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Excess backgrounds in Dark Matter detectors and physics of glasses
by Sergey Pereverzev
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|Authors (as registered SciPost users):
|SciPost Physics Proceedings
|14th International Conference on Identification of Dark Matter (IDM2022)
|Theoretical, Experimental, Phenomenological
Multiple mechanisms allow energy accumulation in materials and later releases. Interactions between excitations, defects, or other configurations carrying excess energy can lead to avalanche energy releases and more complex phenomena like self-organization and self-replication effects in systems with energy flow. Exact theoretical models of these phenomena are often impossible because of insufficient knowledge of interactions inside materials. Still, comparison and analogies between excess low-energy backgrounds in solid-state detectors, mechanisms of inelastic deformations of crystals, and relaxation processes in glasses allow essential predictions for the background properties and insight into the physics of glasses. The presence of avalanche-like releases of stored energy is expected for NaI(Tl). We observed energy accumulation and release processes in the form of delayed random luminescence. We also demonstrated suppression of delayed luminescence by exposure to red light. We cannot yet clearly confirm or refute the presence of small avalanches or excess coincidences in the delayed photon flux. An interplay of fast and delayed luminescence response depends on the type of particles and temperature. One can expect that other environmental factors could cause modulations of low-energy background. More studies of material responses to radiation are required. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-PROC-840806
Submission & Refereeing History
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Reports on this Submission
- Cite as: Anonymous, Report on arXiv:scipost_202210_00066v1, delivered 2022-11-24, doi: 10.21468/SciPost.Report.6189
•I like the proposed idea that excesses in solid-state detectors could be related to metastable states that cause avalanche effects. This could potentially predict the power-law energy spectrum and certainly is an important first step in building models for the excesses observed with cryogenic solid-state detectors.
• The parallels with the theory of self-organized criticality are very interesting, especially because this theory seems to appropriately model the behavior of earthquakes (Gutenberg-Richter law), and a parallel to relaxation effects in solids in thinkable.
• The argument that delayed luminescence could lead to delayed increases of the event rate in NaI(Tl) detectors, caused by an environmental impact that happened already earlier, is interesting and if this was not studied previously, it could be a nice check to perform for experiments using NaI(Tl).
• The author is unspecific about which excesses he wants to explain in this work, and switches between the wording “all detectors” and the DAMA excess specifically. While some of the referenced excesses can have a common origin (cryogenic solid-state detectors), others were already shown to have qualitatively different properties (CCD detectors, gaseous targets). This was shown in the references the author cites. How the DAMA excess, which is mainly characterized by its annual modulation, is related to the others is not clear to me and not motivated in the manuscript in way that I could comprehend.
• The proposed theory is a purely heuristic model and does not include any concrete physics (which states and excitations, etc). Thus, it does not constrain any existing hypothesis for the origin of the excesses, nor does it build a predictive and testable model.
• There are some major issues with scientific reasoning. The author generalizes from some sources to others without argument, draws conclusions without basing them on experimental evidence or calculations, and does not reference some of his statements (see more detailed comments below).
• The author explains the time-dependency in the event rate of low energy excesses with the physics of glasses, and the power law-ish energy spectrum with the theory of self-organized criticality. The connection between these two phenomena is not clear to me and the author does not provide any evidence that either of the phenomena is realized in the nature of the detectors observing the excesses.
• While the studies of delayed luminescence are certainly interesting, the connection to the modulation in the DAMA event rate is not properly argued and attributed to “environmental effects”.
• The structure of the text is more that of a speech, than that of a scientific paper, which makes it very hard to read.
The author discusses low energy excess backgrounds observed by multiple low threshold experiments. He relates the phenomena to the theory of self-organized criticality and the physics of glasses. He also discusses the modulation in the event rate observed by the DAMA experiment and relates it to the phenomenen of delayed luminescence.
In addition to the listed strengths and weaknesses, I have some detailed comments. Due to the lack of line numbering I can only reference parts of the manuscript and not individual sentences, I hope the comments are still useful.
• Liquid noble detectors are mentioned in the introduction but then never again.
• Reference : As I understand “Scientific American” is a popular science journal. Please reference scientific information from scientific journals.
• Headline of section 2: please use shorter headlines
• In several places in the manuscript are spaces missing, this should be easy to fix with a spellchecking program.
• Beginning of section 2: I think 10-1 eV should mean 10-1000 eV.
• Right after this sentence: The claim, that many experiments observe an increase of background with thermomechanical stress dearly needs references. To the best of my knowledge, this is at the moment a hypothesis and only confirmed in one measurement (Ref.  in the manuscript). In this paragraph, the author needs to be very specific about which observations were made in which measurements, instead of generalizing from one measurement to an unspecified number of excesses.
• “Thus, the low-energy background we see is a natural relaxation process of mechanical stress release in single crystals and solids in general.” This is a hypothesis but presented in the manuscript as if it was a result.
• Beginning of section 3: are the used target materials all amorphous? To the best of my knowledge the target crystals are monocristalline. Please specify for which detectors the theory of glasses is applicable. Also, please add some references for the statements in this paragraph.
• Ref. [14, 15, 16]: They are listed together with phenomena that seem unrelated to the content of the text above and below. Please specify how they are relevant for the arguments.
• Section 3, start of third paragraph: I don’t see on which arguments the conclusion that is drawn here is based. Please explain in detail and avoid basing scientific statements on “parallels and analogies”.
• Section 4: This section seems very important in the line of arguments that interpret the experimental results that are presented later on. However, there is not a single reference listed in this section, and the statements that are made are not trivial.
• “The author posits that exposure of the solid-state detector to ionizing radiation or UV light should lead to delayed low-energy background events with events spectrum and decay time resembling those caused by mechanical stress.” This hypothesis was rejected for at least one experiment (arXiv:2207.09375), that does not see an increase in the low energy background after intervals of strong exposure to calibration sources. Please specify which excesses should be explained by this.
• Section 5: This section contains very interesting information about current open questions about glasses. However, the relevance of this information to the rest of the manuscript is not clear to me and should be explained in more detail.
• Section 6: Please describe your experimental setup, your analysis chain, and your results in more detail, quantitatively and in this order. Were the experiment and analysis performed by yourself alone? Otherwise, I recommend referencing/acknowledging collaborators.
• Section 6: A reference to the criticized Saint-Gobain analysis would be helpful.
• Section 6: “The photon flux detected by DAMA-LIBRA and similar experiments consists of delayed luminescence photons (mostly random) and bursts of photon emission produced as a fast/immediate response to external particles.” Is this a result or a hypothesis? If the former, then please reference, if the latter, please formulate as such.
• Section 6: “This suggests that other environmental factors also can affect delayed luminescence response and cause modulation of the DAMA-LIBRA signal; the time of the maximum intensity of random/uncorrelated delayed luminescence can be different from the maximum of muon flux which pumps energy into the system (this was not yet checked).” This argument seems to be one of the most central ones of the manuscript and deserves some more discussion. Which environmental impacts? Could the delay really shift the peak of the event rate? How would this work? In your experimental results the delayed luminescence is released on an exponential scale after the UV irradiation, how would that shift a peak?
• Section 7: The conclusion does mostly state that further work needs to be done, but does not summarize the results, statements, and significance of the work at hand. I strongly recommend adding this. In the last sentence, the decoherence in quantum sensors is mentioned for the first time in the manuscript, I would either explain this claim earlier or remove it.
• Page 6: I recommend putting the figures in the position in the document where they are discussed, not at the end of the document. Also, a smaller figure size would still transport all the information but free up space for text.
• Fig 1: Was the figure made specifically for this paper or published elsewhere? In the latter case, please reference.
• Fig 1: I don’t understand the second plot (b). What decay time is expected of the scintillation light pulses from this target? Are these pulses from particle hits not always several tens of milliseconds long? How can a statistic be made for the number of pulses per mus? Are these pulses really so fast?
• References: I recommend using BibTex or a similar references manager. Otherwise, please implement common formatting, indentations, border width, etc.
I summarize that the intent of this work is certainly important and the ideas have the potential to make a difference in the field. The current state of the manuscript is hard to comprehend, contains incorrectly generalized and unreferenced statements that could cause dangerous confusion among readers, and assertions that are not fully based on the scientific method. I recommend a comprehensive revision based on the below suggestions and a subsequent second round of review.
• I recommend adopting a structure that is easier to comprehend for readers. I especially suggest:
⁃ Start the introduction by specifically describing the problem (which excesses, their properties, references), explain that no expected phenomenon fits the observations (can’t be radiation backgrounds, dark matter, …), motivate your explanation for them, then give an overview of the sections in the paper and what you will discuss/show there.
⁃ After the introduction, first explain the theory (self-organized criticality, properties of glasses), and which behavior of the detectors/excesses it would predict. Do this in a scientific way: make predictions that are testable with data.
⁃ Then continue with your sections, discussing individual observations of experiments and their fit to your theory in detail.
⁃ End with a conclusion where you summarize the results, statements, and impact of this work.
• Please reference the individual experiments that observe the excesses.
• Statements/results need to be clearly distinguishable from assumptions/hypotheses. Former ones have to be referenced properly.
• The detailed comments from above should be addressed.
• The content of the manuscript is very advanced and brings together concepts from different fields. The more important is the preparation of this information in a way that is accessible to readers without extensive previous knowledge. Adding more figures that describe the discussed phenomena could make the reading easier.
• Please add line numbers for the next round of review.