4.4 Shower Calorimeter
The shower calorimeter, consisting of 240 crystals made of pure CsI, is the
heart of the pion beta detector. Its geometry is spherical with two openings
for beam entry and readout of the inner detectors as displayed in Fig. 4.7. The
inner radius is 260 mm, the outer radius is 480 mm, the solid angle
coverage of the calorimeter is ~77% of 4 p ; the total weight is ~1600 kg.
The thickness of the calorimeter in radial direction is 12 radiation
lengths (X0); it was chosen as a compromise between energy
resolution and the volume, i.e. the costs for the CsI. The modularity of the
calorimeter is obtained from a class II geodesic subdivision of an
icosahedron[8]. As a result of the geodesic
breakdown, there are nine distinct modular shapes which make up the 240 CsI
crystal detectors (see Table 4.2).
Module
Type
|
Name
|
Number
|
Volume
[cm3]
|
Weight
[kg]
|
Pentagon
|
PENT
|
10
|
1002
|
4.54
|
Hexagon
A
|
HEXA
|
50
|
1444
|
6.54
|
Hexagon
B
|
HEXB
|
50
|
1576
|
7.14
|
Hexagon
C
|
HEXC
|
50
|
1721
|
7.80
|
Hexagon
D
|
HEXD
|
40
|
1633
|
7.40
|
Half
Hexagon D1
|
HEXD1
|
10
|
799
|
3.62
|
Half
Hexagon D2
|
HEXD2
|
10
|
799
|
3.62
|
Veto
1
|
VETO1
|
10
|
1174
|
5.32
|
Veto
2
|
VETO2
|
10
|
1174
|
5.32
|
Table
4.2:
The 9 module types which make up the shower calorimeter.
The dimensions of the nine different shapes were calculated in such a way as to
leave a gap of 200 µm between adjacent modules, in order to account for
the wrapping of the modules.
Figure
4.7:
The pion beta shower calorimeter and its segmentation as a result of a class II
geodesic subdivision of an icosahedron. The nine different module types listed
in Table 4.2 are indicated.
Each of the crystals is read out by a 3-inch photomultiplier tube (PMT) with
quartz window except for the modules of type HEXD1, HEXD2, and VETO1, VETO2,
which are read out by 2-inch PMT's with quartz windows[9].
The mechanical support of the calorimeter with the requirement of easy access
to inner detectors (the target, the MWPC's and the plastic veto) and upstream
detector readouts was designed by Mr. H. Obermeier. All the parts are currently
under construction.
[8] For a detailed discussion of the geodesic
subdivision used to produce the modules for the pibeta calorimeter see [ASS
95]
[9] The light produced by pure CsI peaks in the
UV region to which quartz is transparent