Geant4 Simulation of Muon Absorption in Concrete Layers

Round 1

Reviewer 1 Report

The discussion section opens the main objective of the paper, which is comparison of the energy loss of muons calculated by a method developed using Geant4 simulations with referred publication by D.E.Groom et.al. They have found good agreement with mentioned reference in case of 0.2 and 0.3 cm thick conrete samples and explain convincingly (lines 223-225) why the disagreement is raising for thicker concrete samples. Though Geant4 allows also direct output information of the energy loss for particles passing materials, the authors have chosen the method described in section 3 and results presented in section 4.  However, referee do not see any reasoning or discussions in in the paper on how the method could be applicable and related to in different applications of muography as mentioned in the abstract (lines 10-12). 

Referee would like to request minor revision and add into Introductory section description that relates the existing text in the introduction with the objectives of this paper and describes how the study and method described in this work relates or could be applicable in muography. 

Author Response

Thank you very much for your feedback; we have expanded both the introduction and discussion sections to better describe how this work relates to muography. In terms of your question about why we did not simply output the information from Geant4 regarding muon energy loss, the answer is that we wanted to investigate not only how much energy the muons would lose in a certain amount of material, but whether this information could possibly be found experimentally, which is why the simulation was extended to simulating the effects of the muons passing through a simple ionization detector.

Reviewer 2 Report

Dear Authors,

I found interesting your manuscript that fits to the scope of this special issue. I believe that the work could be communicated clearer and the results could be more directly compared to the reference data. I suggest a major revision of this manuscript.

I provide a few suggestions as follows.

- The introduction should point out the goals: accurate simulation of muon absorption in thin uniform materials and validation of simulations via alpha source measurements. It would be useful to provide information about the practical applicability and limitations of high-precision muography of thin structures. 

- Simulation and experimental studies should be well distinguished. I provide one possible structure, but I give freedom to you to organize the sections in another order:

1. Introduction
2. GEANT4 Simulations
3. Experimental validation
4. Discussion

- The description of GEANT4 simulation should provide information about the applied physics lists.

- Would it be possible to quantify the consistency between the reference data and the simulation data? For example, can you extract the same quantities (e.g. stopping power) for simulated data and show together with the reference data?

Author Response

Thank you very much for your helpful feedback; we have re-organized our materials and methods section to better distinguish the simulation from the experimental validation as per your suggestion.  We have also adapted the language from your comment about the overall goals of the paper and included it in our introduction. Finally, we included information about the applied physics list we used in our simulation and have expanded the discussion section to better relate our simulation results with the stopping power in the reference data.

Reviewer 3 Report

1.Simulations and experiments with alpha radioactive sources cannot guarantee that Muon's simulations are correct. Simulation involves the setting of many parameters. So the fact that one simulation is correct does not prove that the other simulation is also correct.

2.There is no need to introduce Geant4 software in the paper. There is absolutely no need to tell the reader what your operating system is or what hardware you are using. Such as Figure 15 and Computing Environment.

3.A large amount of simulated data needs to be set with random numbers. I hope you have that set. Figure 17 and Figure 18 should be combined into one figure. And the line in Figure 18 is weirdly over the border!

4.Muon is very penetrating. How will you deal with the Muon coming out of your detector and the energy is not fully deposited? Air absorbs ionized electrons and what do you do with it.

5.Simulations must be compared with experimental results to be believed. I think only the last paragraph is important, but you didn't write enough. I can see from Figure 17 that for 0.2 cm of concrete peak at 7 MeV. But I don't know how to read the stopping power of 8 MeV cm2/g from Figure 19.

6.In the abstract you mentioned Muography, and this is your key word. Maybe you can talk more about how your work help Muography.

Author Response

Thank you very much for your feedback; we have tried to incorporate as many of your suggestions as possible. We have removed some of the generic language describing Geant4 and removed Figure 15 regarding the structure of our computing cluster. We have combined Figure 17 and 18 in the previous version into a single figure with a top and bottom plot, and fixed the problem you pointed out with the line showing the data extending over the x-axis. We have also tried to better relate the results of the simulation with the stopping power by converting the muon kinetic energy corresponding to peak detection to a combined βγ value which is how it is plotted in the figure from the reference.

With regards to your comments about the alphas and muons having quite different physics properties, we agree that they are quite different, but doing an initial study on alphas allowed us a way to experimentally validate in the laboratory that our simulation was working properly in terms of modeling air ionization and charge collection. One of the main differences, as you pointed out, is that while the alphas deposit the majority of their energies in the volume of the detector, muons are much more penetrating and mostly traverse the detector volume. Only in a very specific energy range do the muons ionize an appreciable amount of air in the first few centimeters; this is the reason the small detectors have such a specific energy range for detection.  This has been added to the section introducing the muon simulation.

Concerning random numbers, we agree that high statistics runs of billions of events with the same input parameters would require an extensive random number generation. But the input parameters even for our highest statistics runs are varied every 10,000 events by changing the initial muon energy in 1 keV steps and/or by changing the thickness of the material by 1 mm; we never simulate more than 10,000 events on a single initialized run.

 Finally, we have rewritten our introduction and conclusion in order to better motivate the relevance to muography.  Thank you again for your comments!

Round 2

Reviewer 3 Report

It can be seen that the author is working hard to improve the paper. I hope that when the author writes a paper in the future, the author can consider the meaning of the paper, consider the problem and organize the paper from the perspective of the reader.

Papers about detector simulations are always suspicious. Because simulation is important, but the results of simulation do not necessarily match reality. Simulations of large detectors sometimes require modifications to the Geant4 program to better reflect real results. But the final simulation results will still be different from reality. Writing such paper requires attention to mutual verification with experimental results. I wish you progress!

I suggest improving the typography, don't let a figure take up an entire page.