C.F. Feb 15 2011

A-line PR18

A-line PR18

GDET: the FEE N2 fluorescence X-ray detector

800 eV X-rays, 0.015 torr Upstream counter Downstream counter

Issues: If performance is poor because of photon statistics - - - we removed the UV-pass optical filter on a downstream counter --- light intensity increased by ~ X 20 ! removed it on the other downstream counter Had to reduce gain by ~ X 30 because of PMT/base sag Difficult to find an opportunity to check statistics while it’s in use But: base line fluctuations can lead to 2% fluctuation of summed signal

Time jitter: I seem to see about 1 ns jitter This is measured by the edges of the PMT signals relative to the time axis Is it real (or a ghost of base line fluctuations)? Source? The FADC (Aquiris) has an internal free-running clock. How does it respond to the control system trigger? X-ray jitter relative to control system trigger? Effect: Could be ~3% for high energy short pulses, < 0.5% for low energy long pulses

What next? # Reduce time jitter effect by widening the pulse summation time interval * Could be tested by recording and analyzing traces (as Alan points out). * Ideally use a summation interval tailored pulse width, i.e. pressure * Include the fast early pulse? Does it scale with beam intensity? # Operate top and side PMTs at different operating points * to get a built in handle on linearity of response # Vary the PMT HV as well as the N2 pressure to stay above base line fluctuations as far as possible. Needs an on-line prescription. # Remove the UV-pass filters on the upstream counters

# More invasive changes * Switch to a gated ADC rather than 1 GHz FADC * Replace the multialkali 3/4 –inch PMT with a high peak current, low gain, bialkali tube, and a high current, tapered, high capacitance base *** needs a redesign of the PMT section (don’t forget the mag. field). Yet more invasive changes --- # Use the nitrogen only as a source of ionizing scatter # install a solid scintillator in place of the aluminum liner. # optimize the PMT for the (faster) scintillator # probably can do without the solenoid # N2 jet ???  an R&D project