The two arm trigger configuration requires a signal from at least one of two
superclusters contained in the CsI array, in coincidence with a signal from
the inner NaI detectors, and signals from both beam counters
(B0 and B1). Photons emitted following the charge exchange reaction
are selected by the trigger when they are
approximately anticollinear in the lab frame, along the line between the CsI
and NaI arrays. The spectrum of measured opening angles between the two photons
is shown in Fig. 7.1.
Figure: Spectrum of measured opening angles, with angle-of-incidence (AOI)
corrections, between the two photons originating
from the pion single charge exchange reaction at rest. Charged particle events are
vetoed by the plastic hodoscope.
The momenta of the two photons are determined kinematically from the relative opening
angle 
between them, as discussed in Chapter 1.
As 
, the momenta of the two photons (
and 
) approach 54.9 and 83.0 MeV, respectively, as calculated in
Chapter 1.
By making a cut on the relative angle , one can select a range of photon energies.
The spectra for four different such energy cuts are shown in Fig. 7.2.
Figure 7.2: CsI energy spectrum for the two arm trigger configuration, for four different
minimum relative angles (reconstructed) between the two photons from the charge exchange
reaction. Charged particle events are vetoed by the plastic veto hodoscope.
In order to check the effectiveness of the cut on , one can plot the differences
between the thrown energies of the photons in the simulation, and the monoenergetic
endpoints of the photon box spectrum, for varying values of . As the cut on
becomes tighter, one should see the energies of the two photons approaching
54.9 and 82.9 MeV. Figure 7.3 shows these plots. It
is clear that a cut of 
is sufficient to reduce the spread in
thrown energies to 
.
Figure 7.3: Differences between the 83.0 MeV (top panel) and 54.9 MeV (bottom
panel) endpoint and the thrown energies of the photon box spectrum from the single
charge exchange reaction, for four different minimum relative angles (reconstructed)
between the two photons. Charged particle events are vetoed by the plastic
hodoscope.