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).