This work centers on contributing to the development of the GeMini gamma-ray spectrometer. From 2015-2017, I worked with Dr. Morgan Burks at LLNL and a team of scientists and engineers at Johns Hopkins University Applied Physics Laboratory to help develop the GeMini instrument prototype.
“NASA has awarded $3 million for development of a compact gamma-ray spectrometer by Johns Hopkins University Applied Physics Laboratory (APL) and Lawrence Livermore National Laboratory (LLNL); APL will lead the effort. The miniature instrument, named GeMini Plus, will help reveal the surface elemental composition of planets, comets, and asteroids, information critical to understanding their formation and evolution.”
This detector, which can include an anti-coincidence shield, has been selected as one of the science instruments for the 16-Psyche Mission. The Psyche Mission has been chosen as the 14th Discovery Mission by NASA. The instrument that will fly on Psyche is based on the successful MESSENGER GRS designed by the same group of scientists and engineers from LLNL and JHUAPL.
- Patrick N. Peplowski, Morgan Burks, John O. Goldsten, Samuel Fix, Lena E. Heffern, David J. Lawrence, Zachary W. Yokley,
“Radiation damage and annealing of three coaxial n-type germanium detectors: Preparation for spaceflight missions to asteroid 16 Psyche and Mars’ moon Phobos,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 942, 2019, 162409, ISSN 0168-9002, https://doi.org/10.1016/j.nima.2019.162409.
Three n-type high-purity germanium (HPGe) detectors were irradiated with protons at solar and galactic cosmic ray energies, with fluences corresponding to >=3 years of exposure to the interplanetary radiation environment. Following 1 GeV proton irradiations (8×10^8 protons cm^−2), annealing of the detectors at 105° and 115 °C resulted in full energy resolution recovery after 240 and 140 h of annealing, respectively. A second irradiation, consisting of 7.25×10^9 protons cm^−2 with a solar-proton-like energy spectrum, resulted in full recovery within <316 h of annealing at 105 °C. Prior spaceflight experience raised concerns about annealing-induced leakage currents. During this study, one of the three detectors was annealed for a cumulative time of 57 days, and there was no evidence for increased leakage currents. However, gamma-ray photopeak detection efficiency was found to be strongly anti-correlated with cumulative anneal time for all three detectors. The energy- and time-dependence of the efficiency loss is consistent with the loss of active volume at the inner borehole, which we attribute to thermal diffusion of the lithium contact during annealing. The data collected during this study validate aspects of the design of two in-development planetary gamma-ray spectrometers; one to the asteroid (16) Psyche and one to Mars’ moon Phobos. The annealing strategy and operations concepts for these two instruments are informed by the results of this study.
- Morgan T. Burks, Owen B. Drury, John O. Goldsten, Lisle B. Hagler, Lena E. Heffern, Nathan R. Hines, Geon-Bo Kim, David J. Lawrence, Karl E. Nelson, Patrick N. Peplowski, & Zach W. Yokley. “GeMini: A High-Resolution, Low-Resources, Gamma-Ray Spectrometer for Planetary Science Applications,” Space Sciences Review, 2020 Accepted.
GeMini is a high-resolution gamma-ray spectrometer designed for planetary-science exploration. GeMini serves as the basis for instruments being flown on three upcoming deep-space missions: NASA’s mission to the M-class asteroid (16) Psyche, the Japanese Aerospace Exploration Agency sample-return mission to the moons of Mars, and NASA’s Dragonfly mission that will land on Saturn’s largest moon, Titan. These science missions require high-resolution spectroscopy in a low power, low-mass, rugged design that can survive in a variety of environments. GeMini addresses these needs by providing a cryogenically-cooled crystal of high-purity germanium that can operate with as little as 10 to 20 watts, depending on the implementation and mission, and has a mass of less than 2 kg. GeMini helps determine the elemental composition of planetary bodies by measuring gamma rays emitted from the surface. This work describes the mechanical, thermal, and electrical design of GeMini as well as its performance. It also describes testing that was performed to validate the design with respect to launch loads and radiation damage. Although the basic design of GeMini is common to all three missions, each planetary body has unique environmental conditions and mission specifications. This work concludes by describing these upcoming missions and how GeMini is customized for each.