Introduction

The Belle collaboration evolved from the $B$-factory task force that was organized to study the physics potential of a high-luminosity, asymmetric e$^+$e$^-$ collider operating at the $\Upsilon$(4S) resonance. In particular, the task force addressed the possibilities for experiments that tested the Kobayashi-Maskawa mechanism for $CP$-violation [1]. It was demonstrated that such tests could be done with a data of $\sim$ 10$^7$ $B$-meson decays, corresponding to integrated luminosities at the $\Upsilon$(4S) of order 100 $fb^{-1}$, accumulated with a 4$\pi$ detector with state-of-the-art capabilities [2].
The scientific goals of the Belle collaboration were discussed in a Letter of Intent [3] submitted to the March 1994 TPAC (TRISTAN Program Advisory Committee) meeting. The LoI describes the implications of these goals for the detector and provides a reference detector design based on the R&D program initiated by the task force. With the approval of the LoI the Technical Design Report was written by the Belle collaboration [4].
Figure shows the configuration of the Belle detector. The detector is configured around a 1.5 T superconducting solenoid and iron structure surrounding the KEKB beams at the Tsukuba interaction region [5]. The beam crossing angle is $\pm$ 11 mr. $B$-meson decay vertices are measured by a silicon vertex detector (SVD) situated just outside of a cylindrical beryllium beam-pipe. Charged particle tracking is provided by a wire drift chamber (CDC). Particle identification is provided by $dE/dx$ measurements in CDC, and aerogel Cerenkov counters (ACC) and time-of-flight counters (TOF) situated radially outside of CDC. Electromagnetic showers are detected in an array of CsI($Tl$) crystals located inside the solenoid coil. Muons and $K_L$ mesons are identified by arrays of resistive plate counters interspersed in the iron yoke. The detector covers the $\theta$ region extending from 17$^o$ to 150$^o$. A part of the otherwise uncovered small-angle region is instrumented with a pair of BGO crystal arrays (EFC) placed on the surfaces of the QCS cryostats in the forward and backward directions. The expected (or achieved) performance of the detector is summarized in Table .

Figure: Side view of the Belle detector.

Table: Performance parameters expected (or achieved) for the Belle detector.

Detector	Type	Configuration	Readout	Performance
Beam pipe	Beryllium	Cylindrical, r = 20 mm		He gas cooled
	double-wall	0.5/2.5/0.5(mm) = Be/He/Be
EFC	BGO	Photodiode readout	160 $\times$ 2	Rms energy resolution:
		Segmentation :		7.3 % at 8 GeV
		32 in $\phi$; 5 in $\theta$		5.8 % at 3.5 GeV
SVD	Double	Chip size: 57.5$\times$33.5 mm$^2$	$\phi$ : 40.96 k	$\sigma_{\Delta_z} \sim$ 80 $\mu$m
	sided	Strip pitch: 25 (p)/50 (n) $\mu$m	$z$ : 40.96 k
	Si strip	3 layers: 8/10/14 ladders
CDC	Small cell	Anode : 50 layers	A : 8.4 k	$\sigma_{r\phi}$ = 130 $\mu$m
	drift	Cathode : 3 layers	C : 1.8 k	$\sigma_z$ = 200 $\sim$ 1400 $\mu$m
	chamber	r = 8.3 - 86.3 cm		$\sigma_{p_t}/p_t$ = 0.3 % $\sqrt{p^2_t + 1}$
		- 77 $\leq$ $z$ $\leq$ 160 cm		$\sigma_{dE/dx}$ = 6 %
ACC	Silica	960 barrel/228 end-cap		$N_{p.e.} \geq$ 6
	aerogel	FM-PMT readout		K/$\pi$ separation :
				1.2 $<$ p $<$ 3.5 GeV/c
TOF	Scintillator	128 $\phi$ segmentation	128 $\times$ 2	$\sigma_t$ = 100 ps
		$r$ = 120 cm, 3-m long		K/$\pi$ separation :
TSC		64 $\phi$ segmentation	64	up to 1.2 GeV/c
ECL	CsI	Barrel : $r$ = 125 - 162 cm	6624	$\sigma_E/E$ = 1.3 %/$\sqrt{E}$
	(Towered-	End-cap : $z$ =	1152 (F)	$\sigma_{pos}$ = 0.5 cm/$\sqrt{E}$
	structure)	-102 cm and +196 cm	960 (B)	(E in GeV)
KLM	Resistive	14 layers	$\theta$ : 16 k	$\Delta\phi = \Delta\theta = 30 ~mr$
	plate	(5 cm Fe + 4 cm gap)	$\phi$ : 16 k	for $K_L$
	counters	2 RPCs in each gap		$\sim$ 1 % hadron fake
Magnet	Supercon.	Inner radius = 170 cm		B = 1.5 T

At the time of writing of TDR the detector technologies for particle identification and extreme forward calorimeters were not finalized, and R&D works were continued. All the other detector components entered the full construction stage. After extensive studies and tests of a few options for particle identification techniques the aerogel Cerenkov counter (ACC) system was chosen as the particle identification system. The extreme forward calorimeter system with BGO crystal arrays was also chosen as EFC over the option of a silicon-tungsten sandwich calorimeter. Confronted with various technical difficulties the design of SVD was changed to the present design following the recommendation made by the SVD review committee of June 1997.
Along with development and construction works of readout electronics for all the detector components, the trigger, data acquisition, and computing systems are also developed.
The present report summarizes the results of works by the Belle collaboration during the design, construction, testing, and commissioning stages of the Belle detector.