Speaker
Description
Photosensitive gaseous detectors with a simple photoelectron multiplication mechanism as resistive plate chamber are expected to offer both large photocoverage and excellent time resolution while keeping costs low. We have developed a gaseous photomultiplier (GasPM) and demonstrated that a single-photon time resolution is $25 \pm 0.2$ ps at the gain of 3.3 $\times 10^{6}$ with a $\rm{LaB_{6}}$ photocathode, which has an extremely low quantum efficiency. With a CsI photocathode, GasPM can be used as a picosecond-timing Cherenkov detector. A possible application of this detector is the particle identification in the Belle II experiment, which is an electron-positron collider experiment searching for physics beyond the Standard Model through precise measurements of $B$, $\tau$, and $D$ decays. A picosecond-timing Cherenkov detector can enhance particle identification efficiency through precise time-of-flight measurements.
We developed a second prototype of GasPM with a $\rm{MgF_{2}}$ window as Cherenkov radiator and a CsI photocathode. A mixture of R134a and $\rm{SF_6}$ gases is used, and the gas gap size is 200 $\mu m$. We conducted the first beam test to evaluate the performance of this GasPM using the 3 GeV electron beam at the PF-AR test beamline located at KEK, Japan. We applied up to 2.8 kV across the gap, at which the single-photon time resolution estimated by simulation is $\sigma=60-70~\rm{ps}$, and verified that the time resolution was as expected. However, we observed a secondary avalanche caused by feedback photons from the primary avalanche, which degraded the time resolution due to the overlap of the two pulses. The time resolution can be improved by suppressing the effects of feedback photons and applying a higher voltage.
We will present these results and discuss the plans for the GasPM development and the future implementation at the Belle II experiment.
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