  Motivated by the challenge of identifying intruder ions in a cold ion
crystal, we investigate calorimetry from emitted fluorescence light. Under
continuous Doppler cooling, the ion crystal reaches a temperature equilibrium
with a fixed level of fluorescence intensity and any change in the motional
energy of the crystal results in a modification of this intensity. We
theoretically determine the fluorescence rate of an ion crystal as a function
of the temperature, assuming that laser light is scattered along a two-level
electronic transition, which couples to the crystal's vibrations via the
mechanical effects of light. We analyze how the heat dissipated by collisions
of an incoming intruder ion alters the scattering rate. We argue that an energy
change by an incoming $^{229}$Th$^{10+}$ ion can be unambiguously detected
within 100 $\mu$s via illuminating a fraction of a 10$^{3}$ ion crystal. This
method enables applications including capture and spectroscopy of charged
states of thorium isotopes and investigation of highly charged ions.
