  The full instrument response of a superminiaturised CsI(Tl)-PiN photodiode
radioactivity detector, intended for deployment on a meteorological radiosonde,
has been modelled by combining a physics-based model of the sensor with the
detector circuit response, obtained via an LTspice simulation. The model uses
the incident energy of a gamma ray as an input, and produces the pulse expected
from the detector. The detector response was verified by comparing the
simulated energy calibration with a laboratory source. The measurement circuit
is found to control the minimum detectable energy of 26 keV, and the maximum
detectable energy is ~10 MeV. The energy sensitivity of the PiN detector is
0.29 +- 0.02 mV/keV in the 0-800 keV range. The simulation and laboratory
calibrations were consistent to better than 5% over the calibration range of
the instrument.
