[Mott] Fwd: Comments re tomorrow's call

[Charles Sinclair]

Dear Joe,

Sorry I’ve been so slow on the uptake before tomorrow’s meeting.Getting 
all the last minute things taken care of is proving more time-consuming 
than I had anticipated.Anyway – enough for excuses.My thoughts about the 
coming run are as follows.

·It sounds to me like there is majority sentiment to re-run the 
measurements of asymmetry vs. foil thickness.I’m not sure I sign on to 
Tim’s comment that doing so and getting the same answer for the 
thickness dependence is so overwhelming.I personally characterize the 
asymmetry as A(t) = 1/(1 + a(t)), and if we don’t get the same value for 
a(t) that we got before, we might as well hang up our hats.I guess I’m 
OK with re-doing the thickness run, but it is not my personal highest 
priority.

·I personally would like to see a careful study of singles rates to get 
quality measurements of the foil thicknesses relative to each other.This 
wouldn’t give an absolute measurement of any foil thickness (though one 
could try in this direction – more in a minute), but if carefully done 
should certainly give the ratios of the thicknesses of each foil, useful 
for comparing to Marcy and Mamun’s measurements and Lebow's values.  I 
was quite surprised to see that the values for a(t) were so insensitive 
to the actual thickness numbers used. Many years ago I calculated the 
counting rate expected from the Mott cross section for our particular 
solid angle, and though I can no longer find the calculation, I recall 
that the agreement was quite good.One could easily check this.I think we 
would easily be able to get polarization independent measurements of the 
relative thicknesses of the foils for each of the four channels, and I 
think that is worth doing if it doesn’t consume too much valuable 
machine time (and I believe it should not).

·I would like to see studies with the dump dipole off.I was personally 
bothered by the poor spectrum in one of the left-right channels (right, 
I think I recall) with the thinner foils, and would like to see this 
problem clearly demonstrated to be due to the effect of the dump 
dipole.It might even turn out that running with the dipole off is the 
wisest way to operate – wouldn’t surprise me. Anyway, anything that 
makes our instrument asymmetric is highly undesirable in my mind, and 
should be understood, even if that asymmetry is in the two channels that 
should not in principle affect our asymmetry measurement.

·I think studying the energy cut is the most important thing we can 
focus on.I have spent a moderate amount of time looking at Dan’s 
detailed results of various cuts, and the ONLY thing that affects the 
measured asymmetry is the energy cut.His one sigma cut is probably OK, 
but I’d like more convincing evidence.All the other things – TOF cut, 
rate dependence, position dependence, etc. simply has no statistically 
significant effect on the measured asymmetry or the value of a(t).The 
proposed energy range of study – 4 to 6 MeV – that I heard discussed 
last week seems too large to me.I would settle for something a fair bit 
smaller, but still large enough to more than cover the range of energy 
spread we actually have.I believe it is quite reasonable to put a 
meaningful upper bound on our real energy spread using the 5 MeV 
spectrometer, however poorly characterized it may be.Perhaps the numbers 
have been lost, but that little dipole was in fact well measured many 
years ago, and its field integral for 5 MeV electrons was quite large 
enough to completely dwarf any other small fields that one could 
imagine.If necessary, one could remap the field if the info has been 
lost.Another point is that in fact a hand calculated estimate of the 
field integral actually comes quite close to the measured value – this 
should be in my earliest notebook, if its still around somewhere.Anyway, 
my point is that if the injector is sensibly set up (a) the energy 
spread is not very large, and (b) should be measurable with decent 
precision with that spectrometer and the associated harp.I would then 
choose an energy range to study which would span, say, +/- three to four 
times the actual energy spread, rather than +/- 1 MeV.

·I do not see any value in studying further the effects of spot size or 
emittance.I was with Matt when we measured the asymmetry for a number of 
spots on a +/- 3 mm matrix (I believe I remember that number correctly), 
and we saw absolutely NO dependence of the asymmetry on these various 
position changes.The position changes we used were both very large 
compared to the spot size and very large compared to any expected beam 
motion from setup errors.Unless people have looked at those measurements 
and have found a flaw, this result says that any reasonable changes in 
spot size (from any realistic emittance change) or spot position from 
setup to setup simply do not matter.

·I believe it would be quite interesting to study the asymmetry versus 
beam current.We did some of this in January, but one could be more 
thorough.It would be nice from an operational standpoint to know how 
high one could run without compromising the asymmetry 
measurement.Likewise if it was easy to set up, a 499 MHz measurement 
would be nice.However, for the primary purpose of the paper, this is not 
essential.I certainly would do it last, rather than frog with the laser 
rep rate in the middle of things (unless John has now made things his 
typically bullet-proof way).

So – those are my thoughts.By far I consider the most important aspect 
is a good understanding of the energy spread business to a level where 
we can justify that we can make an energy cutthat will assure our 
asymmetry measurements are valid (and rate independent).Studying the 
singles rate and dump dipole effects might share second place.Repeating 
some of the asymmetry versus foil thickness points (and even all of 
them, if time permits) is OK, but to me not the central focus.

Can you advise me how I should get into the Mott wiki?I seem to be 
unable to get there – it complains that I’m not allowed.Perhaps I have a 
password wrong or something? Also, given the reality of Bluejeans in 
Oregon, I didn’t get any of Dan’s presentation last week, which sounded 
very interesting to me.It was not on the wiki page I had.

Talk to you tomorrow.


Best,
Charlie



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