Photocathode characterisation and ageing studies for precise-timing gaseous detectors

The challenges of future High Energy Physics experiments, including high-pile-up environments expected in the High-Luminosity Large Hadron Collider (HL-LHC), have aroused intense interest in the development of technologies for precise detectors with high time resolution. Within the PicoSec MicroMegas collaboration, a detector that aims at reaching a~time resolution of tens of picoseconds is being developed. The first prototype of the detector demonstrated high time resolution on small scale, however, to make the concept appropriate to physics applications, several developments are required. This includes the optimalisation of the photocathode, which is at the core of the PicoSec detector. The first prototype of the PicoSec detector uses a semi-transparent cesium iodide (CsI) photocathode, but the degradation of CsI quantum efficiency (QE) is a concern in the development of this concept as it directly translates to a degeneration of the detector's timing performance. It is therefore necessary to carry out measurements to quantify the photocathodes degeneration and to find alternative materials that could be used in future versions of the PicoSec detector. The characterisation of the photocathodes was performed with the use of the ASSET measurement setup located in the laboratory of the Gaseous Detector Development group at the European Organisation for Nuclear Research (CERN) in Switzerland. The experimental setup enables QE measurements in the VUV wavelength regime (130 - 200 nm), both in reflective and transmission modes, and allows to study the process of photocathodes accelerated ageing under X-ray irradiation. The measurements of 4 different photocathode materials were made including: CsI, diamond-like carbon (DLC), boron carbide (B$_4$C) and hydrogenated nanodiamonds (HND). Experimental results showed that CsI had the highest quantum efficiency, however, the degeneration of B$_4$C and HND was much slower. This is still not a strong argument to replace CsI photocathodes, but it suggests that these materials may be potential alternatives for CsI in the PicoSec detector. The main achievement is that the setup has been tested and the results are reliable. As a next step, more materials will be examined and evaluated for their suitability for precise-timing gaseous detectors.