{"id":"38ce8be8-ace4-43f1-9fe9-a0beead7e08e","arxiv_id":"2507.01819","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":3.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":1,"one_line_summary":"The FAMU experiment's LaBr3:Ce X-ray detector system with SiPM array readout achieves about 3% energy resolution at 662 keV and shows the muonic oxygen K lines.","lead":"This paper describes the X-ray detector system built for the FAMU experiment, which uses 34 LaBr3:Ce crystals and one HPGe detector to look for muonic oxygen X-rays after a mid-infrared laser excites muonic hydrogen. It reports that the silicon-photomultiplier readout performs comparably to photomultipliers and can see the muonic oxygen signal lines.","discovery_kind":"extension","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The in-beam oxygen-line claim rests on a background subtraction the authors explicitly flag as imperfect; without an HPGe cross-check or alternative background model, the residual K lines may be artifacts.","rationale":"I reviewed the manuscript in good faith. The detector performance claims are well supported by laboratory measurements: the 1-inch SiPM detectors show linear response over 100-1200 keV, FWHM energy resolution of 2.94 +/- 0.14% at 662 keV and 8.03 +/- 0.39% at 120 keV, and the in-situ calibration procedure with Am-241, Ba-133, Cs-137 plus beam-induced C, Pb, and Ag peaks is a reasonable practical approach. The timing claim for the 4-1 readout is less directly demonstrated because there is no same-crystal comparison of 4-1 versus parallel ganging, but this is secondary to the main physics demonstration. The weakest load-bearing point is exactly what the reader identified: the visible oxygen lines in Figure 8 depend on a background subtraction that the authors themselves flag as imperfect. The negative overshoot at the beginning of the spectrum indicates a mismatch between the H2 and H2+O2 datasets, and without a cross-check from the HPGe detector or an alternative background model, the residual peaks are not yet convincingly established as genuine muonic-oxygen X-rays. This does not require rejecting the paper; it justifies the existing CONDITIONAL verdict. The HPGe detector is already part of the setup, so the proposed check is directly feasible.","tokens_in":4886,"tokens_out":3159,"duration_ms":38807,"concrete_test":"Re-analyze the December 2023 beam data using the HPGe inter-calibration detector: reconstruct the background-subtracted HPGe spectrum from the same H2+O2 and H2 runs, using the same normalization procedure, and check whether K-alpha at ~133 keV and K-beta at ~158 keV appear. If the HPGe shows no such lines while the LaBr3:Ce detectors do, the Figure 8 peaks are likely artifacts of the subtraction. In addition, rerun the LaBr3:Ce subtraction with the H2 background scaled by ±3% and with a small energy-calibration shift; genuine lines should persist at stable energies with comparable yields, whereas artifact peaks should shift, invert, or disappear.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The central demonstration that the SiPM-readout LaBr3:Ce system works in the actual physics environment is Figure 8, where muonic-oxygen K-alpha and K-beta/gamma lines appear only after subtracting an H2-only background. The manuscript's own footnote states that negative values at the beginning of the spectrum are due to imperfect background subtraction. That admission matters: it shows the subtraction does not cancel the H2 background cleanly, so the positive residuals at 133, 158, and 167 keV could in principle come from a small gain shift, a normalization mismatch, or a calibration drift between the H2 and H2+O2 runs rather than from genuine muonic-oxygen X-rays. If those peaks are subtraction artifacts, the paper's claim of clear detection of the oxygen signal lines in the beam environment is not supported, even though the laboratory energy-resolution and linearity measurements would remain valid. The concern is not that imperfect subtraction is fatal by itself; it is that the paper provides no independent cross-check, such as the HPGe inter-calibration detector on the same runs, to show the residual lines are robust against reasonable changes in the background model.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The paper describes the fast X-ray detection system built for the FAMU experiment at RAL, which aims to measure the proton Zemach radius via the hyperfine splitting of muonic hydrogen. The system consists of 34 LaBr3:Ce crystals (six with PMT readout, 28 with SiPM array readout) plus one HPGe detector for inter-calibration. The paper reports laboratory measurements of energy resolution and linearity for the SiPM-readout detectors, a dedicated 4-1 readout circuit that reduces signal fall time, a temperature feedback system that controls gain drift, and in-beam data showing muonic-oxygen K-alpha and K-beta/gamma lines after background subtraction. The central conclusion is that the SiPM array readout performs comparably to PMT readout in energy resolution while offering lower cost and flexible geometry.","tokens_in":5060,"tokens_out":6068,"duration_ms":63257,"significance":"If the reported performance is sustained, the FAMU detector system meets the experimental requirements: FWHM energy resolutions of about 3% at 662 keV and 7-8% at 120-142 keV are sufficient to separate the oxygen X-ray lines, and the fall times are compatible with the delayed-signal requirement. The paper contains credible, directly measured laboratory data (linear response over 100-1200 keV, resolution values, and gain-drift reduction from 41% to 5% with temperature feedback) and identifies an operational issue (neutron-induced dark-current increase) together with a recovery procedure. The 4-1 SiPM readout circuit is a useful technical development. These are positive, concrete contributions to detector development for muonic X-ray spectroscopy.","major_comments":[{"comment":"The in-beam demonstration that the system detects the muonic-oxygen signal lines rests entirely on a background-subtracted spectrum with no statistical uncertainties, and the footnote in Figure 8 explicitly states that the background subtraction is imperfect, as evidenced by negative values at low energies. The authors do not provide error bars on the subtracted spectrum, a systematic variation of the background normalization, an alternative background model, or a comparison with the HPGe inter-calibration detector for the same runs. Because a small gain shift or normalization mismatch between the H2-only and H2+O2 runs could in principle produce positive residuals near 133, 158, and 167 keV, the claim of 'clear' detection of the oxygen lines is not yet quantitatively supported. Please provide a robustness study (e.g., varying the normalization by ±1–2%), fit the residuals with known line positions to extract significances, or show the same lines in the HPGe data.","section":"Section 4, Figure 8"},{"comment":"The PMT readout entries (fall time ~60 ns, R 662 keV 3.5–4.6, R 120 keV 7.2–8.1) are quoted as single values or ranges without uncertainties, while the SiPM entries are given as mean ± RMS. Since the paper concludes that SiPM readout 'performs comparably' to PMT readout, the PMT numbers should carry the same statistical treatment (sample mean and spread, number of detectors) or be explicitly referenced to the earlier publication [7]. As written, the comparison between the two readout schemes is not statistically assessable.","section":"Table 1"}],"minor_comments":[{"comment":"The sentence 'scanning the temperature range 10−300C' appears to contain a typo; it should read '10–30 °C'.","section":"Section 3"},{"comment":"The bottom panel's y-axis label 'FWHM Energy Resolution [%]' appears to plot fractional values between 0.02 and 0.2, not percentages; either change the label to a fraction or multiply the plotted values by 100.","section":"Figure 4"},{"comment":"The SiPM model numbers are inconsistent: Section 2 lists Hamamatsu S4161-6050-04-AS arrays, while Section 3 refers to S14161-6050-AS arrays; please unify the notation.","section":"Sections 2 and 3"},{"comment":"For the PMT row, the ranges for R 662 keV and R 120 keV should specify whether they represent the spread over individual detectors, and the number of detectors used for the average should be stated.","section":"Table 1"}],"recommendation":"major_revision","confidential_remarks":"The paper is a conference-style detector report within the scope of physics.ins-det. The laboratory performance claims are credible and well supported. The main weakness is the unquantified background subtraction in the oxygen-line demonstration; this is fixable but needs to be addressed before the in-beam performance claim can be accepted. If the authors provide the requested systematics or an HPGe cross-check, the paper would be suitable for publication."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"You should read this as a status report, not a physics result. What is genuinely new is modest: the 2024 run configuration with the enlarged 1\" detectors, the dark-current problem traced to neutron damage and recovered by baking at 125 C, and the 4-1 summing circuit that cuts signal fall time. Those are real engineering contributions, and the paper is honest about what it does and does not show.\n\nThe lab measurements are the strong part. Energy resolution of 2.94 +/- 0.14% at 662 keV and 7.0 +/- 0.3% at 142 keV for the 1\" SiPM detectors is plausible and consistent with earlier papers from the same group. The linearity plot, the temperature-gain drift correction, and the in-situ calibration procedure are all concrete and reproducible. Credit where due: this is a working detector system, and the performance claims rest on direct measurements, not on circular reasoning.\n\nThe soft spot is exactly what the stress-test note flags. Figure 8 shows K-alpha and K-beta/gamma oxygen lines only after subtracting an H2-only background, and the footnote admits the subtraction is imperfect. Without an HPGe cross-check on the same runs, or a demonstration that the residual peaks survive reasonable changes in background normalization or gain alignment, you cannot fully exclude subtraction artifacts. The authors do mention an HPGe inter-calibration detector in the setup, so asking for that check is reasonable, not speculative. This does not damage the lab-based detector claims, but it does mean the \"clear signal lines in the physics environment\" claim is weaker than the rest of the paper.\n\nMinor issues: Table 1 omits uncertainties for the PMT fall time and gives the 662 keV resolution as a range; the 4-1 circuit's factor-of-four improvement is stated without a direct side-by-side baseline in this paper, though it may be in references [10] and [11]. None of these are load-bearing.\n\nWho is this for? FAMU collaborators and people building fast scintillator detectors with SiPM readout. It is not a breakthrough, but it is a solid engineering record with honest limitations. I would send it to a referee, mainly to force the authors to either add the HPGe cross-check or soften the oxygen-line language. The lab results deserve publication; the beam claim needs a bit more support.","headline":"A useful, honest progress report on the FAMU LaBr3:Ce detector system; the lab measurements carry the paper, the in-beam oxygen-line claim is the soft spot.","tokens_in":5606,"tokens_out":1355,"would_cite":false,"duration_ms":17757,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"The FAMU fast X-ray detector system, built on LaBr3:Ce crystals with SiPM array readout, achieves energy resolution comparable to photomultiplier readout and detects muonic oxygen X-ray lines after background subtraction, supporting the…","keywords":["LaBr3:Ce","SiPM","muonic X-rays","hyperfine splitting","proton Zemach radius","gamma-ray detectors","energy resolution","FAMU"],"falsifier":"Re-analyse the December 2023 H$_2$+O$_2$ beam data using a different background estimate, such as interpolating the H$_2$ continuum from time sidebands before the delayed X-ray window or fitting the continuum shape analytically, and check whether the peaks near 133, 158, and 167 keV persist. Alternatively, examine the same target mixture with the HPGe detector, whose $1.26\\%$ resolution at 142 keV would resolve the lines cleanly if they are real; absence of the lines in the HPGe spectrum would indicate the LaBr3 signal is a subtraction artefact.","tokens_in":4649,"feed_emoji":"⚛️","tokens_out":7273,"duration_ms":67935,"temperature":0.7,"pith_summary":"The FAMU experiment at the RIKEN-RAL muon facility aims to measure the hyperfine splitting of muonic hydrogen and from it the Zemach radius of the proton to better than one percent. The signal is a delayed emission of muonic-oxygen X-rays around 130-170 keV following a mid-infrared laser pulse, so the experiment needs fast detectors with good energy resolution in that range. This paper reports the performance of the experiment's fast X-ray detector system, built from 34 LaBr3:Ce scintillating crystals with silicon-photomultiplier (SiPM) array readout plus one HPGe detector for inter-calibration. The paper's central claim is that the SiPM readout performs comparably to conventional photomultiplier readout (about 2.94% FWHM at 662 keV and 7.0% at 142 keV for the 1-inch detectors), and that a dedicated 4-1 readout circuit shortens the signal fall time by up to a factor of four. It further shows that, after H$_2$ background subtraction, the detectors see clear K$\\alpha$ and K$\\beta$/ $\\gamma$ lines from muonic oxygen, demonstrating that the system works in the physics environment.","feed_headline":"SiPM detector readout matches PMTs and sees muonic oxygen","feed_subtitle":"LaBr3:Ce arrays hit 2.94% resolution at 662 keV and spot muonic oxygen lines at RAL.","key_machinery":"The central mechanism is the LaBr3:Ce scintillator read out by an array of silicon photomultipliers, with the 1-inch detectors using a '4-1' circuit that groups the outputs of four nearby $6\\times6$ mm$^2$ SiPM cells, applies per-group pole-zero compensation and amplification with OPA695 operational amplifiers, and then sums the four sub-array signals and inverts them. This circuit is what compresses the signal fall time by up to a factor of four (used at a conservative factor of two in the final setup) while keeping the energy resolution near 3% at 662 keV, and it is the piece that allows delayed muonic-oxygen X-rays to be separated from the prompt beam background. The system also includes an online gain-drift correction that stabilizes the SiPM response against temperature changes, reducing the variation of the $^{137}$Cs photopeak position from 41% to 5% over a 10-30 C scan.","core_discovery":"The paper establishes, on the basis of laboratory and beam tests, that LaBr3:Ce detectors read by SiPM arrays are a viable replacement for photomultiplier readout in the FAMU fast X-ray system. The 1-inch SiPM detectors show a linear response from 100 to 1200 keV, FWHM energy resolution of $2.94 \\pm 0.14\\%$ at 662 keV and $8.03 \\pm 0.39\\%$ at 120 keV in the laboratory, and $7.0 \\pm 0.3\\%$ at the 142 keV muonic silver peak in beam data, compared with $12.3 \\pm 1.2\\%$ for the PMT-read detectors at that same line. The 4-1 SiPM readout circuit, which groups four $6\\times6$ mm$^2$ SiPMs with individual pole-zero compensation and amplification before summing, reduces the 10-90% signal fall time to $147 \\pm 13$ ns from $372 \\pm 17$ ns for the 1/2-inch parallel-ganged version, a speed-up used conservatively as a factor of two in the experiment. In December 2023 beam data with a H$_2$+O$_2$ (1.5% wt.) gas mixture, the detectors show clear K$\\alpha$ and K$\\beta$/ $\\gamma$ muonic oxygen lines after H$_2$ background subtraction, which is the signature the experiment needs to see the laser-excited hyperfine transition.","pith_inferences":["If the residual lines survive a re-analysis with an independent background model (e.g., time-sideband subtraction), the muonic-oxygen detection would be on much firmer ground; the authors' own footnote about imperfect subtraction leaves this as the main open check.","The fall-time reduction achieved by circuit-level grouping of SiPM cells may generalize to other fast scintillator readouts that need to distinguish prompt and delayed signals in pulsed beams.","The neutron-damage/baking recovery behaviour suggests that SiPM-based detectors operating near muon targets should budget for periodic annealing runs, a practical design consideration for future experiments.","Because the HPGe detector has far better energy resolution, correlating its spectra with the LaBr3:Ce spectra for the same target gas could provide an independent cross-check of the oxygen lines without relying on the LaBr3 background subtraction."],"forward_implications":["With the demonstrated energy resolution and reduced fall time, the FAMU detector system is capable of recording the delayed muonic-oxygen X-rays that encode the laser-driven hyperfine transition, so the 2023-2024 data can be used to extract the proton Zemach radius.","The 4-1 SiPM readout design can be applied to other large-area LaBr3:Ce detectors where PMTs are undesirable, giving comparable resolution with a shorter fall time.","The comparable performance of SiPM and PMT readout means that future detector rings can be built entirely with SiPM readout, simplifying mechanics and lowering cost at equal physics output.","The recovery of neutron-damaged SiPM arrays by a 24-hour bake at 125 C provides a maintenance procedure that keeps the detector system operational over multiple beam campaigns."],"supporting_citations":[{"why":"Supplies the PMT-readout baseline and the digital pulse processor that the SiPM readout compares against.","marker":"[7]"},{"why":"Establishes the laboratory energy resolution and gain-drift correction for the 1-inch SiPM-read detectors.","marker":"[8]"},{"why":"Documents the 1/2-inch parallel-ganged SiPM readout whose fall time the 4-1 circuit improves.","marker":"[9]"},{"why":"Reports the lab measurements of linearity and resolution used for the 1-inch SiPM detectors.","marker":"[10]"},{"why":"Describes the 4-1 readout and the fall-time reduction factor.","marker":"[11]"},{"why":"Provides the temperature-feedback method that stabilizes the SiPM gain during data taking.","marker":"[12]"}],"fun_headline_variants":["SiPM readout for LaBr3:Ce rivals PMTs in FAMU X-ray system","FAMU fast X-ray: SiPMs beat PMTs, spot muonic oxygen","LaBr3:Ce with SiPM readout detects muonic oxygen at RAL","FAMU shows SiPMs match PMTs, catch muonic oxygen signature"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The claim of seeing muonic oxygen signal lines in beam data rests on a background subtraction that the authors explicitly note is imperfect, so the lines could in principle be artifacts of that subtraction rather than genuine X-rays.","fun_headline_variants_meta":{"raw":{"variants":["SiPM readout for LaBr3:Ce rivals PMTs in FAMU X-ray system","FAMU fast X-ray: SiPMs beat PMTs, spot muonic oxygen","LaBr3:Ce with SiPM readout detects muonic oxygen at RAL","FAMU shows SiPMs match PMTs, catch muonic oxygen signature"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.00083,"raw_usage":{"total_tokens":3629,"prompt_tokens":956,"completion_tokens":2673,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":572,"completion_tokens_details":{"reasoning_tokens":2580}},"tokens_in":572,"tokens_out":2673,"duration_ms":24664,"temperature":1.0,"reasoning_tokens":2580,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-06T20:41:07.318758+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Re-analyse the December 2023 H$_2$+O$_2$ beam data using a different background estimate, such as interpolating the H$_2$ continuum from time sidebands before the delayed X-ray window or fitting the continuum shape analytically, and check whether the peaks near 133, 158, and 167 keV persist. Alternatively, examine the same target mixture with the HPGe detector, whose $1.26\\%$ resolution at 142 keV would resolve the lines cleanly if they are real; absence of the lines in the HPGe spectrum would indicate the LaBr3 signal is a subtraction artefact.","supporting_citations":[{"cited_title":"Baldazzi et al., The LaBr3:Ce based detection system for the FAMU experiment, JINST 12 (2017) C03067","cited_arxiv_id":null,"evidence_quote":"Supplies the PMT-readout baseline and the digital pulse processor that the SiPM readout compares against."},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"Establishes the laboratory energy resolution and gain-drift correction for the 1-inch SiPM-read detectors."},{"cited_title":"Bonesini et al., Detection of low-energy X-rays with 1/2 and 1 inch LaBr 3:Ce crystals read by SiPM arrays , PoS (EPS-HEP2021) 770; M","cited_arxiv_id":null,"evidence_quote":"Documents the 1/2-inch parallel-ganged SiPM readout whose fall time the 4-1 circuit improves."},{"cited_title":"Bonesini et al., Large area LaBr3:Ce crystals read by SiPM arrays with improved timing and temperature drift control, Nucl","cited_arxiv_id":null,"evidence_quote":"Reports the lab measurements of linearity and resolution used for the 1-inch SiPM detectors."},{"cited_title":"Bonesini et al","cited_arxiv_id":null,"evidence_quote":"Describes the 4-1 readout and the fall-time reduction factor."},{"cited_title":"Bonesini, R","cited_arxiv_id":null,"evidence_quote":"Provides the temperature-feedback method that stabilizes the SiPM gain during data taking."}],"review_version":1}