Precision fitting function for HPMT spectra

Hybrid Photomultiplier tubes are an ideal tool for detecting small numbers of photons. However, as the number of photons increases, the probability of one or more of the resultant photo-electrons backscattering from the diode increases and eventually the individual photon peaks are no longer resolved. The spectra obtained are complex and the extraction of precise results depends on fitting a complex distribution. A function has been developed that provides an excellent fit to HPMT data and allows the extraction of precision results for the light collection uniformity from lead tungstate crystals for the CMS electromagnetic calorimeter end-caps.