The CRESST experiment monitors 300 \mathrm { g } \mathrm { CaWO _ { 4 } } crystals as targets for particle interactions in an ultra low background environment .
In this paper , we analyze the background spectra that are recorded by three detectors over many weeks of data taking .
Understanding these spectra is mandatory if one wants to further reduce the background level , and allows us to cross-check the calibration of the detectors .
We identify a variety of sources , such as intrinsic contaminations due to primordial radioisotopes and cosmogenic activation of the target material .
In particular , we detect a 3.6 \mathrm { keV } X-ray line from the decay of \mathrm { { } ^ { 41 } Ca } with an activity of ( 26 \pm 4 ) \mathrm { \mu Bq } , corresponding to a ratio \mathrm { { } ^ { 41 } Ca } / \mathrm { { } ^ { 40 } Ca } = ( 2.2 \pm 0.3 ) \times 10 ^ { -16 } .