The barrel calorimeter (BCAL) is a crucial Hall D detector subsystem. This device will be responsible for the detection, identification and total energy measurement of all neutral (photons, neutrons) and charged (protons, pions) particles within its volume. These demands place stringent constraints on its performance characteristics.
Specifically, the BCAL must provide the best possible energy and
timing resolutions, low threshold of detection, and the ability to
completely contain the electromagnetic showers, resulting from the
conversion of photons into electrons and positrons. The dynamic
energy range for photons over the complete list of induced reactions
in the liquid hydrogen target spreads from as low as 20 MeV to as high
as 1 GeV, as a result photons tend to emerge at much larger angles
than heavier particles. Since the total energy of all the final state
particles must be determined, sufficient energy from electromagnetic
showers produced must be contained within the barrel calorimeter to
allow the correct reconstruction of the photon's energy. To achieve
this, the barrel calorimeter must be designed in such a way so that it
covers its entire allocated solid angle confined by
and
. As well, a high Z material
(e.g., lead sheets) and sufficient thickness in radiation length (
) are demanded so as to provide the means for containing the
electromagnetic showers from escaping from the rear of the BCAL.
These requirements are coupled to the minimum inner radius of the
BCAL, which will allow for the placement of the interior subsystems
(chambers, start/vertex counter and target), in setting the radial
dimensions of the BCAL (0.65 m - 0.90 m). It's longitudinal dimension
is largely dictated by the length of the solenoid magnet, resulting in
a length of 4.5 m.
The basic design of the calorimeter follows closely that of the KLOE calorimeter [4]. The design envisions a matrix consisting of lead sheets of 0.2 to 0.5 mm thick and 1 mm diameter scintillating fibers. The lead sheets will be ``grooved'' and the fibers will be glued in these grooves, parallel to the central axis of the Hall D detector. The scintillation photons will travel down the scintilating fiber, to Winston-cone light guides and eventually to photomultiplier tubes attached at the ends of them, thus producing an electrical signal. Consequently the inherent properties of scintillating fibers play a crucial role. The criteria which must be evaluated include: