n_TOF: Measurements of Key Reactions of Interest to AGB Stars

Round 1

Reviewer 1 Report

The article presents an overview of the s-process data results obtained with the  n_TOF facility at CERN. The article reads well and contains all the background, the application in astrophysical objects/processes, and the main results from n_TOF.  Readers in the field of nuclear astrophysics will be interested.

Author Response

We are thankful to the referee for the referee report.

Reviewer 2 Report

THe paper under review presents an overview over the achievements of the n_TOF facility at CERN. The structure of the paper is well-defined and helps the reader to find the relevant information easily. However, I miss a discussion of the following two physics items, and I suggest to improve the presentation in several cases.

Physics items:

1) Very recently, a new systematics for the MACS at 30 keV was suggested by Couture et al., PRC 104, 054608 (2021). That work asks for verification of their predictions where n_TOF could make a significant contribution. Furthermore, the new approach of Couture predicts a 30-keV-MACS of 925 mb for 151Sm with an uncertainty of 25% which is in clear contradiction to the experimental result at n_TOF of 3100(160) mb.

I think that the new approach of Couture should be briefly mentioned, e.g. in the introduction or in the outlook; and a short comment on the discrepancy should be added for 151Sm.

2) n_TOF and time-of-flight data in general are an excellent tool for the measurement of resonances. However, the authors point out several times in the draft that (semi-)magic nuclei with their typically small capture cross sections should be investigated in further detail. Here the time-of-flight method may miss non-resonant contributions of direct neutron capture which may contribute significantly to the MACS (in particular for lighter nuclei). This general limitation of the time-of-flight approach should at least be mentioned somewhere in the draft.

Improved presentation: (Here I feel that the paper should have been read more carefully by the authors before submission.)

A) Typo in Eq.(6):
(kT)^1/3 should read (kT)^3/2

B) line 52:
The statement "the neutron spectrum should be shaped to reproduce a Maxwellian spectrum" is too strong. Alternatively, capture data with any well-defined neutron spectrum can help to constrain calculations of MACS.

C) footnote p.2, last line: "d" at the end should read "dv".

D) line 156:
Delta E/E \approx 10^{-3} instead of $10^3$

E) Fig.4, caption:
The entrance channel is 25Mg+n, the exit channel is 22Ne+alpha (as also written in the figure). "energy" should be more precise "excitation energy".

F) lines 301-314:
The authors point out the spread of literature data from 878(27) mb to 1278(102) mb and claim that this is resolved now. But they omit to present the result 880(50) mb from their measurement [64].

G) The wording around line 341 is unclear/misleading. Probably the "prompt gamma-ray cascade" should mean the gamma-ray cascade after the beta-decay of the residual nucleus.

H) lines 406-409:
References should be given for the n_TOF neutron capture data of 24,25,26Mg.

I) Fig.7:
The photograph is confusing for me. For the non-expert reader some explanations would be very helpful (e.g., arrows for the sample, beam direction, C6D6 detectors, Compton imagers).

J) Fig.8:
The caption of Fig.8 points to the text for more details. However, I cannot find any explanation of the various lines. Perhaps the first two lines with the simulated neutron spectrum and the Maxwellian neutron spectrum are sufficient and reduce the confusion for the reader.

Author Response

We acknowledge the reviewer for his/her criticisms that we accounted for improving our manuscript and submitting a revised version to this journal. We are thankful to the referee for spotting several typing errors and mistakes in the manuscript.

REFEREE

1) Very recently, a new systematics for the MACS at 30 keV was suggested by Couture et al., PRC 104, 054608 (2021). That work asks for verification of their predictions where n_TOF could make a significant contribution. Furthermore, the new approach of Couture predicts a 30-keV-MACS of 925 mb for 151Sm with an uncertainty of 25% which is in clear contradiction to the experimental result at n_TOF of 3100(160) mb.

I think that the new approach of Couture should be briefly mentioned, e.g. in the introduction or in the outlook; and a short comment on the discrepancy should be added for 151Sm.

AUTHORS: We agree with the referee that the proposed systematics by Couture and collaborators represent a viable option to estimate MACS at 30 keV when no experimental data is available. And we also agree that it is surely worth comparing its predictions against experimental data. However, since we have not compared n_TOF results presented here to any other particular theoretical / empirical predictions  we prefer to keep consistency for the case of 151Sm.

REFEREE

2) n_TOF and time-of-flight data in general are an excellent tool for the measurement of resonances. However, the authors point out several times in the draft that (semi-)magic nuclei with their typically small capture cross sections should be investigated in further detail. Here the time-of-flight method may miss non-resonant contributions of direct neutron capture which may contribute significantly to the MACS (in particular for lighter nuclei). This general limitation of the time-of-flight approach should at least be mentioned somewhere in the draft.

AUTHORS: We agree with the referee on this point. In particular, the direct capture contribution has been often included in published MACS. We added this sentence in Section 5: “For small cross sections, the contribution from direct neutron capture (DRC) may become important. As this component is non-resonant, it may be difficult to determine it with the time-of-flight technique, as it is hard to disentangle from the background. In these cases, measured MACSs need to be complemented by model calculations to account for DRC (this is the case e.g. for Mg isotopes [43] and ⁵⁸Ni [46])”

REFEREE

A) Typo in Eq.(6):

(kT)^1/3 should read (kT)^3/2

AUTHORS: We have corrected Eq. (4), according to the referee remark.

REFEREE:

B) line 52:

The statement "the neutron spectrum should be shaped to reproduce a Maxwellian spectrum" is too strong. Alternatively, capture data with any well-defined neutron spectrum can help to constrain calculations of MACS.

AUTHORS: We agree with the referee, and changed the sentence: “… Therefore, shaping the neutron spectrum to resemble a Maxellian spectrum at a certain temperature makes the measurement of the MACS straightforward.”

REFEREE

C) footnote p.2, last line: "d" at the end should read "dv".

AUTHORS: ok, changed

REFEREE

D) line 156:

Delta E/E \approx 10^{-3} instead of $10^3$

AUTHORS: ok, changed

REFEREE

E) Fig.4, caption:

The entrance channel is 25Mg+n, the exit channel is 22Ne+alpha (as also written in the figure). "energy" should be more precise "excitation energy".

AUTHORS: ok, changed

REFEREE:

F) lines 301-314:

The authors point out the spread of literature data from 878(27) mb to 1278(102) mb and claim that this is resolved now. But they omit to present the result 880(50) mb from their measurement [64].

AUTHORS: ok, we added the values of the MACS at kT= 30 KeV deduced from the n_TOF measurement.

REFEREE

G) The wording around line 341 is unclear/misleading. Probably the "prompt gamma-ray cascade" should mean the gamma-ray cascade after the beta-decay of the residual nucleus.

AUTHORS: Indeed, it is confusing because in our mind the sentence should refer to TOF measurement, however after the referee’s comment we realized that sentence seems to refer to activation measurements. In order to avoid confusion, we added “in TOF experiments” at the beginning of the sentence, now it reads: “Finally, in TOF experiments the decay activity of the sample itself represents a conspicuous source of background. In fact, (n,γ) reaction cross section measurements on radioactive isotopes are challenging because the emitted γ rays originating from the decay cannot be easily disentangled from the promptγ-ray cascade following the neutron capture reaction.”

REFEREE:

H) lines 406-409:

References should be given for the n_TOF neutron capture data of 24,25,26Mg.

AUTHORS: ok, added

REFEREE:

I) Fig.7:

The photograph is confusing for me. For the non-expert reader some explanations would be very helpful (e.g., arrows for the sample, beam direction, C6D6 detectors, Compton imagers).

AUTHORS: CESAR→ Done.

REFEREE:

J) Fig.8:

The caption of Fig.8 points to the text for more details. However, I cannot find any explanation of the various lines. Perhaps the first two lines with the simulated neutron spectrum and the Maxwellian neutron spectrum are sufficient and reduce the confusion for the reader.

AUTHORS: ok, we changed the figure as suggested by the referee.