THE LHCB VELO DETECTOR –
AN APPLICATION OF A SILICON STRIP DETECTOR
Tomasz Cybulski
University of Liverpool , UKThe Cockcroft Institute, UK
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
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OUTLINE Intoroduction to radiotherapy Quality Control Teaser Quality Control for Medical Accelerator Semiconductor Detection Principles LHCb VELO Architecture LHCb VELO Electronics Summary
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
INTRODUCTION TO RADIOTHERAPY
Fig. 3. Double strand DNA brake. [2]
Fig. 2. Single strand DNA brake. [2]
Fig. 1. Principles of conformal radiotherapy. [1]
- X – ray photon IntRo
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6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
Fig. 4. Dose depth distribution for different types of radiation.
[3]
INTRODUCTION TO RADIOTHERAPY
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6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
Fig. 6. Sagittal colour-wash dose display for the treatment on meduloblastoma. [4]
Fig. 5. Energy deposition by proton beam as a function of depth - Bragg peak. [4]
INTRODUCTION TO RADIOTHERAPY
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6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
Parameters determining the quality and effectiveness of radiotherapy treatment:
1. DOSE – determines energy deposited in a target (tumour) volume – number of ionisation events
Parameter of importance:Beam current2. Tumour coverage – irradiation of tumour volume and
protection of healthy tissuePenetration depth - determines distal tumour
coverageParameter of importance: Energy
Lateral spread – determines accuracy of lateral irradiation accuracy
E1 E2 E3
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A QUALITY CONTROL TEASER
Fig. 8. LHCb VELO module at the Clatterbridge Centre for Oncology. [5]
Fig. 10. Divergence of the beam halo as a function of distance from the collimator. [5]
Fig. 9. Beam halo hit map on the LHCb VELO at the distance d = 110 mm from the collimator. [5]
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6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
Fig. 11. Treatment room set up at Clatterbridge Centre for Oncology. [3]
QUALITY CONTROL FOR MEDICAL ACCELERATOR
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SILICON DETECTOR PRINCIPLESIntRo
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Diffusion
𝜎=√ 2𝑘𝑇𝑥𝑒𝜀 𝜎 ≤100𝜇𝑚
Charge movement in semiconductors
Drift in electric field
𝜗h=𝜇h ∙𝜀𝜗𝑒=𝜇𝑒 ∙ 𝜀
𝜇𝑒=2.1 ∙104𝑐𝑚2/𝑉𝑠
𝜇h=1.1 ∙104𝑐𝑚2/𝑉𝑠
– mobility in Si (77K)
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
STRIP SILICON DETECTORIntRo
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Fig. 13. Cluster shapes for a underdepleted and fully depleted silicon strip detector. [6]
Fig. 14. TDR LHCb VELO detector sensor structure. [6]
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ARCHITECTUREIntRo
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LHCb VErtex LOcator (VELO) – reconstruction of vertices tracks of decays of beauty- and charm- hadrons in LHCb experiment.
Fig. 15. LHCb VELO modules in cross section in LHCb experiment. [7]
Detector design and construction requirements:
Performance
Geometrical
Environmental
Machine integration
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ARCHITECTUREDETECTOR REQUIREMENTS
Performance:
Signal to noise ratio: S/N aimed to be greater than 14 to ensure efficient trigger performance
Efficiency: the overall channel efficiency at least 99% for a signal to noise ratio cut S/N > 5
Resolution: a spatial cluster resolution of about 4 µm for tracks 100mrad in the region with the pitch region for 40 µm
Spill over probability: fraction of the peak signal remaining after 25ns shall be less than 0.3 to keep the number of remnant hits at the level acceptable for the HLT
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6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
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LHCB VELO ARCHITECTUREDETECTOR REQUIREMENTS
Geometrical:
Polar angle acceptance: down to 15 mrad for all events with a primary vertex within ± 2σ of the nominal reaction point with no more than 8mm distance from the beam
The track angular acceptance: a track of angular acceptance of 300 mrad should cross at least 3 VELO modules
Covering full azimuthal acceptance
Environmental:
Sustain 3 years of nominal LHCb operation: damage to silicon in the inner region for one year should stand the irradiation of 1MeV neutrons with a flux of 1.3 x 1014 neq / cm2
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ARCHITECTUREIntRo
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Fig. 16. rφ geometry of the LHCb VELO sensors (n-on-n).
Tab. LHCb VELO sensors parameters
R – sensor strip pitch
φ – sensor strip pitch
37.7+(79.5−37.7 ) 𝑟 −817017250−8170
37.7+(79.5−37.7 ) 𝑟 −1725042000−17250
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ARCHITECTUREIntRo
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SumFig. 17. Layout of the LHCb VELO module. [7]
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ELECTRONICSIntRo
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SumFig. 18. LHCb VELO readout electronics. [7]
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ELECTRONICSIntRo
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Beetle chip
CMOS technology, 0.12 µm, radiation hard ASIC, analogue
Fig. 19. Beetle chip architecture and pulse shape. The Spill over has to be lower than 0.3 of the peak value after 25ns.[8] The Response of the Beetle to the test-pulse: the measured rise time is 14.7 +/- 0.5 ns and the spillover (26 +/- 0.6%).
Noise Equivalent Charge ENC = 790e +17.5e /pF
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ELECTRONICSIntRo
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Repeater Boards
Functions:
1. Repeater for differential signals
2. Time Fast Control
3. Beetle Chips configuration signals
4. Carrier of voltage regulators for Beetle Chips and L0 electronics service systems
5. ECS card: repeats the signals for the I2C configuration bus and controls and monitors the LV regulators
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ELECTRONICSIntRo
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TELL 1 cards for VELO
Functions:
1. Digitization of the data – 10 bit digitizers sample at the frequency of 40 MHz: 4 A-Rx cards, 16 channels each card
2. Pedestal subtraction
Fig. 20. Pedestal subtraction from the signal determined for two chips. The ADC count corresponds to the charge of approx. 450 electrons, thus the signal is of about 50 ADC counts. The noise is of about 2 – 3 ADC counts. [9]
6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
2. Cross – talk removal
3. Channel re-ordering
4. Common mode suppression
5. Clustering – up to four strips: seeding treshold, inclusion treshold cut.
LHCB VELO ELECTRONICS
Fig. 21. ADC noise before and after channel reordering in Phi – sensor. [9]
Fig. 22. Common noise suppression for signal from each Beetle Chip.
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6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
LHCB VELO ELECTRONICSSummary
1. Proof of principle measurements indicate that the LHCb VELO is capable to measure proton beams
2. It seems possible to qualitatively estimate the proton beam halo divergence by use of the VELO detector
3. Further studies will investigate into potential correlations between beamcurrent and halo signal
4. The possible use of the VELO detector as a non-invasive method for beam QC will be assessed
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6th DITANET Topical Workshop on Particle Detection Techniques - Seville 08.11.2011
Any questions?
Thank you