Development of a novel charge readout for a liquid Argon MeV Gamma-Ray Telescope

This proposal is for initial technology development for a proposed new MeV Gamma Ray instrument based on a liquid argon (LAr) time projection chamber (TPC). Compared to other wavelengths, the MeV portion of the photon sky is effectively unexplored, in large part because of the instrumental challenges in this energy range. It is widely recognized that a powerful new instrument would have a rich set of astrophysics topics to explore. TPC detectors have two primary advantages compared to segmented detectors such as the Si-strip or Ge-strip based detectors that are the current leading MeV technology. The first is that the active detection medium is nearly monolithic, with very little dead material. This gives highly efficient event reconstruction and powerful background rejection. The second is that full 3D readout of the active material is accomplished using only 2D instrumentation on the periphery. This sharply increases the achievable detector mass for a given power and channel budget, which then holds the promise of a very large and still affordable instrument in the current era of reduced costs of launching mass to space. All of this is enabled by substantial developments in liquid noble TPCs over last 20 years in the fields of dark matter detection and neutrinos, and in particular the major and ongoing DOE investment in DUNE A core challenge for measuring MeV Gamma Rays is that they scatter several times, and the location of each scatter must be accurately measured at the sub-mm level. Each scatter creates a recoil electron, and so the goal is to determine the head of each electron track, and, ideally, it's initial direction. In a TPC this is by far best done with a fine-grained pixel readout of charge, but the naive power from doing this exceeds what is allowable in space by a factor of 10^4 or more. We propose a new readout architecture that combines fine-grained pixels with a set of coarse induction grids that provide a trigger to power cycle the pixel readout, thus meeting the power requirements. Pixel readout provides true 3D imaging of electron recoil tracks that is substantially more powerful than the pseudo-3D imaging provided by strip readout (either in a TPC or a semiconductor-based detector). The focus of this proposal is development and demonstration of this novel pixel readout scheme. The main effort is the design of a custom CMOS ASIC which contains the power-cycled front end pixel readout, switched capacitor memory, digitization and logic, along with several cycles of prototype fabrication. We will also develop the coarse grids and a focussing electrode for the pixels. Prototypes of the combined readout will be tested in an existing small scale liquid Ar system. We will also will study the instrument's sensitivity and science reach. An initial assessment compared to a Si-strip based instrument suggests at least comparable point spread function, comparable energy resolution, and the promise of a much larger effective area and sensitivity for a modest (e.g., MIDEX) cost. This effort will strongly leverage existing expertise and infrastructure at SLAC, including: the liquid noble test facility developed for the LZ experiment; the LZ, DUNE and nEXO research groups; an electronics group with expertise in the development of cryogenic ASICs for use in liquid Ar and Xe; and the science team at SLAC and KIPAC that was involved in FERMI. The core charge readout technology could have a significant impact in neutrino physics, and applications in other areas including homeland security and medical imaging.

The Astrophysics Research and Analysis program (APRA) supports suborbital and suborbital-class investigations, development of detectors and supporting technology, laboratory astrophysics, and limited ground-based observing. Basic research proposals in these areas are solicited for investigations that are relevant to NASA's programs in astronomy and astrophysics, including the entire range of photons, gravitational waves, and particle astrophysics. The emphasis of this solicitation is on technologies and investigations that advance NASA astrophysics missions and goals.

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