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