The GeMini Plus High-Purity Ge Gamma-Ray Spectrometer: Instrument Overview and Science Applications

Introduction: Knowing the elemental composition of a planetary surface is key to understanding its formation and evolution. Planetary gamma-ray spectroscopy is a well-established technique for remotely measuring planetary elemental concentrations for the following elements: H, C, O, Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Fe, Ni, Th, and U. It is unique among the available techniques in that it measures bulk concentrations to depths of tens of cm (in contrast to techniques sensitive only to the top tens of microns), and can quantify compositional layering within this range. Orbital gamma-ray measurements have resulted in significant discoveries from the Moon, Mars, Mercury, and asteroids [1–4]. A Gamma-Ray and Neutron Spectrometer (GRNS) is part of the recently selected Psyche mission, which will orbit the M-class asteroid 16 Psyche [5]. Surface-based, in situ gamma-ray spectroscopy has been relatively limited and not fully realized because the laboratory quality spectrometers needed to achieve full elemental sensitivitity require more resources (e.g., mass, power) than are available for landed missions. In addition, orbital missions increasingly require lower resource instruments, and new instruments need to incorporate lessons learned from prior flight instruments. Here we discuss a new instrument called GeMini Plus, which is a high-purity Ge (HPGe) Gamma-Ray Spectrometer (GRS) that can accomplish laboratory quality, high-precision gamma-ray measurements with the type of low resources needed for landed platforms as well as resource-constrained orbital missions. This abstract provides an overview of the GeMini Plus instrument, including improvements made to prior instruments, as well as future science applications for the use of GeMini-Plus technology. A complementary abstract provides additional information about GeMini Plus, including details of initial performance and expected future tests [6]. GeMini Plus Gamma-Ray Spectrometer: GeMini Plus is based on the MESSENGER GRS [7] and a miniature HPGe instrument known as GeMini (Fig. 1). GeMini was developed by Lawrence Livermore National Laboratory for national security applications [8]. GeMini Plus uses the same HPGe sensor as the MESSENGER GRS and so achieves the same sensitivety. Depending on mission scenario, GeMini Plus may or may not need a plastic scintillator anticoincidence shield (ACS) to reduce background from galactic cosmic rays. An ACS is generally not needed for surface-based measurements, whereas an ACS can significantly increase signal-to-background for many types of orbital measurements [9].