Competition between Increasing and Decreasing Effects of the Afterpulsing Rate of PMTs during Night-Sky Observations

Akira Okumura; Alice Donini; Daniel Mazin; Hideyuki Ohoka; Masahiro Teshima (for the CTAO LST Project); Mitsunari Takahashi; Razmik Mirzoyan; Ryuji Takeishi; Seiya Nozaki; Shunsuke Sakurai

REVIEW 3 major objections 4 minor 13 references

Competition between Increasing and Decreasing Effects of the Afterpulsing Rate of PMTs during Night-Sky Observations

T0 review · 3 major / 4 minor · reviewed 2026-08-09 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗

Pith's one-line read Operated photomultiplier tubes did not accumulate afterpulsing over five years of LST-1 observing; the paper attributes this to a self-cleaning effect that balances helium infiltration.

desk verdict Useful first hard look at PMT afterpulsing aging in a working Cherenkov camera, with a plausible but statistically underpowered self-cleaning story. read the letter →

arxiv 2502.02423 v2 pith:JGWJ5HGL submitted 2025-02-04 astro-ph.IM

Takuto Kiyomoto , Tsutomu Nagayoshi , Shunsuke Sakurai , Mitsunari Takahashi , Tokonatsu Yamamoto , Alice Donini , Yusuke Inome , Yukiho Kobayashi

show 8 more authors

Daniel Mazin Razmik Mirzoyan Seiya Nozaki Hideyuki Ohoka Akira Okumura Takayuki Saito Ryuji Takeishi Masahiro Teshima (for the CTAO LST Project)

This is my paper · ORCID

classification astro-ph.IM

keywords afterpulsingphotomultipliertubeimagingatmosphericCherenkovtelescopeheliuminfiltrationionfeedbackself-cleaningLST-1night-skybackground

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper tests whether the afterpulsing rate of PMTs in the first Large-Sized Telescope grew over five years of operation, as helium infiltration into the vacuum would predict. It removed ten operated tubes and twelve stored spares, measured them in the lab, and compared the results with pre-installation quality-control data. The operated tubes did not increase; they slightly decreased and remained below the 2e-4 specification, while the stored spares increased, with a helium-associated afterpulse peak growing 4.4-fold. The paper's central claim is that normal night-sky observing, which supplies high voltage and constant light exposure, removes residual gas from the tube and offsets the helium-driven increase, keeping long-term performance stable.

What carries the argument

The central comparison is between Used PMTs removed from the telescope after five years of operation and Spare PMTs stored since 2013. The measurement setup is a dark box illuminated by fast light pulses of 800–920 ps width corresponding to roughly 50 photoelectrons, with high voltage set for a gain of 40,000 and afterpulses counted in a 0–3 microsecond window using a charge threshold of 4 photoelectron equivalents. The proposed mechanism is ion-feedback self-cleaning: accelerated electrons ionize residual gas inside the tube, and some ions are captured by the photocathode, thereby reducing the gas content during operation. The afterpulse arrival-time distributions, with peaks near 167 ns and 387 ns, identify the ionic species and let the authors attribute the spare-tube growth to helium.

What would settle it

Track afterpulsing rates of a larger, randomly selected set of PMTs measured before installation and again after another several years of operation; if the operated distribution shifts upward or approaches the 2e-4 limit, the equilibrium claim is wrong.

Watch

Extended reading notes

Core claim

The paper reports that after five years of regular operation in LST-1, the removed PMTs showed afterpulsing rates slightly lower than their pre-installation values and still below the 2e-4 quality criterion. In contrast, spare PMTs from the same production era that were merely stored showed increased afterpulsing rates, most visibly in a late arrival-time peak at about 387 ns that the authors attribute to He+ and which grew by a factor of 4.4. The discovery is that the operating environment itself cleans the tube: the ion-feedback process that creates afterpulses also ionizes residual gas molecules, and some of those ions are captured by the photocathode and removed from the vacuum. Over years of observation, this self-cleaning keeps the net afterpulsing rate roughly constant.

Load-bearing premise

The result depends on the ten removed PMTs being a fair sample of the full camera and on the only meaningful difference between Used and Spare tubes being operating history rather than storage history, selection, or manufacturing batch.

Editorial extensions

If this is right

After five years of regular observing, the ten removed PMTs had afterpulsing rates below the 2e-4 specification, so the telescope's false-trigger background from afterpulsing has not worsened.
Stored spare tubes showed rising afterpulsing, with a helium-associated peak growing 4.4-fold, confirming that helium infiltration is a real long-term aging process when tubes are not operated.
The operational environment of an IACT, high voltage plus night-sky light, appears to remove residual gas from the tube and offset helium entry.
The equilibrium between helium infiltration and self-cleaning implies that LST-1's PMT performance can be maintained over the telescope's intended operating lifetime under similar observing conditions.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

Editorial inference: If the self-cleaning effect can be reproduced on the bench, stored spare tubes could be rejuvenated by operating them under high voltage and illumination before installation, potentially extending the usable lifetime of PMT stockpiles.
Editorial inference: The equilibrium interpretation depends on regular operation; extended downtime, bright-moon shutdowns, or periods of low night-sky light could let helium infiltration dominate and push afterpulsing upward.
Editorial inference: The arrival-time peak positions offer a per-tube helium monitor; comparing the 167 ns and 387 ns peak heights in future removals would test whether the helium equilibrium shifts as tubes age beyond five years.

Share X Bluesky LinkedIn Reddit HN

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

You should read this as a solid empirical datapoint with an interpretation that outruns the data. The authors pulled ten PMTs out of LST-1 after five years on the sky, measured their afterpulsing in the lab, and compared them to twelve spares that sat on a shelf. The used tubes are still below the 2e-4 spec; the spares have drifted up, with a helium-associated peak growing by a factor of 4.4. As far as I know this is the first public check of afterpulsing on field-returned IACT PMTs, and the result matters for the LST cameras' energy threshold over a 20-year lifetime.

The measurement itself is straightforward and the paper is honest about its own limits — it flags the mechanism as unclear and the ion-capture explanation as a hypothesis. That is good practice.

The soft spots are statistical and one control issue. The scatter plot has no error bars, no significance test backs the "no increase" claim, and the selection of the removed PMTs is not described. Ten tubes is a small sample. More importantly, the Spare sample is not a clean helium-aging control: the spares were stored roughly nine years, presumably near sea level, while the used tubes spent five years at 2200 m. The helium dose is not the same, and the paper does not correct for exposure or report storage conditions. The stress-test note overreaches, though, when it says the exposure difference alone could explain the absence of an increase: the used tubes actually decreased slightly, and the dose difference is only a factor of about two, not enough to erase a 4.4x growth. But the comparison is still confounded enough that the equilibrium interpretation is underdetermined. A careful referee should ask for the storage history, a helium-dose argument, and a significance test.

Who gets value? Anyone working on vacuum photodetector aging, especially for Cherenkov telescopes. It is a single-telescope study, so within-field impact is modest, but it is the kind of real-operations check that only comes along rarely.

I would send it to review. The measurement deserves to be in the literature with the caveats made explicit. If I were editing, I'd ask for a revision that tightens the statistics and the control discussion, then accept.

Referee Report

3 major / 4 minor

Summary. The paper reports afterpulsing-rate measurements of ten photomultiplier tubes removed from the CTAO LST-1 telescope after about five years of operation, compared with twelve unused spare PMTs and with 2014-2015 quality-control data. The spare PMTs show an increased afterpulsing rate, concentrated in the second arrival-time peak attributed to He+, while the used PMTs show no increase and remain below the 2e-4 specification. The authors interpret this as evidence that operation at high voltage with light exposure removes residual gas molecules, offsetting helium infiltration and keeping the afterpulsing rate stable over the telescope's lifetime.

Significance. If established, the result is important for CTAO operations and for the general understanding of PMT aging in IACTs: it provides direct field evidence on the balance between helium ingress and operation-induced cleaning, and it suggests that the stringent afterpulsing specification can be maintained over years. The paper's strengths are the direct measurement on PMTs from an operating telescope, the use of the original QC baseline, and the arrival-time decomposition that links the spare-tube increase to a specific ion species. However, the central attribution of the used-spare contrast to an operation-driven removal effect is not yet quantitatively supported, because the two samples differ in helium exposure, batch, and selection, and because no controlled high-voltage/light-aging experiment is presented.

major comments (3)

[§2 and §4] The comparison that drives the conclusion is confounded by helium exposure. The spare tubes were produced in 2013 and measured after roughly nine years of storage, while the used tubes were installed around first light in December 2018 and measured after about five years at 2200 m altitude. Helium ingress scales with external partial pressure and time, yet the paper does not report the storage conditions of the spares, the manufacturing batches of either sample, or how the ten used PMTs were selected for removal. If the spares were stored near sea level, their integrated helium exposure would be approximately 9 yr x 1.0 atm versus 5 yr x 0.78 atm for the used tubes, roughly a factor of two higher. The observed contrast (factor-of-4.4 increase in the second peak for spares, no increase for used tubes) is therefore quantitatively consistent with a lower helium dose alone, and the equilibrium claim in Section 4 is underdetermined. Please report the relevant exposure parameters and selection criteria, or limit the conclusion to the observation that the used PMTs remain below specification.
[§3.1 and Figure 1] The central claim that the used PMTs show "no increase" rests on a visual comparison without error bars or a statistical test. The histograms overlap and the scatter plot has no uncertainty per point; the statement that the used rate "slightly decreased" has no confidence interval. With n=10, a paired comparison of the 2014-2015 and 2023 values should be reported with a confidence interval on the mean change, and a predefined equivalence bound or significance test should justify the phrase "no increase." Without this, the reader cannot distinguish a real null result from a small sample that lacks power.
[§4 and Introduction] The mechanism that operation at high voltage with light exposure removes residual molecules is asserted but not demonstrated in this paper. No controlled experiment is shown in which a PMT is operated under high voltage and light and its afterpulsing rate is observed to decrease; the only evidence is the spare-used contrast, which is confounded as discussed above. The statement that "residual molecules are removed from the vacuum during regular observations" goes beyond what the data can show, and the open question about where the molecules go (photocathode trapping) is speculative. Moreover, a single before/after pair cannot establish that an equilibrium has been reached; at most it bounds the net change. Either present a controlled HV and light aging measurement or weaken the conclusion to "the used PMTs did not increase above specification" and present the removal mechanism as a hypothesis.

minor comments (4)

[Figure 1] The color coding is inconsistent between the histograms (Spare red solid, Used blue dashed) and the scatter plot (Used red circles, Spare blue squares), which makes the figure hard to read.
[§2] The timeline should be stated unambiguously: the QC data are from 2014-2015, the used PMTs were installed around first light in December 2018, and the removal was in October 2023. Please clarify whether the "before" values of the used PMTs were taken at production QC or at installation, and whether the pairing in Figure 1 is per serial number.
[§3.2] The Gaussian fits are performed only on the Spare-after distribution; the statements that the first peak increased by about 8% and the second by a factor of 4.4 should specify that these are based on binned rates rather than on fitted peak amplitudes, and should give uncertainties for those factors.
[Throughout] Typos should be corrected, including "eailer" for "earlier" in Section 4, "PMT sampleset" and "abscissaisinthelogarithmicscale" in the Figure 2 caption, and "IfNN" for "INFN" in the acknowledgments.

Circularity Check

0 steps flagged · score 2.0 of 10

No circular derivation: central claim is a direct before/after measurement; self-citations are method/baseline only and not load-bearing.

full rationale

The paper's central result is a direct before/after measurement of afterpulsing rates on the same PMT samples, with the baseline taken from QC data acquired in 2014-2015. No parameter is fitted to a subset and then renamed as a prediction; the equilibrium conclusion is an inference from the Used-versus-Spare contrast, not a derived consequence of the input data. The self-cited references [5,6,9] supply the PMT development, the QC method, and the baseline data, but the new comparison is independent of those citations. The helium-increase premise rests on external references [10,11], and the self-cleaning effect is the paper's own observation rather than an imported assumption. The Gaussian peak identification and the sqrt(m/q) comparison are descriptive/interpretive and do not feed back into the central claim. The main vulnerability is experimental confounding, such as storage history, selection of the removed PMTs, and differing helium exposure between Used and Spare tubes; that is a correctness risk, not a circularity. The paper itself acknowledges open questions about the fate of trapped molecules. No circular step can be exhibited by quoting an equation or a fitted quantity that reduces to itself.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

The paper introduces no new entities. The free parameters are descriptive (threshold and peak fits) rather than load-bearing. The central assumptions are standard PMT physics, helium permeation, and the comparability of the two datasets.

free parameters (2)

afterpulse charge threshold = 4 photoelectrons
Defines which pulses count as afterpulses; adopted from prior LST QC work, not fit here, but it sets the measured rates and the comparison to the specification.
Gaussian peak fit positions for afterpulse arrival times = 166.6 +/- 1.7 ns and 386.6 +/- 1.7 ns
Fitted to the arrival-time distributions for the Spare PMTs after storage; used to identify ion species but not load-bearing for the central claim.

assumptions (4)

domain assumption Accelerated electrons ionize residual gas; resulting ions drift back and trigger afterpulses
Standard PMT model from Coates [10] and HPK handbook [3], used to interpret the peaks in Figure 2.
domain assumption Helium permeates the glass envelope and raises the afterpulsing rate over time
From Bartlett et al. [11] and the QC observations in [9]; motivates the need for the long-term check.
domain assumption Afterpulse arrival time scales as sqrt(m/q) for a given ion in the tube's electric field
Used in Section 4 to assign the 167 ns peak to H+; the paper acknowledges the field is non-uniform, so the comparison is only approximate.
domain assumption The 2014-2015 QC dataset and the 2023 measurements are comparable despite different setups
The paper states both were analyzed with the same method, but no systematic uncertainty is quantified for the comparison.

how reviews work

0 comments

Cite this review

Pith. "Pith review of Competition between Increasing and Decreasing Effects of the Afterpulsing Rate of PMTs during Night-Sky Observations." pith.science (2026). https://pith.science/paper/JGWJ5HGL

@misc{pith2026250202423,
  author       = {Pith},
  title        = {Pith review of: Competition between Increasing and Decreasing Effects of the Afterpulsing Rate of PMTs during Night-Sky Observations},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/JGWJ5HGL}},
  note         = {Machine review of arXiv:2502.02423}
}

read the original abstract

Photomultiplier tubes (PMTs) have been widely used in imaging atmospheric Cherenkov telescopes (IACTs). The Large-Sized Telescopes (LSTs) of the Cherenkov Telescope Array Observatory (CTAO), the latest-generation IACTs, are optimized for challenging observations of low-energy gamma rays, specifically in the 20 to 150 GeV range. To this end, PMTs with an exceptionally low afterpulsing probability have been developed and installed. However, the afterpulsing rate increases over time due to the infiltration of atmospheric molecules, particularly helium, into the tube. Interestingly, we found that the afterpulsing rate decreases when PMTs are operated at high voltage and exposed to light -- a condition naturally met during IACT observations. To evaluate the latest instrument response, after five years of operation, we removed several PMTs from the first LST, which is currently the only operational telescope among the CTAO instruments. Our laboratory measurements showed no increase in afterpulsing compared to pre-installation values. This suggests that the decrease in afterpulsing during operation offsets the increase, thereby maintaining the long-term performance of the PMTs.

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

13 extracted references · 12 canonical work pages

[1]

Bernlöhr,Impact of atmospheric parameters on the atmospheric Cherenkov technique, Astroparticle Physics12(2000) 255

K. Bernlöhr,Impact of atmospheric parameters on the atmospheric Cherenkov technique, Astroparticle Physics12(2000) 255

work page 2000
[2]

The CTA Consortium,Science with the Cherenkov Telescope Array, WORLD SCIENTIFIC (Feb., 2018), 10.1142/10986

doi:10.1142/10986 2018
[3]

Hamamatsu Photonics K

E.C. Hamamatsu Photonics K. K.,PHOTOMULTIPLIER TUBES –Basics and Applications–, Hamamatsu Photonics K. K., Electron Tube Devision, fourth ed. (2017). – 5 –

work page 2017
[4]

Benn and S

C. Benn and S. Ellison,Brightness of the night sky over La Palma,New Astronomy Reviews42(1998) 503

work page 1998
[5]

Mirzoyan, D

R. Mirzoyan, D. Müller, Y. Hanabata, J. Hose, U. Menzel, D. Nakajima et al.,Evaluation of Photo Multiplier Tube candidates for the Cherenkov Telescope Array, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment824 (2016) 640

work page 2016
[6]

Mirzoyan, D

R. Mirzoyan, D. Müller, J. Hose, U. Menzel, D. Nakajima, M. Takahashi et al.,Evaluation of novel PMTs of worldwide best parameters for the CTA project,Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment845(2017) 603

work page 2017
[7]

Toyama, Y

T. Toyama, Y. Hanabata, J. Hose, U. Menzel, R. Mirzoyan, D. Nakajima et al.,Evaluation of the basic propertied of the novel 1.5in. size PMTs from Hamamatsu Photonics and Electron Tubes Enterprises, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment787 (2015) 280

work page 2015
[8]

Measurement of performance of the NectarCAM photodetectors

A. Tsiahina, P. Jean, J.F. Olive, J. Knödlseder, C. Marty, T. Ravel et al.,Measurement of performance of the NectarCAM photodetectors,Nuclear Instruments and Methods in Physics Research A1007 (2021) 165413 [2110.06030]

work page Pith review arXiv 2021

Show all 13 references

[9]

Saito, M

T. Saito, M. Takahashi, Y. Inome, H. Abe, M. Artero, O. Blanch et al.,Development and quality control ofPMT modulesfor thelarge-sized telescopesof theCherenkov TelescopeArray Observatory, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometer...

2025
[10]

Coates,The origins of afterpulses in photomultipliers,Journal of Physics D: Applied Physics6 (1973) 1159

P.B. Coates,The origins of afterpulses in photomultipliers,Journal of Physics D: Applied Physics6 (1973) 1159

1973
[11]

Bartlett, A.L

D.F. Bartlett, A.L. Duncan and J.R. Elliott,Afterpulses in a photomultiplier tube poisoned with helium,Review of Scientific Instruments52(1981) 265 [https://pubs.aip.org/aip/rsi/article-pdf/52/2/265/19306319/265_1_online.pdf]

1981
[12]

Inome, T

Y. Inome, T. Yamamoto, M. Teshima, H. Ohoka, D. Nakajima and R. Mirzoyan,Development of a hundred-picoseconds pulse laser as a calibration source, in2017 IEEE Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), pp. 1–2, 2017, DOI

2017
[13]

Shah, D.S

M.B. Shah, D.S. Elliott, P. McCallion and H.B. Gilbody,Single and double ionisation of helium by electron impact,Journal of Physics B: Atomic, Molecular and Optical Physics21(1988) 2751. – 6 –

1988

Pith tools

Reviewed August 9, 2026 · model on record in the stance chip above.

[1] [1]

Bernlöhr,Impact of atmospheric parameters on the atmospheric Cherenkov technique, Astroparticle Physics12(2000) 255

K. Bernlöhr,Impact of atmospheric parameters on the atmospheric Cherenkov technique, Astroparticle Physics12(2000) 255

work page 2000

[2] [2]

The CTA Consortium,Science with the Cherenkov Telescope Array, WORLD SCIENTIFIC (Feb., 2018), 10.1142/10986

doi:10.1142/10986 2018

[3] [3]

Hamamatsu Photonics K

E.C. Hamamatsu Photonics K. K.,PHOTOMULTIPLIER TUBES –Basics and Applications–, Hamamatsu Photonics K. K., Electron Tube Devision, fourth ed. (2017). – 5 –

work page 2017

[4] [4]

Benn and S

C. Benn and S. Ellison,Brightness of the night sky over La Palma,New Astronomy Reviews42(1998) 503

work page 1998

[5] [5]

Mirzoyan, D

R. Mirzoyan, D. Müller, Y. Hanabata, J. Hose, U. Menzel, D. Nakajima et al.,Evaluation of Photo Multiplier Tube candidates for the Cherenkov Telescope Array, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment824 (2016) 640

work page 2016

[6] [6]

Mirzoyan, D

R. Mirzoyan, D. Müller, J. Hose, U. Menzel, D. Nakajima, M. Takahashi et al.,Evaluation of novel PMTs of worldwide best parameters for the CTA project,Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment845(2017) 603

work page 2017

[7] [7]

Toyama, Y

T. Toyama, Y. Hanabata, J. Hose, U. Menzel, R. Mirzoyan, D. Nakajima et al.,Evaluation of the basic propertied of the novel 1.5in. size PMTs from Hamamatsu Photonics and Electron Tubes Enterprises, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment787 (2015) 280

work page 2015

[8] [8]

Measurement of performance of the NectarCAM photodetectors

A. Tsiahina, P. Jean, J.F. Olive, J. Knödlseder, C. Marty, T. Ravel et al.,Measurement of performance of the NectarCAM photodetectors,Nuclear Instruments and Methods in Physics Research A1007 (2021) 165413 [2110.06030]

work page Pith review arXiv 2021