Microparticle Hybrid Target Simulation for keV X-ray Sources

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors suggest to overcome thermal limitations of rotating anode X-ray tubes by combining it with a  tungsten microparticle stream in close proximity to the anode. Thereby, a large proportion of the incoming electron beam energy is converted to X-rays by the microparticle stream. The remaining electron beam energy which passes the microparticles is then converted to X-rays (and heat) by the rotating anode. The authors claim, that a potential X-ray output gain of at least 85% could be achieved while maintaining a small focal spot size.

The paper is well written and the work seems to be solid and well conducted. Overall, the considerations and conclusions of the paper seem convincing. If their idea worked in practice, it would be of major interest to the CT community. However, there are some aspects that were not totally clear to us.

Major comments:

1)     It seems that the work presented in the paper is only simulation based. If so, then this should be made clear in the title, e.g. by having the words “a simulation study” in the title.

2)     Section 2.2: Why does the potential shift caused by the charging of the microparticles matter? The x-ray spectrum is polychromatic anyway. The shift in potential appears to me as being similar to convolving the bremsstrahlung part of the unshifted spectrum with some smoothing function, e.g. with a rectangle function. Please plot a spectrum without the shift and the superposition of many spectra with shifts. Please explain why this would be a problem for CT imaging and give evidence for this claim. Note that one can control the position of the spectra (be it with or without shifts) by adapting the acceleration voltage. Please describe in more detail why the mentioned effects of charge accumulation impair the X-ray source.

3)     Can you plot a spectrum of the X-ray source that is impaired by the microparticle charging? You could argue with the spectrum why you need mechanisms for charge balancing.

4)     Please reason why you need the hybrid target concept. Why don’t you use a microparticle stream only?

5)     Lines 193-197: I needed to read the geometry description several times to understand it. Can you reference to a figure to help the description?

6)     Section 2.2: The simulation setup from the description is not clear to me. What is the general case of 50 micrometers?

7)     Figure 4: Is b) the same as a) just from another view? Consider adding small coordinates to the subplots to show the viewing angle. It would be helpful if some of the coordinate axes would also show up in other figures.

8)     How does the acceleration of the microparticles with the funnels on the anode work? Are there a lot of funnels on the anode? If the funnels are rotating with the anode, is there a microparticle stream all over the anode? How do you achieve a microparticle stream over the focal spot only?

9)     How many monolayers does a realistic microparticle beam correspond to? Do you intend to perform multilayer simulations?

Minor comments:

1)     Line 26 and others: “of nominal size 0.3”: what unit? mm?

2)     Line 40: Does the term “braking radiation” exist? Else, I suggest to just use “bremsstrahlung”, which is the correct english word (stemming from the German “Bremsstrahlung”).

3)     Figure 2 looks somewhat weird since the left hand side is cut straight off while the right hand side has a 3D appearance. I recommend to also have that 3D appearance on the left hand side.

4)     Figure 2 is not true to scale, which is somewhat confusing. Please adding example measures to the figure (dimensions, velocities).

5)     What average mass density will the particle stream have in comparison to solid tungsten? Can you give a rough estimation?

6)     Figure 5: Please add the electron beam to the figure.

7)     Figure 5: According to the figure description the power distributions are integrated along y, but it seems like just the central layer. If it is integrated, why don’t you see the absorption at the y-poles (e.g. 30 keV)? Maybe the power distributions are averaged along y?

8)     Figure 6: Is this showing just a central slice through the sphere or a projection along the dimension perpendicular to the page?

9)     Figure 8: Please use the same y-range for all three plots? Please use the same orientation of the numbers on the x-axis for all plots. I also recommend to number only multiples of 50 kV on the x-axis.

10)  Figure 8: I suppose that the red curve is the same for the three plots. Please do explicitly mention that in the subfigure.

11)  Figure 8: Curves should be thicker. Backround grid should be sparser.

12)  Lines 398-400: How does the described issue provoke a fluctuation? In my understanding the charging of the microparticles would be constant for each x-position and therefore the resulting X-ray spectrum would be an overlay of the steady spectrums of a range of tube voltages. See also my comment above.

Comments on the Quality of English Language

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Author Response

Dear Reviewer 1,

Thank you for your valuable suggestions and comments. Please find our corrections and other response attached.

Best regards,

Rolf Behling

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

 

High flux X-ray sources with small focal spots are demanded in clinical context, nondestructive testing, and X-ray science. However, the flux of X-ray tubes cannot be easily increased by increasing the power of the electrons hitting the anode target. Due to the bad conversion of electron power into X-rays but a much better conversion into heat, the anode would melt.

The authors mention several existing alternative methods to produce X-rays of high flux and small focal spot size and propose an additional one, “A microparticle hybrid target concept for keV X-ray Sources.” A stream of fast micrometer-sized tungsten particles moves over the surface of a rotating anode X-ray tube through the focal spot of electrons from the cathode. Part of the electrons interact with the particle stream, and part interacts, as usual, with the rotating anode. This concept is proposed to allow higher power because of the shielding of the anode by the particle stream, giving higher X-ray flux by keeping an acceptable power to the anode and a small spot size.

 

The manuscript is well-written and discusses several details of the pros and cons of the hybrid concept. Monte Carlo calculation setups are described, and their results are clearly presented. An outlook for future work on this project is presented in the end.

 

The manuscript needs some minor corrections in some figures for better understanding.

 

Figure 4(e)

The layer thickness H, as given in the graphic, is only 45 mu, not 50 mu, as shown in line 201 and the figure caption. Even if the empty volume of 5 mu lengthens H in the figure for the other four layers, the length of d_gap would be too long. The geometry of this figure should be appropriately adjusted.

 

Figure 5

Compared to the diameter of 5 mu of the W spheres in the upper part of the figure, the distance to the dashed line is more than the 10 mu given in the figure. This distance should be corrected.

 

Figure 7(b)

I have several comments on this figure.

a) The distance h=0 mu (the vertical axis)  starts with the upper edge of the sphere here but was defined to begin with the surface of the solid anode in Figure 2. That should be adjusted.

b) The stopping power starts at about 1% for h=0. Why? Are the values those at ½ steps?

c) If the first data point is at h=0 and the last at h=10, the data point at the lower edge of the sphere is not at h=5 mu but at 4.4 mu.

d) The integral of the stepping power is not 100% but a little more than 60%. It would be helpful to mention in the figure caption (and maybe refer to Table 1) that the missing part is due to the backscattered electrons.

 

Figure 11(b)

The label of the vertical axis is given as h but should be -h (with a minus).

 

Comments for author File: Comments.pdf

Author Response

Dear Reviewer 1,

Dear reviewer 2,

Thank you for your valuable suggestions and comments. Please find our response attached.

Sincerely,

Rolf Behling

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed the concerns during the first revision. Hence, I do not have any major comments to add. The paper is well written and the revised graphics are illustrative.

One more question came up concerning figure 9:

Plots a), b), and c) differ regarding the velocity of the microparticles. However, the entry temperature is 100 °C for b) and c), but 800 °C for a). Should the entering temperatures not be the same for a fair comparison regarding microparticle velocity? I could imagine that the temperature difference is justified because of different acceleration mechanisms (You mentioned that the microparticles heat up to 800 °C when being accelerated by the rotating anode). However, this makes it hard to conclude if the different power limits of a) and b) are mainly due to different microparticle velocity or initial temperature. Consider commenting on that in the description.

Also, although not of direct necessity for the paper, it would be interesting to know how the microparticles are collected and returned to the microparticle feed at the anode. Do you have enough microparticles for one scan or do they need to be used several times per scan?  

Author Response

Dear Reviewer 1,

Thank you for your question for review in round 2. Thank you again for your review, your positive response, and the time that you devoted to this.

Please find our response in the attached file in blue.

Sincerely,

Rolf Behling

(for the authors)

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors,

thank you for meeting all my suggestions.

Author Response

Dear Reviewer 2,

Thank you again for your review, your positive response, and the time that you devoted to this.

Sincerely,

Rolf Behling

(for the authors)