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.