  CALIFA is the high efficiency and energy resolution calorimeter for the R3B
experiment at FAIR, intended for detecting high energy light charged particles
and gamma rays in scattering experiments, and is being commissioned during the
Phase-0 experiments at FAIR, between 2018 and 2020. It surrounds the reaction
target in a segmented configuration with 2432 detection units made of long
CsI(Tl) finger-shaped scintillator crystals. CALIFA has a 10 year intended
operational lifetime as the R3B calorimeter, necessitating measures to be taken
to ensure enduring performance. In this paper we present a systematic study of
two groups of 6 different detection units of the CALIFA detector after more
than four years of operation. The energy resolution and light output yield are
evaluated under different conditions. Tests cover the aging of the first
detector units assembled and investigates recovery procedures for degraded
detection units. A possible reason for the observed degradation is given,
pointing to the crystal-APD coupling.
