Challenges and Requirements in High-Precision Nuclear Astrophysics Experiments

Round 1

Reviewer 1 Report

The paper presents a detailed and enjoyable overview on experimental techniques developed in the decades to arrive at high precision nuclear astrophysics experiments. The article is characterized by the proper level of details for the journal, is clear and well written in my view and I have no major objections. However, before recommending for publication, I have some observations and minor corrections.

General comments:

In the description related to the density measurement for gas targets the beam heating problem is described and the only mentioned solution is the measurement of elastically scattered beam particles on the target gas. In fact, there are also other solutions when a resonance is available for the gas under study or in one of the isotopes of the same gas. This method has several experimental advantages compared to the measurement of elastic scattering. I think the author is familiar with this technique since he is co-author of some papers describing the method; for this reason, I suggest to briefly describe also this possibility.

In the description of the determination of the number of reactions (215-216) I don’t agree with the problem of the knowledge of branching ratios in the reactions used to determine the gamma detection efficiency. The author is right in mentioning this problem, however, in several cases is possible to avoid the use of branching ratios. This is true for instance in the case of the 1:1 cascade for the well-known resonance at 278 keV in 14N(p,g)15O. There are several LUNA papers describing this technique where the author of the present paper is also co-author.

Minor corrections:

5: experiment -> experiments

6: reactions -> reaction

21: element -> elements

72: beam of projectiles seems redundant

111: in mentioned -> is mentioned

163: pressure and temperature measurement

204: complicates -> complicated

205: detector material -> detector materials

231: setup an?

235: from the studies reactions -> from the studied reactions

246-247: measurement geometry ? unclear

259: product -> products

Author Response

I thank the referee for the careful reading of the manuscript and for the positive opinion. I have implemented all the minor corrections and extended the text by mentioning the two issues (beam heating determination by narrow resonances and 1:1 cascades for efficiency measurement) raised by the reviewer. Two additional references are also included.  

Reviewer 2 Report

The paper presents a nice overview of the main sources of uncertainty in typical direct cross section measurements in the field of nuclear astrophysics. 

Although the paper is well written overall, I recommend the following minor corrections:
- Figure 1: the unit of the y axis should be years (plural). I would suggest also to add a label on the x axis.
- lines 86-92: I would keep the discussion more general. Sometimes experiments are performed in inverse kinematics, and the beam charge is not always 1+
- line 162 - 166: I believe temperature measurements are also needed to asses the gas density
- line 194: Please check the LUNA acronym

The authors should also read carefully the text and correct typos or grammatical errors. Below is a non-exaustive list of typos:
- I believe the word Universe should have a capital "U"
- line 48: represent -> represents
- line 49: propagate -> propagates
- line 57: measurement -> measurements
- line 63: motivate -> motivated
- line 96: remain -> remains
- line 103: technique -> techniques (or "a calorimetric technique")
- line 109: lead -> leads
- line 111-113: sentence is unclear
- line 130: causes -> cause
- line 139: inevitable -> inevitably 
- line 143: I believe nitrogen does not need the capital N
- line 195: reaction -> reactions
- line 202: complication -> Complications
- line 204: complicates -> complicated
- line 211: are -> is
- line 228: After emitted -> after being emitted
- line 235: studies -> studied
- line 278: technique -> techniques

Author Response

I thank the referee for the careful reading of the manuscript and for the positive opinion. I have corrected all the typos and followed the other recommendations. The only exception is the x axis of fig.1. If there were a scale indicating the publication year of the values, the points would be distributed very unevenly. Therefore, no x axis label is included.

Reviewer 3 Report

The paper considers an accuracy of theoretical and experimental studies of nuclear reactions relevant for nuclear burning in stars. Although the subject and issues addressed are important, I cannot recommend publication in present form.

In particular.

The author presents general discussions of reactions and measurements without the actual data and accuracy, contrary to what is announced in the abstract.

Some statements and definitions are very confusing. For instance, the definition of the nuclear reaction cross section σ(E)  (Eq. (1)) is difficult to understand. It is hard to figure out what are the respective units.

Besides, there is significant discrepancy between nuclear reactions in the Earth based and stellar environment. For instance, electronic screening effects are quite different for bare nuclei in electron gas of stars and for atoms. Some information regarding these properties can be revealed by making use of indirect methods, e.g., the Trojan horse method (Spitaleri et al. PhAN 2011; 74, 1725–1739), the method of asymptotic normalization coefficients ((Mukhamedzhanov et al. Phys. Rev. C 2011; 84, 024616). The author should at least mention these features.

The neutron-induced reactions can be also affected by, e.g., stellar magnetic fields (Kondratyev, Phys. Rev. C 2004, 69, 038801) which modifies r-process (Kondratyev, Universe, 2021; 7, 487-493) as well. Such phenomena should be also discussed. 

Author Response

The reviewer is right that the manuscript could be extended in several different directions. However – being the guest editor of this special volume of Universe – my aim was to give a short review about typical experimental uncertainties in areas where I have my personal experience and expertise. I have included some statements about this in the paper. I hope the reviewer can accept my manuscript without further details about those topics, partly also because three other referees gave absolutely positive evaluations with phrases like “The paper presents a nice overview…”, “The text by G. Gyurky gives an excellent overview…” and “The paper presents a detailed and enjoyable overview…”.

Units for Eq. 1 are included in the text discussing it.

Reviewer 4 Report

The text by G. Gyurky gives an excellent overview of the current status in experimental nuclear astrophysics. This field faces many challenges which can be addressed now by modern techniques. The various experimental difficulties are clearly explained. The text is well written with an excellent coverage of the recent literature. I therefore recommend the publication of the manuscript.

I found some typos that the author should correct:
L6: "in a nuclear reaction study" --> "in nuclear reaction studies"
L21: elementS
L63: motivateD
Eq(1): is it a strict equality or a proportionality?
L96: remainS
L103 "calorimetric technique"--> "a calorimetric technique" or "calorimetric techniqueS"
L180: "of" --> "on"
L195: "many key reaction of astrophysics" --> "many key reactionS of astrophysical relevance"
L204: "complicates"--> "complicated"
L217: "is recommended"--> "are recommended"
L222: sourceS
L245: "have non-isotropic" --> "have a non-isotropic"
L257: probably it is section 2.4, not 2.3.1

Author Response

I thank the reviewer for the careful reading of the manuscript and for recommending it for publication. I have corrected all the typos (for which I apologize). I prefer to keep section 2.3.1 as a sub-section of 2.3 as it discusses a special case of the “determination of the number of reactions”. Eq. 1 is a strict equality if the various quantities are given in the correct units. The units are now included in the text explaining the formula. 

Reviewer 5 Report

Dear editor, 
the manuscript is a concise outline of a few experimental problems. Even if it is well written, it is not clear what its purpose is. In detail, the manuscript is neither a presentation of novel results nor a review of recent achievements in the field. The author seldom delves in the subject, rather dwells on a few cases, often not even the most critical ones. For instance, no perspectives in detector development are mentioned, in a moment when the ECFA roadmap has issued the final document. Aren't new detector crucial for high precision nuclear astrophysics experiments?
Also, the manuscript is not taking into account the broad variety of experimental approaches that are presently used in nuclear physics, rather focuses on the standard direct methods. The only alternative approach that is presented is the activation technique. It is recommended to discuss, or at list mention, the broad range of high precision nuclear physics techniques that can successfully used to gather information on the cross section at astrophysical energies, such as the measurement of spectroscopic factors that is routinely used to calculate resonance strengths at astrophysical energies.
An additional critical point is the number of self citations, that is testifying that little room is left to represent the whole community active in the field of nuclear astrophysics (while the manuscript is supposed to be a review).
I recommend a thorough revision of the manuscript. It is a good starting point, yet to endorse its publication it should fulfil two points:
1. it should be a real summary or review, as it is written in the first line before the title, providing a brief though complete view of the field.  
2. it should really report on "Challenges and requirements in high precision nuclear astrophysics experiments", tackling the critical points such as systematic errors.

In what follows, some specific comments the author should address are listed. These could be suggestions to help improving some specific points in the manuscript.

1. In the introduction, the role of nuclear theory is quite minimised. Just an example makes it apparent that the role of nuclear theory is of paramount important and should be underscored in the manuscript: the pp reaction governing low-mass star evolution. Many other precesses for which nuclear theory is necessary can be cited, from the triple alpha to the r-process. Therefore, the author should rephrase the relevant sections of the introduction. On the contrary, relevant nuclear experimental data are often missing or affected by large and unknown systematic errors owing to the low energies at which the cross section should be measured, often unreachable at present-day facilities. Also this matter-of-fact should be stated in the introduction and critically analysed in the manuscript, along with other typical drawbacks of experimental measurements.

2. In section 2.1, photodissociation reactions are not mentioned. Forthcoming gamma beam facilities are opening a new frontier of precision measurements as well. In the cosmos, other processes such as neutrino induced reactions are also very important; since they cannot be measured, again theoretical physics is playing a pivotal role as well. Also, at the end of this section the problem of the accuracy of beam energies is mentioned. The author is supposed to discuss the problem of energy loss in the target, introducing very large uncertainties, first of all the one linked to the choice of the interaction energy (since, of course, targets are thick at low energies). Though this discussion is briefly given in the next section, it is not explained what the uncertainty on the effective energy is. This is strongly model dependent and simple formulas are often used, assuming e.g. a parabolic trend for the cross section, which is often far from true. Such topic perfectly fits with the purpose of the present manuscript. 

3. In section 2.3 the author tackles the problem of detection efficiency. In the case of gamma rays, and the same with neutrons, the detection efficiency is energy dependent. Thus the use of calibration sources can be useful to validate simulations at few energy points, usually quite different from the energies of interest. This should be specified as it might lead to large systematic errors. Also, energy resolution of gamma ray detectors should be mentioned. Broad peaks might conceal unknown background lines, with significant impact on the accuracy of the measurement. Would novel scintillation materials help improving such situation? LaBr3:Ce or CeBr3 or CLYC? Later, when discussing about neutron detection efficiency, many effects are not mentioned. For instance, fast neutron might be scattered by walls surrounding the scattering chamber and be considered as slow neutrons because of the longer flight path. Another example is that neutron sources are also intense gamma sources, so in efficiency measurements gamma background rejection is very important. The author is right when the difficulties are mentioned in the manuscript. However, only limited account of possible sources of uncertainties is given. 

Minor points.
Line 176: such measurements are often dubbed inclusive. Of course, you can measure more particles in coincidence, or catch a gamma-ray cascade. The one mentioned here is one of the many cases and, often, the least accurate in comparison to exclusive measurements. 

Line 204: "complicates" should read "complicated"

Line 210: "usally" should read "usually"

Line 228: "After emitted" should read "After emission"

Author Response

The reviewer is right that the manuscript could be extended in several different directions. However – being the guest editor of this special volume of Universe – my aim was to give a short review about typical experimental uncertainties in areas where I have my personal experience and expertise (this explains also the high number of self-citations). Being an experimentalist, I am not competent to discuss nuclear theory, some topics – like for example photodisintegration – are far from my field, and so on. I have included some statements about this in the paper. I hope the reviewer can accept my manuscript without further details about those topics, partly also because three other referees gave absolutely positive evaluations with phrases like “The paper presents a nice overview…”, “The text by G. Gyurky gives an excellent overview…” and “The paper presents a detailed and enjoyable overview…”.

The listed typos are corrected, thank you for the careful reading.

Round 2

Reviewer 3 Report

In the revised version of the manuscript the author made some minor clarifying corrections. As is stated in the author message it is worthless even to mention the effect of differences in the Earth and stellar environment for nuclear processes. Although I have different opinion, is this case I do not feel confident to object to the positive Editor decision for the manuscript publication.

Author Response

I thank the reviewer that in spite of his/her different opinion, does not make objection for the publication of the manuscript.

Reviewer 5 Report

Dear editor, the author did not make significant improvements, therefore the manuscript is not suitable for publication yet. In particular, it is not clear to me how a review work can be based on the personal experience of the author. Though the author has excellent mastery of his/her own research field and the paper is clearly written, this is not enough for a review titled "Challenges and requirements in high precision nuclear astrophysics experiments". The paper is focused on a narrow group of experiments, so the title is definitely misleading.

Author Response

As the reviewer - as opposed to the other four reviewers - does not accept my manuscript, I leave the final decision to the journal editors.