  Counting the microwave photons emitted by an ensemble of electron spins when
they relax radiatively has recently been introduced as a sensitive new method
for electron paramagnetic resonance spectroscopy at millikelvin temperatures.
Here, we apply this spin fluorescence method to a scheelite crystal of CaWO4,
finding some known ($\mathrm{Er}^{3+}$, $\mathrm{Yb}^{3+}$, $\mathrm{Nd}^{3+}$
and $\mathrm{Fe}^{3+}$) and other unknown paramagnetic impurities.
Investigating the zero nuclear spin isotope ($I=0$) transition of
$\mathrm{Er}^{3+}:\mathrm{CaWO}_4$ as a model system, we provide a quantitative
analysis of the time-dependent photon counting rate following an excitation
pulse, as a function of its power. The achieved signal-to-noise ratio is found
to be an order of magnitude higher than the one obtained by
inductively-detected Hahn echo under identical conditions. Finally, we use spin
fluorescence spectroscopy at low excitation power to probe the properties of
rare-earth-ions close to a metallic wire deposited on the surface; our data
reveal line distortion caused by the mechanical strain imparted by the thermal
contractions of the metal relative to the underlying crystal. Coherent
oscillations are also observed for the most highly strained ions.
