Optical Radiation during Sputtering of Lithium into a Noble Gas Using a Nanosecond Electron Beam

Round 1

Reviewer 1 Report

The reviewer would like to thank the authors for the submission of their manuscript titled "Optical radiation during sputtering of lithium into a noble gas by nanosecond electron beam".

 The authors present their findings of luminescence effects during irradiation of lithium with electrons in a noble gas medium. They compare their results with outcomes of preliminary studies, where the excitation of the lithium was realized by collisions with nuclear reaction products.

This is a topic of interest to the researchers in the related areas and this paper can be published after major revisions. The following points require consideration:

 (1)   The importance and necessity of the work should be emphasized in the Introduction section. In particular, the motivation and ambition for using electron bombardment for excitation of the lithium as opposed to preliminary studies should be presented more clearly.

 (2)   The preparation method and design of the lithium target should be described in more detail. A drawing or scheme of the clamped setup consisting of the capillary porous structure would be beneficial. It was mentioned that the CPS matrix was mounted inside of the cuvette loaded with lithium using spot welding (line 109). Please clarify if the mesh was spot welded before adding the lithium or in what sequence the procedure is carried out. Handling pure lithium is related to some chalenges because of its high reactivity with constituts of ambient air such as humidity, oxygen or nitrogen. What efforts have been carried to prevent reaction during preparation? Was the preparation done under inert conditions such as noble gas atmosphere?

Within Figure 1, a collimating lens was place behind each optical window. Please specify where focuses of the lenses are adjusted in relation to position within the chamber. If the focus was adjusted to the surface of the target, how was the light path to the other window realized despite the inclination of the target?

 (3)   The reviewer's main point of criticism addresses the statement that lithium atoms are sputtered by the electron bombardment, which is in particular pronounced in the paper’s title. The appearance of the sputtering effect is convincible for the bombardment with high-energy nuclear reaction products in the MeV energy range. However, a sputtering effect would not be expected during the collision of low-energy electrons in the energy range of about 100 keV. For this situation, the momentum transfer of the light electrons to the comparatively heavy lithium atoms is very small and almost negligible. Indeed, sputtering by electrons and photons was already observed and the low energy mechanisms, which lead to sputtering, can operate either by excitation or ionisation processes. Nevertheless, to the best knowledge of the reviewer these effects are confined only to atomically clean surfaces of insulators and semiconductors like alkali halides. Can the authors cite relevant references from literature, which evidence the occurrence of sputtering effects on metals such as lithium by bombardment of low-energy electrons?

 As an indication for the claimed sputtering effect, the authors state that compared to preliminary studies involving excitation by nuclear reaction products, an intensity of 4 orders of magnitude higher for the luminescence of lithium was observed (line 204). This high intensity is justified by a lithium partial pressure above the saturation vapor pressure (line 234).

In fact, however, a different experimental setup was used in comparison to preliminary studies and a possibility of comparing the absolute luminescence intensity is not discussed. In addition, the intensity of the luminescence of noble gases won’t be a suitable reference for intensity normalization, since other excitation mechanisms may occur.

 Furthermore, the sputtering effect by the electrons should have less dependence on the temperature of the lithium target. Consequently, luminescence of lithium with remarkable intensity should already occur at low temperatures such as 300 K. Furthermore, for long-term operation at low target temperatures a remarkable deposition of lithium layer on surrounding faces should be detectable for validation of the sputtering effect. This was not shown or discussed by the authors.

 The authors mentioned an estimation of partial vapor pressure for lithium due to electron irradiation (line 238). This estimation is not descried and the mentioned result of “a few tens of Pascals” is inexplicable.

Overall, the claims related to sputtering of lithium atoms by irradiation of low-energy electrons should be evidenced as proposed. Otherwise, the association to sputtering has to be removed within the whole paper.

 With above issue well addressed, the reviewer would be glad to recommend it for publication in applied science.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

This paper, reporting the results of a study of the radiation during sputtering of lithium into a noble gas by nanosecond electron beam, is concise, clearly written and pleasant to read. The motivations of the work, as well as the method used and the results obtained are presented in an appropriate way. My opinion is that this paper deserves to be published because it brings new results concerning the population processes of Li levels during the radiation pulse. I just have one minor point to make before final acceptance. In section 2, it is stated that 'calculations were performed using the Casino software to estimate the energy of the electrons at the output from the Be window'. It would be useful for the reader to have some details about this software (or at least a reference describing it).

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The reviewer would like to thank the authors for addressing all issues of the review report.

It was a hasty misconception of the reviewer that the sputtering effect have less dependence on the temperature of the lithium target. After all, the temperature dependence of the erosion rate for Li during bombardment with ions was already pointed out in literature (see for example Martin J. Neumann, Lithium Sputtering, Deposition and Evaporation: Controlled Thin Film Engineering, VDM Verlag 2008, ISBN-10:‎ 9783639041897, p. 77). A compareble relation is conceivable during bombardment with electrons. The reviewer recomments to address this aspect to make the findings more apparent.    

By the way, it is hard to understand that the dusting of the window should had no noticeable effect on the light transmission.

Nevertheless, the manuscript in the present version is suitable for publication in applied science.