Title: Thanks to R' Snyder, L' Kaufman, D' Lhuillier, and S' Bailey
1Luminosity Monitor 250 Hz Reversal R.
Suleiman Qweak Collaboration Meeting, 14 October
2005
1. Lumi Photodetector Photodiode vs. PMT Base
with Gain 1 2. PMT Linearity Studies 3. 250
Hz Reversal and the Pockels Cell Response
Thanks to R. Snyder, L. Kaufman, D. Lhuillier,
and S. Bailey
2Gain 1 Photodetector
- R2046 PD (with quartz window) or PD025 will cost
more than 4,000 each. - Instead use a PMT Base will operate at gain1
and 1,000,000 - Hamamatsu R375 2 inch, quartz window,
multi-alkali, 10-stage, 1,100 each. - Photonis and Electronic Tubes have similar PMTs.
3PMT Base with Gain 1
- Signal picked off from the first dynode
4 Linearity Requirement
The luminosity monitor should measure an
asymmetry null with an absolute precision that
is better than the error we want for
helicity-correlated beam properties in our
systematic error table
Thus we want any false asymmetry resulting from
non-linearities to be smaller than this.
Assuming that the run averaged helicity-correlated
charge asymmetry is lt 0.1 ppm (from Qweak beam
specs), then we require linearity
5PMT Linearity Test
- A non-linear response of a PMT to a given
signal N is parameterized as
where the refers to the two helicity states,
then
with
6Linearity Test Procedure
- Continuous diode simulates the beam nominal
running. - Pulsed diode is turned on/off at the helicity
reversal frequency generating an asymmetry of the
size of the ratio of the two diodes amplitude. - The PMT output is integrated and is measured
for 4 different filter transmissions 80, 60,
40, 20. - A perfectly linear PMT will measure the same
for all filter settings.
7HAPPEX Linearity Test Setup
- Tested HAPPEX (main detector and Lumi) PMTs
- Results look like
8 vs. continuous Diode Signal in ADC channels
- Problem with the
- non-homogeneity
- of filters
- Will improve setup to measure linearity to at
least 0.1 - Will test R375 Base (gain1) at 30 Hz and 250
Hz - Will use the TRIUMF (old-style) I/V preamp box
plus - V/F plus scaler. Could try out the new Qweak I/V
if available.
9 250 Hz Reversal the Pockels Cell Response
- 250 Helicity signal is now available in the laser
room. - T-Settle available 500 us, 100 us, 80 us or 60
us. - Pockels Cell alignment HV, Degree of Circular
Polarization (99.8) are identical to 30 Hz
reversal. - Did oversampling (OS) runs.
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13250 Hz OS 1 T 80 us
30 Hz OS 1 T 500 us
14250 Hz OS 13 T 80 us
30 Hz OS 13 T 500 us
15250 Hz OS 13 T 80 us
30 Hz OS 13 T 500 us
16Large angle luminosity monitor
If there is a 2 foot sewer pipe connecting the
SAMPLE scattering chamber to the most upstream
collimator, then we can insert large angle
luminosity monitors in cups. The monitors
shown would have a count rate of 39 GHz (1 cm x
5 cm x 2 cm quartz block, 15 cm air light guide,
2 inch PMT in unity gain mode) The large angle
( 9-12o) monitors are complementary to the
small angle ( 0.8o) monitors.
17Conclusions and Outlook
- Tests of air light guide transmission at
Virginia Tech. - Prepare a lumi prototype (with quartz, air light
guide, and phototube picked off at first
dynode) for use sometime during the G0 backangle
run (summer - fall 2006). - Study R375 PMT Base (gain 1) linearity.
- More work in the laser room to study 250 Hz
helicity reversal. Figure out the best T_settle
(probably 200 us 5 of the time) that
minimizes charge asymmetry and position
differences.