  The alpha decay of $\n{{}^{210}Po}$ is a dangerous background to rare event
searches. Here, we describe observations related to this alpha decay in the
Cryogenic Rare Event Search with Superconducting Thermometers (CRESST). We find
that lead nuclei show a scintillation light yield in our $\n{CaWO_4}$ crystals
of $0.0142\pm0.0013$ relative to electrons of the same energy. We describe a
way to discriminate this source of nuclear recoil background by means of a
scintillating foil, and demonstrate its effectiveness. This leads to an
observable difference in the pulse shape of the light detector, which can be
used to tag these events. Differences in pulse shape of the phonon detector
between lead and electron recoils are also extracted, opening the window to
future additional background suppression techniques based on pulse shape
discrimination in such experiments.
