Preliminary Results from Integrating Compton Photon Polarimetry in Hall A of Jefferson Lab Page: 4 of 6
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t 0.06
E
E 0.05
a
< 0.04
0.03
0.02
0.01
0
-0.01
-0.02* Measured Asymmetry I
- Simulated Asymmetry .
-
5000 10000 15000 20000 25000 30000
Photon Signal (FADC units)Figure 4. The simulated theoretical
asymmetry As as a function of scattered
photon energy k', compared to the observed
counting asymmetry in successive photon
energy bins. As rapidly approaches zero as
the observed photon energy exceeds k'ma$.ouu
a 500
3 400
0
300
200
100Compton Cavity State
----- Right-Circularly Polarized
Left-Circularly Polarized
-- Out of Resonance
0.1 0.2 0.3 0.4
Compton Signal AsymmetryFigure 5. Integrated, energy-weighted
Compton asymmetries computed over elec-
tron beam helicity pairs (each 67 ms in du-
ration) over the course of a two-hour run.
The sign flip between the asymmetries for
the two photon polarization directions is
based on the conventional definition of the
asymmetry.the photon detector is sampled at 200 MHz and integrated in a digital summing accumulator
for a single helicity window, an interval in which the electron beam helicity is well-defined;
the helicity flips at fixed rates that range from 30 Hz to 1 kHz, depending on experimental
requirements. (The results shown here are all from 30 Hz running.) We then form an asymmetry
in the pedestal-subtracted, summed signal between intervals where the incident particles have
opposite polarization configurations, according to Equation 2; this energy-weighted integral is
proportional to the degree of longitudinal polarization of the electron beam.
In addition to an accumulator that sums all signal over an interval, auxiliary hardware
accumulators allow us to examine only contributions from particular types of signal, such as
very small signals (e.g. pedestal noise) or very large ones. These regions are delineated by
setting appropriate values for two programmable thresholds. We may also program the FADC
to store a certain number of samples in memory; for example, when the signal crosses a threshold,
the Nefore samples prior to the threshold crossing (and the Nafter samples subsequent to crossing
the threshold in the other direction) can also be added to the integral, allowing integration over
the entire pulse rather than its tip alone. Altogether, the various combinations of thresholds
and timing data yield five auxiliary accumulators, each integrating over a particular subset of
the signal.
We are also able to store a small number of digitized, 500-ns waveforms per interval, both
from prescaled triggers (Figure 2) and from random intervals, as well as the numerical integrals
of up to 300 prescaled photon pulses per integral, depending on the rate of helicity flips. This
capability allows us to perform analyses on individual photon pulse data (such as the Compton
spectrum plotted in Figure 3 and the energy-binned counting asymmetry plotted in Figure 4) in
addition to the primary, integrating analysis. For example, the counting asymmetry may be fit
to a Monte Carlo asymmetry to make an independent measurement of the beam polarization.
4. Analysis and Preliminary Results
The asymmetry defined in Equation 2 may be formed between any two intervals with opposite
spin configurations for the incident particles. The shortest timescale on which we can form an- -
- f
-0.2 -0.1 i
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D. Parno, M. Friend, F. Benmokhtar, G. Franklin, R. Michaels, S. Nanda, B. Quinn, P. Souder. Preliminary Results from Integrating Compton Photon Polarimetry in Hall A of Jefferson Lab, article, September 1, 2011; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc834328/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.