Nearly Monochromatic Bremsstrahlung of High Intensity via Microparticle Targets: A Novel Concept

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper by Behling et al. describes an X-ray source based on bremsstrahlung of tungsten microparticles targets, where the emitted spectra depend on particle size. In particular, the authors perform Monte Carlo simulations to determine the spectrum emitted by single and multiple microparticles.

The article is in general well written and the conclusions are supported by the numerical results.

However, I believe the following points should be addressed in order for the paper to be published on "Instruments"

 

1- In my opinion, in the abstract it should be made clear that this is a simulative work

2- The introduction beings with "A major shortcoming of the spectra of unfiltered bremsstrahlung is their wide ener-31 getic bandwidth". 

I'd explain this point better, since there are also applications, e.g. the spectroscopic ones, in which a large bandwidth is useful. The authors write then that the monocromaticity is important for medical imaging  and I agree on this, but I'd suggest not to start with such a decontextualized statement.

3- Still in the introductions, the authors write about rotating anode sources, but they do not consider liquid metal jet sources. It'd be important to add a sentence and a reference to this class of sources.

4- At line 97, the authors write that they are "... convinced that the technical hurdles can be overcome". I think they should at least motivate this statement.

5- They write that "The integral is numerically evaluated using Fig. 5b of ref. [3]". Since this is supposed to be a stand-alone paper, it is ok to cite a figure of a referenced paper, but it'd be important for the reader to provide some details also here.

6- Line 210 "The effect is not very pronounced". Is the effect of the simulated filters? "Not very pronounced" should be quantified in the text.

Author Response

Please see the attached rebuttal letter.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors reported a detailed assessment of the particle-based bremsstrahlung X-ray source which has the potential to produce high intensity and quasi-monochromatic X-ray. I think this manuscript fits this journal and is worth publishing but there are major and minor points that should be revised or reconsidered before publication.

Major comments:

1. There are no references in Sections 1.1 and 1.2. Some parts need references. For example, lines 37-41: need to refer to the source of the information. Line 50: Need reference for “the Duane-Hunt limit”. There could be some references for lines 53-56, lines 56 to 58, and lines 60-62. Adequate amounts of references should be added in Sections 1.1 and 1.2.
2. In Section 3.1, the author claimed that multispectral imaging can be done with a particle-based X-ray source by varying the particle size. However, the given information in the manuscript is only the mean energy. To support the claim, the X-ray spectra with different microparticle radii should be shown. In my expectation, the amount of energy variation due to the microparticle size is smaller than the spectral width of the X-ray beam. In this case, the impact of the energy variation on the image could be small.
3. In Section 3.1, the author claimed, “that may also help controlling the output intensity.” However, no results are shown for this point. Need some graphs, calculations, or descriptions that support the claim.
4. In Table 2, the target current monotonically increases as the particle size increases for the tube voltage of 30 kV. However, it has minimum value at 2.5 micro-m in the cases of tube voltage above 80 kV. Are those calculated values correct? If those values are correct, please add some description of why this phenomenon happens.
5. In conclusion, the authors wrote, “We have shown in this study the benefits for spectral X-ray imaging and the generation of high intensity, narrow bandwidth X-rays that could replace electron synchrotrons for many applications.” However, no discussion is given about this point. If the authors conclude that the particle-based X-ray sources could replace the electron synchrotrons for many applications, some discussions supporting this conclusion should be added to the manuscript.

Minor comments:

1. Figure 1 caption: MC is not defined in the manuscript.
2. Line 139: Need a period after [14].
3. Figure 3 caption: (a) “X-rays mitted” could be “X-rays emitted.”
4. Line 286: In the description “In a previous publication[2] We have…”, “We” should be “we”.
5. Line 296: FEM is not defined in the manuscript.
6. The authors wrote, “We have demonstrated through previous MC and FEM simulations the benefits of replacing the classical rotating X-ray anode with a stream of microparticles.” This description is rather confusing. Does this description mention a previous research work reported in another paper? If not, please remove the word “previous” from the description.

Author Response

Please see the attached rebuttal letter.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors properly addressed the points I raised and, in my opinion, the paper is now suitable for publication in "Instruments".

Author Response

Thank you for your valuable comments and suggestions. The authors

Reviewer 2 Report

Comments and Suggestions for Authors

The authors responded to all of my comments and questions. However, several major comments remain.

 

1. In Figure 3, the authors show the X-ray energy spectrum. As I expected, the peak energy change (~3 keV) is much smaller than the energy spread (~30 keV). I’m not so sure that this relatively small variation of the mean energy against the energy spread can give a significant difference in the obtained image. The authors should add more description on this point.

 

2. There could be different definitions of the word “monochromatic.” In my sense, light having an energy spread larger than 10% should not be categorized as “monochromatic.” In the X-ray field, is it OK to call the X-ray beam having more than 30% energy spreads a monochromatic X-ray? I often use the term “quasi-monochromatic” for light has a lather high energy spread. I think that the X-ray from the particle target with proper filtering should be categorized as a “quasi-monochromatic” X-ray.

 

3. Against my previous major comment #5, the authors replied that they added a reference to the introduction. However, no new discussions supporting this point were added. To support the conclusion, the authors must add some discussions supporting this conclusion based on the results shown in the manuscript. In the reference [5], the used X-ray energy spread looks very narrow and less than 1%. In contrast, the energy spread of the X-ray discussed in this article is >10%. I’m not so sure whether the particle-based X-ray source can replace the electron synchrotron in the application that  the authors pointed out.

 

Minot comment:

1. line 37: “tuneable” should be “tunable”.

Author Response

Thank you for your valuable comments and thorough analysis. We revised the manuscript by softening a claim according to your suggestions and changed a citation to more precisely piointing to the application that we are envisioning. We corrected the minor error,. Thank you for pointing to it.

In X-ray literature, the term "monochromatic" is used in a rather fuzzy manner. Some authors accept 10% FWHM and still characterize the radiation as "monochromatic". Therefore, we prefer to add the tinting particle "nearly" and keep the present title of the paper using the notion "nearly monochromatic". 

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors answered all of my previous comments, however, there is an unsolved concern.

The statement in the conclusion must be supported by the description in the manuscript. The authors claimed, “We have shown in this study the benefits for spectral X-ray imaging and the generation of high intensity, narrow bandwidth X-rays that could replace electron synchrotrons for many applications.” As I mentioned in the previous review report, in my opinion, the authors should discuss this point in the discussion section. The addition of some description to the introduction is not sufficient to support this claim. If the authors compared various X-ray sources, including a Thomson scatter source (MuCLS), please describe the result of the comparison in the discussion section. Please show that your source can fulfill the requirements of various important applications in terms of bandwidth and fluence in the discussion section.

In addition, the newly cited paper (Brummer et al) regarding the Thomson X-ray source is a state-of-the-art source based on a laser wakefield accelerator. The paper reported their simulation study and not the experimentally obtained one. Therefore, the paper is a not good reference to an existing source.

Author Response

Please see the attached rebuttal letter.

Author Response File: Author Response.docx

Round 4

Reviewer 2 Report

Comments and Suggestions for Authors

The authors responded to my previous comments and made sufficienty modifications to the manuscript. Therefore, I have no additional comments on this manuscript.