The LHCb VELO detector – an application of a Silicon Strip detector

THE LHCB VELO DETECTOR –

AN APPLICATION OF A SILICON STRIP DETECTOR

Tomasz Cybulski

University of Liverpool , UKThe Cockcroft Institute, UK

[email protected]

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


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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Fig. 4. Dose depth distribution for different types of radiation.

[3]


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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]


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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

INTRODUCTION TO RADIOTHERAPYIntRo

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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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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)


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]


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

Tomek


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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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



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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



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SumFig. 17. Layout of the LHCb VELO module. [7]


LHCB VELO ELECTRONICSIntRo

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SumFig. 18. LHCb VELO readout electronics. [7]



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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



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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



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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]


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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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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Any questions?

Thank you

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The LHCb VELO detector – an application of a Silicon Strip detector

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