Radiation Damage of 2×2×1 cm32 \times 2 \times 1 \ \mathrm{cm}^3 Pixelated CdZnTe Due to High-Energy Protons

Pixelated CdZnTe detectors are a promising imaging-spectrometer for gamma-ray astrophysics due to their combination of relatively high energy resolution with room temperature operation negating the need for cryogenic cooling. This reduces the size, weight, and power requirements for telescope-based radiation detectors. Nevertheless, operating CdZnTe in orbit will expose it to the harsh radiation environment of space. This work, therefore, studies the effects of $61 \ \mathrm{MeV}$ protons on $2 \times 2 \times 1 \ \mathrm{cm}^3$ pixelated CdZnTe and quantifies proton-induced radiation damage of fluences up to $2.6 \times 10^8 \ \mathrm{p/cm^2}$. In addition, we studied the effects of irradiation on two separate instruments: one was biased and operational during irradiation while the other remained unbiased. Following final irradiation, the $662 \ \mathrm{keV}$ centroid and nominal $1\%$ resolution of the detectors were degraded to $642.7 \ \mathrm{keV}, 4.9 \% \ ( \mathrm{FWHM})$ and $653.8 \ \mathrm{keV}, 1.75 \% \ (\mathrm{FWHM})$for the biased and unbiased systems respectively. We therefore observe a possible bias dependency on proton-induced radiation damage in CdZnTe. This work also reports on the resulting activation and recovery of the instrument following room temperature and$60^{\circ}\mathrm{C}$ annealing

Development of Dual-Gain SiPM Boards for Extending the Energy Dynamic Range

Astronomical observations with gamma rays in the range of several hundred keV to hundreds of MeV currently represent the least explored energy range. To address this so-called MeV gap, we designed and built a prototype CsI:Tl calorimeter instrument using a commercial off-the-shelf (COTS) SiPMs and front-ends which may serve as a subsystem for a larger gamma-ray mission concept. During development, we observed significant non-linearity in the energy response. Additionally, using the COTS readout, the calorimeter could not cover the four orders of magnitude in energy range required for the telescope. We, therefore, developed dual-gain silicon photomultiplier (SiPM) boards that make use of two SiPM species that are read out separately to increase the dynamic energy range of the readout. In this work, we investigate the SiPM's response with regards to active area ($3\times3 \ \mathrm{mm}^2$ and $1 \times 1 \ \mathrm{mm}^2$) and various microcell sizes ($10$, $20$, and $35 \ \mu \mathrm{m}$). We read out $3\times3\times6 \ \mathrm{cm}^3$ CsI:Tl chunks using dual-gain SiPMs that utilize $35 \ \mu \mathrm{m}$ microcells for both SiPM species and demonstrate the concept when tested with high-energy gamma-ray and proton beams. We also studied the response of $17 \times 17 \times 100 \ \mathrm{mm}^3$CsI bars to high-energy protons. With the COTS readout, we estimate (with several assumptions) that the dual-gain prototype has an energy range of$0.25-400 \ \mathrm{MeV}$with the two SiPM species overlapping at a range of around$2.5-30 \ \mathrm{MeV}$. This development aims to demonstrate the concept for future scintillator-based high-energy calorimeters with applications in gamma-ray astrophysics

Development of a CsI Calorimeter for the Compton-Pair (ComPair) Balloon-Borne Gamma-Ray Telescope

There is a growing interest in astrophysics to fill in the observational gamma-ray MeV gap. We, therefore, developed a CsI:Tl calorimeter prototype as a subsystem to a balloon-based Compton and Pair-production telescope known as ComPair. ComPair is a technology demonstrator for a gamma-ray telescope in the MeV range that is comprised of 4 subsystems: the double-sided silicon detector, virtual Frisch grid CdZnTe, CsI calorimeter, and a plastic-based anti-coincidence detector. The prototype CsI calorimeter is composed of thirty CsI logs, each with a geometry of $1.67 \times 1.67 \times 10 \ \mathrm{cm^3}$. The logs are arranged in a hodoscopic fashion with 6 in a row that alternate directions in each layer. Each log has a resolution of around $8 \%$ full-width-at-half-maximum (FWHM) at $662 \ \mathrm{keV}$ with a dynamic energy range of around $250\ \mathrm{keV}-30 \ \mathrm{MeV}$. A $2\times2$ array of SensL J-series SiPMs read out each end of the log to estimate the depth of interaction and energy deposition with signals read out with an IDEAS ROSSPAD. We also utilize an Arduino to synchronize with the other ComPair subsystems that comprise the full telescope. This work presents the development and performance of the calorimeter, its testing in thermal and vacuum conditions, and results from irradiation by $2-25 \ \mathrm{MeV}$ monoenergetic gamma-ray beams. The CsI calorimeter will fly onboard ComPair as a balloon experiment in the summer of 2023

AXTAR: Mission Design Concept

The Advanced X-ray Timing Array (AXTAR) is a mission concept for X-ray timing of compact objects that combines very large collecting area, broadband spectral coverage, high time resolution, highly flexible scheduling, and an ability to respond promptly to time-critical targets of opportunity. It is optimized for submillisecond timing of bright Galactic X-ray sources in order to study phenomena at the natural time scales of neutron star surfaces and black hole event horizons, thus probing the physics of ultradense matter, strongly curved spacetimes, and intense magnetic fields. AXTAR's main instrument, the Large Area Timing Array (LATA) is a collimated instrument with 2-50 keV coverage and over 3 square meters effective area. The LATA is made up of an array of supermodules that house 2-mm thick silicon pixel detectors. AXTAR will provide a significant improvement in effective area (a factor of 7 at 4 keV and a factor of 36 at 30 keV) over the RXTE PCA. AXTAR will also carry a sensitive Sky Monitor (SM) that acts as a trigger for pointed observations of X-ray transients in addition to providing high duty cycle monitoring of the X-ray sky. We review the science goals and technical concept for AXTAR and present results from a preliminary mission design study.Comment: 19 pages, 10 figures, to be published in Space Telescopes and Instrumentation 2010: Ultraviolet to Gamma Ray, Proceedings of SPIE Volume 773

The Advanced Compton Telescope Mission

The Advanced Compton Telescope (ACT), the next major step in gamma-ray astronomy, will probe the fires where chemical elements are formed by enabling high-resolution spectroscopy of nuclear emission from supernova explosions. During the past two years, our collaboration has been undertaking a NASA mission concept study for ACT. This study was designed to (1) transform the key scientific objectives into specific instrument requirements, (2) to identify the most promising technologies to meet those requirements, and (3) to design a viable mission concept for this instrument. We present the results of this study, including scientific goals and expected performance, mission design, and technology recommendations.Comment: NASA Vision Mission Concept Study Report, final version. (A condensed version of this report has been submitted to AIAA.