We had a contradiction between where we thought the target z was and where the survey group said it was. The survey group said it was 3.8 cm too far upstream. We did not believe we could be off that much, especially because there was a 4 cm offset that was floating around for awhile that might have been the cause of the confusion. Well it was a cause of confusion. So we decided to do a sanity check with a tape measure to see who was right. We don't think it matters really, based on some quick simulations from Katherine, where we actually are at this level as long as we know what the actual number is. We do need to know that- it goes into our Q^2.
Some of the tape measure measurements were made over the past several weeks, see hclog entries 198715, 199345 and 199057. We measured with a tape measure and in some cases from the drawings the following (all in cm, and from tape measure measurements unless otherwise indicated):
14.85 Primary collimator thickness 201.0 dx from US face of primary collimator to DS face of SC window 1.905 thickness of SC window from dwg 67503-00098 31.2 dx from US face of SC window to SC lip 16.2 dx from SC lip to DS face of cell window flange 37.9 length of large diameter portion of the cell 3.9 length of small diameter cell flange plus entrance window flange -5.41 length of entrance window from US face of the entrance window flange to the window itself according to dwg TGT-307-1010-2001 -34.393/2 cell half length as measured with the fero arm in data transmittal C1325 ======= 284.35 total dx from tgt center to DS face of the primary collimator
So how does that compare with what we think it should be? My notes say that the tgt center is supposed to be at -650.0 cm, and that the DS face of the primary should be at -370.72 cm. That means the dx should be 279.28 cm.
So according to the tape measure measurements we are 5.07 cm too far upstream with the target compared with the ideal, and the survey group had it right. The tape measurements say we're an extra 5.07-3.8=1.3 cm further upstream than the survey group says, but I reckon a lot of that discrepancy with the survey group can be attributed to the uncertainty in the tape measure measurements, which were in some cases pretty tough to make accurately. Plus we had to combine a lot of tape measure measurements each with at least a 1 or 2 mm uncertainty, in a couple cases that uncertainty could have been 3 or 4 mm. Still I thought we'd be a few mm closer than we wound up being- perhaps my shitty eyesight is to blame. The results also do not account for thermal contraction, or swelling of the cell due to the pressure across the thin windows (which tends to cancel the thermal contraction expected to be on the order of 1.5 mm). Finally, 6 mm of the difference with them is just due to the fact that the cell is 6 mm shorter than the 350 mm we had planned for.