User name R. Michaels
Log entry time 15:39:59 on June28,2004
Entry number 127245
keyword=HAPPEX alignment & ratesThere was some concern about the HAPPEX DAQ result that the
horizontal detector on the left arm has lower rate even though
it should have higher rate (meanwhile for R-arm the horiz. det.
does have higher rate). This is what Kent told me.
Fig 1 shows the alignment plots for 2 detectors on 2 arms, while
requiring a hit above pedestal in the detector. X is the dispersive
direction (+X = higher momentum), Y is perpindcular. det1 in
my notation is the detector plugged into the 1st channel of
discriminator, det2 is in the 3rd. I'm not completely sure how
this correlates to the HAPPEX detector notation, but Bryan would know.
Since the trigger used is the logical "OR" of the 2 detector segments
the number of entries is, to 1st order, the rate, which is indeed
higher on the low momentum segment on L-arm, and higher on the
high momentum on R-arm. Consistent with HAPPEX DAQ. Origin of
the effect ? It looks like the location of the split between 2
detectors is different in the X,Y plane, so that it bisects the
blob of elastic events differently on the two HRS. But the detector
is big, so between the 2 segments all particles are intercepted,
they just get intercepted in different amounts. I think its no
problem, but am now wondering if we would be better off with just
one detector for Hydrogen ? Isn't one segment big enough ?
Intention is to do a few runs for Q2 with different
septum magnet currents to check our sensitivity. As a bonus this
will verify our assumption that this current moves in Y, while
the momentum setting moves in X, to first order (I think it's true).
BTW, the shadows of events where they should not be are due to
pileup, I'm pretty sure (e.g. 200 kHz x 400 nsec memory time of
VDC = 8% chance of pileup).
FIGURE 1
