-sensing capabilities using pixelated electrodes on a CZT detector can help enable discrimination of background charged particles from gamma-ray events without an anticoincidence shield. The potential for background rejection capability, ambient-temperature operation, gamma-ray coded-aperture and Compton imaging, and near High Purity Germanium (HPGe) energy resolution motivate the use of large-volume 3D CZT imaging spectrometers in future space missions.

Gamma-rays, the most energetic photons, carry information from the far reaches of extragalactic space with minimal interaction or loss of information. They bring messages about particle acceleration in environments so extreme they cannot be reproduced on earth for a closer look. Gamma-ray astrophysics is so complementary with collider work that particle physicists and astroparticle physicists are often one in the same. Gamma-ray instruments, especially the Fermi Gamma-ray Space Telescope, have been pivotal in major multi-messenger discoveries over the past decade. There is presently a great deal of interest and scientific expertise available to push forward new technologies, to plan and build space- and ground-based gamma-ray facilities, and to build multi-messenger networks with gamma rays at their core. It is therefore concerning that before the community comes together for planning exercises again, much of that infrastructure could be lost to a lack of long-term planning for support of gamma-ray astrophysics. Gamma-rays with energies from the MeV to the EeV band are therefore central to multiwavelength and multi-messenger studies to everything from astroparticle physics with compact objects, to dark matter studies with diffuse large scale structure. These goals and new discoveries have generated a wave of new gamma-ray facility proposals and programs. This paper highlights new and proposed gamma-ray technologies and facilities that have each been designed to address specific needs in the measurement of extreme astrophysical sources that probe some of the most pressing questions in fundamental physics for the next decade. The proposed instrumentation would also address the priorities laid out in the recent Astro2020 Decadal Survey, a complementary study by the astrophysics community that provides opportunities also relevant to Snowmass.

Observing cosmic sources in the medium-energy gamma-ray regime (~0.4 – 10 MeV) requires an 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 solid-state detectors (SSDs), but with faster rise times (~1 ns), improved radiation hardness, and are generally insensitive to light and temperature. Here we present work preliminary to the pairing of artificial single-crystal diamond detectors (SCDDs) with CeBr3 calorimeters to produce a prototype Compton telescope.

The Experiment for Space Radiation Analysis (ESRA) is the next in the series of Demonstration and Validation (DemVal) projects the Los Alamos National Laboratory (LANL) program will fly. The ESRA program will mature technologies such as the novel Wide-field-of-view Plasma Spectrometer (WPS) and the Energetic Charged Particle (ECP) telescope, along with high voltage power supply, a 3U Eurocard single board computers, flight software architecture, and analog-to-digital electronics. The WPS and ECP sensors are intended to actively monitor the local space environment, thus allowing for the attribution and rapid anomaly resolution of unanticipated instrument or spacecraft behavior as a result of space weather effects.

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.

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.

The origin of the cosmic diffuse gamma-ray (CDG) background in the 0.3–10 MeV energy range is a mystery that has persisted for over 40 years. The Mini Astrophysical MeV Background Observatory (MAMBO) is a new CubeSat mission under development at Los Alamos National Laboratory with the goal of addressing this longstanding puzzle. The concept is motivated by the fact that, since the MeV CDG is relatively bright, only a small detector is required to make high-quality 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 MeV 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. Los Alamos is partnering with commercial vendors for the 12U CubeSat bus and ground station network, which we expect will become a new paradigm for low-cost, fast-turnaround space science missions. We describe the MAMBO instrument and mission concept in detail and present the expected scientific return.

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 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.

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 <10 m and energies >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. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

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.

The mergers of compact binaries with at least one neutron star component are the potential leading sites of the production and ejection ofr-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 theLunar Occultation Explorer(LOX), theAll-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.