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PARTICLE DETECTORS. Günther Dissertori CERN-EP CERN Teachers Seminar July 2001. Outlook. Introduction What to measure, why? Basic Principles Tracking Calorimetry Particle Identification Large detector systems Conclusions. Introduction.

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particle detectors

PARTICLE DETECTORS

Günther Dissertori

CERN-EP

CERN Teachers Seminar

July 2001

outlook
Outlook
  • Introduction
  • What to measure, why?
  • Basic Principles
    • Tracking
    • Calorimetry
    • Particle Identification
  • Large detector systems
  • Conclusions
introduction
Introduction
  • HE physics experiments study interaction of particles
    • by scattering of particles on other particles
  • Results of these interactions are
    • change in flight direction/energy/momentum of original particles
  • production of new particles
introduction4

Detector elements

p1 = -p2

1

2

p2 = 0

1

2

Introduction...
  • These interactions are produced in
  • Goal : measure as many as possible of the resulting particles from the interaction
    • put detector “around” the interaction point
what to measure why
What to measure, why?
  • If we have an “ideal” detector, we can reconstructthe interaction, ie. obtain all possible information on it. This is then compared to theoretical predictions and ultimately leads to a better understanding of the interaction/properties of particles
  • “Ideal detector” measures
    • all produced particles
    • their energy, momentum
    • type (mass, charge, life time, Spin, decays)
measured quantities

Electronic equipment

eg. Geiger counter

  • Its four-momentum

E

p

Energy

momentum in x-dir

momentum in y-dir

momentum in z-dir

=

px

p = py

pz

Measured quantities
  • The creation/passage of a particle ( --> type)
  • Its velocity b = v/c
derived properties

E1,p1

m

E2,p2

Derived properties
  • Mass
    • in principle, if E and p measured:E2 = m2c4 + p2c2
  • if v and p measured:p = m v / (1 - b2)
  • from E and p of decay products:
    • m2 c4 = (E1+E2)2 - (cp1+ cp2)2
further properties

Negative charge

Magnetic field, pointing

out of the plane

positive charge

length

Further properties...
  • The charge (at least the sign…)
    • from curvature in a magnetic field
  • The lifetime t
    • from flight path before decay
so how measure the four momentum

Negative charge

Magnetic field, pointing

out of the plane

Lorentz-force

q v B = m v2/R

p2

positive charge

R1

p1

R2

q B R = m v= p

p1<p2 R1 < R2

v t = L

L

So, how measure the four-momentum?
  • Energy : from “calorimeter” (see later)
  • Momentum :
    • from “magnetic spectrometer+tracking detector”
  • velocity :
    • time of flight or Cherenkov radiation (see later)
principles of a measurement

e-

p

p

p

g

e-

p

  • Change of the particle trajectory
    • curving in a magnetic field, energy loss
    • scattering, change of direction, absorption
Principles of a measurement
  • Measurement occurs via the interaction (again…) of a particle with the detector(material)
    • creation of a measureable signal
      • Ionisation
  • Excitation/Scintillation
detected particles
Detected Particles
  • Charged particles
    • e-, e+, p (protons), p, K (mesons), m (muons)
  • Neutral particles
    • g (photons), n (neutrons), K0 (mesons),
    • n (neutrinos, very difficult)
  • Different particle types interact differently with matter (detector) (eg. photons do not feel a magnetic field)
    • need different types of detectors to measure different types of particles
typical detector concept

Interaction

point

Magnetic

spectrometer

tracking

detector

Hadronic calorimeter

Muon detectors

Electromagnetic calorimeter

Precision vertex

detector

Typical detector concept
  • Combine different detector types/technologies into one large detector system
slide15

Electromagnetic

calorimeter

Muon detector system

Tracking system

Hadronic

calorimeter

Electron e-

Photon g

Hadron, eg.

proton p

Muon m-

Meson K0

tracking detectors
Tracking Detectors
  • Basic goal:
    • make the passage of particles through matter visible --> measure the tracks
  • Reconstruct from the measured space points the flight path
  • Extract information on the momentum(see previous transparencies)
  • NOTE: the particle should not be too much affected by the detector: No dense materials !
this is achieved by
This is achieved by
  • Detectors where
  • Ionisation signals are recorded
    • Geiger-Müller counter
    • MWPC(Multi-Wire Proportional Chambers)
    • TPC (Time Projection Chamber)
    • silicon detectors
    • Bubble chambers (see separate lecture)
  • Scintillation light is produced
    • eg. scintillating fibers
principle of gaseous counters

-

+

cathode

-

+

-

+

-

+

-

+

t = 0

signal

Anode Wire

+

Gas-filled tube

+

+ HV

+

+

-

+

-

-

-

-

t = t1

Principle of gaseous counters
  • Track ionises gas atoms
  • electrons drift towards anode, ions towards cathode
  • around anode electrons are accelerated (increasing field strength)
  • further ionisation --> signal enhancement --> signal induced on wire
now tracking

Anode wires

Cathode: pads or wires

Realization:

wire chamber

(MWPC)

Nobel prize: G.Charpak, 1992

y

x

Now : Tracking
  • Basic idea : put many counters close to each other
tracking mwpc

ITC (ALEPH)

Inner Tracking Chamber

Tracking: MWPC
further development time projection chambers tpc

MWPC

gives r,f

E

B

-

-

+

-

+

+

-

+

-

Anode Wires

-

+

+

Gas-filled cylinder

z = vdrift t

Further development:Time Projection Chambers (TPC)

-

-

-

-

-

-

-

limitations

Uncertainties on space points

Uncertainties on track origin and

momentum

Limitations
  • Precision limited by wire distance

Error on space point  d cannot be reduced arbitrarily!

step forward silicon microstrip detectors

0.2 - 0.3 mm

Now precision limited by strip distance

10 - 100 mm

Silicon wafers, doped

Creation of electron-hole pairs

by ionising particle

Same principle as gas counters

Step forward:Silicon Microstrip Detectors
silicon microstrip detectors27

OPAL VDET

Future ATLAS tracking detector

ALEPH VDET

Silicon Microstrip detectors...
increase in precision
Increase in precision

=Beam crossing point

0

1cm

x

scintillating fibers

Total reflection

Photomultiplier: converts

light into electronic signal

PM

Scintillating

material

Put many fibers close to each other

--> make track visible

Scintillating fibers
  • Certain materials emit scintillation light after particle passage (plastic scintillators, aromatic polymers, silicate glass hosts….)
calorimetry
Calorimetry
  • Basic principle:
    • In the interaction of a particle with dense material all/most of its energy is converted into secondary particles and/or heat.
  • These secondary particles are recorded
    • eg. Number, energy, density of secondaries
    • this is proportional to the initial energy
  • NOTE: last year calorimetry was discussed in detail in talks prepared by teachers
electromagnetic showers
Electromagnetic showers
  • Interactions of electrons and photons with matter:

Lead atom

Matter

block, eg.

lead

  • Shower partially or completely absorbed
how to measure the secondaries

Sandwich structure !

Total amount of signals

registered is proportional

to incident energy.

But has to be calibrated

with beams of known

energy!

Detectors, such as wire chambers,

or scintillators

Dense blocks, such as lead

How to measure the secondaries?
  • 1. With sampling calorimeters:
slide36

e+

e-

slide38

ALEPH ECAL

pions

electron

slide39

photons

muons

how to measure the secondaries40

signal

Photo diode

photons

Crystal (BGO, PbWO4,…)

How to measure the secondaries?
  • 2. With homogenous calorimeters, such as crystal calorimeters:

Note : these crystals are also used in other fields (eg. Medical imaging, PET)

hadronic calorimeters

Sandwich structure !

Total amount of signals

registered is proportional

to incident energy. Same energy lost in nuclear excitations!

Has to be calibrated

with beams of known

energy!

Detectors, such as wire chambers,

or scintillators

Dense blocks, such as iron, uranium

Hadronic calorimeters
  • Hadronic particles (protons, neutrons, pions) can traverse the electromagnetic calorimeters. They can also interact via nuclear reactions !
  • Usually: Put again a sampling calorimeter after the ECAL
slide44

iron

ALEPH 

particle identification
Particle Identification
  • Basic principles:
    • via different interaction with matter (see previous transparencies)
    • by measuring the mass from the decay products
    • by measuring the velocity and independently (!) the momentum
    • Observables sensitive to velocity are
      • mean energy loss
      • Cherenkov radiation
mean energy loss

 Elost  / path length = func( particle-velocity v/c )

Bethe-Bloch formula

Mean energy loss
  • Particles which traverse a gas loose energy, eg. by ionization
  •  Elostamount of ionization size of signals on wires

Note : if plotted as a function

of v and not p all the bands

would lie on top of each other!

cherenkov radiation

Cherenkovlight

wavefront

Compare : shock wave of supersonic airplanes

c0 = speed of light in vacuum

Cherenkov radiation
  • Particles which in a medium travel faster than the speed of light in that medium emit a radiation --> Cherenkov radiation

See http://webphysics.davidson.edu/applets/applets.html for a nice illustration

large detector systems
Large detector systems
  • All these concepts have been put together and realized in large detector systems
  • Examples at LEP
    • ALEPH , OPAL , L3 , DELPHI
  • Fixed Target
    • NA48
  • Future experiments at LHC
    • ATLAS, CMS, LHCb, ALICE
summary
Summary
  • I have tried to explain
    • what are the things we want to measure in HEP experiments
    • how we do it (tracking, calorimetry, particle identification)
  • This is an enormously large field, of course many things have been left out
    • DAQ (data acquisition)
    • other detector technologies
    • applications in particle astrophysics(cosmic rays, neutrinos,…)
    • applications outside HEP
  • I invite you to study these points