1. Status of the Crystal Zero Degree Detector (cZDD)
Achim Denig1, Peter Drexler1, Brice Garillon1, Leonard Koch2, Wolfgang Kühn2, Sören Lange2, Werner Lauth1, Yutie Liang2, Torben Rathmann1, Christoph Redmer1, Milan Wagner2
1 Johannes Gutenberg Universität Mainz 2 Justus-Liebig-Universität Gießen
BESIII Physics & Software Workshop 24 September 2016 IHEP, Beijing

2. The crystal Zero Degree Detector Geant4 Simulations
Outline
Physics Motivations
The crystal Zero Degree Detector
Geant4 Simulations
Beam test on LYSO crystals
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3. Physics Motivations
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4. Annihiliation with ISR
Physics motivations
γ 𝐼𝑆𝑅
𝑒 −
𝑒 −
𝑒 − 𝑞 𝑞 𝑞
𝑒 +
𝑒 +
𝑒 + 𝑞
Annihiliation with ISR
γγ collision
Tagged ISR allows measurement of the total hadronic cross section as function of energy up to √s at fixed accelerator energy.
ISR photons and e± from 2γ processes peaked at
θ = 0◦ and 180◦.
Upgrade for existing Zero Degree Detector : -Scintillating crystal (less radiation damage). -New DAQ to provide real time event correlation.
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5. The cZDD
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6. The cZDD Interaction Point (IP) BES beam structure (top view) IPX Z Y
ZDD
ZDD Z
Interaction Point (IP)
BES beam structure (top view) IP
Two calorimeters located at BESIII front ends (z=+/- 335cm).
2 blocks separated by a 1 cm gap. → The gap limits radiative Bhabha (e+e- → e+e-γ) contamination.
Each block divided by 1x1x14 cm3 crystals.
cZDD front view
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7. Materials in front of ZDD (top view)
Side view IP
Y-Type Crotch
ISPB magnet
ZDD
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8. Septum bending magnet (ISPB)X Z Y
ISPB bends outgoing beam line by about 39 mrad.
Field : GeV e+
Curvature radius : ρ=15294 mm
Fringe field extends 10 cm away from ISPB yoke.
→ ZDD (75 cm)
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9. Geant 4 simulations
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10. Geant 4 implementation CZDD made of PbWO4 crystals
Implementation of the beam pipe by L.Koch and Y.Liang .
Uniform magnetic field in ISPB region added : By=-4160 G
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11. Radiative Bhabha (e+e- → e+e- nγ) : Simulation conditions
Event Generator : Bhlumi
ECMS=3.773 GeV
0.08 <θe+< 5 deg. IP
e+ beam
Undeviated outgoing beam (w. crossing angle) x=3.68 cm
Without two beam pipes (Y-Type Crotch+ISPB+outgoing pipe+ window) : “BesSim.FullBeamPipe = 0”
BESIII detectors (MDC,EMC,TOF,MUC)+Central beam pipe + SCM solenoid + ZDD included
With magnetic field (ISPB included)
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12. Radiative Bhabha : Energy Spectrum
5% of events results in energy deposition.
Half of the sample deposits 0<E≤60 MeV.
Energy is either deposited in left part (z<0) or right part (z>0) of the ZDD.
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13. Effect of radiative processes
Non radiative Bhabha events (e+e- → e+e- ) deposits energy in ZDD only for 0.2<θ<1° .
Radiative Bhabha events deposit energy down to minimum scattering angle. → Need for generating at zero angle.
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14. Radiative Bhabha (e+e- → e+e- nγ) : effects of beam pipe material
Interaction rate with ZDD slightly enhanced by the beam pipe material → Secondary interactions with the beam pipe increases the background.
Larger energy deposition in low energy region ( 0<E<500 MeV). → γISR with E<500 MeV contaminated by Bhabha events for Ψ(3773) run.
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15. e+e- → ppγISR : simulation conditions
MC event generator : Phokhara
Decay mode : ee → ppγ
ECM=3.773 GeV
Born, no FSR.
0.0855<Θγ<0.689˚ (Polar angle acceptance of ZDD)
0.<Θhadron<180˚.
ISPB magnetic field included.
Beam pipe material included.
ZDD
Y-Type Crotch
Beam pipe
ISPB
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16. e+e- → ppγISR : Energy spectra
Energy deposition for half of the sample.
At best ~80% of ISR photon energy deposited
Large fraction of events deposits small energy, even for Eγ=1.4 GeV.
Reconstruction of γISR is not possible with current ZDD design. → Closed gap configuration might improve energy deposition, as γISR passes through less pipe material.
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17. Beam test
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18. Scintillating crystals
PbWO4
LYSO
Density (g/cm3)
8.28
7.10
Radiation Length (cm)
0.89
1.15
Moliere radius (cm)
2.00
1.9
Decay time τ (ns)
6.5 (30.4) 40
Lightyield (ph/MeV)
100(31)
32 000
Fragile Optimal light yield at T=-25°C
Expensive (x2 as PbWO4) Radioactive (~kBq)
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19. Trigger (scintillator)
Setup (1)
Beam MAMI : 195 and MeV e- beam at 1kHz.
1x1 cm2 LYSO crystal read out by 4 SiPMs.
SiPM : SensL C-series (6x6mm2).
Acquisition triggered by a scintillator placed before the crystal.
PMT e-
Trigger (scintillator)
Crystal+readout
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20. Output signal (Single SiPM)
Setup (2)
Output signal (Single SiPM)
Crystal
Output signal (Sum)
Filter
SiPM array
Power supply e-
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21. QDC spectra (E=195 MeV) SiPM#4 SiPM#5 ΣSiPM
Energy=195 MeV Crystal : 1x1 cm2 Filter : T=0.25 SiPM voltage=26V LG : 26 fC/channel HG : 200 fC/channel
SiPM#7
SiPM#8
→ Clear but smeared peak visible for a single SiPM.
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22. QDC spectra (E=855 MeV) SiPM#4 SiPM#5 ΣSiPM
Energy=855 MeV Crystal : 1x1 cm2 Filter : T=0.05 SiPM voltage=26V LG : 26 fC/channel HG : 200 fC/channel
SiPM#7
SiPM#8
→ Clear and tighter peak visible at higher energy.
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23. Thanks for your attention.
Conclusions
Summary
ISPB magnetic field implemented.
Radiative Bhabha : → Few non radiative events hits ZDD at (relatively) wide angle. → Energy deposition even for low scattering angle. → Secondary particles from the beam pipe increases the background.
ISR photon : → Reconstruction not possible with current design.
Beam test on LYSO :
→ Clear signal observed with a single SiPM. → Good energy resolution in energy range of interest (E~1 GeV).
Prospect
Implementation of the fringe field from ISPB.
Closed-gap configuration →BUT MC generator at very small angle scattering (possibly θ=0 deg) needed.
Interaction rate comparison between background Bhabha and processes of interest (ISR, γγ)
Beam test on a cZDD prototype with LYSO crystals
Implementation of ZDD simulation with LYSO material.
Thanks for your attention.
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24. Backup slides
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25. Particle gun (γ)
ZDD
γ particle gun shot at crystal #104 with energy E=1.5 GeV
Magnetic field disabled.
No materials from the beam pipe.
γ interacts with ZDD through pair creation (γ → e+e-) → Photons ends inside the ZDD.
At best 80% of initial γ energy recorded.
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26. Particle gun (e+)
ZDD
e+ particle gun shot at crystal #104 with energy E=1.5 GeV
Magnetic field disabled.
No material from the beam pipe.
e+ interacts with ZDD crystals through brehmstrahlung ( e+ →e+γ) → Few positrons ends beyond ZDD region.
At best 80% of initial γ energy recorded.
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27. Radiative Bhabha : MC Truth final position (e+)
ZDD
ZDD e+ e+
e+ interacts mainly by Bremsstrahlung process.
Most positrons are bended outward by the ISPB magnet.
Few positrons ends in the ZDD. e+
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28. Babayaga NLO : Simulations conditions
Born mode ECMS=3.773 GeV 0+ε<θ<180-ε (ε=0.08º) Beam Energy spread= GeV. IP
Undeviated outgoing beam (w. crossing angle) x=3.68 cm
Without two beam pipes (Y-Type Crotch+ISPB+outgoing pipe+ window) : “BesSim.FullBeamPipe = 0”
BESIII detectors (MDC,EMC,TOF,MUC)+Central beam pipe + SCM solenoid + ZDD included
With magnetic field (ISPB included)
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29. Bhabha : MC Truth final position (e+)
ZDD
ZDD
Most positrons are bended outward by the ISPB magnet.
Few positrons ends in the ZDD.
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30. Bhabha : MC Truth energy spectrum
1% of events results in energy deposition
Energy is either deposited in left part (z<0) or right part (z>0) of the ZDD.
→ Only one of the two particles hit the ZDD!
Particles depositing energy are emitted away from the beam direction (θ>0.20 degree)
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31. Bhabha : Effect of beam pipe materials
Material added : Y-type crotch, ISPB wall and outgoing beam pipe.
Interaction of e+/e- with beam materials increase the interaction rate in ZDD.
Energy deposition is very small for a majority of events.
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32. Bhabha : Correlation between e+ and e- calculated hit
e- hits the detector
Both e+ and e- hits the detector
e+ hits the detector
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33. Bhabha : Correlation between e+ and e- calculated hit
e+ hits top,right ZDD e- hits bottom, left ZDD
e+ hits top,right ZDD e- hits top, left ZDD
e+ hits bottom,right ZDD e- hits bottom, left ZDD
e+ hits bottom,right ZDD e- hits top, left ZDD
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34. Bhabha : Hit position extrapolated from IP
θ>0.2 deg.
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35. Bhabha kinematics e+ e- Without beam crossing angle e+ e- e+ e- e+ e-
With beam crossing angle
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36. Dual Range Q-ADC : -100 pC/3850 chn -800 pC/3850 chn
Readout
Trigger PMT
Crystal
e- beam 5 4 8 7
2x2 SiPMs
x10 PreAmp
Dual Range Q-ADC : -100 pC/3850 chn -800 pC/3850 chn Σ
Delay & Gate
LED
VME
Oscilloscope
Delay ~120ns
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