Towards an Intrinsic Doppler Correction for X-ray Spectroscopy of Stored Ions at CRYRING@ESR

Round 1

Reviewer 1 Report

The improvement of the spectral resolution for x-ray line measurements from few-electron heavy ions below 1 eV opens very attractive possibilities for test of quantum electrodynamics in fundamental atomic systems. The combination of microcalorimeter detectors in forward and backward position in ion beam experiments, combined with cooled ion beams in a storage ring, makes arguably the best use of presently available technology for achieving this goal. The present manuscript presents first data demonstrating the high resolution and typical count rates achievable with a beam of U90+, that is, reaching directly the relevant ion-charge range, and using strong deceleration from the ion-production energy.

The paper is very clearly presented and instructively discusses the very promising perspectives opened by the measurement. Publication in the Journal "Atoms" is recommended. Before publication, the authors should consider the remarks below and optionally apply revisions.

Figure 2:  1) Since one of the achievements is so reach sufficient count rate, it would be interesting to know some key parameters of the measurement shown in the upper frame: approximate stored ion number, data acquisition time, electron density. (There should be enough space to give these few data here so that they do not have to be looked up from (maybe) cited others papers.) 2) It would be helpful to explicitly give the energy bin size. The counts scale gives "per 20 eV", and likely this is the bin size, but only an explicit statement is unambiguous. 3) It is explained that two calorimeters at 0 and 180 degrees are used. Was the 180 degree spectrum also observed already and does it look similar, as one should expect?

Figure 3 and text around: Is the energy resolution in the model calculation assumed to be the same near 25 keV and near 100 keV? Was the required resolution deduced in the end (line 155) already reached in the present experiment (it actually seems so, as near-25 keV is concerned).

Predictability of "calibration" lines (lines 157 ff). Many electron effects should become smaller and smaller if the lower n of the used transitions is chosen as high as possible. Taking the observed intensities (Fig. 2) and the accessible spectral range , would it be feasible to use N->M transitions instead of M->L? - Or, as another point, is the difference between N->L and M->L better predictable than their absolute values, thus better for calibration?

Finally, while eq. (2) is somewhat more simple than the double use of eq. (1), there is also the other combination:

sqrt(E_lab(0)*E_lab(180))=E_emit

which was of course used in laser spectroscopy (e.g., PRL 91, 190403 (2003)). Hence, just the direct observation of a single line in forward and backward direction (with sufficient angular definition) would work without calibration. This expression seems to be a more relevant simplification than eq. (2). Is there any reason that it cannot be applied in the arrangement with the two microcalorimeters so that calibration can indeed be avoided?

 

 

 

 

 

 

Author Response

"Please see the attachment."

 

Thank you very much.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript describes a new experimental approach for the Doppler correction of x-rays emitted from fast-moving heavy ions using detectors with high energy resolution and broad energy. This approach are very important for high-precision measurements of K→L transition energy of high-Z ions in emitter-frame. The manuscript is well written and organized. I recommend the manuscript for publication in Atoms.

Still, there are some following suggestions:

1)      The authors assumed that the energies of M, N→L transitions were precisely known. I suggest that the authors could add some comments on the influence of the accuracy of known M, N→L transitions energy on the simulated uncertainty in Figure 3.

2)      In figure 2 (b): the authors have pointed out that figures (a) and (b) were obtained from ions with different energies, hence the ion energy used in Figure 2(b) is suggested be also listed in figure note or text.

3)      Line 50: “50 Mev/u” should be “50 MeV/u”

4)      Line 169: “instead” should be deleted.

Author Response

"Please see the attachment."

 

Thank you very much.

Author Response File: Author Response.pdf

Reviewer 3 Report

This conference contribution describes the first steps towards a novel technique of using MMCs to perform intrinsic Doppler corrections using high-resolution X-ray detection of cooled, relativistic heavy ions in a storage ring.  This work is timely and the technique is well motivated.

I have a few small comments:

1) In the introduction, it would be nice to have one or two references for justifying the first two sentences on QED studies in high/extreme EM fields.  Please could the authors add something here.

2) The discussion on page 2 regarding the challenges of performing Doppler shift measurements was very useful, however there are quite a number of assumptions that are involved for the usual technique (as discussed in the paragraph that starts on line 45).  I think it would be helpful to give the reader an additional sentence at the end of this paragraph that gives some idea of the confidence level of these assumptions, if possible.

3) The motivation for using MMCs is well laid out, however there is little-to-no discussion of the sensors themselves.  I know the "reader is directed to Ref [8]", however I think here there needs to be some basic information added at the start of page 4 such as what was the absorber material used and what temperature was required for cooling.  This at least gives a sense of the experiment a bit better.

4) Figure 2 is excellent, and really is the main headline of this article, however some more detail on how these data were collected would be helpful.  This could go in the body text or caption, but should at least include length of running time for both the MMC and HPGe detectors to provide some kind of efficiency/rate comparison estimate.  This could go at the end of the sentence that starts on line 116.  Even though this is preliminary data, I think this type of information should be straight forward to include.  Further, it would be great to see an inset of the MMC sensor in the top panel.  I think having a photograph of the device would be nice for some to see for the first time.

5) I found Figure 3 (and the monte carlo work in general) somewhat vague.  I think it would help if an "experimentally optimal" or "experimentally viable" region was highlighted somehow.  The two things that are relevant are what is the statistical uncertainty on the COM that is required and what FWHM resolutions are realistically achievable with MMCs.

6) Related the the last point, I think a sentence on what the sensor development groups ultimately expect to achieve for energy resolution of the MMCs at 100 keV for this work should be included.  For example, is 10 eV achievable with these types of MMCs?  If so, it helps put Figure 3 into much better context with regards to future plans.

Overall, the article is well written, the research is of high quality, and the groups doing this work are the world leaders in these areas.  I recommend publication after the few small comments above are addressed.

Author Response

"Please see the attachment."

 

Thank you very much.

Author Response File: Author Response.pdf