Figure 2.3: Array of 44 pure CsI crystals used during the 1997 beam
time.
This array can be divided into 21 ``outer'' crystals, which comprise the outer ring of
detectors, and 23 ``inner'' crystals.
These 44 crystals are a subset
of 240 crystals which, when assembled, form a spherical detector covering
of 4
steradians (see Fig. 2.4).
Figure: The full PIBETA calorimeter of 240 pure CsI
crystals.
The completed sphere has an inner radius of 26.0 cm and an outer radius of 48.0 cm. The individual CsI crystal shapes are calculated from a class II geodesic breakdown of an icosahedron [10]. From the breakdown, nine different crystal shapes of varying frequency result (see Tab. 2.2).
Table: Nine CsI crystal volume types, and frequency of appearance in the
completed PIBETA calorimeter.
Light output from the CsI crystals is collected and amplified with photomultiplier tubes of three inch and two inch diameter, for whole and half crystals, respectively. The photomultiplier tubes, Thorn EMI 9822QKB and 9923QKB, have quartz windows which are transparent to the UV light emitted by the CsI crystals. The tubes are coupled to the CsI crystals with optically transparent Dow Corning SYLGARD glue.
In order to identify charged particles, an approximately half-cylindrical thin plastic scintillator veto detector was placed in front of the 1997 CsI array (see Fig. 2.3). The detector is comprised of eight thin Bicron BC-400 [3] plastic staves coupled to light guides and one-inch Burle S83062E [4] photomultiplier tubes. The staves have a thickness of 3.2 mm, a width of 4.0 cm, and a length of 300 mm.