  The Gamma-ray Module, GMOD, is a miniaturised novel gamma-ray detector which
will be the primary scientific payload on the Educational Irish Research
Satellite (EIRSAT-1) 2U CubeSat mission. GMOD comprises a compact (25mm
$\times$ 25mm $\times$ 40mm) cerium bromide scintillator coupled to a tiled
array of 4$\times$4 silicon photomultipliers, with front-end readout provided
by the IDE3380 SIPHRA. This paper presents the detailed GMOD design and the
accommodation of the instrument within the restrictive CubeSat form factor. The
electronic and mechanical interfaces are compatible with many off-the-shelf
CubeSat systems and structures. The energy response of the GMOD engineering
qualification model has been determined using radioactive sources, and an
energy resolution of 5.4% at 662keV has been measured.
  EIRSAT-1 will perform on-board processing of GMOD data. Trigger results,
including light-curves and spectra, will be incorporated into the spacecraft
beacon and transmitted continuously. Inexpensive hardware can be used to decode
the beacon signal, making the data accessible to a wide community.
  GMOD will have scientific capability for the detection of gamma-ray bursts,
in addition to the educational and technology demonstration goals of the
EIRSAT-1 mission. The detailed design and measurements to date demonstrate the
capability of GMOD in low Earth orbit, the scalability of the design for larger
CubeSats and as an element of future large gamma-ray missions.
