SPring-8 レーザー電子光ビームラインでの

タギング検出器の性能評価

核物理研究センター三部 勉

LEPS collaboration

日本物理学会 近畿大学 2000.4.1

１．レーザー電子光２．タギング検出器３．実験セットアップ４．エネルギー分解能５．検出効率とバックグラウンドレート６．まとめ

Laser 3.5eV

Scattered photon(Laser Electron Photon) Energy Max 2.4GeV

Intensity 1×107 /sec Linear/Circular polarization

Backward Compton scattering

Electron 8GeV

Laser Electron Photon

Position of Scattered electron （ SSDch ）

Pho

ton

Ene

rgy

（G

eV）

Photon energy

E = Ee - Ee’

Initial electron energy

Scattered electronenergy

Analyzed by Bending Magnet

Photon Tagging

Measured by PWO calorimeter

Tagging counter (top view)

Silicon Strip Detector ×2(Analog readout,100m pitch 640 ch)

1cm8GeV electron beam

Scattered electron

Plastic scintillator ×20(3mm(T),7.4mm(W),10 mm(H))

Energy Counter Timing Counter

Accelerator chamber wall

LEPS spectrometer

• Analyzing magnet (0.7T)• Vertex Detector (SSD)• Charged particle tracking

– MWDC (3 chambers)

• Particle Identification– TOF wall (40 slats)

Experimental Setup (Top view)

TOF

Dipole Magnet （０．７ T)

MWDC

Bremsstrahlung Photon

Target（ Al ２ mm width, １ mm

thick ）＋

Scintillation fiber（ 1mm ２）

E

Tagging Counter

Spectrometer （ e+e- ）

e-

e+

Energy ResolutionTagging efficiencyBackground rate

Plastic Scintillator

e+e- Momentum Reconstruction

E(T

aggi

ng)

GeV•e+e- production point

→ localized in wire target x , z ～ 0.3mm

•One to one correspondence pe xe

Monte CarloSimulation

pe ～ 8 MeV/mm @1GeV

E （ e+e- ） GeVPe : e+/e- momentumxe : e+/e- x-position after magnet

Energy Resolution

2 = Meas2 ｰ e+e-

2

Energy resolution of Tagging

Resolution of momentum reconstruction

Width of Experimental Data Monte Carlo

50‐50（ MeV

）0

E(Tagging) - E(e+e-)

Cou

nts

1.60<E<1.88 GeV Meas=23MeV

Ave.15.3 MeV

(

MeV

)

E (GeV)Systematic error

Electron Track in Tagging counter

ScatteredElectron

SSD

PlasticScintillator

PlasticScintillator

Event Selection / Analysis 1. Pick up two clusters in up and down stream SSD.

2. Angle of track |e - e| < 4 3. Number of corresponding hit in the scintillator array (n)

e

→ Hit pattern ‘quality’ = 0 SSD+ Scintillator(n = 0) =1 SSD+ Scintillator(n = 1) =2 SSD+ Scintillator(n = 2)

Tagging Efficiency and Background rate

Background rateHit pattern(quality)

SSD+ Scintillator(n = 2)

SSD+ Scintillator(n 1)≧

SSD+ Scintillator(n 0)≧

86.5± 0.6%

Efficiency

90.6± 0.5%

91.0± 0.5%

2.3 ± 0.1%

2.6 ± 0.1%

2.8 ± 0.1%

Efficiency ＝ Number of tagger hit(≧1 track) ÷ Number of e+e- pair in Tagging acceptance

Background rate ＝ Number of tagger hit(≧2 tracks) ÷ Number of tagger hit(≧1 track)

Definitions

Condition : Bremsstrahlung photon ＠ maximum electron current

Summary

Performance of Tagging system

1. Experimental method•e+e- momentum reconstruction using wire target.

2. Energy resolution• Photon Tagging Resolution 15 MeV

3. Efficiency and background rate (Bremsstrahlung photon)•Efficiency 90% [SSD+ Scintillator (n 1)≧ ]

•Background rate 3 %.

4. Further study is on going with Laser Electron Photon ...

Photon Energy Spectrum

1st beam observation (1999.July)

Photon energy (GeV)1 2 3

Cou

nts

Tagging counter

PMT

SSD

Beam

PlasticScintillator