Antideuteron Identification in Space with Helium Calorimeter

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript describes development of a detector for cosmic anti-deuterons based on helium calorimeter. It is a great idea to use the metastable state of negative particles in helium as a way to uniquely distinguish anti-deuterons from protons. 

 

However, I cannot recommend publishing this manuscript in the present form. I would like to see the following issues addressed.

 

Major issues:

1)    In section 3, the design of the detector is described. In Figs. 4-6, the authors show some plots that show the expected performance of the detector. At the first reading, it was not clear to me that they were simulated results, as the authors use the word “measured” repeatedly. But these must be simulated results. Otherwise, there would no reason to do the tests with a B-470 detector described in section 4. It needs to be clearly stated that these results are from a simulation. 

2)    The relevant parameters used for the simulation need to be clearly listed with their values and where they came from. 

3)    The purpose of the B-470 detector test should be clearly stated. If they are to confirm some assumptions that went to the simulation, it needs to be stated.

4)    It appears that the timing and energy resolutions are two of these parameters for which the assumptions are studied in the B-470 detector test. The relationship between the timing and energy resolutions measured in B-470 and those expected for the final detector needs to be made clearer, considering the deference in the geometry.  Both timing and energy resolutions depend on the amount of light signal received, which depends on the detector design, including the detector geometry. It is clear from Figs 3 and 7, the geometry of the final detector is very different from that for B-470. The way it is written, it is not clear how the B-470 test informs the detector simulation performed for the final detector geometry.

 

Minor points:

5)    What is the helium gas pressure for the final detector? In some places, it says 200 bar and, in some places, it says 400 bar.

6)    What are the wavelengths of the UV light from helium scintillation? What type of WLS is used? What wavelengths does it convert to what wavelengths?  

7)    Line 102: “fast neutrons detector” -> “fast neutron detector”

8)    In many places, the authors use the word “ToF” to mean “ToF detector”. For example, in lines 140-141, it says “… three prompt hits in the ToF and one prompt energy release in the HeCal.” This is confusing. It should say “… three prompt hits in the ToF detector and one prompt energy release in the HeCal.” I urge the authors to fix this throughout the manuscript. 

9)    Figure 4: what is the double peak structure of S1 coming from? 

10) Figure 4: is the delayed scintillation discussed in Section 4 included in the simulation? 

11) Line 156: “release” -> “deposit”. There are many other occurrences where the word “release” is used when the word “deposit” should be used. 

12) Figures 5 and 6: the figures are blurred and are hard to read.  I cannot read the legend in Figure 6 nor the axis titles. 

13) Line 191: what is “PMT crossing”? It is not clear why one of the PMT had to be replaced with a SiPM array.

14) Line 287: “an hint” -> “a hint”

15) Line 289: Helium -> helium

16) Line 290: Helium -> helium

 

17) Line 304: “An Helium” -> “A helium”

Comments for author File: Comments.pdf

Comments on the Quality of English Language

I recommend that the authors have the manuscript checked by a native English speaker. 

Author Response

We thank reviewer 1 for the useful suggestions. 
Here the answers for all the questions raised by reviewer 1.

Question 1)    
In section 3, the design of the detector is described. In Figs. 4-6, the authors show some plots that show the expected performance of the detector. At the first reading, it was not clear to me that they were simulated results, as the authors use the word “measured” repeatedly. But these must be simulated results. Otherwise, there would no reason to do the tests with a B-470 detector described in section 4. It needs to be clearly stated that these results are from a simulation.
Answer 1) 
The referee has well argued that the plots in fig. 4-6 must be obviously MC simulations since an antideuteron beam is not existing and a test to an antiproton beam is already a very hard task. 
However the captions of fig 4-6 have been modified to clarify this point.

Question 2) 
The relevant parameters used for the simulation need to be clearly listed with their values and where they came from.
Answer 2) 
The velocity and energy resolutions adopted in the simulation are already written in the manuscript (Lines 132 to 134). A sentence is added in the manuscript to further clarify the parameter adopted for the HeCal: “A time resolution of 1 ns and energy resolution of 10% have been assumed in the simulation for the HeCal detector, these assumptions are supported by preliminary measurements on an HeCal prototype (see next sections)”.

Question 3)    
The purpose of the B-470 detector test should be clearly stated. If they are to confirm some assumptions that went to the simulation, it needs to be stated.
Answer 3) 
In the top of the section 4:  “Test of HeCal performance with Arktis B-470 detector” at line  182 
It is already written: 
“The timing and energy resolutions of the HeCal detector are key parameters for the PHeSCAMI project. Some measurements on a prototype based on the fast neutron detector B-470 Arktis Radiation Detectors [ 20 –22 ,29 ] have been conducted at INFN-TIFPA
laboratories in order to test the response of pressurized Helium gas as a scintillator.”
Moreover at the end of sub-section 4.4 it is already written:
“As a summary, the energy resolution tested with the Arktis B-470 Helium scintillator confirms the PHeSCAMI hypothesis that HeCal detector would be able to measure energy depositions larger than 10 MeV with an energy resolution better than 10%”.
A similar sentence is written at the end of sub-section 4.5:
“As a summary, the time resolution tested with the Arktis B-470 detector confirms the capability 
of a pressurized Helium calorimeter to detect the >50ns delayed annihilation, that is the signature for antinuclei of the PHeSCAMI technique.”
Therefore we think that the purpose of the B-470 detector tests are already clearly stated.
 
Question 4)    
It appears that the timing and energy resolutions are two of these parameters for which the assumptions are studied in the B-470 detector test. The relationship between the timing and energy resolutions measured in B-470 and those expected for the final detector needs to be made clearer, considering the deference in the geometry.  Both timing and energy resolutions depend on the amount of light signal received, which depends on the detector design, including the detector geometry. It is clear from Figs 3 and 7, the geometry of the final detector is very different from that for B-470. The way it is written, it is not clear how the B-470 test informs the detector simulation performed for the final detector geometry.
Answer 4)
The B-470 test was the first preliminary test on a (cheap) commercial detector.
The B-470 detector test measured the fast component of He scintillation. It is very fast, therefore we expect no issues to detect a S2 signal that is delayed more than 50ns.
Moreover we verify the 10% energy resolution assumption of the MC simulation. 
It is true that the performance of the improved PHeSCAMI prototype depends on our ability to collect the light signal on the PMT placed outside the COPV vessel. 
This is the reason why we are developing this new prototype in the next two year project:
PRIN-2022 n.2022LLCPMH “PHeSCAMI - Pressurized Helium Scintillating Calorimeter for AntiMatter Identification" as anticipated in the conclusion of the manuscript.

Question 5) 
What is the helium gas pressure for the final detector? In some places, it says 200 bar and, in some places, it says 400 bar.
Answer 5) 
The MC simulation assumes the ArianeGroup 400bar vessel. This is written in line 129.
The B-470 detector used for these preliminary test is filled with 200 bar 
(this is a commercial detector and we cannot modify this pressure).
There is not a “final detector” mentioned in the manuscript.

Question 6)    
What are the wavelengths of the UV light from helium scintillation? What type of WLS is used? What wavelengths does it convert to what wavelengths? 
Answer 6) 
The wavelength of the helium scintillation is 80nm as stated in the cited references. 
We did not measure this number and we did not construct the B-470 detector, 
the details of used WLS system in the B-470 detector are covered by an Arktis patent.
However B-470 detector works, thus a WLS system from 80nm to visible light is feasible. 
When we will build and test our detector we will publish details about used WLS.
Question 7)    
Line 102: “fast neutrons detector” -> “fast neutron detector”
Answer 7) Done. 

Question 8)    
In many places, the authors use the word “ToF” to mean “ToF detector”. For example, in lines 140-141, it says “… three prompt hits in the ToF and one prompt energy release in the HeCal.” This is confusing. It should say “… three prompt hits in the ToF detector and one prompt energy release in the HeCal.” I urge the authors to fix this throughout the manuscript.
Answer 8) Done

Question 9)    
Figure 4: what is the double peak structure of S1 coming from?
Answer 9)
As written in line 155: “The duration of S1 signal is of the order of few ns that is the particle slowdown time in He.”

Question 10) 
Figure 4: is the delayed scintillation discussed in Section 4 included in the simulation?
Answer 10)
No, the long scintillation tail is still not included in this preliminary MC simulation. 
In an “old-style” DAQ, with a correct choice of the duration of the ADC gate width (<30ns), the effect of this tail is negligible as can be argued by fig 13. Obviously, with a “modern” digitizing DAQ, the small contribution due to the long scintillation tail can be inferred for each event. 

Question 11) 
Line 156: “release” -> “deposit”. There are many other occurrences where the word “release” is used when the word “deposit” should be used.
Answer 11) Done

Question 12) 
Figures 5 and 6: the figures are blurred and are hard to read.  I cannot read the legend in Figure 6 nor the axis titles.
Answer 12)
Referee is right. Picture resolution is now improved. 

Question 13) 
Line 191: what is “PMT crossing”? It is not clear why one of the PMT had to be replaced with a SiPM array.
Answer 13)
“avoiding PMT crossing” replaced with “avoiding the passage of the particle through the PMT”

Questions/Suggestions 14-15-16-17)  Changement done in the manuscript.

Best regards

Francesco Nozzoli

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript describes the design, operation, and preliminarily projected performance of a future high-pressure He calorimeter targeting anti-deuteron detection to probe dark matter models.  The design has interesting features, especially in the anti-proton/anti-deuteron separation power provided by the mass-dependent metastable state lifetime of the exotic atom formed by anti-particle capture on the He.  There are, however a few items that I would like clarified or expanded on.

- Where are the data in Fig. 2 taken from?  The only reference provided ([11]), which would appear to be the source of the data, only features measurements taken at considerably lower temperatures.  What kind of scaling are the authors using to produce the data points in these plots?  It is not clear that the results have been studied at near-room temperatures.  Are there any concerns that the conclusions of the authors of [11] might not be valid at 300+ K?  It seems at the very least that the kinematics my be affected by average He energies being one to two orders of magnitude higher.  I also would like the meaning of the lines on the plot to be clarified, as they are drawing attention to a difference in the data, but are never discussed in the text.

- For the COPVs that are mentioned, what are the dimensions?  Are they near-spherical, or extended cylinders?  Geometric considerations affect the grammage that incident particles will pass through, and thus the functional energy range of the instrument and the backgrounds on the measurement.

- For the Geant4 simulations, it's not clear what effects are included in the simulations themselves and what is added post-hoc based on the expectations of the authors.  What physics list is being used for the study?  If the annihilation and creation of decay products is included in the simulations, why simply mention the difference in p-bar and d-bar signals instead of showing the simulated results?

- With regards to the sufficiency of the trigger scheme to reject background events, have the authors considered Solar protons?  For a space-based measurement, depending on orbit and instrument geometry, Solar particles could conceivably satisfy the "start trigger."  Although it seems likely that the time requirement in the "stop trigger" could reject many of these, it would be interesting to see the rejection power for electrons, protons, and He at different primary energies.

- With regards to the energy resolution, significant position-dependent effects were accounted for in the measurement, and the SiPMs demonstrated worse performance than PMTs.  Given these features, are the authors concerned that scaling up to a significantly larger instrument might come with worsened energy resolution?  How does this test inform the readout scheme and electronic design of the future design?

Comments on the Quality of English Language

Overall the English in the paper is very good.  Trusting that a few typos will be found by editors, I have just one significant note:

- On line 183, presumably the steel cylinder isn't 5 meters thick!

Author Response

We thank reviewer 2 for the useful suggestions. 

Here the answers for all the questions raised by reviewer 2.

 

Question 1)

- Where are the data in Fig. 2 taken from?  The only reference provided ([11]), which would appear to be the source of the data, only features measurements taken at considerably lower temperatures.  What kind of scaling are the authors using to produce the data points in these plots? It is not clear that the results have been studied at near-room temperatures.

Are there any concerns that the conclusions of the authors of [11] might not be valid at 300+ K?

It seems at the very least that the kinematics my be affected by average He energies being one to two orders of magnitude higher. I also would like the meaning of the lines on the plot to be clarified, as they are drawing attention to a difference in the data, but are never discussed in the text.

Answer 1)

The referee is right. The source of data plotted in fig.2 was not clear.

Citation to Reference 9 and 10 is now included in the caption of fig.2. 

The caption is improved by mentioning the dashed lines.

The effect is existing and measured in He gas at room temperature as summarized in fig. 2.

Question 2)
- For the COPVs that are mentioned, what are the dimensions?  Are they near-spherical, or extended cylinders? Geometric considerations affect the grammage that incident particles will pass through, and thus the functional energy range of the instrument and the backgrounds on the measurement.
Answer 2):
As written in line 129 the ArianeGrup COPV is considered in the MC simulation. 
And as written in line 116 the volume of the ArianeGroup vessel is 300L.
The ArianeGroup vessel is spherical, as shown in fig3.right panel. However also cylindrical shapes are produced for the automotive market, as shown in the example of fig.3.left panel.

Question 3)
- For the Geant4 simulations, it's not clear what effects are included in the simulations themselves and what is added post-hoc based on the expectations of the authors.
What physics list is being used for the study?  If the annihilation and creation of decay products is included in the simulations, why simply mention the difference in p-bar and d-bar signals instead of showing the simulated results?
Answer 3)
The updated Geant4 version and used FTFP_BERT physics list was added to text.
The annihilation and creation of decay products is included in this physics list.
The meta-stability effect is non included in the physics list but is simply modeled by tagging all the particles produced by the annihilation.
The discrimination of p-bar is shown as this is the most important background for d-bar search.

Question 4)
- With regards to the sufficiency of the trigger scheme to reject background events, have the authors considered Solar protons? For a space-based measurement, depending on orbit and instrument geometry, Solar particles could conceivably satisfy the "start trigger." Although it seems likely that the time requirement in the "stop trigger" could reject many of these, it would be interesting to see the rejection power for electrons, protons, and He at different primary energies.
Answer 4)
This manuscript summarizes the preliminary simulations and prototype results of the project: INFN (Grant73/2018) "ADHD - Anti Deuteron Helium Detector".
The detailed study of all the background sources and improved prototype test will be addressed in the next two years with the project: "PRIN-2022 n.2022LLCPMH “PHeSCAMI - Pressurized Helium Scintillating Calorimeter for AntiMatter Identification".

Question 5)
- With regards to the energy resolution, significant position-dependent effects were accounted for in the measurement, and the SiPMs demonstrated worse performance than PMTs.
Given these features, are the authors concerned that scaling up to a significantly larger instrument might come with worsened energy resolution?
How does this test inform the readout scheme and electronic design of the future design?
Answer 5)
The referee is right. SiPM performance was worse than PMT, mainly due to the smaller total surface. This SiPM array device was necessary to match the pipe geometry of the ARKTIS B-470 detector and was shaped as a ring to allow the beam entrance.
We will test the resolutions of the improved (large COPV) prototype in the next two year project:
PRIN-2022 n.2022LLCPMH “PHeSCAMI - Pressurized Helium Scintillating Calorimeter for AntiMatter Identification". We are planning to use a PMT for this project.

Best Regards

Francesco Nozzoli

Reviewer 3 Report

Comments and Suggestions for Authors

Figures Understanding and Referencing (Line 87-98):

Fig2 appears to derive from FIG.9 of reference 11, but the discussion lacks clarity regarding the differences and the authors' contributions. It would be beneficial to elaborate more on these modifications, making it easier to understand the original work from reference 11.

Additionally, the absence of access to reference 20 makes it challenging to identify the presence of the PHeSCAMI project within it. It would be beneficial for the authors to provide additional details or context related to PHeSCAMI within the paper.

Figure 3 Details (Line 99-106):

In Fig. 3, particularly in panel a), there is a lack of size information. Including specific length measurements would enhance the comprehensibility of the depicted data. Similarly, the sizes in the right panel of Fig. 3 need better clarification.

Unexplained Terminology and References (Line 107-115, 121-133):

The usage of S1 and S2 at lines 138-149 lacks detailed explanations regarding their origins. Additionally, from lines 154-170, more descriptions about the backgrounds referenced would enhance the reader's understanding. Furthermore, at line 172-173, defining "TOF activity classifier" would be beneficial for a clearer grasp of the context.

 

Clarity on Experiment Details (Line 183-196):

 

There might be a typographical error at line 183 regarding "5m thick." It would be helpful to clarify this statement.Regarding the Arktis B480 at line 197, providing information about its relevance to the experiment would be valuable for readers' understanding.The transition from two PMTs to one PMT and SiPM readout at line 200 requires more explanation, along with the results comparison from the two PMT version.

Missing Data and Results (Line 204-213):

Section 4.1, from line 196-204, lacks any presented data or results. Incorporating relevant findings or an update on the current status would enhance this section's comprehensibility.

 

A question, the Arktis B480 is made for this experiment ?

Detailed Figure Explanations (Line 214-228):

Fig. 9 requires descriptions of the Trigger T0 and veto T2, along with information about the timing resolution of T0 for better contextual understanding.The asymmetry in data in Fig. 10’s right panel requires discussion or an explanation to offer clarity on the matter. At line 226-228, elucidating on the measured energy at the forward PMT and rear SiPM sensors, including energy distributions, would enrich the data interpretation.

Understanding Experimental Concepts (Line 235-243):

Line 235-243 mentions P(d) without clarifying its derivation from the actual experiment. Providing insights into how P(d) was determined in the real experimental setup would be valuable.

Figure Details and Representation (Line 244-266):

In Fig. 12, a clarification about how the timing resolution of the trigger counter T0 was considered would help in interpreting the data.Similarly, Fig. 13 requires details on whether the figure represents averaged pulse shapes or singular snapshots. Additionally, the treatment of data on the vertical axis labeled in A.U needs clarification, especially considering they represent event entries.

Completeness of Data (Line 267-282):

Regarding Fig. 14, the interpretation indicates that the data point at 200 bar stands alone and serves as an upper limit. It would be advantageous to confirm if there are no data from other experiments at different pressures to complete the analysis.

 

Conclusion and Future Directions (Line 283-298): 

In the conclusion, expanding on "the confirmation of the PHeSCAMI" by providing a more precise description of the project status and outlining future activities needed would significantly enhance the paper's conclusion.

Author Response

We thank reviewer 3 for the useful suggestions. 
Here the answers for all the questions raised by reviewer 3.

Question 1)
Figures Understanding and Referencing (Line 87-98):
Fig2 appears to derive from FIG.9 of reference 11, but the discussion lacks clarity regarding the differences and the authors' contributions. It would be beneficial to elaborate more on these modifications, making it easier to understand the original work from reference 11.
Answer 1)
The referee is right. The source of data plotted in fig.2 was not clear.
Citation to Reference 9 and 10 is now included in the caption of fig.2. 
The caption is improved by mentioning the dashed lines.

Question 2)
Additionally, the absence of access to reference 20 makes it challenging to identify the presence of the PHeSCAMI project within it.
It would be beneficial for the authors to provide additional details or context related to PHeSCAMI within the paper.
Answer 2)
This conference proceeding is the first manuscript related to the PHeSCAMI project.
Ref. 20 was not related to PHeSCAMI project but was just one paper related to He scintillation.
Ref. 20 has been replaced with a more accessible one.  

Question 3)
Figure 3 Details (Line 99-106):
In Fig. 3, particularly in panel a), there is a lack of size information. Including specific length measurements would enhance the comprehensibility of the depicted data.
Similarly, the sizes in the right panel of Fig. 3 need better clarification.
Answer 3)
Fig.3.left is just an Example, the caption is improved adding "Example".
Regarding Fig.3.right to clarify the size information, in addition to "1.5m" written in the picture, also the information of the volume of the Vessel (300 L) is added in the caption.
As written in line 129 of the text the ArianeGrup COPV is considered. And as written in line 116 the volume of ArianeGroup vessel is 300L.

Question 4)
Unexplained Terminology and References (Line 107-115, 121-133):
The usage of S1 and S2 at lines 138-149 lacks detailed explanations regarding their origins.
Answer 4)
line 143 is simply the definition of S1
line 149 is simply the definition of S2
The origin of the double scintillation signal in He is related to the delayed annihilation.

Question 5)
Additionally, from lines 154-170, more descriptions about the backgrounds referenced would enhance the reader's understanding.
Answer 5)
Citation to [27] is added for the description of the main background sources in Cosmic Rays.

Question 6)
Furthermore, at line 172-173, defining "TOF activity classifier" would be beneficial for a clearer grasp of the context.
Answer 6)
The description of the information used in the “ToF Activity Classifier” is in the caption of fig.6
However reference [28] to multivariate classifiers is added in the caption and in the text.

Question 7)
Clarity on Experiment Details (Line 183-196):
There might be a typographical error at line 183 regarding "5m thick." It would be helpful to clarify this statement.
Answer  7)
The referee is right. The wall thickness is only 5mm. Typo corrected.

Question 8)
Regarding the Arktis B480 at line 197, providing information about its relevance to the experiment would be valuable for readers' understanding.
Answer 8)
The lines 183-190 and fig.7 describe the Arktis B-470 neutron detector. Ref [20-22] are citing papers using this kind of detector.
Citation 29 is the Arktis website. We think this description is enough.

Question 9)
The transition from two PMTs to one PMT and SiPM readout at line 200 requires more explanation, along with the results comparison from the two PMT version.
Answer 9)
Lines 194-200 explain well why we had to replace one of the two pmt with an hollow/ring SiPM array. We improve this explanation:
"To allow the detection of the charged particles in the calorimeter avoiding the passage of the particle through the PMT, one PMT of the Arktis B-470 detector was replaced with an array of 8x Silicon Photo-Multipliers (SensL MicroFJ-60035 6x6mm2 Fill Factor 65%) see left panel of Fig. 8. The SiPM circular array is shielded by 20 cm of iron and a central hole, ∅ ≈1 cm, allows the particles to enter in the Helium target crossing only the (≈ 2.5 cm thick) fused silica optical window."

Question 10)
Missing Data and Results (Line 204-213):
Section 4.1, from line 196-204, lacks any presented data or results.
Incorporating relevant findings or an update on the current status would enhance this section's comprehensibility.
Answer 10)
Fig.10 is showing the gray points (MIP \mu in the legend) related to muon data taking
Fig.11 and 12 yellow and light green points (\mu in the legend) are related to muon data taking
This sentence is added at the end of "muon calibration" subsection to further convince the reader we really used these muon acquired events to infer the detector performance:
"The muon calibration is analyzed along the proton calibration to measure the detector performances described in the following sections (figures 10,11,12)."

Question 11)
A question, the Arktis B480 is made for this experiment ?
Answer  11)
This detector is a commercial fast neutron detector Arktis B-470,  see Answer 8)

Question 12)
Detailed Figure Explanations (Line 214-228):
Fig. 9 requires descriptions of the Trigger T0 and veto T2, along with information about the timing resolution of T0 for better contextual understanding.
Answer 12)
A sentence is added as suggested:
"Two plastic scintillators (4mm thickness) are placed in front of the detector, to provide the DAQ trigger, T$_0$ and behind the detector, to provide a crossing/veto trigger T$_2$."
Intrinsic time resolution of the plastic scintillators is very good.
Our measurement of time resolution of Arktis B-470 assumes cautiously that the contribution from the plastic scintillator is negligible, we add another sentence about this in line 267.
For our system time resolutions are limited by the "typical non-uniform sampling time step, varying from cell to cell, of the CAEN DT5742B 5GS/s digitizer based on the DRS4 chip" 
as written in the text.

Question 13)
The asymmetry in data in Fig. 10’s right panel requires discussion or an explanation to offer clarity on the matter. At line 226-228, elucidating on the measured energy at the forward PMT and rear SiPM sensors, including energy distributions, would enrich the data interpretation.
Answer  13)
The asymmetry in data in Fig. 10's right panel is already explained in the caption of fig.10 and in text: "Position resolution for particles passing near the SiPM (right) side is worse than the one for particles passing near the PMT (left) side due to the smaller detection surface of the 8xSiPM array (1.9 cm2 vs 3 cm2 considering the 65% SiPM Fill Factor and the 27% PMT Quantum Efficiency)."

Question 14)
Understanding Experimental Concepts (Line 235-243):
Line 235-243 mentions P(d) without clarifying its derivation from the actual experiment.
Providing insights into how P(d) was determined in the real experimental setup would be valuable.
Answer 14)
The referee is right. The left/right amplitude ratio shown in the previous version does not allow a simple visualization of P(d). To help the reader the Left plot of fig.10 is replaced with a direct measurement of P(d) based on the same data.

Question 15)
Figure Details and Representation (Line 244-266):
In Fig. 12, a clarification about how the timing resolution of the trigger counter T0 was considered would help in interpreting the data.
Answer 15)
In the text is already written:
"measuring the time difference of the Helium scintillation and the plastic scintillator signals."
We add also the "obvious" clarification:
"The time resolution of the plastic scintillator is negligible in this difference."

Question 16)
Fig. 13 requires details on whether the figure represents averaged pulse shapes or singular snapshots. Additionally, the treatment of data on the vertical axis labeled in A.U needs clarification, especially considering they represent event entries.
Answer 16)
The sentence "obtained as the average of many different scintillation pulses" is added.
The vertical axis is normalized, thus "normalized amplitude" or A.U. are already the correct units.

Question 17)
Completeness of Data (Line 267-282):
Regarding Fig. 14, the interpretation indicates that the data point at 200 bar stands alone and serves as an upper limit.
It would be advantageous to confirm if there are no data from other experiments at different pressures to complete the analysis.
Answer 17)
We are sorry, we cannot find other data from other experiments at different pressures.

Question 18)
Conclusion and Future Directions (Line 283-298):
In the conclusion, expanding on "the confirmation of the PHeSCAMI" by providing a more precise description of the project status and outlining future activities needed would significantly enhance the paper's conclusion
Answer 18)
We prefer to keep a cautious attitude in the conclusion, as written the improved PHeSCAMI prototype is still under development. We prefer to avoid adding a list of future measurements that we have still not planned in detail.

Best regards

Francesco Nozzoli

 

Reviewer 4 Report

Comments and Suggestions for Authors

Dear Author, the paper is well presented and deserve to be published. 

I have some comments on how the discussion is presented, for your consideration. 

In general you frequently make use of very long sentences, which I suggest to break in two shorter ones for a better reading.

 

 

Abstract

l 5: A pressurized Helium calorimeter in space -> A space-based pressurized Helium calorimeter

 

paper:

l 32: x-rays -> X-rays

l 37: The 1440 Si(Li) sensors of the GAPS tracker needs to be cooled to -> The 1440 Si(Li) sensors of the GAPS tracker are cooled down to

l 42: space spectrometers -> space-base spectrometers

l 45: provides -> provide

l 54: atom [18], thus a slightly -> atom [18].A slightly (break the sentence)

l 71: of the single remaining electron -> of the remaining electron

l 84: remove one 'the'

l 88: He density, the measurements -> He density. The measurements (break the sentence)

l 108: allowing the dectection -> allowing for the detection

l 115: gas cylinders, they are -> gas cylinders. They are (break the sentence)

l 156: are relativistic protons, they would release 10 MeV crossing the HeCal diameter, this energy -> are relativistic protons, thus they would release 10 MeV crossing the HeCal diameter, and this energy

l 162: nuclei and would open a 50 ns -> nuclei opnening a 50 ns

l 163: The delayed annihilation signal is also providing a relatively large energy released -> The delayed annihilation signal provides a relatively large energy 

l 169: the offline event analysis, in particular figure 5 -> the offline event analysis. In particular figure 5 (break the sentence)

l 173: allows -> allow

l 173: this is shown -> as shown

l 181: in INFN -> at INFN

l 188: Response -> Responses

l 199: The data was -> The data were

l 202: bar Helium), conversely the -> bar Helium. Conversely the (break the sentence)

l 209: 7), in -> 7). In (break the sentence)

l 212: protons entering in the detector longitudinally -> protons entering longitudinally in the detector 

l 213: In this last case -> In this latter case

l 214: the Helium scintillator is following the typical -> the Helium scintillator follows the typical

l 215: Waveforms was -> Waveforms were

l 217: The measurement the Arktis -> The measurement of the Arktis

l 222: is an photon -> is a photon

l 235: the rough approximantion -> a rough approximation

l 247: , 4.5% is -> , 4.5%, is 

l 258: Where sigma 1 can be -> Sigma 1 can be

l 276: The signal grow time -> The signal rise time

l 277: the measured Tau -> The measured Tau (break the sentence)

l 285: for nuclear recoils, for this reason -> for nuclear recoils. For this reason (break the sentence)

l 287: an hint -> a hint

l 288: delete 'precisely'

l 291: gas impurities, in figure 14 -> gas impurities. In figure 14 the (break the sentence)

l 293: Since our system is not directly measuring the UV scintillation -> Since our system does not directly measure the UV scintillation

l 304: provide a suitable energy -> provide suitable energy

 

Comments for author File: Comments.pdf

Author Response

We thank reviewer 4 for the useful suggestions. 
All the suggested corrections have been applied in the new manuscript version.

Best regards

Francesco Nozzoli

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Author Response

Reviewer1 report2) 

We thank the Reviewer1 report for this proceedings of the ASAPP2023 conference.

 

Question1)

My main issue remains to be associated with the authors’ claim that a 10% energy

resolution was demonstrated. To get a 10% energy resolution, one needs to get at

least 100 photoelectrons per event. I maintain that the authors have not demonstrated

this. In fact, as I point out below, there is a serious flaw in the assumption that went into

the design of the experiment, which would likely cause the number of photoelectrons to

be zero.

There are many factors that go into the number of photoelectrons detected:

1. Energy deposition per event
2. EUV photon yield per energy deposition
3. The solid angle subtended at the location of the event by EUV photon detector

(WLS)

4. The conversion efficiency of the WLS
5. Transmission efficiency of the wavelength-shifted light
6. Detection efficiency of the photon detector for the wavelength-shifted light

From the description in the manuscript and from the authors’ response to my questions,

the authors do not have sufficient knowledge and understanding of some of these

factors needed to make an estimate of the expected number of photoelectrons for the

interested events nor to make a solid connection between the B-470 based

measurement and the MC simulation. Below are some details:

1. In their response to my question, the authors state that the wavelength of the

EUV photon they wish to detect is 80 nm. This is the dominant EUV emission in

liquid helium. An 80 nm photon is emitted when an excited helium molecule

decays to the unbound ground state. There are several nuclear and particle

experiments that use this EUV (extreme ultraviolet photons) and there have been

extensive studies of their formation mechanism and EUV photon yields per

energy deposition for different type of ionizing radiation in liquid helium (see e.g.

Refs. [Ito2012, Phan2020]). Note that the photon yield depends on the types of

the radiation (e.g. fast electron vs heavy particles such as protons and alphas).

The formation rate of the excited molecules naturally depends on the helium

density. Also, there are non-radiative decays (i.e. quenching) of these helium

excimers, caused by Penning ionization, which depends on the ionization density

of the particle track [Ito2012]. For gas, therefore, I would expect that the

scintillation yield per energy deposition would depend on the helium gas

pressure. What is the EUV photon yield for 200 bar and 400 bar? Is the

difference in the EUV photon yield taken into account when connecting the B-470

based measurement to the MC?

1. For gas helium, there are other scintillation lines that originates from atomic

process (instead of excited molecules). See e.g. Ref. [Kaji2014].

1. The authors do not know what kind of material is used as wavelength shifter in

the B-470 detector. How can they connect the results from the B-470 based

measurement to the MC? By the way, there are not many WLS materials that

work for 80 nm EUV photons. People who detect liquid helium scintillation

typically use tetraphenyl butadiene (TPB) (see e.g. Gehman2013). People either

evaporate it on a substrate or put it into a polystyrene matrix.

1. Figure 3 shows that the simulated detector uses WLS fibers. Are they assumed

to be commercially made WLS fibers? If so, which brand and model? I have not

seen any commercially made WLS fibers that work at 80 nm. The materials used

for optical fibers are opaque to 80 nm light. Therefore, 80 nm light cannot even

penetrate the cladding of WLS fibers, keeping the photons from reaching the

core and thereby resulting in zero detected photons. One could consider making

cladding-less fibers from TPB loaded polystyrene. In this case, the difference in

the conversion efficiency between evaporated TPB (what B-470 is likely using)

and TPB loaded in a polystyrene matrix need to be taken into account. In any

case, the authors need to discuss what is assumed for the WLS fiber drawn in

Fig. 3 and how the detector performance is affected by the differences in the

geometry and materials between B-470 and what is depicted in Fig 3 .

In conclusion, I am forced to conclude that the MC results that assume a 10% energy

resolution are not justified and that the manuscript does not meet the standard for

publication.

 

Answer1)

I thank the Reviewer1 for the useful suggestions regarding the design of the WLS system and I am also thankful about the suggestions about the evaluation of the number of expected  photoelectrons. The WLS system developed by Arktis is mentioned in ref [22]. It seems feasible.

As mentioned in the caption, the Left panel of fig 3 is just the “Example of a possible light readout system for the COPV”. The topic of this manuscript is not the design of the WLS system that is still ongoing and we are aware of all the issues suggested by the Reviewer1. 

 

I understand that Reviewer1 is concerned that a 10% energy resolution is not feasible with He scintillators. However we bought the Arktis B-470 detector and we achieved a 10% resolution.

The energy resolution of the Arktis B-470 Helium scintillator measured at the proton beam and with atmospheric muons are shown in fig 11.

Left plot of fig.11 shows that the measured energy resolution is worse than 10% only for small energy depositions (in the range 0.25MeV-1.1MeV) but reaches 10% when 10 MeV energy is deposited in the Helium, following the expected Poisson statistics with 56 photoelectrons/MeV detected by the PMT. Right plot of fig.11 shows that an energy resolution better than 10% is obtained for a 16MeV energy deposition. This is a reasonable demonstration that with Arktis B-470 helium scintillator an energy deposition in the range of 25-250MeV (the one of fig. 5) would be affected by an energy resolution better than 10%.

 

In the MC we assumed for HeCal an energy resolution of 10%. This is a fair assumption.

This MC is considering just a spherical vessel filled with Helium; there is no simulation of a WLS system for the COPV that is still in the design phase. This is the usual approach in science, one performs a simulation with some assumptions and then tests the assumptions on prototypes.

This test with the Arktis B-470 prototype was the first successful confirmation that this 10% assumption is feasible, we will further verify this assumption also by building/testing the next prototype using an automotive COPV as mentioned in the conclusions.

 

We didn’t assume an optimistic 1%, but a fair 10% that is supported by the prototype measurements. What should we do? Never shoving or mentioning a MC simulation?

Should we use 15% resolution or 30% or 50% resolution? And why not 10?

To give a practical example, the simulated performances of the hypothetical future ALADInO project are much more aggressive/optimistic however this manuscript was published to the cover of Instruments https://www.mdpi.com/2410-390X/6/2 and this is true for 90% of projects for future detectors. Reviewer1 would reject them all? With this attitude science will stop.  

 

To accept in some way the Reviever1 comment, the strongly criticized sentence (Line 260-262):

“As a summary, the energy resolution tested with the Arktis B-470 helium scintillator

confirms the PHeSCAMI hypothesis that HeCal detector would be able to measure energy

depositions larger than 10 MeV with an energy resolution better than 10%.”

has been changed to:

“As a summary, the energy resolution tested with the Arktis B-470 helium scintillator

supports the PHeSCAMI hypothesis that HeCal detector would be able to measure energy

depositions larger than 10 MeV with an energy resolution better than 10%.”

 

Question2) 

1. i) I understand that the B-470 comes with 200 bar helium gas. But what is the reason for choosing 400 bar for the simulated detector?

Answer2)

This is another assumption. As written in line 129 we considered the ArianeGroup vessel that is a COPV that works with 400 bar pressure, has thinner walls and is space qualified. 

 

Question 3)

1. ii) The double peak structure of S1 cannot be explained by the finite slowing-down time. 

There needs to be a mechanism that causes two distinct distributions.

Answer 3) 

To satisfy Reviewer1 we remove the sentence: “The duration of S1 signal is of the order of few ns that is the particle slowdown time in He” from the manuscript since this tail has a negligible impact on the whole manuscript.

As requested by the other reviewers in this manuscript version we fold the signal amplitude distributions of fig.4 considering the effect of the scintillation decay components of Helium.

Question 4)

iii) The authors did not answer why one of the PMT had to be replaced with a SiPM.

Answer 4)

We already answered to the same question in the previous report.

Moreover it is also clearly explained in the manuscript (lines 193-199):

“To allow the detection of the charged particles in the calorimeter avoiding the passage of the particle through the PMT, one PMT of the Arktis B-470 detector was replaced with an array of 8x Silicon Photo-Multipliers (SensL MicroFJ-60035 6x6mm 2 Fill Factor 65%) see left panel of Fig. 8. The SiPM circular array is shielded by 20 cm of iron and a central hole, ∅ ≈ 1 cm, allows

the particles to enter in the Helium target crossing only the ( ≈ 2.5 cm thick) fused silica

optical window”.

 

Question 5) 

1. iv) Refs 20 and 21 are identical. Refs 20, 21, and 22 do not mention the

wavelength of scintillation from helium gas.

Answer 5)

We thank the Reviewer1 for pointing out the wrongly duplicated ref. 20-21. 

Ref. 20 is now pointing to the correct reference.

The wavelength of scintillation from Helium gas is never quoted in the manuscript since the measurements summarized in this manuscript are not describing the WLS system.

However cited ref. [22] mention the 80nm value and the Arktis WLS system: “The interior surface of the stainless-steel cylinder was coated with a PTFE-based diffuse reflector [29] which was itself coated with an organic phosphor that converted the wavelength of the scintillation light from 80 nm to 430 nm.”

 

Best regards

Francesco Nozzoli

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

No further comments.

Author Response

Thank you

Reviewer 3 Report

Comments and Suggestions for Authors

    1    Line 233: Why is the square of (d/d0) used in the exponential function? It requires further clarification.
    2    Line 270: The usage of (Nph*Eside) seems redundant. Could you elaborate on why it is employed twice?
    3    Line 276: Is the "relatively fast single photoelectron signal shape" detected with both the PMT and the SiPM? Could this be attributed to differences in light acceptance between the PMT and SiPM systems?
    4    Line 278: Could you provide more information about what is meant by "the typical non-uniform sampling time step"?

Author Response

Reviewer3 report2)

We thank Reviever3 for the report of this proceeding of ASAPP2023 conference.

 

Question 1)

Line 233: Why is the square of (d/d0) used in the exponential function? It requires further clarification.

Answer 1)

This is the function parameterizing the measurements shown in the left panel of fig. 10.

As written in the manuscript this effect might be attributed to the peculiar B-470 detector geometry (a pipe) and most probably also the geometry of the WLS system have some impact. 

 

Question 2)

Line 270: The usage of (Nph*Eside) seems redundant. Could you elaborate on why it is employed twice?

Answer 2)

n_ph was defined at line 251. It is the number of measured photoelectrons per unit of energy deposited at d=0. We prefer to factorize out P(d) that is not dependent on the photodetector.

Thus (n_ph*Eside) is the number of measured photoelectrons at the considered side.

It is expected that the time resolution (and also the energy resolution) are following a function that is dominated by the Poisson statistics, thus decreasing as the inverse square root of the number of photoelectrons measured by the photodetector. 

This is compatible with our measurements.

 

Question 3)

Line 276: Is the "relatively fast single photoelectron signal shape" detected with both the PMT and the SiPM? Could this be attributed to differences in light acceptance between the PMT and SiPM systems?

Answer 3) 

As described by the Poisson dominated model, the overall time resolution is mainly dependent on: 1) the number of detection photons (described by n_ph*Eside); 2) the width of the single photoelectron time distribution whose effect is accounted for by sigma_1 parameter; 3) the electronic noise and digitizer time scale non uniformity that asymptotically dominates the time resolution at large number of detected photons and are described by the sigma_0 parameter. 

Both for SiPM and PMT, sigma_1 has the common contribution due to the characteristic decay time of scintillation light in He. However the signal distribution of our SiPM array has a fast rise-time but a long tail due to a large parasitic capacitance of the junctions, this explains the larger sigma_1 for the SiPM channel. On the contrary our PMT signal is very fast both in rise-time and in decay time and for PMT channel sigma_1 is dominated by the He scintillation.

Thus also factoring out the different acceptances (that are encoded in n_ph) our PMT signal is still “better” than our SiPM signal.

  

Question 4)

Line 278: Could you provide more information about what is meant by "the typical non-uniform sampling time step"?

Answer 4)

The nominal time step of a 5Gs/s digitizer is 200ps. Avoiding some complex procedure of time scale (cell) calibration, that we didn't apply, this is also the uncertainty in the sample timestamp.

We are plotting a time difference, thus the contribution of the order of 200*sqrt(2)=280 ps to the time resolution is simply coming from our digitizer time scale. This is the dominating contribution to sigma_0 value measured for the PMT channel.

 

Best regards

Francesco Nozzoli

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

I thank the authors for implementing all suggestions. Best regards

Comments on the Quality of English Language

no comments

Author Response

Thank you

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

see attachment

Comments for author File: Comments.pdf

Author Response

Reviewer1 report3)
We thank Reviewer 1 for further reviewing our manuscript.

1) Geometry
Answer1) The geometry of the ArianeGroup vessel is assumed in the MC simulation. As usual in all existing projects for future experiments we are free to assume our preferred geometry in our MC simulations for future detectors. 
We will test this geometry in the next prototypes.

2) Pressure
a. The authors never answered my question about the physics considerations that go into determining the operating pressure.
b. I am not sure if one knows how the scintillation yield depends on the pressure.
Answer2.a) This is already explained in the Answer2 to Question2 of the previous report.  The 400bar working pressure of the ArianeGroup vessel is assumed in the MC simulation.  We preliminary test the functionality of He scintillation at 200bar due to limited funds.
Answer2.b) In literature there is description of He scintillators working at different pressures. 
Helium at pressure lower than 200 bar is a good scintillator and liquid helium also is a good scintillator therefore interpolating we can argue that helium at 400 bar is a good scintillator. However we will test and publish the scintillation of Helium at 400bar when we build and test the next prototypes as written in the conclusion. 
This is usual in Physics. People do the simulations of future detectors hypothesizing some assumptions and eventually get funds to build prototypes to test the assumptions. 

3) The two rounds review revealed that the authors do not know the wavelength of the scintillation, the yield of the helium scintillation, how that depends on the pressure, nor the nature of the wavelength shifter used in the Arktis detector. (If they don’t know what were used in the Arktis detector how can they argue that )
Answer3) 
The two rounds revealed that the beliefs of Reviewer1 about the authors are wrong/biased. 
3.1) We know the wavelength of He scintillation, the spectrum is shown in fig.3 and fig.4 of the cited ref. 20, the main 80nm emission is quoted in cited ref. 22. We don’t quote these numbers in this conference proceeding since they are off-topic, they are not used and not required by the analysis of our measurements/simulation.
3.2) See previous answer2 for the pressure argument. 
3.3) We didn’t make any assumptions about the WLS system, for this reason we do not give an off-topic description of the future prototype WLS system in this conference proceeding.
However in the Answer5 to the Question5 of the previous report, we already report a sentence from our cited ref. [22] that is describing the Arktis WLS system: “The interior surface of the stainless-steel cylinder was coated with a PTFE-based diffuse reflector [29] which was itself coated with an organic phosphor that converted the wavelength of the scintillation light from 80 nm to 430 nm.”  This description of the Arktis WLS system was enough for the published paper [22]. 

Modification to the manuscript:
As required by Reviewer1 we replace lines 259-262 with the suggested sentence:
“As a summary, there are a lot of unknowns and uncertainties when relating our measurements based on the Arktis B-470 detector to the expected performance of PHeSCAMI detector. These include: the nature of helium gas scintillation (wavelength, photons yields, and their pressure dependence etc), the wavelength shifter used in the Arktis B-470, and the differences in the geometry. Despite these, we feel that our measurements based on the Arktis B-470 detector shows that it is plausible to achieve the assumed hypothesis that the HeCal detector would be able to measure energy depositions larger than 10 MeV with an energy resolution better than 10\%. It is our future project to study the unknowns and uncertainties mentioned above.”

Best regards
Francesco Nozzoli

Reviewer 3 Report

Comments and Suggestions for Authors

I have asked some questions and got reasonable responses. There will be similar  questions. I believe to describe your arguments as detail as you can. This  helps readers understanding rapidly and smoothly.

Author Response

We thank Reviewer3 for the manuscript review.

Best regards

Francesco Nozzoli

Round 4

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have not addressed my concerns in a sufficient manner.