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
GDET 241 signal - baseline, no beam, Feb 7 2011
Width = 5391 counts
5
number of cases
4
3
2
1
0
-20000
-15000
-10000
-5000
0
5000
10000
signal - baseline (counts)
15000
20000
•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
Gas detector. Distribution of time difference between
the two PMT pulses: leading edge half height points.
Width = 1.2 ns
number of traces per 0.5 ns
5
• 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
4
3
2
1
0
-6
-4
-2
0
time between PMT signals (ns)
2
4
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