  Segmented electromagnetic calorimeters are used to determine both the total
energy and direction (momentum components) of charged particles and photons. A
trade off is involved in selecting the degree of segmentation of the
calorimeter as the spatial and energy resolutions are affected differently.
Increased number of individual detectors reduces accidental particle pile-up
per detector but introduces complications related to ADC pedestals and pedestal
variations, exacerbates the effects of electronic noise and ground loops, and
requires summing and discrimination of multiple analog signals. Moreover,
electromagnetic showers initiated by individual ionizing particles spread over
several detectors. This complicates the precise gain-matching of the detector
elements which requires an iterative procedure. The PIBETA calorimeter is a
240-module pure CsI non-magnetic detector optimized for detection of photons
and electrons in the energy range 5-100 MeV. We present the
computer-controlled, automatic, in situ gain-matching procedure that we
developed and used routinely in several rare pion and muon decay experiments
with the PIBETA detector.
