Comparative Measurements of the Photon Detection Efficiency of KETEK SiPM Detectors for the LHCb SciFi Upgrade Project

The LHCb SciFi detector is conceived to employ arrays of SiPM detectors to detect scintillation light from ribbons of 2.5 m long scintillating fibres of 250 $\mu$m diameter. The fibres of type Kuraray SCSF-78 are blue emitting with an emission maximum at 440 nm. However, as a consequence of the radiation damage mainly from charged hadrons in the LHCb experiments, the effective emission spectrum at the end of the fibre will shift to longer wavelengths. A simulation of the light absorption in the fibre, assuming an ionizing dose distribution along the fibre as predicted by the FLUKA code, is able to predict the emission spectrum. Fig. 1 shows the emission spectra (in arbitrary units) for 10 cm intervals along the fibre. At 250 cm, where the ionization dose is expected to reach over the full lifetime of the upgrade LHCb detector about 30 kGy, the average wavelength of emission is approximately 500 nm. The sensitivity spectrum of the SiPM detector should be tuned to match this emission spectrum, i.e. the PDE should be as high as possible in the range from 400 to 600 nm with a maximum around 500 nm. This working document summarizes preliminary results of comparative measurements of the Photon Detection Efficiency (PDE) of KETEK SiPM detectors, produced in 2012 for the LHCb SciFi upgrade project (LHCb 1$^{\rm{st}}$ prototyping). The aim of these measurements is to provide feedback to the supplier in order to tune the sensitivity spectrum for the LHCb second prototyping, which is foreseen for the second half of 2013.