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

Simone Garnero, Kenji Inami, Kodai Matsuoka, Ryogo Okubo, Koichi Ueda

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.

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

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 gain, using a picosecond-pulse laser and a LaB photocathode. However, a 2023 beam test with electrons impinging on a MgF 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 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.
Paper Structure (7 sections, 5 figures)

This paper contains 7 sections, 5 figures.

Table of Contents

  1. Introduction
  2. Design
  3. Previous results
  4. Photon feedback study in a beam test
  5. LaB$_6$ photocathode study
  6. Summary and prospects
  7. Acknowledgement

Figures (5)

  • Figure 1: Design of the GasPM prototype (left) and a photo of the prototype (right).
  • Figure 2: Single-photon time resolution from laser test (left) gaspm and a schematic of the photon-feedback mechanism (right).
  • Figure 3: Schematic of the beam-test setup at KEK PF-AR test beam-line.
  • Figure 4: Waveforms from two beam-test events with polynomial fit projections overlaid and the relevant derivatives in the bottom panels. One is tagged as single avalanche (left) and the other as photon feedback (right).
  • Figure 5: Distribution of the pulse rise time from 50% to the peak after applying the algorithm. Events in cyan and orange are respectively tagged as single avalanche and photon-feedback events.