  The Gravitational wave high-energy Electromagnetic Counterpart All-sky
Monitor (GECAM) , composed of two small satellites, is a new mission to monitor
the Gamma-Ray Bursts (GRBs) coincident with Gravitational Wave (GW) events with
a FOV of 100% all-sky.Each GECAM satellite detects and localizes GRBs using 25
compact and novel Gamma-Ray Detectors (GRDs) in 6 keV-5 MeV. Each GRD module is
comprised of LaBr3:Ce scintillator, SiPM array and preamplifier. A large
dynamic range of GRD is achieved by the high gain and low gain channels of the
preamplifier. The energy response of GRD prototype was evaluated using
radioactive sources in the range of 5.9-1332.5 keV. A energy resolution of 5.3%
at 662 keV was determined from the 137Cs pulse height spectra, which meets the
GECAM requirement (< 8% at 662 keV). Energy to channel conversion was evaluated
and a nonlinearity correction was performed to reduce the residuals (< 1.5%).
Also, a Geant4-based simulated in-flight background and a measured GRD LaBr3:Ce
intrinsic activity were used to evaluate the capability of in-flight
calibration. These results demonstrate the design of GRD.
