Design and Construction of ECL

The overall configuration of the Belle calorimeter system, ECL, is shown in Fig. . ECL consists of the barrel section of 3.0 m in length with the inner radius of 1.25 m and the annular end-caps at $z$ = +2.0 m and $z$ = $-$1.0 m from the interaction point. The geometrical parameters of each section are given in Table .
Each crystal has a tower-like shape and is arranged so that it points almost to the interaction point. There is a small tilt angle of $\sim 1.3^o$ in the $\theta$ and $\phi$ directions in the barrel section to avoid photons escaping through the gap of the crystals. End-cap crystals are tilted by $\sim 1.5^o$ and $\sim 4^o$ in the $\theta$ direction in the forward and backward sections, respectively. The calorimeter covers the polar angle region of $17.0^o < \theta < 150.0^o$, corresponding to a total solid-angle coverage of 91 % of $4\pi$. Small gaps between the barrel and end-cap crystals provide a pathway for cables and room for supporting members of the inner detectors. The loss of solid angle associated with these gaps is approximately 3 % of the total acceptance. The entire system contains 8736 CsI($Tl$) counters and weighs 43 tons. The size of each CsI($Tl$) crystal is determined by the condition that approximately 80% of the total energy deposited by a photon injected at the center of the crystal is contained in that crystal. Crystals with smaller cross sections would have somewhat improved position resolution and two-photon separation but at the cost of an increased number of channels and poorer energy resolution. The latter is caused by the increase of gaps and inactive materials between crystals. The transverse dimensions of the crystals vary depending upon their polar angle positions; typical dimension of a crystal being 55 mm$\times$ 55 mm (front face) and 65 mm $\times$ 65 mm (rear face) for the barrel part. Dimensions of the end-cap crystals have a large variation. The dimensions of the front and rear surfaces vary from 44.5 mm to 70.8 mm and from 54 mm to 82 mm, respectively. The number of crystals having different dimensions is 39, 29, and 30 for the forward end-cap, barrel, and backward end-cap sections, respectively. The 30 cm length (16.2 $X_0$) is chosen to avoid deterioration of the energy resolution at high energies due to the fluctuations of shower leakages out the rear of the counter. A beam test result indicates that the energy resolution for 25 cm CsI($Tl$) counters is $\sim$2.5 % for electron energies above 1 GeV [59].
The energy resolution of this system at low energies is dominated by electronic noises and the fluctuations of lateral shower leakages. In order to accommodate increases in the lateral shower size for low-energy photons, more counters must be used in the summation for the energy calculation. This puts a premium on low electronic noise for each channel. We aim at around 200 keV incoherent thermal noise and less than 100 keV coherent pickup noise in the whole electronics chain.
The linearity and energy resolution of the calorimeter will deteriorate if there is significant nonuniformity in light collection efficiency in a crystal. This effect as well as the influences of calibration and inactive materials in front of the calorimeter have been quantified by means of a Monte Carlo simulation, the results of which are presented later.

Figure: Overall configuration of ECL.

Table: Geometrical parameters of ECL.

Item	$\theta$ coverage	$\theta$ seg.	$\phi$ seg.	No. of crystals
Forward end-cap	12.4$^o$ - 31.4$^o$	13	48 - 144	1152
Barrel	32.2$^o$ - 128.7$^o$	46	144	6624
Backward end-cap	130.7$^o$ - 155.1$^o$	10	64 - 144	960