[PPT] - Calibration of Magnetic Distortions in the LHCb-RICH1 Photon PowerPoint Presentation

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Calibration of Magnetic Distortions in the LHCb-RICH1 Photon Detectors

Fatima Soomro

Imperial College London

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March 6, 2008 Fatima Soomro 2

A Single-arm spectrometer for precision measurements of CP violation in B-hadrons search for new physics in rare b decays

Imperial College London

The LHCb Detector

b bbar Cross section = 500 μb 1012 b bbar pairs in one year

Vertex Locator (VELO) Tracking Stations Calorimeters Muon Chambers

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Imperial College London

Ring Imaging Cherenkov Ring Imaging Cherenkov (RICH) Detectors (RICH) Detectors for Particle Identification (PID) in LHCb:

RICH2 RICH1 Polar angles vs momenta RICH1: upstream of the magnet. momentum range: ~2-60 GeV/c radiators: Aerogel, n=1.03, L=5 cm C4F10 ,n=1.0014, L= 95 cm RICH2: downstream of the magnet momentum range 60 to beyond 100 GeV/c radiator: CF4, n=1.0005, L=180 cm

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Working Principle: Cherenkov Radiation

Imperial College London

Opening angle of the cone

f Cherenkov Photons θ

θc

c

A charged particle travelling in a medium, at a speed faster than the speed of light in that medium, emits Cherenkov Photons. Speed of the

charged particle β

β

Refractive index

f the medium n

n cos cos θ θc

c = 1/ nβ

= 1/ nβ

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The RICH1 design was motivated by the following considerations:

Available space Minimize material within acceptance Access to beam pipe

Imperial College London

The RICH1 Detector The RICH1 Detector

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Photon Detectors for the RICH Detectors:

Imperial College London

High Quantum efficiency High granularity 2.5 x 2.5 mm2 High active to total area 64% after close packing Good signal to noise ratio Single photoelectron detection efficiency ~85% Readout compatible with 25 ns bunch crossing of LHC Operable in magnetic field B < 50 Gauss local shielding

ffline correction

Withstand radiation dose of 3kRad/yr ability demonstrated

The Requirements The Choice

Pixel Hybrid photon Detectors (HPDs)

signal 5000 e noise 160 e Threshold 1200 e

(RMS spread 100e)

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The HPD – Internal structure and Working

Imperial College London

Vacuum tube Quartz window S20 multi-alkali photocathode(-20 kV) Cross focusing electron optics Si pixel anode at ground (1024 elements) Pixel anode bump bonded to readout chip.

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The HPD – Internal structure and Working

Imperial College London

Vacuum tube Quartz window S20 multi-alkali photocathode(-20 kV) Cross focusing electron optics Si pixel anode at ground (1024 elements) Pixel anode bump bonded to readout chip.

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March 6, 2008 Fatima Soomro 9

Imperial College London

The magnetic field at the HPD plane and Local Shielding for the HPDs.

Notice that the Magnetic field is not uniform and varies from tube to tube.

Test pattern measurements with locally shielded HPD

Distortion patterns for 50 Gauss transverse field (top) and 50 Gauss axial field, overlapped with the reference 0 Gauss image.

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Characterizing the magnetic distortion (My Future Work)

Scan a collimated light source over the entire HPD plane. Find a relationship between the position of light source on the HPD window and the signal on Si anode. Develop a map or look up table.

Imperial College London

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The magnetic distortion system

Imperial College London

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The magnetic distortion system The LED matrix

A calculation assuming the LED to emit photons at a rate of 1Mhz shows that to scan the entire HPD plane with a resolution of 0.5 mm2 will take 6 months!! It is very important to develop a strategy and pattern, which has an adequate resolution and is less time consuming.

Imperial College London

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The HPDs are extremely elegant photon detectors providing good quantum efficiency good signal to noise excellent resolution Their operating conditions in LHCb: not optimal Performance can be restored with local shielding calibration

ffline correction

Imperial College London

Conclusion and current status Conclusion and current status

The distortion system is in advance stages of manufacture and will be delivered to CERN around Easter. The DAQ and analysis software is yet to be written.

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Spare slides

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Imperial College London

Particle Identification (PID) in LHCb: Ring Imaging Cherenkov (RICH) Ring Imaging Cherenkov (RICH) Detectors Detectors

π/K Separation by different PID methods RICH2 RICH1 Polar angles vs momenta

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(a) Response curve shows excellent signal to noise separation (b) QE of a single HPD (c)The average QE(%) at 270 nm versus the HPD batch number

(a) (b) (c)

Imperial College London

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⊗

B┴ B║

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R. Mountain, Syracuse University

LHCb RICH1 Mag Cal Meeting, 05 Sep 2007 5

OPTICAL COLLIMATION

E F D C L x PMT (3″) PA / ADC / PC Moving Screen D d

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25 February 2008 V.Gibson SUSSP57 St Andrews 104

VI.1: LHC

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) ( 1

cos

λ β

n

∗

= Θ

2 2

sin

λ λ Θ ⋅ ⋅ =

l N d dN

The angle of emission is given by: and the number of photons by:

[ ]

Θ ⋅ ⋅ − ⋅ ⋅ =

2 ) ( 1 ) ( 1 6

sin ) ( 10 6 . 4

1 2 2 1

cm l N

A A

λ λ λ λ

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The same, but let us consider how a charged particle interacts with the medium

k  

⋅ = β ω

Conservation of energy and momentum The behavior of a photon in a medium is described by the dispersion relation

2 2

= − ε ω

k

ε β

1 cos

= Θ

p m k E m

= < < = < < β γ γ ω If: then:

β

m

k

ω

q

The Cherenkov radiation condition:

ε real

and

0≤ cos(Θ )≤ 1

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RICH1 Design

4m Upper Magnetic Shielding Protecting HPDs, mounted on cavern wall, supports upper HPDs Spherical Mirrors Lightweight carbon fibre mirrors 1.5% radiation length Flat Mirrors Glass mirror planes Beampipe made from beryllium VELO Exit Window 2mm aluminium Photon detector plane 14 by 7 Hybrid Photon Detectors (HPDs) Quartz Windows UV transparent Gas Enclosure supports mirrors and aerogel, contains C4F10 RICH1 Exit Window Carbon fibre Lower Photon detector plane Mounted on lower shield Lower Magnetic Shielding mounted

n cavern floor,

supports lower HPDs and Gas Enclosure

Radiation length(total) of RICH1 is 8 X0

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p distribution for B tracks

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1/pt distribution for B tracks

Impact Parameter Resolution

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40

Project Costs (kCHF)

Item RICH1 RICH2 Mechanics, Optics 527 1204 Photodetectors 1473 2290 Electronics 537 814 Gas system, monitoring 365 365 Aerogel 102

Total:

3004 4673 Total Cost (incl. spares) 7677 kCHF

Why silicon

Low ionization energy ( good signal )
Long mean free path ( good charge

collection efficiency )

High mobility ( fast charge collection )
Low multiple scattering
Little cooling required

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