Improvements of the optical detection setup of COLLAPS

In Collinear Laser Spectroscopy of rare isotopes it is crucial to keep the background of the measurements low. Using photomultiplier tubes for those measurements, the dark count rate should be as low as possible and the signal gain high. Therefore, it is important to use the right type of photomultiplier tubes for the purpose of single photon detection with a low signal rate. Thus, I tested several different types of photomultiplier tubes for their dark count rates and noise to signal separation. The results have identified the most suitable type of photomultiplier tubes, but further measuring for verification might be necessary. Therefore, it is very important to keep the photomultiplier tubes shut from light. Exposing the photocathode to light implies life-long damage and increased background for several days. So far, when dismounting one of these photomultiplier tubes from the setup, a big cage around the optical detection setup was covered into black sheets to keep it as dark as possible inside. Still, the photomultiplier tubes got exposured by some light. To avoid this, a shutter system was developed. This system contains a slit for reducing stray light, a system for adjustment of the distance between the photomultiplier tubes and some focusing lenses and the actual shutter, that can be closed in case of dismounting a photomultiplier tube. This shutter already has been tested and the results are promising.