tracking detectors for particle physicsepweb2.ph.bham.ac.uk/user/thompson/lectures/2007/... ·...

Birmingham 2007 Tracking for HEP Paul Thompson 1

Tracking Detectors for Particle Physics

Paul Thompson

Trajectory measurement

Basic multi-wire chambers

Drift Chambers

Silicon Detectors


Introduction Literature on particle detectors

Lecture Courses- C. Joram CERN Summer Student Lectureshttp://atlphy01.kek.jp/~atlasjpn/higgs-work/lecture/2002/SSL_PD_II.pdf

/SSL_PD_III.pdfCopied to /home/pdt/joram/2004 -O Ullalandhttp://agenda.cern.ch/tools/SSLPdisplay.php?stdate=2004-07-05&nbweeks=7

Text Books- W.R. Leo, Techniques for Nuclear and Particle Physics Experiments,1994-W.Blum, L. Rolandi, Particle Detection with Drift Chambers, 1994R.C. Fernow, Introduction to experimental particle, physics, Cambridge University Press, 1989Others-Particle Data Book-R.Bock, A. Vasilescu, Particle Data Briefbookhttp://www.cern.ch/Physics/ParticleDetector/BriefBook/


Multi-Detectors in HEP

-Tracking integral part of HEP “multi-detectors”

-Provide triggering information and efficient, accurate particle momentum and vertex measurements

-Focus on only most common types of tracking detector


Tracking Detectors in HEP

-Reconstructed B meson decayin the DELPHI (LEP) micro-vertex detector

Decay cτ (μm)D0 123D+/- 312B0 461B+/- 501

-For fully reconstructed vertexneed resolution of σ(z)~80 μm And σ(rφ)~100 μm

Beam profile or `spot’ ~

100 μm x 25 μm


Tracking Detectors in HEP

LHC detectors require precise muon detectors


Trajectory Measurement-For constant B solution to equation of motion is a helix

-Require 5 independent measurements

|κ| = 1/R, dca, φ, θ, z0

measured at distance of closest approach to 0,0,0

-Solve equations using e.g. least squares, Kalman filter

-Improve measurement by constraining to vertex

z(s) = z0 + s . cot ϑ

s

s is the path length here


Error on Measurement

s is the sagitta here

e.g. 3 measurements of s

For N equidistant measurements:


Multiple Scattering(MS)Case of light particles through heavy medium-undergo multiple elastic Coulomb scattering -> deflection in angle no energy loss

-Error on the momentum measurement is independent of p

(I.e. MS dominates error at low p)

e.g.

X0 is the radiation length

of material


Energy Loss Heavy particles incident on light target - electromagnetic energy loss due to ionization dominates. Only slightly deflected.

Mean energy loss dE/dx of particle given by Bethe-Bloch formula

Electrons require Bremsstrahlung terms in addition

Analyse signal amplitude +combine with p measurement

-> Particle ID at low p

Corrections required for:-

Changes in gas pressure temperature with time

Differences in cell geometry

Space charge effects


Wire Chambers

Fast charged particle ionizes the atoms of the gas.

Electrons drift towards anode along field lines

Close to the anode wire field is large so that primary electrons ionize other atoms. Secondary electrons ionize further pairs leading to gas multiplication

Exponential increase in number of electron-ion pairs -> avalanche

Choose dense noble gases as gas e.g. Argon. Add poly-atomic gases as quenchers (absorb photons discharged by gas atoms) e.g. methane

Ionization


Operation ModesIonization mode:No gas

multiplication, weak output signal from primary electrons

Proportional mode:Voltage above threshold gas multiplication starts. Signal proportional to energy loss of particle (dE/dx). Gain of order 10^5.

Limited proportionality:Space charge effects of ions alter effective electric field.

Geiger Mode: Electric breakdown of gas. Recombination of ions result in photo-emission. Avalanches merge ->spark emission.


Multiwire Proportional Chamber(MWPC)

G. Charpak et al. 1968, Nobel Prize 1992Typical values

d=2mm,w=8mmw

-Operate in proportional mode. Analysis of pulse height, possible particle i.d.

-Digital readout of anode pulses

-Fast response, ideal for triggering

-But, poor spatial resolution 300μm, require multi-layers for more precision


Limited streamer mode

Iron of return yoke of magnet instrumented with limited streamer tubes

Operate in Limited streamer mode

using thicker wires or highly quenched gas

Gives large signal -> simple electronics


Drift Chambers

High spatial resolution achieved by measuring time electrons need to reach anode wire

Wider wire spacing means fewer read-out channels

Electron drift velocity νD= 5cm/μs


Drift Chambers

Require constant, predictable drift field -> Field Shaping wires

Planar Drift Chamber gives coordinates of track intersection with a plane. (Electrons drift in the plane)

Multiple layers are needed to give trajectory information.

Different orientations of wires provide information on θ or φ.

5cm

8cm

Planar Drift Chamber


Jet Chambers

-Multi-independent cells

-Optimized for max. points measured on track in radial direction

-Left-right ambiguity resolved e.g. by staggering

-E.g. Incline to radial direction so that drift direction perpendicular to wire plane (drift times = curvature)

-Spatial resolution e.g. 150 μm

-Two track resolution 1-3 mm


z-measurement

-z measurement from charge divisionσ(z) ~1% of track length e.g. 2cm

-Similarly, timing differenceQL QR

z

L-z

z/L = QR / (QR+QL)

Improve resolution

-z-chambers: with sense wires perpendicular to beam-pipe (σ(z) ~ 200μm)

-stereo chambers:rotate end points of one end of wire by angle α

x-y measurement becomes a function of z -> better estimation of z

δ(z) = d(rφ)/α σ(z) ~ 1mm

Make α as large as possible but keep down field inhomogeneties


0

50

100

0.1 1 10 100 1000

X/p

%

ElasticExcitation

IonizationKinetic

Argon

0

50

100

0.1 1 10 100 1000

X/p

%

Elastic

Excitation

IonizationKinetic

VibrationN2

Detector Gas

Approximate computed curves showing the percentage of electron energy going to various actions at a given X/p (V/cm/mmHg)

Elastic: loss to elastic impactExcitation: excitation of electron levels, leading to light emission

and metastable statesIonization: ionization by direct impactKinetic: average kinetic energy divided by their “temperature”Vibration: energy going to excitation of vibrational levels

Noble Gases (e.g. Ar) chosen –

gas multiplication from excitation

at lower energy compared with

complex gases.


Detector Gas

-Not possible to operate at high enough gains due to permanent discharge.

Emission of photons (e.g. Ar 11.6 eV) above ionisation threshold for other molecules e.g. H2 causes new avalanches.

-Add polyatomic gases as quenchers. Absorb photons through elastic collisions or dissociation to smaller molecules e.g. methane absorption band 8-14 eV

-Typical mixture Ar-CO2-CH4 (90:10:1)


Time Projection Chamber

Rather stringent requirement on homogeneity of E and B field

Long drift times means a slow readout

High event rates lead to high charge densities and gas breakdown leading to distortions in E-field

Time Projection ChambersLarge volume active detector.- full 3-D track reconstruction (no ambiguities)- x-y from wires and segmented cathode of MWPC- z from drift time (electrons drift along B-field)- dE/dx


Silicon DetectorsSolid state detectors history of Si, Ge, Li for energy measurement

We require precision tracking

Why silicon?-Requires only 3.6 eV to produce an electron-hole pair (~30 eV for gas detectors) -> high energy resolution-High specific density (2.33 g/cm3) gives good spatial resolution(reduce range of secondary electrons) -Small dimensions -> fast charge collection (5ns) -Progress in micro-technology allows large-scale production of sophisticated designs at acceptable cost

Challenges-High density leads to significant energy loss (390 eV/μm) -> Typical thickness ~300 μm -3.2x104 e-h pairs -> low-noise electronics-No charge multiplication!


Obtaining a signal

In a pure semiconductor electron density n is equal to hole density p

In this volume 4.5 x 108

free charge carriers, but only 3.2x104 e-h pairs produced by MIP

Reduce number of carriers –deplete the detector


Doping

Doping concentration 1012 cm-3(n) c.f. 1017 cm-3 for electronics. Resistivity factor 5 higher

Silicon is group IV


p-n junction


Microstrip detectorSpatial information by segmenting p doped layer Pitch,σ = pitch/ 12


Double-sided microstripSegmenting also the n doped layer, but:

Either add p+ doped blocking strips or

Add negative biased Al to repel e-

Positive charges in SiO2 attract e- in n- layer -> short circuit


Pixel DetectorsSegment silicon to diode matrix ( e.g 50 x 500 μm)

Readout electronics with same geometry

Connection by bump bonding techniques

Requires sophistic readout architecture

Used in LHC experiments (ALICE,ATLAS,CMS)

‘Pad’ detectors (~mm x mm)


Micro vertex detectors

Babar SVT: Inner 3 layers give vertex information (spatial σ~10-15 μm), outer 2 layers Pt measurement (σ~40 μm)

-2 track resolution ~ 10 μm

Double layer orthogonal strips (φ and θ)

σ(d0) = 25 μm

σ(z0) = 65 μm

Pt = 3 GeV


Radiation DamageA major issue for LHC detectors

Caused by Non-Ionizing Energy Loss (NIEL)

-Bulk effects. Displacements from lattice positions, vacancies

-Surface effects. Oxides and surface interfaces trap charges

Lead to following

1) Increase of sensor leakage current (decreases signal to noise ratio)

2) Change of depletion voltage (increases with radiation)

3) Decrease of charge collection efficiency

Solutions:

1) Build sensors which stand high depletion voltage (500 V)

2) Keep at lower temperature (-10 C)

3) Advanced methods e.g. new materials – diamond no depletion or doping required (Beam monitoring at LHC /BaBar)


Summary

Met at an introductory level the most common tracking detectors

Hopefully, some of the acronyms/terminology will be relevant to understanding your detector.

Detector R&D continuous and fast moving field in HEP. E.g. on-going LC research – no time (or expertise!) to touch upon those subjects here.

Some of the concepts you have learnt today will be relevant/expanded upon in the calorimeter lecture.

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