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Lecture 9 PARTICLE DETECTORSDetlev GottaInstitut für Kernphysik, Forschungszentrum Jülich / Universität zu Köln

GGSWBS'12, Batumi, Georgia5th Georgian – German School and Workshop in Basic Science

August 16, 2012

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EXAMPLES OF COMBINED DETECTION SYSTEMS

HOW TO DETECT?

INTERACTION OF CHARGED PARTICLES WITH MATTER

“ MASSIVE NEUTRAL PARTICLES WITH MATTER

“ RADIATION WITH MATTER

DETECTOR PRINCIPLES

WHAT TO DETECT ?

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WHAT TO DETECT ?

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PARTICLES

Light

Heavy

particle detector registration

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PARTICLES

What characterizes a particle?

mass M

charge Q

Spin intrinsic angular momentum S

life time t0

shape (for extended particles) <r2>

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RADIATION

fluid

gas

„light“

fundamental constant: c = speed of light in vacuum ( 30 cm / ns)

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RADIATION

What characterizes waves?

wave propagation velocity c = ln

wave length l

frequency n

particle physics

usually electromagnetic radiation

wave propagation velocity in vacuum c = l n “ “ “ in medium c‘ = l‘n < cindex of refaction n = c / c‘

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CONSTITUENTS OF MATTER I

atoms

10-10 m

atomic shells nucleus

electron proton neutron e p n

Q 1 + 1 0

M Mp / 1836 Mp Mp

size < 10-18 m 0.8 10-15 m

life time t0 > 1026 y > 1029 y 886 s

decay - - n p e n

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CONSTITUENTS OF MATTER II

pions kaons many more

p K …

Q 0, 1 2 0, 1

M Mp / 7 Mp / 2

size 0.6 10-15 m 0.6 10-15 m

life time t0 p 2610-9 s K 1210-9 s

p0 810-17 s K0S,L

910-10 / 510-8 s

decay p m n K m n, …

p0 g g K0 p + p , p 0 p0 ,...

new particles – unstable being free

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PARAMETERS

total energy

rest mass m0 ≠ 0 range in matter = 0 attenuation in matter

charge Q ≠ 0 deflection in el.-mag fields = 0 no deflection

life time t = gt0 decay length l = v t =

relativistic factor

massive particles el.-mag. radiation

0mtotalEkinE

2c0m

4c20m2c2ptotalE

+

γkinE

hpctotalE

ν

cv

limcv,

21

1 γββ

γ

h Planck constant

= minimal action

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HOW TO DETECT ?

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FORCES

• nuclear force keeps protons and neutrons together

• electromagnetic force keeps electrons around the nuclei

• weak force makes the (free) neutron to decay

• gravitation keeps us on the ground

stre

ngth

Stan

dard

Mod

el

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ELECTROMAGNETIC FORCE

a force is mediated

classical picture quantum world

by field around a source field quanta = particles

„light“ particles = photons g2

21

0Coulomb r

QQ4

1F πε

electromagnetic radiation = E and B fields interacts with electric charges

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DEFLECTION OF CHARGED PARTICLES IN EL.-MAG. FIELDS

• electric field

• magnetic field BvQxmF

EQxmF

B = const.

circular motion B plane of projection

T2B

MQ πωω

MQ

rBQpBvQr/mv2

p

r

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SIGNAL CREATION

• via electric charges

• measure the electric current I or voltage U

resistor R UI

capacitor CU

Q

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INTERACTION OF

CHARGED PARTICLES

WITH MATTER

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before after collision

1. Mparticle 1 >> Mparticle 2

2. Mparticle 1 = Mparticle 2

CHARGED PARTICLES

interaction happens by collisions of particles type 1 and 2

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CHARGED PARTICLES I: ENERY LOSS BY COLLISIONS

1. Mparticle >> Melectron

e.g. protons, deuterons, …

2. Mparticle = Melectron

electrons or positrons

collisions create electron ion pairs

strongly ionising

weakly ionisingexponential attenuation with depth x

µ: material dependent attenuation coefficient

%31RR

Δ

for all elements

µxe)x(N

no defined range R!

Bragg peak

well defined range R!

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CHARGED PARTICLES II: ENERY LOSS BY RADIATION

the charge polarizes the medium

emission under specific angle C

Radiation if vparticle > cin medium Cerenkov 1930s

C measures the velocity of the particle

electrons „radiate“

in the water above

the core of

a nuclear power plant

cos C = 1 / n

n = index of refraction

(small) dispersion !

Cerenkov wave front

acoustics analogue: Mach‘s cone for supersonic source

„light“ blue!

collisionxE

radiationCxE

<<

ΔΔ

ΔΔ

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INTERACTION OF

MASSIVE NEUTRAL PARTICLES

WITH MATTER

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neutrons – no defined range

detection by recoil of protons (from hydrogen)

MProton MNeutron

i.e. good shieldings are water concrete (15% water) paraffin ( (CH)n) …

NEUTRONS

collisions create recoil particles

maximum energy transfer for Mneutral = Mrecoil

central collision all energy is transferrednon central all energies according to scattering angle

cloud chamber picture

neutrons

energy transfer DE per collision

DE

prob

abili

ty

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INTERACTION OF

RADIATION WITH MATTER

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RADIATION I : PHOTO EFFECT

1. photon disappears

photo electron Ee = Ephoton - EB

2. refilling of hole in electron shell by

a) emission of photon or

b) Auger electron emission of

loosely bound outer electron

EAuger EB

detected energy E

photo peak E = Ephoton

= Ee + EB

escape peak E = Ephoton - EKa

example

Argon EKa = 2.95 keV

photon EPhoton = 6.41 keV

photopeak

escape peak

requires particle nature of „light“ Einstein 1905

Energy

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RADIATION II : COMPTON EFFECT

photon does not disappear

recoil electron Ee = Ephoton – Ephoton‘

continuous spectrum

detected energy E = Ee we neglegt EB of the electronand Erecoil of the nucleus because usually EB, Erecoil << Ee

proof of particle nature of „light“ Compton 1922

billard with photons and „quasifree“ electronsΔλ =λ (1− cosθ )

Compton edge= maximum energy transfer

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RADIATION III : BREMSSTRAHLUNG

bending force by Coulomb potential

force acceleration

any distance r continuous spectrum

accelerated charged particles radiate Hertz 1886

electromagnetic waves

characteristic X-raysrefilling of holes in inner atomic shells

a recoil partner (nucleus) is needed to fulfil energy and momentum conservation

rm

2r

nucleusQparticleQ

041

CoulombF

πε

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RADIATION IV : PAIR PRODUCTION

+Ze

Ephoton = hn > 2 melectron

in general > 2 mparticles at very high energies

 

el.-mag shower e+ e – g - cascadepair production and Bremsstrahlung alternateshower may start with photon or electron

radiation length x0

characteristic material dependent constant depth, where about 2/3 of the incident energy is converted

proof of mass-energy equivalence Blackett 1948

conversion of energy into matter

magnetic field Ba recoil partner (nucleus) is needed to fulfil energy and momentum conservation

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CHARGED PARTICLES : SUMMARY I

Fractional energy loss.

MIPsminimum ionsing particles

ρΔΔ 1

xE

dxdE

2M0

...v1

xE

2collision

ΔΔ

T < 2M0

stopping power

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CHARGED PARTICLES : SUMMARY II

Fractional energy loss per radiation length in lead as a function of electron or positron energy.

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RADIATION: SUMMARY I

cross section s Z5

s reaction probability

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RADIATION: SUMMARY II

intensity after layer thickness x

attenuation

x)h(0eI)x(I νμ

Lambert-Beer law

x)h(

0e

I)x(I νμ

Io I

x

dx

transmission

)h()h(i

i νμνμ sum of linear attanuation coeff.

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DETECTOR PRINCIPLES

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(Wilson) cloud chambertypical Open Day presentations

saturated alcohol vapor

a-particle emitting nuclide

overheated LH2

bubble chamber (D. Glaser noble prize 1960)+ magnetic field

"beer" inspired !!!

among othersdiscovery of theweak neutral current

BEBC @ CERN 73 until 80ies 3.7 T, 35 m3 LH2

not only HISTORY

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CHARGE

capacitor

voltage generatorionising

particle

current or voltage detection

charge created by charged particles or by „light“ is collected

by applying a voltage by means of a curent or voltage detection

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SCINTILLATORS produce “LIGHT”

ionisation caused by

charged particles or light

excitation and delayed light emission

usually in the UV range

anorganic NaI(Tl), CSI, BaF2, …

inorganic doped „plastics“

UV light is converted to charge

at a photo cathode and

multiplied by a multi stage

photo „multiplier“

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TIME

10 ns

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WIRE CHAMBERS I

to control avalanchequench gases, e.g. CO2, CH4, C2H6

multiplication avalanchegain 105 - 106

wire chambers tutorial:F. SauliCERN yellow report 99-07

electron multiplicationaround anode (fast)

drift of ions (slow)

typical ion drift velocity: 1 - 10 cm/(µskV)Ar CH4

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WIRE CHAMBERS II

many wires: MWPC = multiwire proportional chamber

position resolution wire distance typically 2 mm

• (x,y) - coordinate per pair of frames

• trajectory from MWPC stacks

field configuration

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tracking: cut on fiducial target volume

example: p-3He pnn or dn

WIRE CHAMBERS III

3He vesssel

pion beam

beam defining countersmainly p carbon reactions

protons

deuterons

MWPC 1

MWPC 2

target

beam defining counters

good bad event

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"simple" mechanics10 MHz rateinside magnetic field

ZEUS - DESY wedge

Type-2 module (520 ‘straws’)

ATLAS at the LHC

individual counters, timing 20 nsHV: coat, ground: sense wire (~ kV)typical size: length 1 - 2 m, f mm - cm

resistive read outIleft Iright

zDz < 1 mm

Monte Carlosimulation

gas fillinge.g., Ar/C2H6

wall: aluminised mylar foilsanode wire: f 20 µm

STRAW TUBES

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time positionexternal time reference, e.g., plastic scintillator

trick: choose field configuration, which keeps the nonlinearity of time-to-position relation small position resolution

DRIFT CHAMBERS I

20 µm

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The wires are arranged in layers that pass through the cylinder at three different angles. The set of wires that give a signal can be used to allow computer reconstruction of the paths (or tracks) of all the charged particles through the chamber.

The "drift" in the name of this chamber refers to the time it takes electrons to drift to the nearest sense wire from the place where the high-energy particle ionized an atom. Any three sense wires are only nearby in one place so a set of "hits" on these three fix a particle track in this region. By measuring the drift time, the location of the original track can be determined much more precisely than the actual spacing between the wires.

improved position resolution by nearest 3 wires method

inclined wires

DRIFT CHAMBERS II

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properties:• full 3-dimensional detector• constant drift velocity due to the collisions

in the gas mixture (typical a few cm/µs). • low occupancy even for high background (high rates)• large dE/dx due to large gas thickness (particle identification)

idea: avoid pile-up many MWPC planes (typical gas thickness of 1 cm) principle: electrons produced follow the constant electric field lines to a single MPWC plane located at one end of the volume ( x-y coordinates on this plane) Third coordinate, z, from the drift time of the electrons to the anode plane

STAR TPC - RHIC, Brookhaven

TPC - time projection chamber David Nygren, 1974

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Single Track

Track ClusterPixel Tracker

• Pixel Size • Occupancy• Charge Sharing• S/N• ExB Drift• Radiation Damage

LHC - 1014 /cm2/yr

vertex resolution(20-30) mm IP

& Trigger

Charge Sharingcharge center of gravity

high position resolution

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+

+

charged particle

principle

pn diode

as almost all semiconductor detectors

miniaturisation

Readout Chip

Sensor

arrays of soldering dots

typical x-y (front-back)arrangements

200 µm stripslayer thickness 300 µm

SILICON MICRO - STRIP DETECTORS I

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CMS - LHC scheme

silicon µ-strip module

semiconductor telescope65/300/300/5500 µm thick double-sided Si-strip detectors

ANKE - COSY

• inner tracker

• vertex detection• recoils • polarisation (left-right asymmetry)

SILICON MICRO - STRIP DETECTORS II

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EXAMPLES

OF

COMBINED DETECTION SYSTEMS

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focal plane

particle identification by dE/dx

2

2

21

pm

Tm

vdxdE

counter number

1

16

FOCAL PLANE SPECTROMETERfor positively charged particles

ANKE@COSY I: SET-UP

aim: measure simultanuously positively and negatively charged particles

e.g., pp pp K+ K

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ANKE@COSY II: FOCAL PLANE DETECTOR

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WASA@COSY I: SET-UP

aim: measure photons from neutral particle decay in coincidence with charged particles

e.g., dd 4He p0

gg

photon detector: calorimeter charged particle detector: forward hodoscope

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WASA@COSY II: CALORIMETER

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WASA@COSY III: FORWARD HODOSCOPE

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• Silicon Vertex Tracker (SVT) - precise position information on charged tracks

• Drift Chamber (DCH) - the main momentum measurements for charged particles and helps in particle identification through dE/dx measurements

• Detector of Internally Refected Cerenkov radiation (DIRC or DRC) - charged hadron identification

• Electromagnetic Calorimeter (EMC) - particle identification for electrons, neutral electromagnetic particles, and hadrons

• Solenoid (not a subdetector) – high magnetic field for needed for charge and momentum measurements

• Instrumented Flux Return (IFR) - muon and neutral hadron identification• and more …

Todays detectors comprise ...

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EXERCISES LECTURE 9: PARTICLE DETECTORS

1. Derive the nonrelativistic relation between kinetic energy and momentum from

the relativistic energy-momentum relation.

2. By which process charged particles loose kinetic energy in matter?

3. Which process dominates – depending on the energy of the radiation – the

attenuation in matter?

4. Which processes are involved in an X-ray session at your medical doctor having

an apparatus labeled 25 keV?

5. Which is the minimum velocity (in units of speed of light c) for particles in order

to produce Cerenkov light in plastic material with index of refraction n = 1.5?

6. Which kind of detector should be used to detect neutral pion decays?

7. How many planes of MWPCs are needed to measure the trajectory of a charged

particle with and without the presence of a magnetic field B.