Design and Construction of TOF

To achieve the design goal of 100 ps, the following design strategies have been adopted:

• use of fast scintillator with an attenuation length longer than 2 m,
• elimination of light guides to minimize the time dispersion of scintillation photons propagating in the counter, and
• use of photo-tubes with large-area photocathodes to maximize photon collection.

These considerations led us to a configuration with fine-mesh-dynode photomultiplier tubes (FM-PMT) [54] mounted directly on the TOF and TSC scintillation counters and placed in a magnetic field of 1.5 T.
The TOF system consists of 128 TOF counters and 64 TSC counters. Two trapezoidally shaped TOF counters and one TSC counter, with a 1.5-cm intervening radial gap, form one module. In total 64 TOF/TSC modules located at a radius of 1.2 m from the interaction point cover a polar angle range from 34$^o$ to 120$^o$. The minimum transverse momentum to reach the TOF counters is about 0.28 GeV/c. Module dimensions are given in Fig. . These modules are individually mounted on the inner wall of the barrel ECL container. The 1.5 cm gap between the TOF counters and TSC counters was introduced to isolate TOF from photon conversion backgrounds by taking the coincidence between the TOF and TSC counters. Electrons and positrons created in the TSC layer are impeded from reaching the TOF counters due to this gap in a 1.5 T field [55]. Table provides parameters for the TOF and TSC counters. The width of the TOF counter is approximately 6 cm.

Figure: Dimensions of a TOF/TSC module.

A Monte Carlo study with a full detector simulation including the effects of the material of the barrel ACC and back-splash from the barrel ECL indicates that the clean hit probability is about 90 % for particles from $B\bar{B}$ events in the 128-segmented barrel TOF solid angle.

Table: Parameters of the TOF and TSC counters. Scintillator materials are Bicron BC408 for TOF and Bicron BC412 for TSC.

Counter	Thickness (cm)	z coverage (cm)	r (cm)	$\phi$ segm.	No. of PMTs
TOF	4.0	-72.5 to +182.5	122.0	128	2
TSC	0.5	-80.5 to +182.5	117.5	64	1