Mark Widholm

First Multipoint in Situ Observations of Electron Microbursts: Initial Results from the NSF FIREBIRD‐II Mission

We present initial dual spacecraft observations that for the first time both constrain the spatial scale size and provide spectral properties at medium energies of electron microbursts. We explore individual microburst events that occurred on 2 February 2015 using simultaneous observations made by the twin CubeSats which comprise the National Science Foundation (NSF) Focused Investigations of Relativistic Electron Bursts: Intensity, Range, and Dynamics (FIREBIRD II). During these microburst events, the two identically instrumented FIREBIRD II CubeSats were separated by as little as 11 km while traversing electron precipitation regions in low-Earth orbit. These coincident microburst events map to size scales >120 km at the equator. Given the prevalence of coincident and noncoincident events we conclude that this is of the same order of magnitude as that of the spatial scale size of electron microburst, an unknown property that is critical for quantifying their overall role in radiation belt dynamics. Finally, we present measurements of electron microbursts showing that precipitation often occurs simultaneously across a broad energy range spanning 200 keV to 1 MeV, a new form of empirical evidence that provides additional insights into the physics of microburst generation mechanisms.

Single-sided CZT strip detectors

We report progress in the study of thick CZT strip detectors for 3-d imaging and spectroscopy and discuss two approaches to device design. Unlike double-sided strip detectors, these devices feature both row and column contacts implemented on the anode surface. This electron-only approach circumvents problems associated with poor hole transport in CZT that normally limit the thickness and energy range of double-sided strip detectors. The work includes laboratory and simulation studies aimed at developing compact, efficient, detector modules for 0.05 to 1 MeV gamma radiation measurements while minimizing the number and complexity of the electronic readout channels. These devices can achieve similar performance to pixel detectors for both 3-d imaging and spectroscopy. The low channel count approach can significantly reduce the complexity and power requirements of the readout electronics. This is particularly important in applications requiring large area detector arrays. We show two single-sided strip detector concepts. One, previously reported, features rows established with collecting contacts and columns with non-collecting contacts. Another, introduced here, operates on a charge sharing principle and establishes both rows and columns with collecting contacts on the anode surface. In previous work using the earlier strip detector concept we reported simulations and measurements of energy and spatial resolution for prototype 5 and 10 mm thick CZT detectors. We now present the results of detection efficiency and uniformity measurements conducted on a 5 mm thick detector using a specific configuration of the front-end electronics and event trigger. We discuss limitations associated with implementation of this approach.

Further Studies of Single-Sided Charge-Sharing CZT Strip Detectors

We report progress in the study of a thick CZT strip detector module designed to perform gamma-ray spectroscopy and 3-D imaging. We report preliminary performance measurements of 7.5 mm thick single-sided charge-sharing strip detector prototype devices. This design features both row and column contacts on the anode surface. This electron-only approach addresses problems associated with poor hole transport in CZT that limit the thickness and energy range of double-sided strip detectors. This work includes laboratory and simulation studies aimed at developing compact, efficient, detector modules for 0.05 to 1 MeV gamma measurements while minimizing the number and complexity of the electronic readout channels. This is particularly important in space-based coded aperture and Compton telescope instruments that require large area, large volume detector arrays. Such arrays will be required for the NASA Black Hole Finder Probe (BHFP)and Advanced Compton Telescope (ACT). This new design requires an anode pattern with contacts whose dimensions and spacing are roughly the size of the ionization charge cloud. The first prototype devices have 125 μm anode contacts on 225 μm pitch. Our results demonstrate the principle of operation but suggest that even finer anode contact feature sizes will be necessary to achieve the desired performance.

Development of CZT strip detector modules for 0.05- to 1-MeV gamma-ray imaging and spectroscopy

We report progress in our study of cadmium zinc telluride (CZT) strip detectors featuring orthogonal coplanar anode contacts. We specifically report on the performance, characterization and stability of 5 and 10 mm thick prototype CZT detectors fabricated using material from several manufacturers. Our ongoing work includes laboratory and simulation studies aimed at optimizing and developing compact, efficient, high performance detector modules for 0.05 to 1 MeV gamma radiation measurements with space-based instrumentation. The coplanar anode strip configuration retains many of the performance advantages of pixel detectors yet requires far fewer electronic channels to perform both 3-d imaging and spectroscopy. Minimizing the channel count is important for large balloon or space instruments including coded aperture telescopes (such as MARGIE or EXIST) and Compton imaging telescopes (such as TIGRE or ACT). We also present plans for developing compact, space qualified imaging modules designed for integration into closely packed large area detector arrays. We discuss issues associated with detector module and array electronics design and development.

Readout and performance of thick CZT strip detectors with orthogonal coplanar anodes

We report progress in the study of CZT strip detectors featuring orthogonal coplanar anode contacts. The work includes laboratory and simulation studies aimed at optimizing and developing compact, efficient, high performance detector modules for 0.05 to 1 MeV gamma radiation measurements. The novel coplanar anode strip configuration retains many of the performance advantages of pixel detectors yet requires far fewer electronic channels to perform both 3-d imaging and spectroscopy. We report on studies aimed at determining an optimum configuration of the analog signal processing electronics to employ with these detectors. We report measurements of energy and spatial resolution in three dimensions for prototype 5 and 10 mm thick CZT detectors using a set of shaping and summing amplifiers.

Readout of scintillating plastic fibers with an APD array and prototype ASIC

We report on the development of custom front-end electronics for use with avalanche photodiode (APD) arrays as part of a NASA technology study for the readout of scintillating plastic fibers. APD arrays featuring 64 1 mm square pixels are used. We demonstrate that a pixel of these APD arrays coupled to relatively thin (0.25 mm) and short (15 cm) scintillating plastic fibers can be used to detect and measure the tracks of even minimum ionizing particles. An application-specific integrated circuit (ASIC) implementation of the electronics is required to produce a detector sufficiently compact for practical use in a space flight experiment featuring many thousands of channels. This paper briefly describes the detector concept and performance and presents the design and performance of a four-channel prototype ASIC fabricated using the 0.35 /spl mu/ Taiwan Semiconductor Manufacturing Company (TSMC) process.

Characterization of Single-Sided Charge-Sharing CZT Strip Detectors for Gamma-Ray Astronomy

We report progress in the study of thick single-sided charge-sharing cadmium zinc telluride (CZT) strip detector modules designed to perform spectroscopy and 3-D imaging of gamma-rays. We report laboratory measurements including spectroscopy, efficiency and 3-D imaging capability of prototype detectors (15 × 15 × 7.5mm3) with 11×11 unit cells. We also report on Monte Carlo simulations (GEANT4 v7.1) to investigate the effect of multihits on detector performance in both spectroscopy and imaging. We compare simulation results with data obtained from laboratory measurements and discuss the implications for future strip detector designs.

A prototype ASIC for APD array readout of scintillating plastic fibers

We report on the development of custom front-end electronics for use with avalanche photodiode (APD) arrays as part of a NASA technology study for the readout of scintillating plastic fibers. APD arrays featuring 64.1 mm square pixels are used. We demonstrate that a pixel of these APD arrays coupled to relatively thin (0.25 mm) and short (15 cm) scintillating plastic fibers can be used to detect and measure the trucks of even minimum ionizing particles (MIPs). An application-specific integrated circuit (ASIC) implementation of the electronics is required to produce a detector sufficiently compact for practical use in a flight experiment featuring many thousands of channels. This paper briefly describes the detector concept and performance and presents the design and performance of a four-channel prototype ASIC fabricated using the 0.35 micron TSMC process.