A Method to Measure Positron Beam Polarization Using Optically Polarized Atoms

Round 1

Reviewer 1 Report

Authors propose an innovative method, based on  atomic interactions, to produce polarized positrons. It is difficult to underestimate a potential importance of an experiment with polarized positrons, as it would elucidate phenomena of electron exchange in collisions, i.e. indirectly Pauli’s exclusion principle.

 Authors show great knowledge on historical development of positron techniques and their current state. I seems that a significant part of the proposed technique have been already tested by the authors.  They give also a precise description of atomic phenomena to occur in the Rb cell and expected yield of the whole source.  

One question: authors state that positrons from Na source are polarized, correct. Obviously, they loose their polarization in the moderation process, especially in the back-scattered (i.e. in Ne) geometry. Do other geometries (i.e. transmission, Venetian blinds) assure maintaining the polarization of positrons? Any reference?

Author Response

Referee #1

We thank the referee for their comments and note the question they raise regarding the maintenance of polarisation when using other forms of moderation.

This is not something we have investigated but can bring the referee’s attention to work by the Michigan group in the late 70’s and 80’s (Zitzewitz et al. PRL 43, 1281 (1979); Van House and Zitzewitz Phys Rev A29, 96 (1984)) where they produced a positron beam with a polarization of 22% from a ⁵⁸Co source with a gold foil/tungsten grid moderator, both of which were coated in MgO.

Reviewer 2 Report

Comments for author File: Comments.pdf

Author Response

Referee #2

We thank the referee for their helpful comments and suggestions.  In response:

1. Point taken. As the method is not, in principle, restricted to Rb atoms we propose adopting a slightly modified version of the referee’s suggestion-
   “A method to measure positron beam polarisation using optically polarised atoms”
   
   
2. The introduction was designed for what we believe is a very broad readership of the “Atoms” journal, many of whom may not be familiar with some of the positron background material. We would prefer to largely maintain that but, following the referee’s suggestion, have removed the equation relating to the decay of ²²
   At line 32 we have removed the first sentence.  We also discuss in the introduction the reason for the relatively low analysing power of positron-nuclear scattering as well as the effect of accelerating the positron beam to higher energy.
   The Referee’s comment about the Rice group seems to be referring to the paper by Oro et al. RSI 63 3519 (1992) in which they calibrated a micro-Mott electron polarimeter using optically-pumped metastable He. This paper seems to be more relevant in the discussion of our optical pumping and probing scheme, and we have now mentioned Oro et al. in that section.
   
   
3. We thank the referee for drawing to our attention the recent work of Kawasuso’s group which we had missed. We have added this reference and a brief discussion in the introductory paragraph to highlight the importance of spin-dependent positron probes in condensed matter systems.  Indeed this work, where o-Ps asymmetries are measured is an excellent example of where a positron polarimetry measurement of the absolute beam polarisation would be an advantage.
   
   
4. We note the referee’s point, but the eigenstate discussion was not included to keep the discussion of magnetic quenching to a minimum as the relative production of o- and p-Ps is the primary quantity relevant to the measurements. We also wanted to keep the discussion focused on polarimetry rather than spectroscopy. Thus, we have specifically focused on collision energies below the Ps(n=2) threshold.
   
   
5. The intent of the original statement was to highlight, in a succinct manner, that all events occurring after 100ns are due to o-Ps decay, ignoring cascade. The choice of a NaI detector was for simplicity and its high efficiency. The referee is correct in suggesting that the energy discrimination, or pulse height analysis, would be better for a NaI scintillator, but only in the case in which one event was detected during each shot of the trap. When multiple events are detected within the time response of the scintillator, pulse height discrimination cannot provide the energy of the gamma ray. Integration of the combined signal at longer times (~100ns) is primarily composed of o-Ps events. Additionally, using a scintillator with a faster time response immediately leads to the question of o-Ps time-of-flight, which will be the topic of a separate manuscript. That said, we have updated the text to include a statement, and reference to the single-shot- PALS work (e.g., LYSO paper), which indicates that there is a trade-off between efficiency and time response.
   
   
6. 
   Line 173. “ … from the BGT to the polarimeter ….”
   
   
7. We agree that this is an optimistic view and have softened the narrative somewhat, including the repetition rate
   
   
8. We agree that some other polarimetric methods can be done at low field, but simply want to point out the advantage of not requiring the high field. As suggested, we have now commented briefly on this point.
   
   
9. “solenoidal coils” inserted at Line 172
   
   
10. As it was difficult to ascertain the nature and extent of the referee’s aesthetic displeasure with Figure 2, we have made attempted to improve the aesthetics and have updated the figure and modified the figure caption accordingly.
    
    
11. Comment noted.  We have changed reference [33] as suggested, removed the sentence referring to strong drive and modified reference [34] to be more appropriate to this section.
    
    
12. The available collisional data for positron-Rb scattering is rather thin and has been summarised by one of us in a recent review [now reference 42]. The essential conclusion to this section, that Ps formation at 2.4 eV is about 20% of the total scattering cross section, is reflected in Figures 4.10.1 and 4.10.2 of [42], so we have added a sentence to this effect rather than include a figure which would essentially be a reproduction of these two figures from [42].
    We are also keen to emphasise that, while Rb is the example atom for this proposed approach, any optically polarizable atomic system with an appropriate Ps production cross section would be suitable, so we don’t want to only focus on Rb.
    
    
13. Agreed, a detailed discussion of the consequences of excited Ps is not warranted if the energy is below the excitation threshold, as we indicate. We have modified the text accordingly with only a brief justification for the choice of the 2.4 eV collision energy. We have also added an appropriate reference [43] for the excited state discussion.
    
    
14. Noted and again we have added a sentence to point out that this is an optimistic assessment with perhaps best possible conditions.