Mechanism for reduction of the afterpulsing rate of PMTs
Pith reviewed 2026-05-15 17:27 UTC · model grok-4.3
The pith
Afterpulsing in photomultiplier tubes decreases only when both illuminated and operated at high voltage because ionization at later dynodes traps residual gas ions.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We found that the reduction of afterpulses requires both illumination and high-voltage operation. Notably, the reduction strongly depends on the applied high voltage and is closely correlated with the integrated anode current. Therefore, we conclude that the reduction of residual gas is mainly caused by ionization occurring at later dynodes of the PMTs, and the ions are trapped by the dynodes.
What carries the argument
Ionization at later dynodes that traps residual gas ions, reducing afterpulsing when both light and high voltage are present.
If this is right
- Afterpulsing rates decrease under combined high voltage and illumination conditions.
- The amount of reduction increases with higher applied voltages.
- The reduction amount scales directly with integrated anode current.
- Ionization and ion trapping occur mainly at the later dynodes.
Where Pith is reading between the lines
- Brief operation with light and high voltage before storage could prevent afterpulsing from rising in idle PMTs.
- Periodic low-level operation of stored detectors might keep afterpulsing rates low without full telescope runs.
- Tube designs that favor ion trapping at later dynodes could improve long-term stability in vacuum detectors.
Load-bearing premise
The correlation between afterpulsing reduction and integrated anode current is caused by ionization specifically at later dynodes trapping ions rather than other effects of voltage or current.
What would settle it
If afterpulsing rates drop in tubes run at high voltage without illumination or with illumination but at low voltage, the claim that both conditions are required would be falsified.
read the original abstract
Photomultiplier tubes (PMTs) are used in Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect Cherenkov light produced by air showers induced by gamma rays in the atmosphere. The afterpulsing rate of the PMTs for the Large-Sized Telescopes (LSTs) of the Cherenkov Telescope Array Observatory (CTAO) was found to increase if they were kept unused in storage. In contrast, PMTs that had been operated in the first LST showed a slight decrease in the rate. This decrease could be explained by a reduction of residual gas caused by ion feedback, although the detailed mechanism remained unclear. In this study, to investigate factors responsible for the evolution in the afterpulsing rate, we operated several PMTs under different high voltage and light illumination conditions. We monitored their rate daily for three weeks to compare their evolution under different conditions. We found that the reduction of afterpulses require both illumination and high-voltage operation. Notably, the reduction strongly depends on the applied high voltage and is closely correlated with the integrated anode current. Therefore, we conclude that the reduction of residual gas is mainly caused by ionization occurring at later dynodes of the PMTs, and the ions are trapped by the dynodes. We also discuss a possible explanation of the reduction of afterpulsing rate by later dynodes.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports results from operating several PMTs under controlled combinations of high voltage and illumination while monitoring afterpulsing rates daily over three weeks. The authors find that afterpulsing reduction occurs only when both illumination and HV are applied, that the reduction rate increases with higher applied voltage, and that it correlates closely with integrated anode current. From these observations they conclude that residual-gas reduction is caused primarily by ionization at later dynodes, with the resulting ions trapped by those dynodes.
Significance. If the reported correlations hold under further scrutiny, the work supplies a practical, low-cost method for conditioning PMTs to lower afterpulsing rates in large IACT arrays such as CTAO LSTs. The empirical demonstration that both light and HV are required, together with the dependence on total anode current, is useful for telescope operations even if the precise dynode-stage attribution remains provisional.
major comments (2)
- [Discussion and conclusions] The inference that ionization occurs specifically at later dynodes rests on the correlation with total integrated anode current. Because anode current is measured after the entire dynode chain, the same correlation is equally consistent with ionization distributed across multiple stages or with non-ionization effects (surface conditioning, getter activation) that scale with total current or HV. No stage-resolved voltage scans, per-dynode current measurements, or ion-trajectory calculations are described that would exclude these alternatives (see the discussion of the mechanism and the concluding paragraph).
- [Results] The results section presents daily rate monitoring but does not report the number of PMTs tested per condition, the precise HV and illumination values, uncertainties on the rate measurements, or quantitative measures of the claimed correlation (e.g., correlation coefficient or fit quality). Without these, the strength of the dependence on HV and anode current cannot be assessed rigorously.
minor comments (1)
- [Abstract] The abstract summarizes the conclusion but omits any numerical indication of the size of the rate reduction or the strength of the current correlation; adding one or two representative numbers would improve readability.
Simulated Author's Rebuttal
We thank the referee for the constructive comments and positive assessment of the work's significance for CTAO operations. We address each major comment below and have revised the manuscript to improve clarity and completeness.
read point-by-point responses
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Referee: [Discussion and conclusions] The inference that ionization occurs specifically at later dynodes rests on the correlation with total integrated anode current. Because anode current is measured after the entire dynode chain, the same correlation is equally consistent with ionization distributed across multiple stages or with non-ionization effects (surface conditioning, getter activation) that scale with total current or HV. No stage-resolved voltage scans, per-dynode current measurements, or ion-trajectory calculations are described that would exclude these alternatives (see the discussion of the mechanism and the concluding paragraph).
Authors: We agree that the integrated anode current does not resolve individual dynode stages and that the observed correlation is in principle consistent with ionization distributed across the chain or with other current-dependent processes. Our attribution to later dynodes is an inference drawn from the strong voltage dependence (higher voltages increase electron energies progressively) together with the physical expectation that ions are trapped by the dynode electrodes. We have revised the discussion section to state this limitation explicitly, to mention alternative mechanisms such as surface conditioning or getter activation, and to note that stage-resolved measurements would be required for definitive confirmation. revision: partial
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Referee: [Results] The results section presents daily rate monitoring but does not report the number of PMTs tested per condition, the precise HV and illumination values, uncertainties on the rate measurements, or quantitative measures of the claimed correlation (e.g., correlation coefficient or fit quality). Without these, the strength of the dependence on HV and anode current cannot be assessed rigorously.
Authors: We thank the referee for noting these omissions. The revised manuscript now reports the number of PMTs tested per condition, the precise high-voltage and illumination settings, the uncertainties on the daily rate measurements, and quantitative measures of the correlation (including correlation coefficients) between afterpulsing-rate reduction and integrated anode current. revision: yes
Circularity Check
No circularity; purely empirical comparison of operating conditions
full rationale
The paper presents direct experimental observations: PMTs operated under controlled combinations of high voltage and illumination, with daily monitoring of afterpulsing rates over three weeks. The reported correlation between afterpulse reduction and integrated anode current, plus the requirement for both illumination and HV, is measured data rather than a derived quantity. The conclusion that ionization occurs at later dynodes is an interpretive inference from these observations, not a mathematical step that reduces to fitted inputs or self-citations. No equations, parameter fits presented as predictions, ansatzes, or load-bearing self-citations appear in the derivation chain. The work is self-contained against external benchmarks as a set of controlled empirical comparisons.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Residual gas inside PMTs produces afterpulses via ion feedback
discussion (0)
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