Positron-to-Photon-to-Analyzer/Detector
J. C. Sheppard
May 13, 2003
I used EGS4 to see if I can understand the 10% Ap. I think that we are in better agreement
than before, The geometry I ran was: 0.175 cm thick W converter; 1.25 cm space
between converter and Fe; 7.5 cm, 15 cm, and 0 cm of Fe; 48.675 cm distance from the
beginning of the Fe (41.25 cm downstream of the exit to the 7.5 cm converter) to the
detector. I used a 7 cm radius for the W, Fe, and the detector plane.
For input, I used 7.0 MeV, P=1, positrons. The table below lists the results of the runs.
Fe Length N+
N
<P> <E>±std(E)
cm
#
#
%
MeV
5
7.5
10
400
38
1.8±1.4
6
15.0
10
513
54
2.5±1.7
0
105 4585
24
1.4±1.4
5
0†
10 135840 18.3
1.0±1.0
† Photon flux at exit of converter target
So, the conversion rate of 7 MeV positrons to photons is 1.35 /e+ just after the target.
For 105 incident positrons, 4585 photons show up within the 7 cm radius aperture of the
detector plane when no iron is present. This number is reduced to 400 photons for 7.5 cm
of Fe. If the Fe is increased to 15 cm, only 51.3 photons show up (referenced to 10 5
incident positrons). The photon polarization averaged over the total sample is shown as is
the average photon energy.
Estimating the Ap:
For the case of 7.5 cm Fe, the peak Compton asymmetry  is 0.03. This is relatively flat
so let’s take an average value of <> = 0.025. I scale this down by the average
polarization 38/100 and scale up by the ratio of 26/2 to get Ap:
Ap= 26/2*0.38*<> = 0.1235.
In the past, I made a similar calculation in which I decide an initial e+ polarization of
53% resulted in a measurement asymmetry of 2.5% for 15 cm of Fe and a 1 mm W
converter. Using the entries in the fourth row, today I get
 = 0.05*0.54*0.53 = 1.4%.
This is lower than before. To do a better job, I will need to average the Compton
asymmetry over the individual photon polarizations and energies. But not tonight.