BESIII TOF Simulation

Deng Ziyan

Jan 10-12, 2006BESIII Collaboration Meeting

2/17

Outline

• TOF geometry

• Geant4 hits recording

• Digitization based on hitsLight productionLight propagationPMT responseDiscrimination

3/17

TOF Geometry

barrel end-cap

Dimensions and materials same as designed

Scintillator (EJ200)

Wrapping materials (Al + PVF)

PMT (R5924)

Barrel TOF

End-cap TOF

EMC

4/17

Geant4 Hits

0.8 GeV/c electron perpendicular incidence

Geant4 hits recorded at discrete steps along track

Global time, step time, position, direction

scintillator ID, energy loss…hit

5/17

Digitization based on hits

Geant4Get hit information: energy loss, time, position

ScintillatorScintillator light emissionLight propagation

PMTPhotoelectrons productionSignal pulse generation

Discriminatoroutput TDC and ADCDigitization

6/17

Characteristic Parameters Scintillator (EJ200)Scintillation Efficiency 8000 photons/MeV

Rise Time 0.9 ns

Decay Time 2.1 ns

No. Atoms H:C 11:10

Refractive Index 1.58

Bulk Attenuation Length 3.8 m

Density 1.032 g/cm3

PMT (R5924)Photocathode Diameter 39 mm

Spectral Range 300-650 nm

Rise Time 2.5 ns

Transit Time 9.5 ns

Transit Time Spread 0.44 ns

Collection factor 0.6

Gain at 1 tesla 2.5 x 105

7/17

Simulation of Light Emission• Light uniformly emitted along step

• Isotropic angular distribution

• Number of photons proportional to energy loss

• Emission time

2 1

3

/ //

2 1 3

1( ) ( )

1

emit emit

emit

t tt

emit

e e RE t e

R

rise time = 0.9 ns

1

2

3

0.9

2.1

14.2

0.27

ns

ns

ns

R

8/17

Simulation of Light Propagation

90 ,trap trap endless reflection

1

int

0 , sin ( ),airtrap trap

sc

n

nreach PMT

int int int/ cos , / , /pro sc sc pro pro scL d v c n t L v

90 ,trap escape

9/17

Collection Factors

/( ) , 3.8pro proL

pro proR l e m

40%

(1) Light atttenuation in scintillator

(2) Mismatch between scintillator and PMT

(3) Response of incident angle at PMT front glass

cosangle iR

Average: 66%

10/17

Collection Factors(4) Quantum efficiency of

PMT photocathode

20%

number of generated photoelectrons

Collection factor of PMT dynode: 60%

(5)

11/17

Time Distributionpe flight deltaT emit pro TTt t t t t t

deltaTtemitt prot

TTt pet

time of photoelectrons

12/17

PMT Response

2 2

2 2

2 /

2 /( )

4.3

t

e t

t ev t GC

t e dt

ns

time response function for single photoelectron signal

1

( ) ( )pen

PMT ii

V t v t

PMT signal pulse

rise time=2.5ns

MeanPulseHeight=50mv

13/17

Preamplifier and Discriminator

electronic noise added

( )V t dtADC

R

LT

TDC

rise time:4ns

Preamplifier:

low threshold (LT) = 50mvhigh threshold (HT)= 250mv

TDC and ADC( ) ( ), 10PMTV t A V t A

MeanPulseHeight=500mv

14/17

Time Walk

before T-Q correction

after T-Q correction

/T a b Q

0.8GeV electron, perpendicular incidence

15/17

Effective speed Veff = 17.7 cm/ns

Effective attenuation length λ= 318cm

Intrinsic time resolution at z=0 : 85 ps

beam test result

16/17

More to do

• Add uncertainty from start pointstart positionstart time t0

• More research on end-cap TOF digitization

The end

Thanks!