Serious Problems with the 900A Be Target Scans
MJL - 5/25/93
1) The 1.5mm Be target (.28 gm/cm2) is of similar thickness to
the Ti window at 10.5" (.07 gm/cm2) and other background sources
(the silicon planes and windows) in the vicinity of the target. This
is clearly seen by looking at e.g. the uniterated Z distribution for
dihadrons or dimuons with no silicon tracking. Since the gold target is
thicker (2.9 gm/cm2) things are ok there.
2) For the same reason AMON has a large background for the Be target
which is not from the target. The geometry of the AMON telescope
and the hole in the shielding it looks through have been put on
a drawing thanks to Martin, Jan, and Chuck. There is no direct
line-of-sight between the AMON telescope and the Ti window so it
is unclear how much response AMON should have to the Ti window and
things downstream of it. From the target scans and beam dump runs at
different times the background appears to vary by a factor of 8-10
and is apparently as high as 18% of the total AMON when the
Be target is in the beam. The variations over time are apparently due
to changes in the beam X position.
*** (see Appendix tables for numbers relating to following discussions)
3) The dump runs, to some extent, serve as empty target runs and give
a measure of the AMON background. However for most of them they give
a value much smaller than that indicated by looking at the outer flat
tails of the target scans. For the dump run taken in the midst of the
900 amp running, however, the background is consistant with the target
scans. If you look carefully at the log book you see that whenever a
dump run was taken the beam was moved in X to give good left/right
symmetry. This probably explains why everytime a dump run is taken
the regular data before and after it shows a different AMON background
and the AMON background is sometimes much different for the dump runs
than for the target scans near them (see the target scans and dump runs
taken at the end of the run). Another aspect of the dump runs is that
most of them are taken at a much higher SM12 magnet current (e.g. 2400
amps instead of 900 amps) and all the target scans and dump runs taken
at the end of the run had the 36" Be dump in place.
4) I have found a number of spills within the 900 amp Be data
where the beam has moved up enough to totally miss the Be target.
These spills have very large SEM and must correspond to some wierd
steering of the beam in the beamline. Using
the processor force-through dihadron events with no silicon tracking
I can clearly see the origin of these events. They come from the Ti window
at about 10.5" downstream of the target with a large and long tail
stretching towards larger Z corresponding to the silicon planes and windows.
Their Y position on the window appears, within the uniterated downstream
track + swim resolution to be about the same as that for normal spills,
i.e. the beam has not moved too far, just enough to totally miss the target.
Silicon tracking to the 10.5" Ti window confirms this.
For the spills where the beam has missed the target completely (i.e. no
events from the Z of the target are seen) an AMON background of about
AMONSB/SEM3SB = 0.8e-3 is seen. This value is a little larger than
the 0.4e-3 to 0.5e-3 seen in the nearby target scans but these are
seperated from these missed target spills by a dump run where the beam
X position was changed then "restored".
5) Scaling of the summed AMON/SEM from the target scans between different
thickness targets. For the Au targets the total AMON/SEM scales with the
target thickness except for the 3mm target which give about 22% less than
expected from the 1.5mm target. The AMON background correction has little
effect. For the Be targets the ratio of the 1.5mm to .8mm which should
be 1.88 is 2.62 to 3.75 without an AMON background correction but with
a background correction is 1.86 as it should be!
Another sanity check is to see whether the AMON/SEM ratio between Au
and Be makes sense. Using the corrected numbers for the 1.5mm/150um target
scans I obtain an alpha for the A-dependence or events into AMON of 0.88;
which seems plausable considering these are particles produced at 90 degress
to the beam (and I have little or no expectation for what their A-dependence
should be.)
6) Scaling of the targeting fraction with target height. It is difficult
to draw definitive conclusions from this since the targeting fraction is
sensitive to the vertical focusing of the beam which cannot be monitored
easily if at all. i.e. silicon tracked events from the 10.5" Ti window.
Three of the Au target scans give a maximum targeting fraction of about
0.4 (.8m/100um, 1.5mm/150um, and 3mm/200um), but a fourth (1.5mm/150um)
gives about .325 instead.
For Be all three target scans (.8mm, and two 1.5mm) give about 0.4, but
only after correction for AMON background. Before the correction they have
targeting fractions of .235, .303, and .41 (each target scan has a quite
different AMON background). Also note that both targets give about 0.4
maximum targeting fraction once AMON background has been corrected for,
except for the one Au scan that has a lower fraction. The puzzling thing
is why the targeting fraction does not scale with the target Y size?
However since this is true for both targets it does not seem to be
directly related to the AMON background issue. Later I will try to
look at the beam Y profile on the 10.5" Ti window using the silicon
for the Au and Be targets; but I'm not sure I will be able to see
anything for the Au target. This might give some idea of what the
targetting fraction should be. Pat also suggests looking at the SWIC
data which might give some information on whether there might be
some kind of long (possibly flat) tail on the beam Y distributions.
I have also plotted the targeting fraction from the 900 amp data versus
run number with and without the AMON background correction. At first
glance the uncorrected fraction looks better since it is more constant
between Be and Au. With the correction I get the first set of gold runs
at about 0.35, then Be runs at about 0.43, then Au starting at 0.43 but
falling later to about 0.35. However the target scan for Au that gave the
anomolously low targeting fraction was taken around this time. So I
think it is plausible that the beam was poorly focused for most of the
Au data here giving the somewhat lower targeting fraction than Be.
Runs on the different targets were seperated by dump runs where the
beam was moved in X and perhaps focusing changed. Also the fact that
the later Au runs' targeting fraction starts at a value the same as
that for the preceeding Be runs gives some credence to this.
7) With these backgrounds the apparent targeting fraction changes (w.r.t.
no background correction) by as much as 39% or 24%. For the
24% change this corresponds to a change in alpha of the A-dependence
from .93 to about 1.01 (for the Ntau=9.5 lifetime cut).
8) For the 3mm gold target 1500 amp B data the AMON background appears to
give a targeting fraction correction of -6%.
9) Even if we can reliably correct for the background in the Be case
the Be target scan statistics are marginal with only about 300 counts
over the best two scans and 60 counts for the peak scan position.
APPENDIX/TABLES:
AMON backgrounds (present gestimate) and chronology:
9/25/91 22:00, R2920, Log V-182, .8mm/100um Au tgt scan
<e-3
9/28/91 00:15, R2940, Log V-196, .8mm/100um Be tgt scan
<0.15e-3
10/5/91 3:23, R3019,22, Log V-227, 1.5mm/150um Au, 2400A Cu dump run
0.025+/-0.02e-3
12/22/91, R3683-3702, 900A Au data
12/22/91 16:00, R3704, Log VI-247, 1.5mm/150um Au tgt scan
<=0.5e-3
12/22/91 15:26, R3703-4, 2400A Cu dump run
0.25+/-0.10e-3
12/22/91 21:15, R3706, Log VI-252, 1.5mm/150um Be tgt scan
0.4+/-0.15e-3
12/24/91 21:19 - 12/31/91 15:30, R3720-47, 900A Be data
12/31/91 Log. VI-284, CNB changed beam X
12/31/91 18:55 - 1/2/92 14:00, R3752-72, 900A Au data
1/7/92 16:17, R3824, Log. VII-23, 900A Cu+Be dump run
0.14+/-0.03e-3
1/8/92 2:37, R3834, Log VII-28, 1.5mm/150um Au tgt scan
1.0+/-0.15e-3
ditto, 1.5mm/150um Be tgt scan
0.9+/-0.20e-3
ditto, 3mm/200um Au tgt scan
1.45+/-0.2e-3
1/8/92 5:38, R3835-6, 900A Cu+Be dump run
0.85+/-0.025e-3
SUMMARY OF THE TARGET SCANS:
--- UNCORRECTED ----- ----- CORRECTED -----
SIZE TGT TF sum(AMON/SEM) TF sum(AMON/SEM)
(x e-3) (x e-3)
---------- --- --- ------------- --- -------------
.8mm/100um Be .41 5.38e-3 .41 5.38e-3
Au .40 84.4 .40 84.4
1.5mm/150um Be .30,.24 14.14,20.31 .40,.39 9.74,10.11
Au .31,.37 160,159 .33,.40 152,146
3mm/200um Au .39 251 .41 232
(the first scans for Be and Au 1.5mm/150um targets are the ones which
were taken during the beginning of the actual 900 amp data taking)