Time of Flight Counter BESIII International Review Sep. 16, 2002 Heng Yuekun

Outline  Functions and target  Analysis of time res.  Two options TOF+TOF TOF+CCT  Scintillator and PMT  Structure and installation  Electronics introduction

Functions of TOF  Particle ID: 2  K/  separation up to 1.0GeV/c  Give a fast trigger

Analysis of time res. (1) 1) :intrinsic time resolution of TOF  Scintillator and PMT time performance  Light transmit spread: scintillator length  Number of pes:light output: light output and thicknesss attenuation and length quantum efficiency

Analysis of time res. (2) 2) :beam bunch center uncertainty Phase stabilization of RF at storage ring:±1º, i.e.,5ps Considering cable transmit and electronics: <20ps 3) :beam bunch length uncertainty beam bunch length: 1.5cm, i.e., 50ps Two bunches colliding: 35ps 4) :Z-position particle impact uncertainty Transit time in the scintillator should be reduced and the resolution is related to the hitting position determination. As MDC track reconstruction simulation: several mms, ~25ps

Analysis of time res. (3) 5) : electronics of time measurement  CERN HPTDC  25ps according to its design 6) : res. of expected time of flight in MDC  Particle ID capability: measured time minus expected time  Expected time in MDC: 30 ps tracking length: several mms momentum: 0.6%

Analysis of time res. (4) 7) : threshold correction: ~10ps  High threshold(~250mv): to give trigger  low threshold(~50mv): to measure time T(ns) V(mv)

Analysis of time res. (5) Non-TOF error totally is over 60ps, TOF intrinsic resolution is 80ps.

Two options  Radius 81cm: 1.0GeV/c K/  time difference is only ~280ps  Two layers, two independent times  Two options: TOF+TOF TOF+CCT

CCT Principle  Improve PID increase time difference  Threshold Cerenkov radiation & Full reflection: Under threshold, TOF layer give trigger Fig. CCT operating principle. Figure Momentum threshold for different particles in CCT.

Comparison of K/  sep.  TOF+TOF  TOF+CCT Fig. K /  separation for Double TOF Fig. K /  separation for TOF+CCT

CCT Material  Better UV transmission  KEK beam test Quartz is best, but expensive Plastic: ps  BC800: Trans. over 70% from 300nm to 400nm

Fig. Transmission spectra of BC800 and BC802

CCT Simu.  Based on GEANT4  Number of pes VS positions: 20~ Number of photoelectrons in right or left PMT versus hit position from CCT simulation.

CCT Simu.: δ-electrons  Number of pes from δ-electron is fewer than that from pions: threshold useful  pes fromδ-electron: earlier for vertical particle Fig CCT simulation ： comparison of hit time and photoelectron yield at the PMT for a 1.0GeV/c π and a δ-electron produced by π. (a) π hits at 90 ° (b) π hits at 30 °

Scintillator  BC404 VS BC408: more light output faster rise and decay times shorter attenuation length Fig. KEK beam test, BC404’s time resolution is better ~10ps than BC408.

PMT R5924 It has 19 fine-mesh dynode stages and high gain. It has high quantum efficiency for the light with 300nm to 500nm wavelengths. It has good timing performance: anode pulse rise time is 2.5 ns, and transit time spread (FWHM) is 0.44ns. Its length is only 50mm, very suited to our limited space.

PMT: R5924

BTOF Dimension  Placed between MDC and EMC  R-direction space: 81cm-92.5cm  Scintillator Length: 2440mm  Coverage: ~82%  Pieces: 88 /layer  Thickness: 50mm /layer

Fig. Assembly of barrel TOF. BTOF installation Fig. BTOF side view. To save space, the base of PMT housing is pentagon-shaped and the inner and outer layer is across. It has four screws to connect the scin.

ETOF structure and installation Fig. TOF structure Fig. Installation of endcap TOF

Monitor system  Amplitude and time performance monitor Fig. TOF monitor system

Schedule

TOF comparison

TOF Elec. Intr. (1)  Details, by Prof. AN Qi  Tasks: Time measurement : <25ps Charge measurement to correct time-walk: 4mv~4V, effective bit:10 Fast trigger signal

TOF Elec. Intr.(2)  Block diagram of Front-End Electronics HT: ADC gate; double end signal to trigger LT: measure time

TOF Elec. Intr.(3)  Time measurement: CERN HPTDC, Very high reso. mode(25ps), no time stretcher High reso. mode(100ps) with time stretcher(1:4)  Charge measurement: Pulse Waveform Digitization: ATWD of 1GSPS (analog transient waveform digitizer) Pulse amplitude measurement: integrator + FADC  Refer. Time: Use RF 500M Clock to generate a 40M refer. clock, which is precisely synchronized with the beam collision time.

The End Thanks a lot!