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Peer-Review Record

Design and Characterization of the Modified Purdue Subcritical Pile for Nuclear Research Applications

Instruments 2025, 9(2), 13; https://doi.org/10.3390/instruments9020013

by Matthew Niichel^1,2,*

, Vasileios Theos², Riley Madden²

, Hannah Pike²

, True Miller², Brian Jowers² and Stylianos Chatzidakis^2,*

Reviewer 1:

Antonín Krása

Reviewer 2:

Michael Reichenberger

Instruments 2025, 9(2), 13; https://doi.org/10.3390/instruments9020013

Submission received: 30 January 2025 / Revised: 30 May 2025 / Accepted: 3 June 2025 / Published: 6 June 2025

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript has a good potential and it attracted my attention at the beginning. However, after careful reading I am quite dissapointed as the work is rather weak. The description of the updated subcritical pile contains many details that are unnecessary, it repeats notorically known facts and presents experimental results with very high uncertainties, which avoids drawing strong conclusions. Moreover, the manuscript is not written carefully as it contains many mistakes (e.g. in references and captions). The authors should pay more attention to their work before submitting it.

If it was a report on an exercise for students that need to get familiar with activation method, calibration of HPGe and get a feeling of physics trends, it would be OK. However, this is supposed to be a research paper and even numerous Monte Carlo simulations and attempts to treat errors in complex manner are not sufficient. In case the authors significantly improve their work, it can become worth publishing in Instruments.

Major comments

1) Information that is elementary should be omitted, for example most of sections 4.1, 5.1, entire section 4.5. You wrote yourself (line 397) that "the use of the HPGe is standard". No need to write about reactions that were not used (e.g. line 260).

2) Literature review should contain at least basic description of the listed subcritical piles (e.g. fuel, moderator, source). How can it be useful if you write about a results of caculations of a pile and you do not specify its parameters. An overview table would be useful containing the name, location, main materials, keff, purpose.

3) When a cross section value is given (e.g. page 6, line 257, 264), the neutron energy must be stated and the source (evaluated library) must be referenced.

4) In data analysis, did you take into account uncertainties of every quantity that you use? For example, uncertainties of the evaluated cross sections, uncertainty of the foil dimensions (which reflect in the uncertainty of the Gth factor), uncertainty of the efficiency (Table 2).

5) Table 1 - why are the errors so big? 10% is enormously big for this type of measurement! Did you have only 100 counts in the net peak area? Rel.err = 1/sqrt(N)

6) Fig. 9 - I have never seen so terrible calibration of HPGe. How come that the error bars are so big? Show an example of a measured gamma-ray spectrum. I think you should remeasure to reach better counting statistics (at least 1E5 net peak area to get 1% rel. err.). How come that the points around 200-300 keV are so discrepant? How did you fit a calibration curve from which you determine the value stated on line 450?

7) lines 649, 654 - the measured data certainly cannot be considered "accurate". Just look at the enormous error bars in Fig. 10, 11, 12. Should be remeasured to reach reasonably low uncertainties.

8) lines 650-652 are not correct. MC results are in agreement with the Au measurements but not with the In measurements.

9) line 658 is not correct. The MC and experimental results do not agree at the depth 50-60 cm.

10) Captions of Tables in Appendix claim they contain efficiencies. The text in the tables state it is resolution. Anyway, it is irrelevant to show these table whether resolution or efficiency. Just show the efficiency calibration plot with a resonable, physical fit.

Minor comments

1) Values should be rounded to the same decimal place as the uncertainty (e.g. page 2, line 75, Table 1).

2) page 7, line 287 - not clear what "several orientations" mean.

3) page 8, line 318 - you write about "neutron production factor" that is 2.5. It is wrong. "neutron production factor" is eta and it represents the average number of neutrons produced per thermal neutron absorbed in fuel. The quantity you use in eq. 5 is neutron emission multiplicity, nubar, which is the average number of neutrons emitted in fission.

4) Fig. 4 - 4 wide by 7 deep should be shown.

5) Fig. 5 should contain In cross section and a sketch of an In foil (surely it was not levitating in air but it was packed and placed on top of something).

6) Fig. 7 - the image on the left-hand side might be omitted - no clear reason why it is there

7) Fig. 8 - Lines are not physical - remove them or replace them with a fit that is physical.

8) Some figures use cm, some use inch. Use consistently the same unit in all the plots.

9) Line 500 - the statement "OpenMC allows for a validation of the measurements" is wrong. I understand that MC gives you values anywehere you want but at the end it is the experiment that you use to validate your model (as you correctly wrote on line 529).

10) Fig. 14 - what are the sharp structures above 1E7 eV and below 1E-2 eV?

11) line 528 - what do you mean by "error"? Is it the uncertainty of the result or the difference between experiment and calculation?

12) Results in Fig. 15a should equal the sum of the results in Fig. 15b, should not it? It seems they are different.

13) Fig. 18 shows almost constant spectrum of gamma-rays between 100 keV and 5 MeV. That is not possible. The energy binning is clearly not fine enough causing there are no peaks visible.

14) Fig. 20 - how come that the fast and thermal flux maps look so similar?

15) lines 647, 657 - ref. to Fig. 16 should be replaced by Fig. 15.

16) lines 661 - ref. to Fig. 17 should be replaced by Fig. 16.

17) References 13, 22, 23 are incomplete.

18) Correct numbering of references 21 and 22.

Author Response

Reviewer 1:

Sir/Ma’am, thank you for taking the time to review the article. I appreciate the time, the feedback is appreciated. Please excuse the delay in returning the paper. We had to remove one of the members from the paper. Experiments needed to be rerun, and their work removed. This proved to be difficult and time extensive. Nonetheless, we appreciate your patience.

Major comments

1) Information that is elementary should be omitted, for example most of sections 4.1, 5.1, entire section.4.5. You wrote yourself (line 397) that "the use of the HPGe is standard". No need to write about reactions that were not used (e.g. line 260).

Thank you for your comment. We reviewed these sections and determined some of the information would be seen as elementary. However, given this is an instruments journal and not a nuclear journal, some of the information was removed, but the detector calibration was kept for repeatability, and for a more general audience.

This is a fair point. In the interest of space savings, the materials of the pile with geometries have been provided. Any amplifying details can be found in the reference titled “University subcritical assemblies.”

3) When a cross section value is given (e.g. page 6, line 257, 264), the neutron energy must be stated and the source (evaluated library) must be referenced.

Thank you for pointing this out. We have included the energies for thermal neutrons 0.625 eV. This is according to the ASTM standard. As the paper alludes, there are errors introduced in assuming a pure thermal flux.

Thank you for pointing this out. On second review, we used the ASTM standard and collected values provided in the reference. There is some contention with the cadmium covered procedures among three scientists that I spoke to independently at Naval Station Crane and Oak Ridge. However, we used these procedures anyway.

5) Table 1 - why are the errors so big? 10% is enormously big for this type of measurement! Did you have only 100 counts in the net peak area? Rel.err = 1/sqrt(N)

Thank you for point out this mistake. This was an excel error, should have caught that. But it happens.

The good news is at least the data was not forged. Anyhow, this was reconducted three times by different people on our teams. The curve fit can be shown in the paper. Our HPGe did not perform well on energies below 160 keV. These measurements were omitted, as the gamma energies of interest greatly exceed this value.

7) lines 649, 654 - the measured data certainly cannot be considered "accurate". Just look at the enormous error bars in Fig. 10, 11, 12. Should be remeasured to reach reasonably low uncertainties.

Thank you, this has been corrected. Both gold and indium experiments were reconducted several times.

8) lines 650-652 are not correct. MC results are in agreement with the Au measurements but not with the In measurements.

Thank you for your comment. This has been corrected with the redesign of the OpenMC and MCNP models.

9) line 658 is not correct. The MC and experimental results do not agree at the depth 50-60 cm.

Hopefully, this is corrected with the rerun of the models.

Thank you, for the comment. However, with the recent uptick in paper recalls, we wish to present as much of the data as possible. Additionally, it was difficult finding a reference that contained all of these methods and presented the data cleanly. We hope for this paper to be that resource for future researchers.

Minor comments

1) Values should be rounded to the same decimal place as the uncertainty (e.g. page 2, line 75, Table 1).

Thank you for the comment, this has been addressed.

2) page 7, line 287 - not clear what "several orientations" mean.

Axis have been added to images to hopefully make it unambiguous for the referenced orientation.

Thank you, this has been corrected.

4) Fig. 4 - 4 wide by 7 deep should be shown.

Figure updated.

5) Fig. 5 should contain In cross section and a sketch of an In foil (surely it was not levitating in air but it was packed and placed on top of something).

Figure updated.

6) Fig. 7 - the image on the left-hand side might be omitted - no clear reason why it is there.

Figure updated.

7) Fig. 8 - Lines are not physical - remove them or replace them with a fit that is physical.

Figure updated.

8) Some figures use cm, some use inch. Use consistently the same unit in all the plots.

Thank you for the catch. Those pesky units will get you! International efforts are always a pleasure.

Hopefully, this has been better been explained.

10) Fig. 14 - what are the sharp structures above 1E7 eV and below 1E-2 eV?

The data from these plots have been removed. They were the work of the removed author.

11) line 528 - what do you mean by "error"? Is it the uncertainty of the result or the difference between experiment and calculation?

The data from these plots have been removed. They were the work of the removed author.

12) Results in Fig. 15a should equal the sum of the results in Fig. 15b, should not it? It seems they are different.

The data from these plots have been removed. They were the work of the removed author.

13) Fig. 18 shows almost constant spectrum of gamma-rays between 100 keV and 5 MeV. That is not possible. The energy binning is clearly not fine enough causing there are no peaks visible.

The data from these plots have been removed. They were the work of the removed author.

14) Fig. 20 - how come that the fast and thermal flux maps look so similar?

The data from these plots have been removed. They were the work of the removed author.

15) lines 647, 657 - ref. to Fig. 16 should be replaced by Fig. 15.

The data from these plots have been removed. They were the work of the removed author.

16) lines 661 - ref. to Fig. 17 should be replaced by Fig. 16.

The data from these plots have been removed. They were the work of the removed author.

18) Correct numbering of references 21 and 22.

Thank you for keeping us honest. This has been corrected.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript describes the requalification of the Purdue Subcritical Pile after a substantial reconfiguration to accomodate a larger irradiation chamber. The overall approach presented herein is appropriate for this endevour, however the author incorrectly concludes that the measurements and model agree, neglecting an obvious oversight in the measurements. Please see the attached red-line for specific comments, however the major revisions required for this article should at least address these following concerns:

1) Overall the claims in the abstract vastly over-value the impact of this work. Although I recognize the importance of this study, the fact remains that the neutron and gamma flux in the facility is comparable to typical beam port facilities. Therefore, I disagree that this represents a "uniquely large volume" or "significant neturon and gamma fluxes". Furthrmore, I am unaware of any significant isotope production activities that could be undertaken with these levels of irradiation. Finally, the speculation that this facility could be used for non-destructive imagine is also unfounded. Imaging applications require a directional beam and one advatage of this facility is the isotropic nature of the irradiation. These arre in contrast with one another.

2) Generally speaking, the author goes into far too much background in this manuscript that undervalues the intelligence of the reader. Section 2 could be reduced to 1 paragraph as part of the introduction, a detailed review of all subcritical piles is not necessary for the context of this work. Many sub-sections within section 4 are also unnecessary (see redline). The breadth of the background is such that it prevents much depth in any one area and as such does not provide any meaningful value to the manuscript. It's important to be concise in techincal writing and a big part of that is to understand the knowledge of the reader. Citations can be presented to provide a means to find more information. I think this research could be sufficiently addressed in 8-10 pages instead of the 24 that were presented here.

3) Several citations are incorrect (I believe this was just an editorial problem but it must be resolved), see redline.

4) The technical quality of the gamma-ray spectroscopy must be improved. The efficiency calibration plot in Fig. 9 does not illustrate the fit that was used to determine the efficiencies in Tables 2 and A1 and the uncertainties for the calibration are much larger than they should be. Either a higher activity calibration source or longer measurement is required to improve this calibration.

5) The biggest shortcoming of this work has to do with the determination of thermal neutron flux by Indium activation. ASTM standard test method E262 clearly describes the importance of self-shielding corrections for both gold and indium (thie methods used in this work). There was no discussion of this correction being made, and it's clear to me in Figure 10 that this was not accounted for. What's worse is that the author claims that the "results agree within the measured data points", waving aside the disparity in results as "within each other's error allowance, allowing them to be considered similar". If this were truely the case you would expect variation on both sides of the average. However Fig. 10 clearly indicates a bias with all of the In results significantly lower than the Au results.

This manuscript certainly has value, and although the above concerns are significant, they are also very achievable. I am encouraged to see such validation measurements. It is all too common to relie entirely on simulation and modeling in this case. However it's important that we all conduct reactor dosimetry measurements in line with the accepted standards and practices. I would encourage the author to consider participation on the ASTM International E10 committee if this is an area of continued interest.

Comments for author File: Comments.pdf

Author Response

Reviewer 2:

Sir/Ma’am, thank you for taking the time to review the article. I appreciate the time, the feedback is appreciated. Please excuse the delay in returning the paper. We had to remove one of the members from the paper. Experiments needed to be rerun, and their work removed. This proved to be difficult and time extensive. Nonetheless, we appreciate your patience.

Overall the claims in the abstract vastly over-value the impact of this work. Although I recognize the importance of this study, the fact remains that the neutron and gamma flux in the facility is comparable to typical beam port facilities. Therefore, I disagree that this represents a "uniquely large volume" or "significant neturon and gamma fluxes". Furthrmore, I am unaware of any significant isotope production activities that could be undertaken with these levels of irradiation. Finally, the speculation that this facility could be used for non-destructive imagine is also unfounded. Imaging applications require a directional beam and one advatage of this facility is the isotropic nature of the irradiation. These arre in contrast with one another.

Thank you for the comments. We believe the abstract has been updated with claims that are more reasonable.
Generally speaking, the author goes into far too much background in this manuscript that undervalues the intelligence of the reader. Section 2 could be reduced to 1 paragraph as part of the introduction, a detailed review of all subcritical piles is not necessary for the context of this work. Many sub-sections within section 4 are also unnecessary (see redline). The breadth of the background is such that it prevents much depth in any one area and as such does not provide any meaningful value to the manuscript. It's important to be concise in techincal writing and a big part of that is to understand the knowledge of the reader. Citations can be presented to provide a means to find more information. I think this research could be sufficiently addressed in 8-10 pages instead of the 24 that were presented here.
Thank you for your comments. We agree that the literature review and the diffusion theory was a bit much. We have removed these sections to keep the paper more concise. We felt that the inclusion of details surround HPGe detectors is useful to help explains the methods used, allow for reproducibility, and most importantly appeal to a general audience as this is in Instruments and not a nuclear journal.

3) Several citations are incorrect (I believe this was just an editorial problem but it must be resolved), see redline.

Thank you for pointing this out. These have been corrected.

Thank you for pointing this out, the quality of the plot was a concern for the other reviewer. This data for the updated plot was taken three times by three people. Hopefully, the results are more appropriate.

Overall, the feedback was fair and appreciated. Thank you for taking the time to read the paper. Hopefully, the revised manuscript meets addresses your concerns.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I appreciate that the authors did the measurements again with a better precision. However, treatment of some of my comments is careless.

My previous major comment No. 5 was answered and stated that it was an error. However, values in Table 1 remain unchanged!

My previous major comment No. 6 was answered and stated that the calibration was reconducted by three times. Great! Show the results of the three measurements!
I agree with omitting the low-energy part as it is anyway not used further. However, the efficiency values in Table 3 cannot be correct. They are about 5% of the original ones and they certainly do not agree with the values plotted in Fig. 11. Uncertainties of the efficiency should also be reported in Table 3.

My previous major comment No. 7 was answered and stated that the experiments were reconducted several times. Great! Show all the results. That is what is valuable.
Moreover, Fig. 12 showed values larger for Au198 than In166m in the original version. Now it is opposite. Such a change in the trend must be discussed.

My previous major comments No. 8-9 were answered and I appreciate that the new Monte Carlo results are so precise that the error bars are invisible. Now Fig. 18-20 clearly show the underestimation in OpenMC compared to MCNP and experiment. Discuss it. Is the MCNP model better than the OpenMC model?

The point of my previous major comment No. 10 was not answered. Table A1 was changed but it still states "Resolution", while the caption states "Absolute Photopeak efficiency". Which values do you show?

My previous minor comment No. 5 was answered as "Figure updated", however, the part of the figure I was asking to update was simply deleted.

My previous minor comment No. 6 was answered as "Figure updated", however, my comment was not taken into account at all. The only change made was conversion of inches to cm.

Minor comments
1) Due to changes, typing mistakes appear at some places, sometimes full stops missing. Read the manuscript again and do your job finally carefully.
2) Distinguish measurement from calculation. For example, caption of Fig. 12 claims "calculated by gold and indium foils." It should be "measured with gold and indium foils."
3) Fig. 21 is not a figure, it is a table.
4) Fig. 22a shows neutron flux spectrum normalized per unit energy. That is not a good way to present plots in log scale because it creates a fake impression that the low-energy region is more important than it actually is. Instead, plot the spectra normalized per unit lethargy. That will give the correct shape.
5) References 19 and 20 should be completed.

Author Response

Reviewer 1:

I appreciate that the authors did the measurements again with a better precision. However, treatment of some of my comments is careless.

Thank you for taking the time to reconsider the work and provide comments. We appreciate the honesty and the critical feedback. We are your future replacements in the industry.

My previous major comment No. 5 was answered and stated that it was an error. However, values in Table 1 remain unchanged!

Thank you for pointing this out. You are correct, this table was remade but was overlooked in updating the manuscript. This version now has the updated table. It is now labeled as Table 2. Our test sources were calibrated in 2003. To reduce error (1/sqrt(N)) we doubled up on the test sources to increase total counts, ultimately reducing the error.

This was a bit confusing in the paper. The table was for the indium absolute efficiencies and the plot was for gold. This has been revised to include figure 9 with both indium and gold values. Table 3 now reflects this change as well. Yes, the measurements were conducted multiple times, however, not all of the data is good as you correctly pointed out in the first version. We saw no added value in the paper to include this incorrect data, or data with high error.

The change in values can be attributed to the use of the ASTM tabulated values as opposed to the ENDF cross-sections for thermal neutrons. Again, this was conducted multiple times. However, not all data meets the quality threshold. Most of the data points are within the error bars. We maintain that the use of the ASTM standard overshoots and undershoots the values of the cross-section data depending on the location. The position which exhibits the most error and largest discrepancy is the foils measured at 80 cm. This is relatively far from the sources.

Please see the addition of sections 6.2 and 6.3 addressing this concern. Thank you.

It was absolute photopeak efficiency, mislabeled as resolution. Appendix A was removed because Figure 9 now shows the values for the Endcap. The inclusion of Appendix A was to provide this information. Hopefully, it is clear now that the gold experiments with its absolute efficiency is separate from the indium and its absolute efficiency.

My previous minor comment No. 5 was answered as "Figure updated", however, the part of the figure I was asking to update was simply deleted.

Anything that was deleted was due it having been created by the redacted author. To ensure that there were no conflicts of interests, or intellectual data in the paper this was removed. As far as the “floating air comment,” we agreed and removed this image. While it did explain the thickness of cadmium, we feel that the ASTM standard is quite clear on the dimensions of cadmium to be used. Should the reader wish to get this information we refer them to the standard.

My previous minor comment No. 6 was answered as "Figure updated", however, my comment was not taken into account at all. The only change made was conversion of inches to cm.

The gold and indium experiments were conducted separately. These were initially two separate experiments with no intention of joint work. It wasn’t until afterward that the paper was considered and the data combined to discuss the entire pile. As a result, the indium foils were measured from the platform and the gold foils were measured from the endcap. A line has been added to clarify this point. (Yes, bad lab practice for contamination on the detector.) However, to properly display the methods we have included both images for others to reproduce the data should they repeat the experiment.

Minor comments
1) Due to changes, typing mistakes appear at some places, sometimes full stops missing. Read the manuscript again and do your job finally carefully.

Thank you, I will be more careful next time.
2) Distinguish measurement from calculation. For example, caption of Fig. 12 claims "calculated by gold and indium foils." It should be "measured with gold and indium foils."

We respectfully disagree with this statement. In this paper, any value that was collected through an instrument is considered a measurement. Any value that required a formula to derive is a calculation. In this case, the gamma counts were measured with the HPGe. The measured counts were used to calculate the flux. No neutron instruments were used; therefore, we maintain that the flux is calculated and not measured.
3) Fig. 21 is not a figure, it is a table.
We believe this is in reference to Figure 19, not 21 the cross-section values. This is a technicality. We agree it is a Table. However, per the MDPI Instruments journal formatting guideline the table has no color and has the general format used elsewhere in the paper. Regardless, they have been updated to a standard table.
4) Fig. 22a shows neutron flux spectrum normalized per unit energy. That is not a good way to present plots in log scale because it creates a fake impression that the low-energy region is more important than it actually is. Instead, plot the spectra normalized per unit lethargy. That will give the correct shape.
This figure has been updated. However, it does not show the epithermal/fast flux that was observed in the other figure. As a result, the right figure was maintained to show that there is a 200% increase in non-thermal flux at various regions in the pile when compared to the testbed.

5) References 19 and 20 should be completed.

Thank you, I use Zotero for citations. Sometimes it auto updates.

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for this opportunity to reconsider this work that I think is very important to share. All too often, experiments like this are disregarded in lieue of simulation and modelling. This work points out importance and challenges associated with these measurements. I would encorage the authors to consider presentation of this type of work in the future at the International Symposium on Reactor Dosimetry (ISRD) conference to suplement journal submission.

This revised manuscripts represents a dramatic improvement from the original submission. Nearly all of the concerns from my prior review have been addressed, but I have a hand-ful of comments that I believe can be easily considered to further improve the narrative. Details are provided in the attached markup, but are summarized below:

1) A citation for the claims concluding in lines 91 and 181 are necessary.

2) The statment in line 221 regarding saturation of the gold foils after 14 days of irradiation is could be slightly modified to be more technically correct to: "... 14 days to approach the saturation condition."

3) I have a bit of an issue with the curve-fit used in Fig. 11 for the HPGe efficiency. ASTM E181 suggests a polynomial fit would be used for a near-point source like this (Fig. 2 of that standard). The energy range used for calibration does not extend low enough to peg the low-energy so it may be that an expoential fits better in this case (as shown in eq. 11). The outlying point from Cs-137 also creates some uncertainty in the fit for the 411-keV peak from Au. It might be worth including the uncertainty from this curve in the calculated flux values for Au and In respectively. I think the methods used herein are okay but a bit more justifcaiton for the fit would go a long way, especially considering the audienace of this paper may not be as familiar with HPGe measurements.

4) This revised analysis has gone a long way to resolve the dispartiy between Au and In measurements however a clear bias still exists in Fig. 12. I think this could be related to the cross section analsysi in Fig. 21 and a couple of sentences connecting these explicitly could go a long way in resolving this bias.

5) In figure 14, it's interesting htat there is a large discrepancy in the final measurement locaitons between Au and In. I did not see this addressed in any way but suspect it has to do with some variation in the physical characteristics of the foil (position probably). I'd just like this observation addressed, even if there is no smoking gun as it were.

6) Is there any speculation of the source of the additional gamma peaks in Fig. 16(a)?

7) ASTM E181 suggests more than 3 energies be used for this type of calibration. In fact, scintillators can exhibt strong non-linearities in the energy calibration that can not be observed with only 3 points.

8) In Fig. 17 it is strange that the positions of the measurements and simulation do not align. This makes it hard to assess the quality of the results. Was this on purpose for some reason or could the simulation locations be chosen to align with the experiment?

9) In line 488, please change "error" to "uncertainty".

10) It would be very interesting to know the average value and standard deviation of the corrected cross-sections illustrated in Fig. 21. Then, a comparison to the "book value" presented in the ASTM standard E232 could help to further drive home your conclusion that this approach leads to descrepancies between the experiment and simulation results.

11) There is an extra tab in line 545

12) The claim in line 571, 'slightly lower neutron flux' should be quantified instead of using qualitative language if possible.

13) Where, precisely, was the maximum flux from line 591? I assume in the irradiation cavity but this could be incorrect.

The method described in section 4.3 to account for self-shielding is clever but not entirely novel as claimed. This method of effectively diluting the samples is typical for such calculations but generally the density is artifically modified instead of using a thin source. This is fundamentally a sound method and does not need to be changed.

Comments for author File: Comments.pdf

Author Response

Reviewer 2:

Thank you for taking the time to reconsider the work and provide comments. We appreciate the honesty and the critical feedback.

A citation for the claims concluding in lines 91 and 181 are necessary.
- Thank you! Yes, I agree.
The statment in line 221 regarding saturation of the gold foils after 14 days of irradiation is could be slightly modified to be more technically correct to: "... 14 days to approach the saturation condition."

A mathematician and an engineer are placed on opposite sides of a room. They're told that every minute, they'll be allowed to move halfway toward each other.

The engineer immediately starts walking, getting closer and closer.

The mathematician just stands there.

The engineer says, “Why aren’t you moving?”

The mathematician replies, “Because we’ll never actually touch — it’s a converging series that never reaches the limit.”

The engineer shrugs and says, “Well, I figure I’ll get close enough for all practical purposes.”

Anyhow it’s been updated.

I have a bit of an issue with the curve-fit used in Fig. 11 for the HPGe efficiency. ASTM E181 suggests a polynomial fit would be used for a near-point source like this (Fig. 2 of that standard). The energy range used for calibration does not extend low enough to peg the low-energy so it may be that an expoential fits better in this case (as shown in eq. 11). The outlying point from Cs-137 also creates some uncertainty in the fit for the 411-keV peak from Au. It might be worth including the uncertainty from this curve in the calculated flux values for Au and In respectively. I think the methods used herein are okaybut a bit more justifcaiton for the fit would go a long way, especially considering the audienace of this paper may not be as familiar with HPGe measurements.
- We agree that a 5^th order polynomial is typically used to conduct this calibration. However, we removed the low energy gamma peaks because they were not relevant in this analysis. As a result, fitting with an exponential curve provided a better fit.

6) Is there any speculation of the source of the additional gamma peaks in Fig. 16(a)?

It is most likely due to the Aluminum activation from the testbed. See added paragraph at the end of section.

We agree. However, the use of a NaI detector and the very small focus on the gamma profile of the pile in this particular paper is incredibly light. This section simply serves as a ‘we should investigate this more.” We just need to set up the HPGe in a way that won’t damage it.

This has been better explained in lines 506-512. The locations selected were based on where there were indium foil cavities. The MC data tallies were based on a 4 inch^3 definition. If the foil location intersected the tally volume, this value was used. However, the tally placement is the centroid of the cube. This is the cause for the discrepancy.

9) In line 488, please change "error" to "uncertainty".

Thank you.

Please see added sections 6.2-6.3. Yes, part of it was due to the cross-section data. This location was also under the testbed support beam. We intend to publish another paper using openmc/mcnp to optimize the testbed for the fuel placement to maximize flux in the testbed. This will certainly be used in the next paper.

11) There is an extra tab in line 545

Thank you.

12) The claim in line 571, 'slightly lower neutron flux' should be quantified instead of using qualitative language if possible.

Removed the subjective language.

13) Where, precisely, was the maximum flux from line 591? I assume in the irradiation cavity but this could be incorrect.

We clarified that this is above the AmBe source. However, this is as close as we could physically get to the source based on the physical design. Another paper is planned for optimizing the pile layout for maximum flux within the useable volume of the testbed.

Thank you for the information!

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

The authors do not lack confidence and I appreciate they can defend themselves. At the same time they should search for appropriate balance between confidence and humbleness. I hope they will support their confidence with better quality of their future research output. I hope that once the authors will become researchers and start reviewing manuscripts of others, they will do their reviewer job properly and they will not be satisfied with work that deserves to be improved.

So far, according to Scopus, it is only the last author (out of seven) who has non-zero h-index. The authors are likely students. That was the reason why I did not reject the manuscript at the beginning because I wanted to give them at the beginning of their career a chance to work on it and improve it to an acceptable level. And that is the reason why I have given them not only comments what to improve but also recommendations on how to improve the manuscript, which is normally not a reviewer's job but a supervisor's job.
The authors have done quite some job but it is not yet sufficient. When improving the manuscript, they created new mistakes and confusions.

My new comments:

1) Table 2 - finally it is updated. Good that there are more lines and the errors have decreased considerably. However, is it really possible so much? If I consider 1/sqrt(N), then the error of 356 keV decreased from 0.038 to 0.002, i.e. 19 times. That would mean 361 times more counts. Really?

2) Table 2 - number of digits of the resolution and error should be consitent. For example, 276 keV line has resolution 0.49% with error of 0.0054. Does it mean (0.49 +/- 0.0054)%? Then there should be two more significant digits in the resolution.

3) Correct numbering of figures (Fig. 8 twice).

4) Line 611 - replace "MeV" with "meV"

5) Line 616 - replace "under thermal energies" with "in the thermal region"

6) Fig. 19 left part was updated. Now it shows very strange structures, while in the previous version it was nicely smooth. I guess there is a mistake in the normalization per unit lethargy. Check that. The spectrum shape should be without any structures in the thermal region.

7) Fig. 19 - the vertical axis was changed to linear scale, which causes that the epithermal+fast flux is not visible. Return the vertical axis to log scale.

8) Again concerning caption of Fig. 10. I understand your reasoning that measured values are only detector count rates, while if you use the count rate and, let's say multiply it with nuclear data and detector efficiency, you do not get measured values but calculated values. It sound reasonable but there are three issues.
Firstly, the caption says: "Comparison of average neutron flux in the testbed calculated by gold and indium foils."
That is not quite correct, because it suggests that the foils themselves did the calculation, which is neither logically nor grammatically accurate.
Secondly, it is not consistent terminology, compare to Fig. 13 "Flux map of the entire pile as measured by indium foils", Fig. 15 "Comparison of measured thermal flux."
Thirdly, in your manuscript you compare results of Monte Carlo calculations with experimental results. That's why it might be confusing to call those values that were determined from the measured values as calculated. Do you feel it?
I suggest you use "Experimental neutron flux" which refers to value inferred from measurements, as opposed to theoretical or simulated values.
Thus, caption of Fig. 10 would be for example "Experimental neutron flux in the testbed determined from measurements with gold and indium foils."
In a similar way, correct captions of Fig. 11-13, 15-18.
I hope you understand that "Normalized In-116m foils" is not a quantity and it is not correct caption of a plot showing neutron flux.

9) Add dimensions or scale to Fig. 2 and indication of materials (think of usefulness of such a figure).

10) Add necessary information to references 20-22 (think whether it is possible for a reader to find the publication you are refering to). Each of these reference claims they are from ENDF/B-VIII.0 but each of them from a different author and different year. Weird.

11) Add uncertainties to Table 4 and 5.

Author Response

My new comments:

Table 2 - finally it is updated. Good that there are more lines and the errors have decreased considerably. However, is it really possible so much? If I consider 1/sqrt(N), then the error of 356 keV decreased from 0.038 to 0.002, i.e. 19 times. That would mean 361 times more counts. Really?
1. Table 2, has been updated once again. We wanted to ensure there was enough counts. Each ROI was conducted for 60 minutes. Our sources are old (2003). Table 2 is now updated with the counts for each ROI.
Table 2 - number of digits of the resolution and error should be consitent. For example, 276 keV line has resolution 0.49% with error of 0.0054. Does it mean (0.49 +/- 0.0054)%? Then there should be two more significant digits in the resolution.
1. We see that mixing percent with numbers in Table 2 may have been confusing. The table with updated to include the numbers and not percent. Figure 10 has been updated accordingly. (Energy resolution plot).
Correct numbering of figures (Fig. 8 twice).
1. Thank you for pointing out the oversight. Figure 8 was removed from being displayed twice. All Figures beyond the true Figure 8 have been updated. Their values are the Figure # plus 2.
Line 611 - replace "MeV" with "meV"
1. Thank you for pointing this out. Was labeled 2.5E-06 MeV corrected to 2.5E-08 MeV. Given that neutrons are typically discussed in energies of eV or MeV, we left it as standard MeV notation.
Line 616 - replace "under thermal energies" with "in the thermal region"
1. Line has been updated to read, "in the thermal region."

19 left part was updated. Now it shows very strange structures, while in the previous version it was nicely smooth. I guess there is a mistake in the normalization per unit lethargy. Check that. The spectrum shape should be without any structures in the thermal region.
1. Given this is MCNP and the tally is a function of the input SDEF card, it makes no difference if it is normalized or not. We removed the normalization.
19 - the vertical axis was changed to linear scale, which causes that the epithermal+fast flux is not visible. Return the vertical axis to log scale.
1. Updated now in a log-log format. Though I have found some differences among publications from LANL and Sandia. It seems that Sandia uses lethargy more frequently. For future reference, I am curious as to what benefit this way of plotting has? My understanding of lethargy is related to slowing down power, but that requires some discrete birth energy. More of a curiosity than anything.
Again concerning caption of Fig. 10. I understand your reasoning that measured values are only detector count rates, while if you use the count rate and, let's say multiply it with nuclear data and detector efficiency, you do not get measured values but calculated values. It sound reasonable but there are three issues.
Firstly, the caption says: "Comparison of average neutron flux in the testbed calculated by gold and indium foils."
That is not quite correct, because it suggests that the foils themselves did the calculation, which is neither logically nor grammatically accurate.
Secondly, it is not consistent terminology, compare to Fig. 13 "Flux map of the entire pile as measured by indium foils", Fig. 15 "Comparison of measured thermal flux."
Thirdly, in your manuscript you compare results of Monte Carlo calculations with experimental results. That's why it might be confusing to call those values that were determined from the measured values as calculated. Do you feel it?
I suggest you use "Experimental neutron flux" which refers to value inferred from measurements, as opposed to theoretical or simulated values.
Thus, caption of Fig. 10 would be for example "Experimental neutron flux in the testbed determined from measurements with gold and indium foils."
In a similar way, correct captions of Fig. 11-13, 15-18.
I hope you understand that "Normalized In-116m foils" is not a quantity and it is not correct caption of a plot showing neutron flux.
1. The Figure descriptions have been updated to address this comment.
Add dimensions or scale to Fig. 2 and indication of materials (think of usefulness of such a figure).
1. Figure updated with minor dimensions and high-level bill of materials.
Add necessary information to references 20-22 (think whether it is possible for a reader to find the publication you are refering to). Each of these reference claims they are from ENDF/B-VIII.0 but each of them from a different author and different year. Weird.
1. ENDF Citations updated.

11) Add uncertainties to Table 4 and 5.

Uncertainty is determined to change at the 1E-04 placeholder. The largest source of uncertainty was among the ENDF files.

Reviewer 2 Report

Comments and Suggestions for Authors

All comments have been addressed satisfactorily.

This is a very nice paper!

I will recommend it to students and peers alike in this area.

Author Response

Good afternoon,

Thank you for the compliment and the review. We had a few more items to check off for Reviewer 1.

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Design and Characterization of the Modified Purdue Subcritical Pile for Nuclear Research Applications

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