Speaker
Description
The Cherenkov Telescope Array is the new gamma-ray observatory consisting of three types of telescopes to cover the energy range from 20 GeV to 300 TeV. In the current design of medium- sized and large-sized telescopes, photomultiplier tubes (PMTs) are used as photon detectors in camera modules. Initially they were chosen due to their low cost and quantum efficiency (QE) curve corresponding well to the Cherenkov spectrum. The main disadvantage of PMTs is the degradation of their performance over time. The larger is the total charge emitted from the last dynode of a PMT, the more its gain decreases, thus it is not advisable to operate PMTs under high night sky background (NSB) conditions. Due to the improvement of silicon photomultipliers’ (SiPMs) performance and a decrease of their price in recent years it is now viable to consider them as an alternative option. Unlike PMTs, SiPMs are highly NSB tolerant, so by using them instead of PMTs we might be able to extend the duty cycle of the telescopes thus increasing sensitivity and energy range.
A single pixel of a camera module consists of a photon detector and a special reflective cone (a light concentrator) that is made to increase the collection area of a detector. Said cones are not designed to be used with SiPMs and introduce a wide spread of the photons’ angle of incidence at SiPMs’ surface, which is expected to cause reflection, refraction and interference effects that are not well studied at the moment. Furthermore, SiPMs’ spectral sensitivity differs from that of PMTs, which gives even more reason to carefully examine the behaviour of new camera pixels and compare it to the current design.
We are going to present measurement data of SiPM- and a PMT-based MST camera pixels’ response at 8 different wavelengths as well as our simulated comparison of these camera pixels for wavelengths of interest and provide an estimate of the total Cherenkov light collection efficiencies at camera modules. These results will allow us to make preliminary conclusions regarding the viability of such an upgrade and discuss its advantages and disadvantages.
