Post on 09-Feb-2016
description
C.F. Feb 15 2011
A-line PR18
A-line PR18
GDET: the FEE N2 fluorescence X-ray detector
Upstream counter Downstream counter
800 eV X-rays, 0.015 torr
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
GDET 241 signal - baseline, no beam, Feb 7 2011Width = 5391 counts
signal - baseline (counts)
-20000 -15000 -10000 -5000 0 5000 10000 15000 20000
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•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
Gas detector. Distribution of time difference between the two PMT pulses: leading edge half height points.Width = 1.2 ns
time between PMT signals (ns)
-6 -4 -2 0 2 4
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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
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
# 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).