Recent GasPM advances: photon-feedback mitigation and LaB$_{6}$ photocathode studies

TL;DR

This paper reports recent advances in GasPM technology, focusing on photon-feedback mitigation and LaB$_{6}$ photocathode studies, with improvements in time resolution and understanding of feedback effects for particle detection applications.

Contribution

It introduces an improved GasPM setup with a new digitizer and investigates LaB$_{6}$ photocathodes for enhanced performance and feedback suppression.

Findings

Achieved 25 ps single-photon time resolution with LaB$_{6}$ photocathode in 2022.

Observed degradation to 70 ps in 2023 beam test with MgF$_2$ window and CsI photocathode.

Developed a new digitizer to discriminate primary and secondary signals, addressing photon feedback issues.

Abstract

We report 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, offering excellent time resolution and cost-effective scalability. In addition, the GasPM provides precise and efficient Cherenkov-based charged-particle identification if combined with a radiator. Our primary use case aims at an upgrade of the Belle II detector to suppress beam-induced background photons, preferably detected off-collision time, that degrade the performance of the electromagnetic calorimeter. In 2022 we achieved a promising single-photon time-resolution of 25 ps at 3.3 x 10$^6$ gain, using a picosecond-pulse laser and a 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 work aims at addressing 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). We design and execute an improved beam test. Along with several GasPM configuration changes, we introduce a new 10 GSPS frequency digitizer to better discriminate primary from secondary signals thus enabling the study of photon feedback. We also conduct a cosmic-ray test using a LaB$_6$ 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.