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4.3.2 On-line Results

After equalizing the gains of the 44 CsI-modules the energy resolution was determined using a 70 MeV positron beam. Two plane MWPCs were used to accept only positrons of proper momentum by excluding beam divergencies of more than 2%. Sum histograms were generated for the innermost 18 crystals by adding all CsI modules whenever the crystal of interest got the main fraction of the shower energy. The differences in the obtained energy resolutions reflect the measured number of photoelectrons and non-uniformities. The average contribution from the noise of approximately 0.1 MeV is negligible.

Crystal Channel
Crystal Name
Nph
Uniformity [%/cm]
FWHM in %
s [MeV]
calculated s
C11
S161
78
0.055
5.4
1.6
1.4
C12
S064
78
-0.035
5.5
1.6
1.4
C13
S028
71
0.660
5.9
1.8
2.4
C14
S035
81
-0.175
5.2
1.5
1.4
C15
S068
50
0.045
6.9
2.1
1.7
C20
S115
66
0.065
5.3
1.6
1.5
C21
S112
69
0.355
5.8
1.7
1.8
C22
S029
79
0.075
5.7
1.7
1.4
C23
S002
116
0.370
5.3
1.6
1.6
C24
S021
79
-0.145
5.3
1.6
1.4
C25
S114
81
0.320
6.3
1.9
1.6
C26
S117
70
0.190
6.0
1.8
1.5
C29
S165
89
0.585
7.5
2.2
2.1
C30
S062
83
0.055
6.3
1.9
1.3
C31
S031
51
0.050
5.8
1.7
1.7
C32
S061
93
0.135
5.9
1.8
1.3
C33
S162
70
-0.015
5.4
1.6
1.4
C37
S130
64
0.025
5.9
1.8
1.5
Table 4-2 Summary of the obtained energy resolutions of the inner CsI modules during the 1997 beam period obtained with 70 MeV positrons directly impinging the array. The figures are obtained by summing over the whole array, when the crystal of interest achieves the main fraction of an electromagnetic shower.

The average energy resolution (FWHM) of the inner part of the array for 70 MeV positrons was determined to be 4.2 MeV. Most of that amount can be directly attributed to the optical properties of the individual modules. A major contribution comes from shower spread over several modules with different uniformities and intercalibration uncertainties. Minor contributions are due to electronic noise.


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