Integration and testing of the background-subtracting gamma-aay spectrometer for the MAMBO CubeSat mission

Abstract

The origin of the cosmic diffuse gamma-ray (CDG) background in the 0.3 – 10MeV energy range is a mystery that has persisted for 50 years. The best existing measurements have large systematic uncertainties, and the latest theoretical models based on emission from active galactic nuclei and supernovae differ significantly from these data below 1MeV. The Mini Astrophysical MeV Background Observatory (MAMBO) is a CubeSat space mission under development at Los Alamos National Laboratory with the goal of making high-quality measurements of the MeV CDG to help solve this puzzle. The MAMBO concept is motivated by the fact that the sensitivity of space-based MeV gamma-ray instruments to the CDG is limited not by size, but by the locally generated instrumental background produced by interactions of energetic particles in spacecraft materials. The MAMBO mission will provide the best measurement ever made of the MeV CDG spectrum and uniformity across the sky by 1) flying on a comparatively tiny 12U CubeSat platform, which provides a uniquely quiet environment relative to previous large gamma-ray science missions; and 2) deploying a unique shielded spectrometer that simultaneously measures signal and background, enabling the remaining instrumental background to be subtracted away. Primary detector, background monitor, and active shield volumes made of bismuth germanate (BGO) scintillator are read out by custom silicon photomultiplier (SiPM) arrays to enable an efficient, compact instrument suitable for CubeSat deployment. A tagged calibration source maintains constant gain across detector elements despite expected temperature variations. We describe the integration and testing of the MAMBO payload and satellite bus and present calibration measurements which verify the required energy range, energy resolution, and stable performance over the expected operating temperature range. These measurements validate a Monte Carlo simulation model, enabling the prediction of detector counting rates and the instrument’s scientific performance in orbit.