Notes from Meeting Feb 27, 2009

• ADC Testing

• Discussed Luis's latest ADC tests of non-linearity in HAPLOG.
• Linearity for ADC16 better than 1.5% (typ. 1%), the ADC18 nonlinear term (beta * N0) is twice as large.
• Next step : perform tests suggested by Kent to determine origin of nonlinearity (I wasn't too clear on what these tests were but everyone else seemed to know. Also unclear was what is our criteria for acceptability, though it seems 0.1% would be nice).

• PREX Collimator

• Another iteration on the dimensions after Al Gavalya confronted geometric realities.
• Alignment criteria and procedure discussed. Detailed e-mail was sent around. Basically, we make the apparatus as symmetric as possible (at sub-1 mm level) using these collimators.

• Compton Electron Detector

• New plots of hit maps from Alex in HAPLOG, showing non-uniform response and some holes, spawned a lot of discussion.
• Holes in the strips (skips 3 strips every 16), possibly a screw-up in FPGA programming. Engineers to be consulted.
• The results were obtained with a simple standalone DAQ in polling mode. The old Compton DAQ isn't working: truncates the Edet data severely. VME bus analysis shows the DMA transfers work, but apparently the data gets "lost" in the software (a mystery). However, the standalone DAQ seems to work, adding fuel to the idea of incorporating this in our FADC DAQ as a way to survive.
• When putting near the beam we get a too-huge rate; may need shielding (e.g. thin Alum.)
• Question of how pulser works and what rate to expect: would like to get an absolute efficiency. Plane A seems less efficient than B,C. May be a discriminator amplifier card, will try to swap it.
• Future plan: run at low current, try to see tracks, try to do e-gamma coincidence.

• More on Compton

• Diana and Gregg put in new PMT and base. Trying to reduce the problem of gain shifts (10% gain shift for factor of 7 change in rate).
• The "busy" bit on FADC is stuck on. Power-cycling did not help. Question whether reloading FPGA may help, e.g. maybe it got zapped in the radiation environment. Raises the question of possibly necessary shielding.
• Gregg: Moving between 4-pass and 5-pass has been useful to move Compton edge and calibrate.
• Abdurahim reports on laser room status. Trying different things to improve cavity power.
• Circularization of the beam to reduce wavefront distortion did not help much.
• Tried to improve mirror-cleaning techniques, testing on old or broken mirrors.
• Mirror-cleaning leads to small improvement (15% increase in power) but on a microscope one can still see residual "dirt"; it may be residual chemicals.
• The electronics board (Dan Sexton) for stabilizing power has problems: integrator in feedback loop, switch, 16-bit DAC (not sure I understood all this). Looked to me like we could use more of Dan's time here, but that's just my take.
• The hope is to obtain a production version of the board in time for installation. Yes, we have our fingers crossed.