Double Photodiode Readout System for the Calorimeter of the HERD Experiment: Challenges and New Horizons in Technology for the Direct Detection of High-Energy Cosmic Rays

Round 1

Reviewer 1 Report

This paper documents an important step in the development of the heart of the Chinese-European HERD mission. HERD is the most important next space detector for high energy cosmic rays, extending the detectable energy range to the so-called knee(s) in the spectrum. It will thus be able to make contact with indirect detection results of UHECR using air showers. 

The paper concentrates on the discussion of test beam results concerning the double readout of the 3D calorimeter with WLSF and double PD, the latter being at prototype stage. A limited matrix of crystals (1x3x21) was readout by the double system allowing a systematic study of the performance. However, the study was hampered by an imperfect alignment of beam and calorimeter leading to lateral leakage. Thus only upper limits on non-linearity and resolution were obtained, and a plot of the resolution as a function of energy is not shown. This will be improved in the next version exposing  a much larger fraction of the calorimeter and using surrounding tracking devices. 

The presentation of the set-up and the results are satisfactory in detail and depth of discussion. There is only one source of confusion. If the misalignment of the calorimeter is in the vertical direction, there will be lateral leakage in the up-down (z) direction, no additional longitudinal leakage in the beam direction (y). It cannot be recovered given that only one layer of crystals has double readout. The text should be corrected in this sense. 

A few minor corrections/suggestions:

Line 25: "very larger" should be "very much larger"

Line36: "first idea of this" should be "first idea for this"

Line 44: "the reconstruction" should be "the 3D-reconstruction"

Figure 1: The WLS fibers are hard to see, the photo should be enlarged.

Line 179: "has not" should be "does not have"

Line 208: "quiet" should be "quite"

Line 227: "calorimeter to explore" should be "calorimeter explore"

Author Response

Dear referee, thank you very much for your comments.

I agree with you that the misalignment of the calorimeter in the vertical direction causes lateral leakage (and not longitudinal like I stated in the first version of the manuscript), and I've corrected the text in this sense.

I've enlarged a little bit the foto in figure 1 on the top right corner in order to make the WLS fibers more visible.

Finally, I've implemented the English corrections.

Reviewer 2 Report

The paper presents and describes the double PD system for the calorimeter of HERD. However, the reader is referred to refs. [11] and [13] for the details of the double PD system. The main new results presented in this paper are summarized in section 3 and concern the measurements and calibrations performed at the SPS2021 beam. Both abstract and introduction should be more specific about the original contents of this paper. Section 4 is more of a description of future plans and therefore no concrete results are presented. 

Major recommendations: 

- One of the main results regarding the non-monotonicity of the energy resolution is not well described in the paper (paragraph starting at line 130). Why not show a graph with all the results? At what energy does the non-monotonicity appear? What is the worst energy resolution observed? 

- In Section 4, the explanation of the development from the "homemade" package to the first prototype needs improvement. The "project" referred to in Fig. 6 and line 188 is not welll described and needs to be explained better. 

- In the last section the description of "future plans" can be shortened as the details should be deferred to a future paper once those future plans are realised. 

Minor recommendations: 

- Scales in Figs 1 would help to understand the images. 

- Line 104: why "preliminary" main results? If only preliminary, the authors should await the final results before publishing. 

By line number: 

3: electron+positron flux MEASUREMENT 

24: eliminate "indeed" 

25: much larger 

37: which demonstrated 

43: less than 

62: rewrite, "studies"... "that were measured" 

Fig. 1 caption: coming out of the reflective.... 

79: has been ---> was 

all over the text in lines 97-103: have been ---> were 

99: through their correlation

105: In particular, ... 

115: particle that produced the shower 

137: by less than 0.2 degrees 

137: it can cause a decrease 

145: with respect to the y axis 

150: prototype's response 

169: these two independent 

172: in one of the two systems 

190: will be tested 

193: prototype will have 

194: particles parallel 

194: as in the flight model 

202: carried out 

206: resolution is 

208: quite 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

No more comments

Reviewer 2 Report

Comments and suggestions were correctly implemented in this second version. 

Line 195: "a homemade..."