Universit`a degli studi Roma Tre Measurement of the KL meson ...

28 CHAPTER 3. THE KLOE DETECTOR
However, to calibrate the DC response and to monitor the 232 space-time relations
there is need for tracks with all possible orientations to simulate the KL decay
products. This is done with cosmic rays. A calibration program, incorporated into
the KLOE online system, automatically starts at the beginning of each run and
selects about 80,000 cosmic-ray events. These events are tracked using the existing
space-time relations and the average value of the residuals for hits in the central
part of the cells is monitored. If the residuals exceed 40 µm then about 300,000
cosmic-ray events are collected and a new set of calibration constants is obtained.
3.2 The Calorimeter
The calorimeter [33] has been designed to satisfy a set of stringent requirements
such as providing a hermetic detection of low energy photons with high efficiency,
a reasonably good energy resolution and an excellent time and space resolution to
reconstruct the vertex for the KL neutral decays. Then it is not only a calorimeter,
but also a rough tracker and a time of flight detector. The calorimeter response has
also to be fast since its signals are used to provide the main first-level trigger for the
events.
The chosen solution for the EMC is a finely segmented sampling calorimeter,
composed of lead passive layers and of scintillating fiber sensing layers. It is made
of cladded 1 mm diameter scintillating fibers sandwiched between 0.5 mm thick lead
foils. The foils are imprinted with grooves wide enough to accommodate the fibers
and some epoxy, without compressing the fibers. This precaution prevents damage
to the fiber-cladding interface. The epoxy around the fibers also provides structural
strength and prevents light travelling in the cladding.
About 200 such layers are stacked, glued, and pressed, resulting in a bulk material.
The resulting composite has a fibers:lead:glue volume ratio of 48:42:10 which
corresponds to a density of 5 g/cm 3 , an equivalent radiation length of X0 = 1.5
cm and an electromagnetic sampling fraction of ∼13%. The large ratio of active
material to radiator, the frequent signal sampling and the special fiber arrangement
result in a factor √ 2 improvement in energy resolution with respect to calorimeter
with slabs of equivalent scintillator to lead ratio.
This material is shaped into modules 23 cm thick (∼15 X0). 24 modules of
trapezoidal cross section are arranged in azimuth to form the calorimeter barrel,
aligned with the beams and surrounding the DC. An additional 32 modules with
rectangular cross section, are wrapped around each of the pole pieces of the magnet
yoke to form the endcaps, which hermetically close the calorimeter up to ∼98% of
4π, see Fig. 3.6. The unobstructed solid-angle coverage of the calorimeter as viewed
from the origin is ∼94% [33].
The fibers run parallel to the axis of the detector in the barrel, vertically in the
endcaps, and are connected at both ends to lucite light guides of area of 4.4×4.4
cm 2 . The light guides are shaped as Winston cones with the other area coupled to
the photocathode of fine mesh photomultipliers of 1.5 inches diameter with quantum
efficiency of ∼20%. Thus each module is segmented in five consecutive layers
each made of 4.4×4.4 cm 2 calorimeter cells. 4880 photomultiplier tubes view the
24+32+32 modules. The total length of scintillating fibers is 15,000 km.