particle detectors - dipartimento di fisica e geologia · 2007-10-12 · particle detectors history...

Particle Detectors

History of Instrumentation ↔ History of Particle Physics

The ‘Real’ World of Particles

Interaction of Particles with Matter, Tracking detectors

Photon Detection, Calorimeters, Particle Identification

Detector Systems

Summer Student Lectures 2007Werner Riegler, CERN, [email protected]

1W. Riegler/CERN

Particle Detectors

Detector-Physics: Precise knowledge of the processes leading to signals

in particle detectors is necessary.

The detectors are nowadays working close to the limits of theoretically

achievable measurement accuracy – even in large systems.

Due to available computing power, detectors can be simulated to within 5-

10% of reality, based on the fundamental microphysics processes (atomic

and nuclear crossections).

2W. Riegler/CERN

Particle Detector Simulation

Very accurate simulations

of particle detectors are

possible due to availability of

Finite Element simulation

programs and computing power.

Follow every single electron by

applying first principle laws of

physics.

Electric Fields in a Micromega Detector

Electrons avalanche multiplication

Electric Fields in a Micromega Detector

3W. Riegler/CERN

I) C. Moore’s Law:

Computing power doubles 18 months.

II) W. Riegler’s Law:

The use of brain for solving a problem

is inversely proportional to the available

computing power.

I) + II) = ...

Particle Detector Simulation

Knowing the basics of particle detectors is essential …

4W. Riegler/CERN

http://upload.wikimedia.org/wikipedia/commons/c/c5/PPTMooresLawai.jpg

Interaction of Particles with Matter

Any device that is to detect a particle must interact with it in

some way almost …

In many experiments neutrinos are measured by missing

transverse momentum.

E.g. e+e- collider. Ptot=0,

If the Σ pi of all collision products is ≠0 neutrino escaped.

Claus Grupen, Particle Detectors, Cambridge University Press, Cambridge 1996 (455 pp. ISBN 0-521-55216-8)

5W. Riegler/CERN

Elektro-Magnetic Interaction of Charged Particles

with Matter

1) Energy Loss by Excitation and Ionization

2) Energy Loss by Bremsstrahlung

3) Cherekov Radiation and 4) Transition Radiation are only minor

contributions to the energy loss, they are however important effects for

particle identification.

Classical QM

6W. Riegler/CERN

Classical Scattering on Free Electrons:

A charged particle with mass M, charge Z1e and velocity v

passes an electron at distance b .

Classical Scattering on free Electrons

Energy Loss by Excitation and Ionization 7W. Riegler/CERN

Targetmaterial: Atomic Mass A, Periodic Number Z, Density [g/cm3]

‘A’ gramm ‘NA‘ Atoms: Number of electrons/cm3 ne=NA Z/A [1/cm3]

With E(b) db/b = -1/2 dE/E Emax= E(bmin) Emin = E(bmax)

dx

Emax 2mev2 (M>>me), Emin I (Ionization Energy)

Classical Scattering on Free Electrons


= const. * 1/v2 * ln v

The relativistic form of the electro-

magnetic field doesn’t change the

energy transfer, is just changes Emax.

This formula is up to a factor 2 and the

density effect identical to the precise

QM derivation:

Bethe Bloch Formula:1/

Energy Loss by Excitation and Ionization

Classical Scattering on Free Electrons

9W. Riegler/CERN

Density effect. Medium is polarized

Which reduces the log. rise.

Kaon

Pion

Pion

Pion

W. Riegler/CERN 10

Discovery of muon and pion

Cosmis rays: dE/dx α Z2

Large energy loss

Slow particle

Small energy loss

Fast particle

Small energy loss

Fast Particle

Für Z>1, I 16Z 0.9 eV

For Large the medium is being polarized by the

strong transverse fields, which reduces the rise of

the energy loss density effect

At large Energy Transfers (delta electrons) the

liberated electrons can leave the material.

In reality, Emax must be replaced by Ecut and the

energy loss reaches a plateau (Fermi plateau).

Characteristics of the energy loss as a function

of the particle velocity ( )

The specific Energy Loss 1/ρ dE/dx

• firts decreaes as 1/2

• increases with ln for =1

• is independent of M (M>>me)

• is proportional to Z12 of the incoming particle.

• is independent of the material (Z/A const)

• shows a pleateau at large (>>100)

1/

Bethe Bloch Formula


Bethe Bloch Formula, a few Numbers:

For Z 0.5 A

1/ dE/dx 1.4 MeV cm 2/g for ßγ 3

Example 1:

Scintillator: Thickness = 2 cm; ρ = 1.05 g/cm3

Particle with βγ = 3 and Z=1

1/ρ dE / dx ≈ 1.4 MeV

dE ≈ 1.4 * 2 * 1.05 = 2.94 MeV

Example 2:

Iron: Thickness = 100 cm; ρ = 7.87 g/cm3

dE ≈ 1.4 * 100* 7.87 = 1102 MeV

Example 3:

Energy Loss of a Carbon Ion with Z=6 and

Momentum of 330 MeV/c/Nukleon

in Water, i.e. βγ = p/m = 330/940 ≈ .35

β ≈ .33

dE/dx ≈ 1.4 Z2 /β2 ≈ 460 MeV/cm→

Cancer Therapy !

1/

This number must be multiplied

with ρ [g/cm3] of the Material

dE/dx [MeV/cm]


Bethe Bloch Formula

12W. Riegler/CERN

Energy Loss as a Function of the Momentum

Energy loss depends on the particle

velocity and is ≈ independent of the

particle’s mass M.

The energy loss as a function of particle

Momentum P= Mcβγ IS however

depending on the particle’s mass

By measuring the particle momentum

(deflection in the magnetic field) and

measurement of the energy loss on can

measure the particle mass

Particle Identification !


W. Riegler/CERN 14

Measure momentum by

curvature of the particle

track.

Find dE/dx by measuring

the deposited charge

along the track.

Particle ID

Energy Loss as a Function of the Momentum

Particle of mass M and kinetic Energy E0 enters matter and looses energy until it

comes to rest at distance R.

Bragg Peak: For >3 the

energy loss is constant

(Fermi Plateau)

If the energy of the particle

falls below =3 the energy

loss rises as 1/2

Towards the end of the track

the energy loss is largest

Cancer Therapy.

Independent of

the material

Range of Particles in Matter


Average Range: Example

Kaon, p=700 MeV/c in Water:

=1 [g/cm3], Mc2=494 MeV

=1.42 /Mc2 R = 396g/cm2 GeV

R=195cm

In Pb: =11.35 R=17cm



Average Range:

Towards the end of the track the energy loss is largest Bragg Peak

Cancer Therapy

Photons 25MeV Carbon Ions 330MeV

Depth of Water (cm)

Rela

tive D

ose (

%)



17W. Riegler/CERN

Intermezzo: Crossections

Crossection : Material with Atomic Mass A and density contains

n = NA /A [Atoms/cm3], where NA is the Avogadro Number.

Considering the Atom to be a shere with Radius R: Atomic Crossection = R2

A Volume with surface A1 and thickness A1dx contains N=nA1dx Atoms.

The total area of these Atoms is A2=N .

Ratio of the areas p = A2/A1 = NA /A dx =

Probability that an incoming particle hits an atom in dx.

The probability that a particle hits an atom between distances m*dx and m*dx+dx is:

p(1-p)m p exp(-mp) = NA /A dx exp(- NA /A mdx)

With x=m dx and = A/ NA we have the probability P(x)dx that a particle hits an atom between x and

x+dx

P(x)dx = 1/ exp(-x/) dx

Mean Free path = x 1/ exp(-x/) dx = = A/ NA

Average number of interactions per unit of length = 1/ = NA /A

A1 dx

Fluctuation of the Energy Loss:Up to now we have calculated the average energy loss. The energy

loss is however a statistical process and will fluctuate.

A2

Fluctuations of the Energy Loss


Intermezzo,:Differential Crossection:

d (E,E’)/dE’ Crossection that an incoming particle of Energy E loses the energy E’ in the

interaction.

Total Crossection: (E)= d (E,E’)/dE’ dE’

Probability that in an interaction an amount of energy between E’ and E’+dE’ is lost: d

(E,E’)/dE’/ (E)

Average number of collisions per unit of length in which the particle loses and energy

between E’ und E’+dE’: NA /A d (E,E’)/dE’

Average Energy loss per unit of length: dE/dx = NA /A E’d (E,E’)/dE’ dE’



19W. Riegler/CERN

X X X X X

XX X X

X X X

XXX X XX

X XXX X

E E -

What is the probability that a particle of energy E looses the energy in a

material of thickness D ?

Probability for an interaction between x und x+dx:

P(x)dx = 1/ exp(-x/) dx with = A/ NA

Probability to have n interactions in distance D is

x=0 x=D

Fluctuation of the Energy Loss :

Poisson Distribution



20W. Riegler/CERN

Probability for loosing energy between E’ and E’+dE’ is given by the differential

crossection d (E,E’)/dE’/ (E)

From P(n) and d (E,E’)/dE’ we can calculated P(E,),

the probability that a particle of energy E loses the

energy when traversing a material of thickness D.

P(E,) cannot be expressed on closed form, but just

as an integral:

Landau Distribution

d

(E,E

’)/d

E’/

(E

)

Scattering on free electronsExcitation and ionization

P(E,)



21W. Riegler/CERN

P(): Probability for energy loss

in matter of thickness D.

Landau distribution is very

asymmetric.

Average and most probable

energy loss must be

distinguished !

Measured Energy Loss is usually

smaller that the real energy loss:

3 GeV Pion: E’max = 450MeV A

450 MeV Electron usually leaves

the detector.

Landau Distribution

Landau Distribution


LANDAU DISTRIBUTION OF ENERGY LOSS:

For a Gaussian distribution: N ~ 21 i.p.

FWHM ~ 50 i.p.

00 500 1000

6000

4000

2000

N (i.p.)

Counts 4 cm Ar-CH4 (95-5)

5 bars

N = 460 i.p.

PARTICLE IDENTIFICATION

Requires statistical analysis of hundreds of samples

0 500 1000

6000

4000

2000

N (i.p)

Counts

0

protons electrons

15 GeV/c

I. Lehraus et al, Phys. Scripta 23(1981)727

FWHM~250 i.p.


Landau Distribution

23W. Riegler/CERN


Particle Identification

‘average’ energy loss

Measured energy loss

In certain momentum ranges,

particles can be identified by

measuring the energy loss.

24W. Riegler/CERN

Up to this Point we have learned the following:

A charged particle leaves a trail of electrons and excited atoms along it’s path.

We have also quantified the amount of energy, which is given by the Bethe Bloch formula.

This deposited energy is (to first order) only depending on the particles’ Z12 and it’s velocity v. It is

(almost) independent of the particle’s mass.

1/ dE/dx is independent of the material and has a minimum at 1.4MeV cm2/g at 3.

We can identify three characteristic ‘regions’ as a function of the particle’s velocity (or better )

1) For non-relativistic velocities (v<<c) the deposited Energy goes as 1/v2

2) After this minimum For v c the energy loss rises according to ln (relativistic rise)

3) For very large of the particle the energy loss shows a plateau due to polarization of the medium.

The energy loss is a statistical process ‘Landau Distribution’.

The energy loss is strongly asymmetric due to rare ‘large’ energy transfers (delta electrons).


Detectors based on registration of

excited Atoms Scintillators

26W. Riegler/CERN Scintillator Detectors

Detectors based on Registration of excited Atoms Scintillators:

Charged particles leave a trail of excited Atoms (and ions) along their path.

Emission of photons of by excited Atoms, typically UV to visible light.

a) Observed in Noble Gases (even liquid !)

b) Inorganic Crystals.

c) Polyzyclic Hydrocarbons (Naphtalen, Anthrazen, organic Scintillators)

ad b) Substances with largest light yield. Used for precision measurement of energetic Photons. Used in Nuclear Medicine.

ad c) Most important cathegory. Large scale industrial production, mechanically and chemically quite robust. Characteristic are one or two decay times of the light emmission.


Light yield is typically proportional to the energy loss. Saturation effects described

by Birks‘ law.

kB = 10-2 … 10-4

Typical light yield of scintillators:

Energy (visible photons) few of the total energy Loss.

z.B. 1cm plastikscintillator, 1, dE/dx=1.5 MeV, ~15 keV in photons; i.e. ~ 15 000

photons produced.


LHC bunchcrossing 25ns LEP bunchcrossing 25s

Organic (‘Plastic’) Scintillators

Low Light Yield Fast: 1-3ns

Inorganic (Crystal) Scintillators

Large Light Yield Slow: few 100ns


Photons are being reflected towards the ends of the scintillator.

A light guide brings the photons to the Photomultipliers where the

photons are converted to an electrical signal.

By segmentation one can arrive at spatial resolution.

Because of the excellent timing properties (<1ns) the arrival time, or time

of flight, can be measured very accurately Trigger, Time of Flight.


Typical Geometries:

31W. Riegler/CERN Scintillator DetectorsFrom C. Joram

UV light enters the WLS material

Light is transformed into longer

wavelength

Total internal reflection inside the WLS

material

‘transport’ of the light to the photo

detector

The frequent use of Scintillators is due to:

Well established and cheap techniques to register Photons Photomultipliers

and the fast response time 1 to 100ns

Schematic of a Photomultiplier:

• Typical Gains (as a function of the applied

voltage): 108 to 1010

• Typical efficiency for photon detection:

• < 20%

• For very good PMs: registration of single

photons possible.

• Example: 10 primary Elektrons, Gain 107

108 electrons in the end in T 10ns. I=Q/T =

108*1.603*10-19/10*10-9= 1.6mA.

• Across a 50 Resistor U=R*I= 80mV.


e-

glass

PC

Semitransparent photocathode

Planar geometries

(end cap)

Circular geometries

(barrel)

(R.C. Ruchti, Annu. Rev. Nucl. Sci. 1996, 46,281)


corepolystyrene

n=1.59

cladding(PMMA)n=1.49

typically <1 mm

typ. 25 m

Light transport by total internal reflection

n1

n2

Fiber Tracking

High geometrical flexibility

Fine granularity

Low mass

Fast response (ns)

From C. Joram


Fiber Tracking

From C. Joram

Readout of photons in a cost effective way is rather challenging.

Magnetic Spectrometers for Momentum Measurement of Charged Particles

Limit Multiple Scattering

A charged particle describes a circle in a magnetic field:

35W. Riegler/CERN Magnetic Spectrometers

ATLAS:

N=3, sig=50um, P=1TeV,

L=5m, B=0.4T

∆p/p ~ 8% for the most energetic muons at LHC

Magnetic Spectrometers for Momentum Measurement of Charged Particles


Statistical (quite complex) analysis of

multiple collisions gives:

Probability that a particle is defected by an

angle after travelling a distance x in the

material is given by a Gaussian

distribution with sigma of:

0 = 0.0136/(cp[GeV/c])*Z1*x/X0

X0 ... Radiation length of the materials

Z1 ... Charge of the particle

p ... Momentum of the particle

Magnetic Spectrometers for Momentum Measurement of Charged Particles:

Multiple Scattering.


‘VERTEX’ – Detectors

By direct measurement of the ‘decay-position’ of the paticle ona can measure the

lifetime and therefore identify the particle.

Mainly for Charm, Beauty Hadron and Tau-Lepton Physics.

Typical Lifetimes: 10-12 bis 10-13 s

Typical decay distances: 20-500 μm

Typical required Resoution: ~10 μm

Silicon Strip or Pixel Detectors.

`

38W. Riegler/CERN Vertex Detectors

Detectors based on registration of Ionization: Tracking in Gas and

Solid State Detectors

Charged particles leave a trail of ions (and excited atoms) along their path:

Electron-Ion pairs in gases and liquids, electron hole pairs in solids.

The produced charges can be registered Position measurement Tracking

Detectors.

Cloud Chamber: Charges create drops photography.

Bubble Chamber: Charges create bubbles photography.

Emulsion: Charges ‘blacked’ the film.

Gas and Solid State Detectors: Moving Charges (electric fields) induce

electronic signals on metallic electrons that can be read by dedicated

electronics.

In solid state detectors the charge created by the incoming particle is

sufficient.

In gas detectors (e.g. wire chamber) the charges are internally multiplied in

order to provide a measurable signal.

39W. Riegler/CERN Tracking Detectors

The induced signals are readout out by dedicated

electronics.

The noise of an amplifier determines whether the

signal can be registered. Signal/Noise >>1

The noise is characterized by the ‘Equivalent

Noise Charge (ENC)’ = Charge signal at the input

that produced an output signal equal to the noise.

ENC of very good amplifiers can be as low as

50e-, typical numbers are ~ 1000e-.

In order to register a signal the registered charge

must be q >> ENC i.e. typically q>>1000e-.

Gas Detector: q=80e- too small.

Solid stated detector: q=200 000e- O.K.

Gas detector needs internal amplification in order

to be sensitive to single particle tracks.

Without internal amplification it can only be used

for a large number of particles that arrive at the

same time (ionization chamber).An amplifier measures the total charge in case

the integration time is larger than the drift time of

the charge.

40W. Riegler/CERN Tracking Detectors

Principle of signal induction by moving charges:

q

q

A point charge q at a distance z0

Above a grounded metal plate ‘induces’ a surface charge.

The total induced charge on the surface is –q.

Different positions of the charge result in different charge distributions.

The total induced charge stays –q.

-q

-q

The electric field of the charge must be calculated with the boundary condition that the potential φ=0 at z=0.

For this specific geometry the method of images can be used. A point charge –q at distance –z0 satisfies the boundary condition electric field.

The resulting charge density is

(x,y) = 0 Ez(x,y)

(x,y)dxdy = -q

I=0

Signal induction by moving charges 41W. Riegler/CERN

qIf we segment the grounded metal plate and if we ground the individual strips the charge density doesn’t change.

-q

-q

V

I1(t) I2(t) I3(t) I4(t)

z0(t)=z1-v*t

The charge induced on the individual strips is now depending on the position z0 of the charge.

If the charge is moving there are currents flowing between the strips and gorund.

The movement of the charge induces a current.

Principle of signal induction by moving charges:


In order to calculate the signals the Poisson equation must be calculated for all

different positions of the charge q difficult task.

Theorem (1) (Reciprocity theorem, Ramotheorem):

The current induced on a grounded electrode (n) by the movement of a charge

q along a trajectory x(t) can be calculated the following way:

One removes the charge q and brings electrode (n) to potential V0 while

keeping all the other electrodes at ground potential.

This defined an electric field En(x) (‘Weighting field’ of electrode n).

The induced current is then


Example: Elektron-Ion pair in gas

The induced charge depends on the position from where the charge starts moving.

The total induced charge on a specific electrode, once all the charges have arrived at the electrodes, is equal to the charge that has arrived at this specific electrode.

I1

I2

q,vI-q, ve

zE1=V0/D

E2=-V0/D

I1= -(-q)/V0*(V0/D)*ve - q/V0 (V0/D) (-vI) = q/D*ve+q/D*vI

I2=-I1

I1

t

Z=D

Z=0

Z=z0

Qtot1= I1dt = q/D*ve Te + q/D*vI*TI

= q/D*ve*(D-z0)/ve + q/D*vI*z0/vI

= q(D-z0)/D + qz0/D =

qe+qI=q

TeTI

E

The induced current is proportional to the velocity of the charges.


Theorem (2):

In case the electrodes are not grounded but connectd by arbitrary active or passive

elements one first calculates the currents induced on the grounded electrodes and

places them as ideal current sources on the equivalent circuit of the electrodes.

-q,v2q, v1

Z=D

Z=0

Z=z0

I2

I1

R

R

C

R

R

V2

V1


By using thin wires, the electric

fields close to the wires are very

strong (e.g.100-300kV/cm).

In these large electric fields, the

electrons gain enough energy to

ionize the gas themselves

Avalanche a primary electron

produces 103-106 electrons

measurable signal.

46W. Riegler/CERN

Detectors based on Ionization

Gas detectors:

• Transport of Electrons and Ions in Gases

• Wire Chambers

• Drift Chambers

• Time Projection Chambers


• Transport of Electrons and Holes in Solids

• Si- Detectors

• Diamond Detectors

47W. Riegler/CERN

Transport of Electrons in Gases: Drift-velocity

Electrons are completely ‘randomized’ in each collision.

The actual drift velocity along the electric field is quite

different from the average velocity of the electrons i.e.

about 100 times smaller.

The driftvelocity v is determined by the atomic crossection

( ) and the fractional energy loss () per collision (N is

the gas density i.e. number of gas atoms/m3, m is the

electron mass.):

Because ( )und () show a strong dependence

on the electron energy in the typical electric fields,

the electron drift velocity v shows a strong and

complex variation with the applied electric field.

v is depending on E/N: doubling the electric field

and doubling the gas pressure at the same time

results in the same electric field.

Ramsauer Effect

Transport of charges in gases 48W. Riegler/CERN

Typical Drift velocities are v=5-10cm/s (50 000-100 000m/s).

The microscopic velocity u is about ca. 100mal larger.

Only gases with very small electro negativity are useful (electron attachment)

Noble Gases (Ar/Ne) are most of the time the main component of the gas.

Admixture of CO2, CH4, Isobutane etc. for ‘quenching’ is necessary (avalanche

multiplication – see later).

Transport of Electrons in Gases: Drift-velocity

49W. Riegler/CERN Transport of charges in gases

An initially point like cloud of electrons will ‘diffuse’ because of multiple collisions and assume a

Gaussian shape. The diffusion depends on the average energy of the electrons. The variance σ2 of

the distribution grows linearly with time. In case of an applied electric field it grows linearly with

the distance.

Electric

Field

x

Thermodynamic limit:

Solution of the diffusion equation (l=drift distance)

Because = (E/P)

1

PFE

P

Transport of Electrons in Gases: Diffusion


The electron diffusion depends on E/P and scales in

addition with 1/P.

At 1kV/cm and 1 Atm Pressure the thermodynamic

limit is =70m für 1cm Drift.

‘Cold’ gases are close to the thermodynamic limit

i.e. gases where the average microscopic energy

=1/2mu2 is close to the thermal energy 3/2kT.

CH4 has very large fractional energy loss low low diffusion.

Argon has small fractional energy loss/collision

large large diffusion.

Transport of Electrons in Gases: Diffusion


Drift von Ions in Gases

Because of the larger mass of the Ions compared to electrons they are not

randomized in each collision.

The crossections are constant in the energy range of interest.

Below the thermal energy the velocity is proportional to the electric field

v = μE (typical). Ion mobility μ 1-10 cm2/Vs.

Above the thermal energy the velocity increases with E .

V= E, (Ar)=1.5cm2/Vs 1000V/cm v=1500cm/s=15m/s 3000-6000 times

slower than electrons !


Gas Detectors 53W. Riegler/CERN


Gas Detectors:


• Wire Chambers

• Drift Chambers




• Si- Detectors


• Principle: At sufficiently high electric fields (100kV/cm) the electrons gain

energy in excess of the ionization energy secondary ionzation etc. etc.

• Elektron Multiplication:

– dN(x) = N(x) α dx α…’first Townsend Koeffizient’

– N(x) = N0 exp (αx) α= α(E), N/ N0 = A (Amplification, Ga)

– N(x)=N0 exp ( (E)dE )

– In addition the gas atoms are excited emmission of UV photons can ionize

themselves photoelectrons

– NAγ photoeletrons → NA2 γ electrons → NA2 γ2 photoelectrons → NA3 γ2 electrons

– For finite gas gain: γ < A-1, γ … ‘second Townsend coefficient’


Detectors with Electron Multiplication

Cylinder Geometry for Wire Chambers

Electric field close to a thin wire (100-300kV/cm). E.g. V0=1000V, a=10m,

b=10mm, E(a)=150kV/cm

Electric field is sufficient to accelerate electrons to energies which are

sufficient to produce secondary ionization electron avalanche signal.

Wire with radius (10-25m) in a tube of radius b (1-3cm):


Proportional region: A103-104

Semi proportional region: A104-105

(Space Charge effect)

Saturation Region: A >106

Independent from the number of primary electrons.

Streamer region: A >107

Avalache along the particle track.

Limited Geiger Region:

Avanache propagated by UV photons.

Geiger Region: A109

Avalanche along the entire wire.




Proportional

Semi proportional

Saturation

Streamer

Limited Geiger region

Geiger region


The electron avalanche happens very close to the wire. First multiplication only

around R =2x wire radius. Electrons are moving to the wire surface very quickly

(<<1ns). Ions are drifting towards the tube wall (typically 100s. )

The signal is characterized by a very fast ‘spike’ from the electrons and a long Ion

tail.

The total charge induced by the electrons, i.e. the charge of the current spike due

to the short electron movement amounts to 1-2% of the total induced charge.



Rossi 1930: Coincidence circuit for n tubes Cosmic ray telescope 1934

Position resolution is determined

by the size of the tubes.

Signal was directly fed into an

electronic tube.



Charpak et. al. 1968, Multi Wire Proportional Chamber


‘Classic‘ geometry (Crossection) :

One plane of thin sense wires is placed

between two parallel plates.

Tyopica dimesions:

Wire distance 2-5mm, distance between

cathode planes ~10mm.

Electrons (v5cm/s) are being collectes

within in 100ns. The ion tail can be

eliminated by electroniscs filters pulses

100ns typically can be reached.

For 10% occupancy every s one pulse

1MHz/wire rate capabiliy !

In order to eliminate the left/right

ambiguities: Shift two wire chambers by

half the wire pitch.

For second coordinate:

Another Chamber at 900 relative rotation

Signal propagation to the two ends of

the tube.

Pulse height measurement on both ends

of the wire. Because of resisitvity of the

wire, both ends see different charge.

Segmenting of the cathode into strips or

pads:

The movement of the charges induces a

signal on the wire AND the cathode. By

segmengting and charge interpolation

resolutions of 50m can be achieved.


Charpak et. al. 1968, Multi Wire Proportional Chamber

1.07 mm

0.25 mm

1.63 mm

(a)

C1 C1 C1 C1 C1

C2C2C2C2

Anode wire

Cathode s trips

(b)

C1

Cathode Strip:

Width (1) of the charge

distribution DIstance

‘Center of Gravity’ defines the

particle trajectory.

Ladungslawine


Drift Chambers 1970:

In an alternating sequence of wires with different potentials one finds an electric field

between the ‘sense wires’ and ‘field wires’.

The electrons are moving to the sense wires and produce an avalanche which induces a

signal that is read out by electronics.

The time between the passage of the particle and the arrival of the electrons at the wire is

measured.

The drift time T is a measure of the position of the particle !

By measuring the drift time, the wire distance can be reduced (compared to the Multi Wire

Proportional Chamber) save electronics channels !

E

Scintillator: t=0

Amplifier: t=T


Drift chambers: typical geometries

W. Klempt, Detection of Particles with Wire Chambers, Bari 04

Electric Field 1kV/cm


The Geiger Counter reloaded: Drift Tube

Primary electrons are drifting to the wire.

Electron avalanche at the wire.

The measured drift time is converted to a radius by a (calibrated) radius-time correlation.

Many of these circles define the particle track.

ATLAS MDTs, 80m per tube

ATLAS Muon Chambers

ATLAS MDT R(tube) =15mm Calibrated Radius-Time correlation


The Geiger Counter reloaded: Drift Tube

Atlas Muon Spectrometer, 44m long, from r=5 to11m.

1200 Chambers

6 layers of 3cm tubes per chamber.

Length of the chambers 1-6m !

Position resolution: 80m/tube, <50m/chamber (3 bar)

Maximum drift time 700ns

Gas Ar/CO2 93/7


ATLAS MDT Front-End Electronics

Single Channel Block Diagram3.18 x 3.72 mm

• 0.5m CMOS technology

– 8 channel ASD + Wilkinson

ADC

– fully differential

– 15ns peaking time

– 32mW/channel

– JATAG programmableHarvard University, Boston University


Position resolution

depends on:

Diffusion (Pressure)

Gas Gain

Fluctuation of Primary

Charge.

Electronics

Diffusion Charge Fluctuation

Gas Gain

Gas Pressure


Simulation of ATLAS

muon tubes:

Switch off individual

components.

Large Drift Chambers: Central Tracking Chamber CDF Experiment

660 drift cells tilted 450

with respect to the

particle track.

drift cell


y

z

x

E

B drift

charged track

wire chamber to detect projected tracks

gas volume with E

& B fields

Time Projection Chamber (TPC):

Gas volume with parallel E and B Field.

B for momentum measurement. Positive Effect:

Diffusion is strongly reduced by E//B (up to a

factor 5).

Drift Fields 100-400V/cm. Drift times 10-100 s.

Distance up to 2.5m !


Important Parameters for a TPC

Transverse diffusion (m) for a

drift of 15 cm in different

Ar/CH4 mixtures


Position resolution momentum resolution.

Number of pad rows and pad size.

Homogeneous, parralel E and B field.

X0 Choice of material to limit multiple

scattering.

Energy resolution particle Identification.

ALICE TPC: Detektorspezifikationen

• Gas Ne/ CO2 90/10%

• Field 400V/cm

• Gas Gain >104

• Position resolition = 0.2mm

• Diffusion: t= 250m

• Pads inside: 4x7.5mm

• Pads outside: 6x15mm

• B-field: 0.5T

cm


ALICE TPC: Konstruktionsparameter

• Largest TPC:

– Length 5m

– diameter 5m

– Volume 88m3

– Detector area 32m2

– Channels ~570 000

• High Voltage:

– Cathode -100kV

• Material X0

– Cylinder from composit materias

from airplace inductry (X0= ~3%)


ALICE TPC: Pictures of the construction

Precision in z: 250m

Wire Chamber: 40m

End plates 250m


ALICE : Simulation of Particle Tracks

• Simulation of particle tracks for a

Pb Pb collision (dN/dy ~8000)

• Angle: Q=60 to 62º

• If all tracks would be shown the

picture would be entirely yellow !


ALICE TPC


My personal

contribution:

A visit inside the TPC.

Solid State Detectors 77W. Riegler/CERN


Gas detectors:


• Wire Chambers

• Drift Chambers




• Si- Detectors


particle detectors - dipartimento di fisica e geologia · 2007-10-12 · particle detectors history...

Documents