Geant4 Simulations of a Scintillator Cosmic-Ray Detector

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The topic of this manuscript is timely and relevant to the interest in distributed cosmic-ray detection. Simulating simple, low-cost detectors for large-scale arrays is highly useful for cosmic-ray research. A good match of physical processes is considered, including the PDE, shielding effects, and angular dependency.

Manuscript design is very interesting, and the results are presented excellently.

The text requires some  ammendements.. (some examples:))

A the detector should be …these detectors

B Due to their low costs  please change to low cost

C “Scintillator detectors it is betted to add an article The scintillator

Fig. 5–6, 10 and 14, please improve captions,

The Introduction need some idea of what specific gap this work is addressing.

The choice of SiPM model must be explained in detail.

I would like to comment on the need for experimental validations. can the authors comment on how real data can be obtained? With this new system? Or what is the idea about building an experimental prototype?

Table 1. is not in the   journal style, no borders

References (check all), some detected mistakes

8. S. Axani, K. Frankiewicz, J. Conrad, Cosmicwatch: The desktop muon detector, J. Instrum 13 270

(2018) 03, https://doi.org/10.1088/1748-0221/13/03/P03019...other used full journal names

1. Radomir, M. Dimitrije, J. Dejan, V. Nikola, D. Aleksandar, U. Vladimir, A. Ivan, 288

On the omnipresent background gamma radiation of the continuous spectrum, 289

https://doi.org/10.1016/j.nima.2014.01.065 (2014). No data about journal or pages?

1. Brun, F. Rademakers, Root - an object oriented data analysis framework, 268

https://doi.org/10.1016/S0168-9002(97)00048-X (Sep. 1996). Journal name?..see

Rene Brun, Fons Rademakers,

ROOT — An object oriented data analysis framework,

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,

Volume 389, Issues 1–2, 1997, Pages 81-86, https://doi.org/10.1016/S0168-9002(97)00048-X.

Comments on the Quality of English Language

almost OK, some minor errors

Author Response

The text requires some  ammendements.. (some examples:))
A the detector should be …these detectors
B Due to their low costs  please change to low cost
C “Scintillator detectors it is better to add an article The scintillator
- Changed.

Fig. 5–6, 10 and 14, please improve captions,
- Captions changed and should now provide more in-depth information about the plots.

The Introduction need some idea of what specific gap this work is addressing.
- Explanation added and the text of Introduction is modified.

The choice of SiPM model must be explained in detail.
- SiPM chosen for analysis has a peak of PDE function close to the wavelength of maximum emission for BC408 scintillator and has the largest area from this family of products. There are SiPMs which provide better temporal parameters but they are more expensive and such design would not benefit much from such improvement.

I would like to comment on the need for experimental validations. can the authors comment on how real data can be obtained? With this new system? Or what is the idea about building an experimental prototype?
- The goal of this work was to provide  a proof, prior to building a prototype detector system, that the proposed detector is capable of detecting a single EAS and distinguish it from background. Such prototype detector is currently under construction and test, but is not ready for measurements that could be presented now. The future tests may include both measurements of the cosmic rays and registration of particles from radioactive sources or accelerators - whichever is available.

Table 1. is not in the journal style, no borders
- Changed to proper style.

References (check all), some detected mistakes
- Checked and corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

applsci-3667610; Review Report

Authors presented the design and simulation of a simple low-cost scintillation detector consisting of the BC-408 scintillator and the SiPM photon detector. They used the Geant4 software for the simulation. Through simulation of the detector, authors have shown the sensitivity to the detection of electrons and muons. My comments are as follows.

1. Please include references for equations used in the manuscript if they are existing ones.
2. Please explain the function of 'Shielding' in Figure 1.
3. Please review existing works related to the scintillator simulation.
4. Line 40-41:

"However, like every tool, Geant4 has some limitations, as it is not designed to simulate signals in electronic of the sensors."

Please check '...in electronic of the sensors'. It seems that '... in the electronics of the sensors' might be more suited.

5. Equation 6, Figure 4:

1) Use a different symbol for the thickness of the detector. 'd' is used for the distance between the shield and the detector in Figure 1.

2) Equation 6 needs to be checked. Please check and make corrections.

A rectangular coordinate system is set up at the xy-plane (top face) center of the detector. Assume that the ray comes from the direction of the unit vector a_r in the spherical coordinate system whose x, y, and z components can be written in a vector form a_r = (sin(theta)cos(phi), sin(theta)sin(phi), cos(theta)). Denote the area of the xy-plane (top face), zx-plane (front face), and yz-plane (side face) of the detector as Az, Ay, and Ax, respectively, whose surface normal vector is (0, 0, 1), (0, 1, 0), and (1, 0, 0), respectively. The capture area is given by the area projected in the direction of the ray. It can be written

A_proj = (Face area)*[(Surface normal vector) dot (Unit vector of the incoming ray = a_r).

The correct formula reads

A_eff = Az*cos(theta) + Ay*sin(theta)*sin(phi) + Ax*sin(theta)*cos(phi)

3) Equation 6 gives an infinite value when theta is pi over 2. The effective or capture area contribution of the top surface (area A0) should be dependent on the direction of the incoming ray.

6. Equation 7:

Has the contribution by front and side faces of the detector been included in Equation 7 as analyzed in Equation 6? The arguments of the integrand eta(x,y) tell a different story. Please check and make corrections.

7. Figure 1 and Geant4 Software:

Please consider the following issues and add some comments.

1) The photon detector SiPM is placed at the center of the scintillator's bottom face. Its area is 6 mm by 6 mm. Please add some comments on the travel direction of photons generated by the scintillator. Do photons travel uniformly in all directions?

2) If the bottom face is fully covered with an 8 x 8 array of SiPM, will the sensitivity be increased 64 times?

3) Is 10 mm an optimum thickness of the scintillator material in regard to the area of the top face of the scintillator and in regard to the placement of the SiPM detector?

8. Why electrons and muons only?

Authors considered the detection of electrons and muons only. Why has the detection of other secondary particles such as positrons, protons, pions, and heavy ions not been considered? 

9. '5. Conclusions'

Conclusions seem to be too succinct. Please expand Conclusions by findings shown in '5. Results'.

 

Author Response

Please include references for equations used in the manuscript if they are existing ones.
- Added a reference to Eq. 2 and 3.

Please explain the function of 'Shielding' in Figure 1.
- The goal of this shielding is to reduce background of low energy particles that do not originate from EAS. Those can be single cosmic-ray particles reaching the ground without producing an extensive cascade or particles from radioactive decays in the air. This explanation is included in the text.

Please review existing works related to the scintillator simulation.
Line 40-41:
"However, like every tool, Geant4 has some limitations, as it is not designed to simulate signals in electronic of the sensors."

Please check '...in electronic of the sensors'. It seems that '... in the electronics of the sensors' might be more suited.
- Changed.

Equation 6, Figure 4:
1) Use a different symbol for the thickness of the detector. 'd' is used for the distance between the shield and the detector in Figure 1.
- Thickness is now indicated by 'r'.

2) Equation 6 needs to be checked. Please check and make corrections.

A rectangular coordinate system is set up at the xy-plane (top face) center of the detector. Assume that the ray comes from the direction of the unit vector a_r in the spherical coordinate system whose x, y, and z components can be written in a vector form a_r = (sin(theta)cos(phi), sin(theta)sin(phi), cos(theta)). Denote the area of the xy-plane (top face), zx-plane (front face), and yz-plane (side face) of the detector as Az, Ay, and Ax, respectively, whose surface normal vector is (0, 0, 1), (0, 1, 0), and (1, 0, 0), respectively. The capture area is given by the area projected in the direction of the ray. It can be written

A_proj = (Face area)*[(Surface normal vector) dot (Unit vector of the incoming ray = a_r).

The correct formula reads

A_eff = Az*cos(theta) + Ay*sin(theta)*sin(phi) + Ax*sin(theta)*cos(phi)

3) Equation 6 gives an infinite value when theta is pi over 2. The effective or capture area contribution of the top surface (area A0) should be dependent on the direction of the incoming ray.
- Area described by Eq. 6 is not a projection onto the plane perpendicular to the vector of the ray. It is a projection of the detector geometry onto an abstract flat 2D plane parallel to the top of the detector. This method of accounting for particles hitting the sides of scintillator is valid only for reasonable values of zenith angle, theta <90 degrees. Indeed, the area calculated using Eq. 6 for theta -> 90 degrees approaches infinity, but this increase is compensated by decreasing to zero density of particles on the flat horizontal plane. 

Equation 7:
Has the contribution by front and side faces of the detector been included in Equation 7 as analyzed in Equation 6? The arguments of the integrand eta(x,y) tell a different story. Please check and make corrections. 
- Yes, it is included as integration is performed over area (S=12x12 cm^2) much larger than that of the top of scintillator (A0 = 5x5 cm^2). In addition it accounts for contribution from particles that should miss the scintillator but are producing signals, because they interact with the material above it and generate secondary particles that hit the scintillator. A parametrization where A0 is replaced by A(theta,phi) is also possible but it seemed less appropriate: particles hitting the sides together with those that should miss it form a "bonus addition" to the particles hitting the top of the scintillator.

Figure 1 and Geant4 Software:
Please consider the following issues and add some comments.

1) The photon detector SiPM is placed at the centre of the scintillator's bottom face. Its area is 6 mm by 6 mm. Please add some comments on the travel direction of photons generated by the scintillator. Do photons travel uniformly in all directions?
- Scintillator photons are generated along the path of particle that passes through it. They are emitted in all directions uniformly and then they scatter from the reflective coating filling more or less the whole volume of the scintillator. 

2) If the bottom face is fully covered with an 8 x 8 array of SiPM, will the sensitivity be increased 64 times?
- Additional SiPM could increase the sensitivity for low energy particles. However, financially it would be more optimal to invest in another scintillator to construct two detectors than to attach two, or especially more, SiPMs to one scintillator.

3) Is 10 mm an optimum thickness of the scintillator material in regard to the area of the top face of the scintillator and in regard to the placement of the SiPM detector?
- This value was taken from the original Cosmic Watch design and seems to be cost/effect optimal. Of course in thicker scintillator more energy is deposited and more photons are produced, thus especially detection of high-energy photons should be more effective. However, the overall efficiency would be larger for two devices with 10 mm scintillators than for one with 20 mm  scintillator.
As presented in Fig. 6, if electrons hit the scintillator further from the SiPM the chance of producing sufficiently strong signal is smaller, therefore putting SiPM in the centre seems optimal.

Why electrons and muons only?
Authors considered the detection of electrons and muons only. Why has the detection of other secondary particles such as positrons, protons, pions, and heavy ions not been considered? 
- Gamma photons are also considered. Those three types of particles account for 98% of all particles from Extensive Air Showers that reach the ground and  in the analysis of performance of a cosmic-ray detector their impact is negligible.

'5. Conclusions'
Conclusions seem to be too succinct. Please expand Conclusions by findings shown in '5. Results'.
- Conclusions were extended.

Reviewer 3 Report

Comments and Suggestions for Authors

The article is both interesting and relevant. Its main contribution is investigating the effectiveness of simple scintillator detectors for detecting secondary cosmic radiation, through simulations in the Geant4 environment. It focuses on very small devices based on plastic scintillators, which are inexpensive and easy to produce in large quantities. The authors demonstrate that these detectors are highly efficient in detecting high-energy muons and electrons. However, they also found that sensitivity to low-energy electrons and photons is limited and must be considered when processing experimental data. Due to their simplicity and low cost, these detectors are well-suited to use in arrays operating on the coincidence principle. Section 4: Results presents ten graphs (Figs. 7–13 and 15–17) showing the sensitivity of the detectors to muons and photons under various conditions.  Therefore, the article is useful. It contains new practical results.

At the same time, I have the following comments on the article:

1) What is the purpose of this article? In section 1, Introduction, the authors should clearly state the purpose of the article. Introduction, the authors should clearly state the article's purpose. In section 5, Conclusions, they should indicate whether they have achieved the study's aim. Conclusions, they should indicate whether they have achieved the aim of the study.

2) The authors focused their attention on the energy parameters of the device signals, particularly the amplitude of the photomultipliers and the detector's sensitivity to muons and photons. However, the paper lacks information on the response time. The fast scintillation process enables accurate measurement of particle arrival times and the use of time correlation to eliminate background noise. Therefore, the authors should include information on the device's response time.

3) What are the impulse and transient responses of the device? What limitations do these responses place on the sensitivity and accuracy of the device? This information should be added to the section '3. Estimation of sensitivity'.

4) The article contains a large number of numerical values of the research results in the form of tables or figures. However, the Abstract and Conclusions do not contain any numerical results. The authors should include the main research results in numerical form in the Abstract and Conclusions.

5) One minor point: the title of the article requires an indefinite article before the word 'scintillator'. It should read 'Geant4 simulations of a scintillator cosmic-ray detector' instead of 'Geant4 simulations of a scintillator cosmic-ray detector'.

Author Response

1) What is the purpose of this article? In section 1, Introduction, the authors should clearly state the purpose of the article. Introduction, the authors should clearly state the article's purpose. In section 5, Conclusions, they should indicate whether they have achieved the study's aim. Conclusions, they should indicate whether they have achieved the aim of the study.
- Introduction and conclusions has been extended.

2) The authors focused their attention on the energy parameters of the device signals, particularly the amplitude of the photomultipliers and the detector's sensitivity to muons and photons. However, the paper lacks information on the response time. The fast scintillation process enables accurate measurement of particle arrival times and the use of time correlation to eliminate background noise. Therefore, the authors should include information on the device's response time.
- The response from SiPM in the mode analysed in this work is a signal with rising time around 10 ns and with a tail hundreds of nanoseconds long. This signal duration is of the order of the difference of arrival time between the fastest and the slowest particles from an EAS, thus it is appropriate for detection of coincidences between such particles. The coincidence in a top-bottom setup that allows to detect a particle (muon) as arriving from above would require measuring time differences less than 1 ns.  Such SiPMs  fast mode which produces much shorter signals is available but it is dedicated to detection of single photons. In such operation mode a very low amplitude and short signals are produced that require very fast electronics which is more complicated and expensive and this design aims to minimize costs.

3) What are the impulse and transient responses of the device? What limitations do these responses place on the sensitivity and accuracy of the device? This information should be added to the section '3. Estimation of sensitivity'.
- Presented analysis was performed under assumption that amplification of the signal is linear, it does not affect its shape and only the amplitude of the signal determines whether it is registered or not. Simulation of behaviour of electronics which processes the signal would require earlier tests with the use of an already existing prototype, which is beyond  the scope of this work.

4) The article contains a large number of numerical values of the research results in the form of tables or figures. However, the Abstract and Conclusions do not contain any numerical results. The authors should include the main research results in numerical form in the Abstract and Conclusions.
- Quantitive results added.

5) One minor point: the title of the article requires an indefinite article before the word 'scintillator'. It should read 'Geant4 simulations of scintillator cosmic-ray detector' instead of 'Geant4 simulations of a scintillator cosmic-ray detector'.
- Changed.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors simulated the performance of a scintillator detector for the detection of cosmic rays using Geant4 software. The article analyzes the detection efficiency of the detector for different types of particles (e.g., muons, electrons, and photons) and explores the effects of detector design parameters and external conditions on its performance. The article has some innovations but needs to be revised before publication:
1. The article provides a detailed description of the background and purpose of the study in the introduction section, but the innovation of the study is not described prominently enough. It is recommended that the methodological or applied innovations of this article be clearly stated in the introduction section, such as the first application of a particular detector design or parameter optimization in simulation.
2. Although the article provides detailed simulation results, they are not compared with actual measurement data for verification. It is suggested to add a comparative analysis of the simulation results with the actual measurement data in the discussion section to enhance the credibility of the study.
3. The sensitivity analysis of the detector to different particles is provided in the results section, but the interpretation of the results is rather general. It is suggested that a specific explanation of the results be added to the discussion section, especially a detailed analysis of the reasons for the lower sensitivity to low-energy particles.
4. The discussion section puts forward suggestions for optimizing the detector design, but they are rather broad. It is recommended that more targeted and operational recommendations be made in the light of practical application requirements, such as the optimal detector configuration under different environmental conditions.
5. It is suggested that the conclusion section should be supplemented with a detailed description of the specific research directions, such as how to further optimize the detector design and how to carry out practical applications under different environmental conditions.

Author Response

1. The article provides a detailed description of the background and purpose of the study in the introduction section, but the innovation of the study is not described prominently enough. It is recommended that the methodological or applied innovations of this article be clearly stated in the introduction section, such as the first application of a particular detector design or parameter optimization in simulation.
- Introduction has been extended.

2. Although the article provides detailed simulation results, they are not compared with actual measurement data for verification. It is suggested to add a comparative analysis of the simulation results with the actual measurement data in the discussion section to enhance the credibility of the study.
-  A prototype is currently under construction and presented results will be tested in the future. Through measurements of muon flux as a function of theta angle of the primary cosmic-ray particle sensitivity to muons can be estimated. Likewise, measurements with gamma and beta radiation sources enable evaluation of the sensitivity to low-energy electrons and photons. Measurements of secondary particles from cosmic rays with the use of different shieldings would be another test of results obtained from the simulations. 

3. The sensitivity analysis of the detector to different particles is provided in the results section, but the interpretation of the results is rather general. It is suggested that a specific explanation of the results be added to the discussion section, especially a detailed analysis of the reasons for the lower sensitivity to low-energy particles.
- Lower sensitivity to low-energy particles is caused by the fact that they deposit less energy in the scintillator material, therefore the number of produced photons is lower. In case of low-energy electrons and photons they may also be stopped in the material of the shielding.

4. The discussion section puts forward suggestions for optimizing the detector design, but they are rather broad. It is recommended that more targeted and operational recommendations be made in the light of practical application requirements, such as the optimal detector configuration under different environmental conditions.
- Recommendation about the configuration of shielding is now more prominent in the conclusions. Recommendations for construction of the detector are that the reflective coating should me as reflective as possible and it should adhere to the surface of the scintillator. The layer of optical gel or Vaseline in this example should be thin and uniform without any air bubbles in it. The temperature of SiPM should also be kept as constant as possible to provide efficiency that is uniform in time. Those conclusions are also added to the paper.

5. It is suggested that the conclusion section should be supplemented with a detailed description of the specific research directions, such as how to further optimize the detector design and how to carry out practical applications under different environmental conditions.
- Conclusion section has been extended.

Reviewer 5 Report

Comments and Suggestions for Authors

I would like to thank the authors for this quality of work. The paper reads well and brings something new to the field of research.  The paper is well-structured, with clear sections that logically flow from introduction to system model, analysis, and numerical results.

However, i have some comments and questions that need to be considered in order to improve the quality of your draft:

• Some figures (e.g., Fig. 5, 6) are referenced before their detailed explanation in the text, which may confuse readers. Consider reordering
• Figures 5, 6, 10, and 14 are helpful but could be improved with clearer labels.
• The histograms in Figures 7–17 are informative, but adding error bars or confidence intervals would strengthen the statistical presentation. 
• How were the refractive indices and other material properties (Table 1) validated?
• For the SiPM model, were temperature-dependent effects explicitly included in the simulations?
• How does the amplifier threshold  compare to typical noise levels in field conditions?     

Comments on the Quality of English Language

Page 1: "It is especially important when detectors are very simple like scintillator detectors considered in this work, which provide only information about the amplitude of signal generated by detected particle." → Missing article ("a detected particle").  

 Page 5: "In a call hit by a photon" → "In a cell hit by a photon."  

Author Response

Some figures (e.g., Fig. 5, 6) are referenced before their detailed explanation in the text, which may confuse readers. Consider reordering

- Figures has been reordered.
     

Figures 5, 6, 10, and 14 are helpful but could be improved with clearer labels.

- Captions have been extended to better describe the plots.
      
The histograms in Figures 7–17 are informative, but adding error bars or confidence intervals would strengthen the statistical presentation.

- Error bars are not shown as their size is smaller or simillar to the size of markers.
     

How were the refractive indices and other material properties (Table 1) validated?
- Their values were taken from database which is cited in the caption of the table.      

For the SiPM model, were temperature-dependent effects explicitly included in the simulations?

 - Different temperatures were not explicitly simulated.  For SiPM two parameters depend on the temperature: gain, G(T), and breakdown voltage, Vbr(T). Those relationships are provided by the manufacturer in the datasheet of considered SiPM model. They were used to estimate the amplitudes of the signals and therefore the efficiencies for different values of T. Presented in Fig. 17.
     

How does the amplifier threshold  compare to typical noise levels in field conditions?   
- Noise in the electronics is on the level of 10 mV which is very low comparing to 1 V threshold in the circuits that process the signal.