Present: Chris Allgower, Andy Bacher, John Olmsted, Tom Rinckel, and Ed Stephenson
SUMMARY OF THE RUN
Abstract: The unpolarized beam is now up above 2 mA, and the luminosity has reached the goal of 3E31 /cm2/s. By removing the copper absorbers, we can now reconcile all measures of the luminosity to within a few percent. PMT sag at the higher rates is manageable. Everything is now ready for production.
Because of the apparently small alpha-pi0 peak in the d+d test running, we had decided to abandon polarized beam running in favor of maximizing the luminosity. Bob Pollock and Terry Sloan worked on this during a development period. A factor of two was realized just from the switch from polarized to unpolarized injected beam. Eventually stores were achieved with 2 - 2.5 mA at the beginning of the flattop.
From the last run, there had been a large change in the position of the Cooler beam in order to overlap the jet target. This time we made scans of the liftime as a function of horizontal position without the jet in order to determine the center of the Cooler acceptance. Using luminosity system rates and lifetime with the jet on, we determined that the jet is centered. That is, the maximum overlap comes at the same T_WIDE setting as maximum lifetime without the jet. So there is no case to be made for moving the location of the jet target. This was checked at injection energy first (110 MeV) and again at the flattop energy.
We again scanned target pushing pressure and flattop time to determine the optimum conditions for maximizing the rate of events recorded. We came to the same conditions as before, a pushing pressure of 220 Torr and a flattop time of 103.5 s. But there was very little change with flattop time. From later luminosity measurements, this yields a target thickness of 3.1E15 /cm2. Together with the increase in beam, we have essentially reached the luminosity that was given in the original proposal as the goal for this experiment.
New mounts were made for the luminosity scintillators at 44 degrees. These incorporate and additional clamp that grabs the light guides about halfway between the PMT and the scintillator, and add extra degrees of freedom for positioning the scintillators. The range of X-position values turned up as a good indication of the correct overlap of the front and back scintillators in each pair.
To better understand the d+p cross section, we added delta-E detectors at 25 degrees to the calibration system. This was specifically to look for proton-proton events between the 44 and 25 degree detectors systems. We found such events, so these detectors will become a part of the calibration hardware.
The main obstacle then became getting the two luminosity systems to agree with each other. As initial run made with a hydrogen target showed a low luminosity from the forward/silicon system, even after all inputs to the calculation were checked against the real geometry and cross sections interpolated to 114 MeV were substituted. The breakthrough came with the removal of the copper absorbers from the forward/silicon system. An elastic peak was still easily extracted. Finally, lumiosities within several percent of one another were measured for the three systems (forward/silicon and two 25-44 scintillators). This means that we can proceed with the HD gas calibration, and we decided that the best time to do this is at the beginning of production running.
Later in the run, there were some changes to the left/right ratio of deuteron counts in the 44-degree system. This is being investigated after the fact to determine whether these systems were changing in small ways with time during the run.
Spectra were added that show the pulse heights in the channel scintillators as a function of time during the flattop. One set shows shifts at the higher luminosity, the two detectors for delta-E2. At the end of the run, we tried to lower the voltage to reduce the gain of the PMT by a factor of two. This seems to have cured the problem, and some offline replay will be devoted to deciding whether this compormises our ability to make good particle ID cuts.
The first MWPC continued to give problems with voltage holding. The voltage was lowered in small steps throughout the run. The plateau is lower for alpha particles, as was measured over a year ago. So this is an acceptable procedure. At the end of the run, we removed the chamber for inspection and are waiting on a report.
Channels dropping out of the time-to-charge converter for the channel scintillators continues to be a problem. Now two channels have been permanently rerouted. There are two spares. In the interests of preserving the calibration we decided not to change the TFC unit. A pulser run was made to document the present condition in case this changes.
The main headache with the run was vacuum. Jack Doskow had installed new cryopumps for the upper and lower stage-2 ports. Even as the run was getting underway, the motor for the upper pump gave signs of failing. By Thursday, this pump was no longer working. Jack purged the helium lines, but in the end swapped to another pump. This pump had too little capacity, and needed regeneration every 12 hours or less. At that point we decided to go back to the pump that had been there originally before the new ones were put in, even though it doesn't get as cold as one would like for pumping deuterium. In the process Jack rearranged some helium lines among the compressors, but didn't check for leaks. There was one, and the compressor unit for two other pumps failed. We were unable Sunday evening to get help, so were off the air until Monday evening. On Monday, it was decided that, in addition to fixing the leak and getting the pumps going again, we would once again swap the pump on stage-2. We traded with the pump in the Cooler G-region, which was of similar type, in order to get one that would cool to 10 degrees K. The present configuration seems solid, but we only had about 14 hours to try it out before the end of the run Tuesday morning. The pump that failed was returned to Austin, and the latest report is that they have replaced the motor and are returning it to us. This will provide us with a spare.
NSF SITE VISIT
The National Science Foundation visited us Monday to review the scientific program in preparation for setting the level of next year's budget. Ed presented a progress report on the CSB experiment. Chris Allgower and John Olmsted presented posters at the afternoon reception.
The IUCF program impressed the reviewers, and a number of favorable comments were made concerning the progress made by the CSB experiment.
REVIEW OF ITEMS FROM THE RUN