At the last analysis meeting it was reported that the ed acceptance was in
good agreement for inbending field but it did not change by reversing the
field to outbending. This was attributed to the TOF threshold of 6.6 MeV
for hadrons.
The predicted MC acceptance for ed elastic is shown in the first plot for
inbending (black) and outbending field (red). My comments are geared to
estimate
1) how we can recover all the predicted data down to 24 deg e- angles
2) how to improve on our set-up to get closer to 20.9 deg, the BLAST
acceptance for outbending electroncs.
>From montecarlo one understands that for inbeding blast field hadron
energy loss and hadron outbending outside the acceptance of the last tof
paddle is the limiting factor for the ed acceptance. In fact, from ep
results, e- below 26 deg are in principle possible for inbending field.
Moreover some ep protons are already lost at low momenta and deuteron
angle is more backward and should get swept aways faster (because of
faster energy loss).
Data showed that after reversing the field our data stops at about 26 deg,
corresponding to T_d = 37.3 MeV. Since montecarlo has really no energy
threshold, and for outbending field you bend hadron inward, I'll assume
that
T_d = 37.3 MeV loose enough energy to be just above hadron detection threshold
at 6.6 MeV in the TOFs
T_d = 32.0 MeV range out completely in blast.
The hadron threshold can be lowered to 5.8 MeV. I estimate that if 37.3
MeV deuterons lose (37.3-6.6 = 30.7 MeV) before reaching the TOF plastic,
then 36.9 MeV deuterons should lose 30.7 * (37.3/36.9) = 31 MeV and
deposit 36.9 - 31.0 = 5.9 MeV in the TOF. Of course this assumes that
energy loss is simply linear on the incident energy, which is not true for
"thick" materials. But the effec is a conservative over-estimate of the
minimum deuteron energy.
Lowering the TOF threshold to 20 mV should bring the acceptance down to
25.8 degrees, which is a minimal gain.
Very optimistically one may think of increasing the TOF pmt gain by a
factor of 2. Then our single PMT threshold becomes 0.6 MeVee, the hadronic
coinc treshold becomes (0.6+1.3)/0.6/0.61 = 5.2 MeV Even then, with the
same algebra, one may expect deuterons down to 36.5 MeV, for theta_e 25.7
deg, again a minimal gain.
In other words, even after lowering the thresholds and increasing x2 the
TOF gain we can only expect to improve the elastic ed Q2 acceptance from
0.14 to 0.137 GeV2 (or 1.9 fm^-1) which is not very significant..
-----------------------------
According to the montecarlo (infinitely small detection threshold) we could
hope for Q2= 0.120 (1.75 fm^-1). The montecarlo includes the effect of a
0.01" Pb sheet in front of the tof's
It seems that significantly smaller angles become possible only after
removal of the 0.01" Pb in front of the TOFs. Pb alone absorbs about 18
MeV of the deuteron energy, that would be otherwise available for light
conversion in the TOF. In that case one could consider that 37.3 MeV
deuterons would now only lose 30.7 - 18 = 12.7 MeV in BLAST before
entering the TOFs, where they would deposit about 37.3 - 12.7 = 24.6 MeV.
With similar argument, one would then expect that 25 MeV deuterons would
lose about 12.7*37.3/25 MeV (actually slight less) and still deposit about
6 MeV inthe TOF and be above threshold.
A 25 MeV deuteron corresponds to 21.1 deg electron angle, which is very
close to the true BLAST minimum angle and corresponds to Q2 = 0.094 GeV2
(1.56 fm^-1) which is a significant improvement from 0.14. The deuteron
cross section would change a factor of about 4, and so the time to measure
T20 asymmetries with equal precision.
It seems to me that in this case the overall gain in cross section+acceptance,
seems very significant and would nicely complement to effort spent in measuring
with outbending BLAST field.
-- ________________________________________________________________________________ Tancredi Botto, phone: +1-617-253-9204 mobile: +1-978-490-4124 research scientist MIT/Bates, 21 Manning Av Middleton MA, 01949 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This archive was generated by hypermail 2.1.2 : Mon Feb 24 2014 - 14:07:30 EST