Speaker
Description
We report the results of recent developments and tests with beams and cosmic rays of the gaseous photomultiplier (GasPM). The GasPM is a photosensor that combines a photocathode with the avalanche-multiplication mechanism of a resistive-plate chamber and offers excellent time resolution and cost-effective scalability. In addition, the GasPM can provide precise and efficient, Cherenkov-based, charged-particle identification if combined with a radiator. The GasPM can find applications in many HEP experiments. Our primary use case would be in an upgrade of the Belle II detector to suppress off-collision-time beam-induced background photons that degrade the performance of the electromagnetic calorimeter. In 2022 we achieved a promising single-photon time-resolution of 25 ps at 3.3 × 10$^6$ gain using a picosecond-pulse laser and a lanthanum hexaboride (LaB$_6$) photocathode. However, a 2023 beam test with electrons impinging on a MgF$_2$ window attached to a CsI photocathode showed a worsening to 70 ps.
This contribution addresses the principal causes of the time-resolution degradation. We primarily target ultraviolet-photon emission during excitation and de-excitation of the gas molecules, which leads to a secondary signal that overlaps the primary signal, spoiling time resolution (photon feedback). To this end, we designed and executed an improved beam test that, along with several GasPM configuration changes, introduces a new 10 GSPS sampling-frequency digitizer to better discriminate primary from secondary signals thus suppressing photon feedback. We also conduct a cosmic-ray test using a LaB₆ photocathode, which is known to have higher than CsI's resistance to ions drifting backwards onto the photocathode and to air exposure, to probe quantum efficiency in view of an upcoming beam test.
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