Present: Chris Allgower, Andy Bacher, Jack Doskow, Anders Gardestig Scott George, Paul Pancella, Mark Pickar, Tom Rinckel, John Schweitzer, Adam Smith
The meeting came to order at 3:05 pm
TARGET POSITION MEASUREMENT
Pancella reported on measurements made by him and Jack of the nozzle position, assumed unchanged since the 3He test run. These measure- ments indicate a significant difference between the assumed nozzle position and its actual location. With an accuracy estimated at plus or minus 1 mm or better, the measured nozzle position is 8.2 mm downstream of where it was thought to be, and 5.6 mm further to beam right, for a total displacement of nearly a centimeter. Cooling the nozzle moves it less than 1 mm to beam left, probably within the uncertainties. This revised position should be accounted for in any simulations being done for the 3He data.
This position error will have an insignificant effect on the Cooler beam energy calibration done earlier (increasing the measured circumference by 2.3 mm where PVP optimistically quoted an error of +/- 7 mm, or a fractional difference of 2.6 x 10e-5). It will have no effect on the time of flight measurements, of course. However, combined with measurements also made of the plate between the 6-degree magnet pole tips, it may have a significant effect on our treatment of the luminosity monitor. The direction and magnitude of the shift are such that this plate probably did not obstruct the solid angle seen by the forward luminosity scintillators, where previously we thought that it did. Thus the solid angle of the luminosity system from the test run is probably known to pretty good precision after all. The acceptance in theta will be determined by the first scintillator, and in phi by the PSD.
The direction of the shift may also account for the limit of the cone size we could observe. Assuming the beam still parallel to the target box axis, but shifted to center on the new nozzle posi- tion, the predicted maximum cone angle before obstruction now matches the observation (1.45 degrees) much closer. This raises the question of where we should mount the nozzle and aim the beam in the new target box. The beam position at the target could be moved horizon- tally during the test run by a few mm to either side of the jet. We will have the capability of moving the nozzle horizontally by more than that amount in the new box. We should follow up on this to pin down a desired beam/target location in the new box.
MAGNETIC SHIELDING
Jack presented the current state of the design for the large magnetic shield to be mounted on the upstream face of the 6-degree magnet. The purpose of this shield is to reduce the magnetic field at the lead glass phototubes. Hermann Nann has calculated a stray field magnitude of less than 5 Gauss at the nearest phototube position on beam right, with the current two-layer design.
In discussion it was noted that the present design may not extend sufficiently close to the yoke on beam left. Jack will measure the location of the power input leads and see if the plates can be extended.
Jack also showed the state of the lead glass support struc- ture, which is getting close but still needs a few changes. Allgower was asked to sketch a pair of cosmic ray trigger scintillators which would do the job, to make sure they will fit in the support before sending any drawings to the shop for fabrication.
Chris showed the results of measurements made by him and Adam and John, investigating the effect of magnetic shielding options on individual phototubes mounted on lead glass bars. Using LED light and a variable magnetic field, the gain of the detector was observed to sag with increasing field in the expected ways. It was not too difficult to prevent this sag for fields up to 30G perpendicular to the tube axis, and up to 8G parallel, with modest amounts of mu-metal. More importantly, the tested configurations will still allow the bars to be stacked as desired. This is regarded as sufficient, and Chris will prepare an order for the necessary material. It will still be a big job to actually assemble the large number of shields from the sheet material.
TARGET BOX
Half of the plates for the target box are clean, having been subjected to a 4-step process, mostly by John. John is leaving tomorrow, so we need a volunteer to complete this job. Adam will do it. It may be possible to start welding the box together while Tom is away next week.
The meeting adjourned at 4:20 pm.