CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/1 Introduction Summer Student...

CERN Summer Student Lectures 2003Particle Detectors

Christian Joram I/1

Introduction

Summer Student Lecture Series 2003

Christian Joram

EP / TA1

From (very) basic ideas

to

rather complex

detector systems

1 + 1 2


Christian Joram I/2

Introduction

Outline + approximate timing

Introduction, basics

Tracking (gas, solid state)

Scintillation and light detection

Calorimetry

Particle Identification

Detector Systems

Discussion session I

Discussion session II

= Detector Exhibition

Thu/Fri (2x45 min)

Mon/

Tue

(2 x45 min)

Wed

(45 min)

Fri, 4 June, 11:15

Tue, 8 June, 11:00


Christian Joram I/3

Introduction

Literature on particle detectorsLiterature on particle detectors

Text books C. Grupen, Particle Detectors, Cambridge University

Press, 1996 G. Knoll, Radiation Detection and Measurement, 3rd

Edition, 2000 W. R. Leo, Techniques for Nuclear and Particle

Physics Experiments, 2nd edition, Springer, 1994 R.S. Gilmore, Single particle detection and

measurement, Taylor&Francis, 1992 W. Blum, L. Rolandi, Particle Detection with Drift

Chambers, Springer, 1994 K. Kleinknecht, Detektoren für Teilchenstrahlung, 3rd

edition, Teubner, 1992

Review articles Experimental techniques in high energy physics, T.

Ferbel (editor), World Scientific, 1991. Instrumentation in High Energy Physics, F. Sauli

(editor), World Scientific, 1992. Many excellent articles can be found in Ann. Rev.

Nucl. Part. Sci.

Other sources Particle Data Book (Phys. Rev. D, Vol. 54, 1996) R. Bock, A. Vasilescu, Particle Data Briefbook

http://www.cern.ch/Physics/ParticleDetector/BriefBook/

Proceedings of detector conferences (Vienna VCI, Elba, IEEE)


Christian Joram I/4

retina

“The oldest particle detector”(built many billion times)

• High sensitivity to photons• Good spatial resolution• Very large dynamic range (1:1014)

+ automatic threshold adaptation• Energy (wavelength) discrimination • Modest speed.

Data taking rate ~ 10Hz (incl. processing)

Introduction


Christian Joram I/5

Use of photographic paper as detector Detection of photons / x-rays

W. C. Röntgen, 1895Discovery of the ‘X-Strahlen’

Photographic paper/film

e.g. AgBr / AgCl

AgBr + ‘energy’ metallic Ag (blackening)

+ Very good spatial resolution+ Good dynamic range- No online recording- No time resolution

Introduction


Christian Joram I/6

From: J.J. Thomson: Cathode Rays. Philosophical Magazine, 44, 293 (1897).

“… The rays from the cathode C pass through a slit in the anode A, which is a metal plug fitting tightly into the tube and connected with the earth; after passing through a second slit in another earth-connected metal plug B, they travel between two parallel aluminium plates about 5 cm. long by 2 broad and at a distance of 1.5 cm. apart; they then fall on the end of the tube and produce a narrow well-defined phosphorescent patch. A scale pasted on the outside of the tube serves to measure the deflexion of this patch….”

J. Plücker 1858 J.J. Thomson 1897

accelerator manipulationBy E or B field

detector

Thomson’s cathode ray tube

Scintillation of glass

Introduction


Christian Joram I/7

E. Rutherford H. Geiger1909

The Geiger counter, later further developed and then calledGeiger-Müller counter

First electrical signal from a particle

pulse

Introduction


Christian Joram I/8

C. T. R. Wilson, 1912, Cloud chamber

The general procedure was to allow water to evaporate in an enclosed container to the point of saturation and then lower the pressure, producing a super-saturated volume of air. Then the passage of a charged particle would condense the vapor into tiny droplets, producing a visible trail marking the particle's path.

First trackingdetector

Introduction


Christian Joram I/9

physics theories

experiments

technologies& materials

knowledge / progress

Introduction

“progress cycle”

detectors


Christian Joram I/10

Introduction

A W+W- decay in ALEPH

e+e- (s=181 GeV) W+W- qq 2 hadronic jets + missing momentum

_



Introduction

Reconstructed B-mesons in the DELPHI micro vertex detector

B 1.6 ps l = c 500 m

Primary Vertex

Primary Vertex



Introduction

A simulated event in ATLAS (CMS)

H ZZ 4

pp collision at s = 14 TeV

inel. 70 mb

Interested in processes

with fb

23 overlapping minimum bias events / BC

1900 charged + 1600 neutral particles / BC

L = 1034 cm-2 s-1, bunch

spacing 25 ns



tim

e

e-

e+ q

q-

Z

Introduction

Idealistic views of an elementary particle reaction

• Usually we can only ‘see’ the end products of the reaction, but not the reaction itself.

• In order to reconstruct the reaction mechanism and the properties of the involved particles, we want the maximum information about the end products !

ion)hadronizat (

0

qqZee

m



Introduction

The ‘ideal’ particle detector should provide…

coverage of full solid angle (no cracks, fine segmentation

measurement of momentum and/or energy detect, track and identify all particles (mass, charge) fast response, no dead time

practical limitations (technology, space, budget)

Particles are detected via their interaction with matter.

Many different physical principles are involved (mainly of electromagnetic nature).

Finally we will always observe... ionizationionization and excitationexcitation of matter.

pppp,

ep,,ee

end products • charged • neutral• photons

detector

m



Definitions and units

Some important definitions and units

energy E: measure in eV momentum p: measure in eV/c mass mo: measure in eV/c2

420

222 cmcpE

1 eV is a tiny portion of energy. 1 eV = 1.6·10-19 J

mbee = 1g = 5.8·1032 eV/c2

vbee= 1m/s Ebee = 10-3 J = 6.25·1015 eV

ELHC = 14·1012 eV

To rehabilitate LHC…

Total stored beam energy:

1014 protons * 14·1012 eV 1·108 J

this corresponds to a mtruck = 100 T

vtruck = 120 km/h

11

110

2c

v

20 cmE cmp 0

E

pc



The concept of cross sections

Definitions and units

Cross sections or differential cross sections ddare used to express the probability of interactions between elementary particles.

Example: 2 colliding particle beams

= N1/t = N2/t

What is the interaction rate Rint. ?

beam spot area A

Rint ·t) = · L

Luminosity L [cm-2 s-1]

Example: Scattering from target

solid angle element d

incident beam

scattered beam

target

Nscat() Ninc· nA · d = dd()·Ninc·nA·

d

has dimension area !

Practical unit:

1 barn (b) = 10-24 cm2

.nA = area density

of scattering

centers in target



Multiple scattering

Bethe-Bloch formula / Landau tails

Ionization of gases

Wire chambers

Drift and diffusion in gases

Drift chambers

Micro gas detectors

Silicon detectors strips/pixels

Momentum measurement

Silicon as a detection medium








the sagitta s is determined by 3 measurements with error (x):

for N equidistant measurements, one obtains (R.L. Gluckstern, NIM 24 (1963) 381)

ex: pT=1 GeV/c, L=1m, B=1T, (x)=200m, N=10

L

s

B

y

x

qBpBvqmv

T )(2

2

23

23.

3.0

8)()()(

BL

px

s

x

s

s

p

p Tmeas

T

T

)4/(720

3.0

)(2

.

NBL

px

p

p Tmeas

T

T (for N 10)

%5.0

.

meas

T

T

p

p

)( 3121

2 xxxs

(s 3.75 cm)

m)(T3.0)cGeV( BpT

Tp

BLL

3.022sin

2

Tp

BLs

22

8

3.0

82cos1



Multiple Scattering

Scattering

An incoming particle with charge z interacts with

a target of nuclear charge Z. The cross-section

for this e.m. process is

Average scattering angle Cross-section for infnite !

Multiple ScatteringSufficiently thick material layer

the particle will undergo multiple scattering.

2sin

14

4

22

p

cmzZr

d

d ee

00

Rutherford formula

d/d

L

plane

rplane

RMSplane

RMSplane space

2

120

P

plane0

Gaussian

sin-4(/2)




Back to momentum measurements:

What is the contribution of multiple scattering to ?

0

1045.0

)(

LXBp

pMS

T

(p)/p

(p)/p

(p)/p

p

MS

meas.total error

independent of p !

%5.0)(

MS

Tp

p

Approximation

X0 is radiation length of the medium (discuss later)

00

1

X

L

p

Tp

p)(

TT

pxp

p )(

)(

px MS 1)( 0

• ex: Ar (X0=110m), L=1m, B=1T

remember

constant)(

MS

Tp

p

More precisely:



Interaction of charged particles

Detection of charged particles

How do they loose energy in matter ?

Discrete collisions with the atomic electrons of the absorber material.

Collisions with nuclei not important (me<<mN).

If are big enough ionization.

densityelectron :

0

N

ddE

dNE

dx

dE

e-

k ,

0,mv

k ,

Instead of ionizing an atom, under certain conditions the photon can also escape from the medium.

Emission of Cherenkov and Transition radiation. (See later).



Bethe-Bloch formula

dE/dx in [MeV g-1 cm2]

Bethe-Bloch formula only valid for “heavy” particles (mm).

dE/dx depends only on independent of m !

First approximation: medium simply characterized by

~ electron density

2

2ln

14 2max

2

222

21

2222

T

I

cm

A

ZzcmrN

dx

dE eeeA

2121 cm MeV g..dxdE

Average differential energy loss

Ionisation only Bethe - Bloch formuladxdE

A

Z

Z/A~0.5

Z/A = 1

2

1

dx

dE

22ln dx

dE

“relativistic rise”

“kinematical term” 3-4 minimum ionizing particles, MIPs

“Fermi plateau”



Landau tails

For thin layers (and low density materials): Few collisions, some with high energy transfer.

Energy loss distributions show large fluctuations towards high losses: ”Landau tails”

For thick layers and high density materials: Many collisions. Central Limit Theorem Gaussian shape distributions.

Real detectors (limited granularity) can not measure <dE/dx> !

It measures the energy E deposited in a layer of finite thickness x.

<E>

E

e-

e-

<E>

E



Ionization of gases

Gas detectors

Fast charged particles ionize the atoms of a gas.

Often the resulting primary electron will have enough kinetic energy to ionize other atoms.

primarytotal nn 43

Primary ionization Total ionization

10 - 40 pairs/cmE/pair ~ 20 - 40 eV

• Assume detector, 1 cm thick, filled with Ar gas:

1 cm

~ 100 e-ion pair

100 electron-ion pairs are not easy to detect!

Noise of amplifier 1000 e- (ENC) !

We need to increase the number of e-ion pairs.



Proportional Counter

Gas amplification

Consider cylindrical field geometry (simplest case):

a

b

r

E

1/r

a

cathode

anode

gas

Ethreshold

a

rCVrV

r

CVrE

ln2

)(

1

2

0

0

0

0

C = capacitance / unit length

Electrons drift towards the anode wire ( stop and go! More

details in next lecture!).

Close to the anode wire the field is sufficiently high (some

kV/cm), so that e- gain enough energy for further ionization

exponential increase of number of e--ion pairs.




0CVkeM

xrxE ennenn 00 or : First Townsend coefficient

(e--ion pairs/cm)

(O. Allkofer, Spark chambers, Theimig München, 1969)

1 : mean free path

Cr

a

drrn

nM exp

0Gain

(F. Sauli, CERN 77-09)



Signal

formation

Avalanche formation within a few wire radii and within t < 1 ns!

Signal induction both on anode and cathode due to moving charges (both electrons and ions).


drdr

dV

lCV

Qdv

0

Electrons collected by anode wire, i.e. dr is small (few m). Electrons contribute only very little to detected signal (few %).

Ions have to drift back to cathode, i.e. dr is big. Signal duration limited by total ion drift time !

Need electronic signal differentiation to limit dead time.



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CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/1 Introduction Summer Student...

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