Sara Abraham a b, Yuefeng Zhu a, Suzanne Nowicki b, Peter Bloser b, James Berry a, Benigno Sandoval b, Sara Lanctot b, Matthew Petryk a, Jonathan Deming b, Alexei Klimenko b, Zhong He a
a University of Michigan, 2355 Bonisteel Blvd, Ann Arbor, MI 48109, USA
b Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Abstract
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.
Introduction
Gamma-ray spectrometers are regularly used in planetary science to probe the subsurface elemental composition of planetary bodies such as the Moon [1], Mars [2], Mercury [3], Venus [4], and asteroids Eros [5], Vesta and Ceres [6], [7]. Gamma rays emitted from planetary materials provide key information on the subsurface elemental composition that can be paired with mineralogical data to help understand the formation and evolution of the planetary body. These gamma rays are the result of both radioactive decay and interactions between the subsurface materials and an activation source (i.e. galactic cosmic rays or a pulsed neutron generator). Currently, the spatial resolution of mineralogical data such as data acquired by infra-red spectroscopy, X-ray diffraction and many other methods, is usually better than that achieved with gamma-ray spectrometers. The diameter of a spatial resolution element with the Mars Odyssey Gamma Ray Spectrometer (GRS) on the Martian surface is about 360 to 450 km [8]. To advance the state-of-the-art and enable the synthesis of elemental and mineralogical data at finer spatial scales, planetary gamma-ray spectrometers with near-HPGe energy resolution, imaging capabilities, high signal-to-noise ratio, high efficiency, low mass, low volume, and no cryogenic cooling are desirable. Large-volume, high resolution 3D CdZnTe (CZT) gamma-ray detectors using pixelated anodes are a suitable candidate to satisfy these requirements.
The wide bandgap (1.6 eV) of CZT, 3D position-sensing, and improvements in low-noise readout electronics and event reconstruction algorithms have enabled CZT detectors to achieve near-HPGe energy resolution at room-temperatures. The 3D position of interactions can be used to correct for electron trapping, material nonuniformity, and weighting potential variation to consistently achieve spectroscopic performance around 0.5% full-width at half maximum (FWHM) at 662 keV for all events. This is comparable to the energy resolution achieved by HPGe used in space applications, which can be on the order of 0.3% FWHM at 662 keV (i.e. for SPI [9] and RHESSI [10]) or even closer to that of CZT (0.5% FWHM at 662 keV for MESSENGER [11]). Unlike HPGe, CZT does not require cryogenic cooling, greatly reducing the mass, volume, and risk associated with a payload.
In space, intense levels of mixed-radiation background can degrade the sensitivity of spectroscopic instruments. The 3D position sensitivity of CZT detectors can help enhance background rejection without an anticoincidence shield. Firstly, the 4-π Compton imaging capability can be used to reject local gamma-ray background from scatter off spacecraft. Secondly, background events due to charged particles can be identified and rejected. While gamma-ray interactions are discrete in interaction positions, charged particles deposit energy starting from one of the out facing surfaces of each CZT detector continuously along their path. The trail of energy deposition due to charged particles will therefore create a spatial signature that can be distinguished from the discrete gamma-ray interactions.
High-resolution large-volume 3D CZT detectors are commercially available and used in many relevant terrestrial applications such as nuclear non-proliferation, defense, medical imaging, and homeland security, but have not yet been used in space applications, putting them at a Technical Readiness Level (TRL) of 4 [12]. CZT has successfully flown in previous space missions such as Swift-BAT [13], AstroSat [14], and NuSTAR [15]; however, the CZT used was much thinner (5 mm or less) and intended for X-ray detection. Virtual Frisch-grid CZT, which can be up to 30 mm in thickness but limited in cross-section area, has been proposed for future gamma-ray telescope GECCO [16], but has not yet been space qualified. Compared to 3D CZT detectors using pixelated anodes, virtual Frisch-grid CZT has poorer energy resolution and position resolution for multi-interaction events and will require an anti-coincidence shield to reject background cosmic rays.
High-altitude balloon flight offered a cost-effective avenue to perform a capability demonstration of large-volume 3D CZT detector technology in a space-like environment for the very first time, raising the TRL to 6 for space applications. The relatively small size and weight of CZT instruments made it a candidate for the relatively new hand-launch capability offered by NASA’s Columbia Scientific Balloon Facility (CSBF). Compared to traditional small “piggyback” experiments hosted on larger balloon missions, hand-launch payloads are easier to integrate and launch, and suffer from less locally generated background due to the reduced overall mass. The high-altitude balloon flight aimed to show that with minor modifications, current 3D CZT systems can be adapted to operate in near-vacuum space-like environments and detect gamma rays while identifying charged particles to aide in the development of background discrimination algorithms.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/capability-demonstration-of-a-3d-cdznte-detector-on-a-high-altitude-balloon-flight.html