7.5 Data Analysis and Software Cuts
Although a relatively clean spectrum was achieved using gainmatching and the
summing algorithm, sources of background and ambiguities had to be removed by
software cuts. A major cut, which affected energy summing, was introduced in
the previous section. The notable reduction of the tail was essential for a
clean separation of the RC-peak and the SCX-distribution on one hand, and a
separation of the SCX-distribution to low-energy background, on the other. The
cut on the tail has different implication for both distributions of interests
since the shower develops differently. The idea of two threshold values to
account for the different energies, as introduced in the previous section, was
not followed in order to avoid (uncorrectable) ambiguities. Consequently, 2 MeV
was the allowed maximum contribution of the outermost crystals to the summed
energy - as obtained accordingly to Figure 7-10. This restriction led to a
reduction to 42% of the events in the case of SCX and to 37.5% in the case of
RC. (These relations are uncorrected for background contributions and therefore
an error is not given.) The benefits are a reduction of the systematical error.
The error for the fit was reduced from 0.11% to 0.22% due to lower statistics.
At the same time the contribution of the RC-peak tail under the
p 0-photons decreased from 0.95% to 0.27%.
The uncertainty of the result due to the applied cuts was determined by a
variation of the cut parameters around the applied value. Then the fractional
change of the obtained result for P beyond the change of the statistical
error was recorded. Since the cuts are influencing one another also the
covariance term was calculated to account for the correlation. This way the
systematical error due to the applied cuts was obtained to be 0.44%. Hereby the
contributions of the subsequently reported cuts are of the same order of
magnitude. The main contribution to this error is due to differences in the
simulated and measured distribution of the p 0-photons. While the
129.4 MeV peak agrees well, the simulated photon distribution appears to be
broader (Figure 7-12). The FWHM was determined to be 6.6% at 129.4 MeV obtained
with a fit of the complete CsI-array sum comprising 4 days of beam time[34].
[34] Without the applied gain matching the
resolution would be 7.7%.