particle detectors günther dissertori cern-ep cern teachers seminar july 2001
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PARTICLE DETECTORSPARTICLE DETECTORS
Günther DissertoriCERN-EP
CERN Teachers Seminar
July 2001
OutlookOutlook
IntroductionWhat to measure, why?Basic Principles
Tracking Calorimetry Particle Identification
Large detector systemsConclusions
IntroductionIntroductionHE 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
Introduction...Introduction...These interactions are produced in
1 2
p2 = 0
GoalGoal : measure as many as possible of the resulting particles from the interaction put detector “around” the interaction point
p1 = -p21 2
Detector elements
What to measure, why?What to measure, why?
If we have an “ideal” detector, we can reconstruct the 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
If we have an “ideal” detector, we can reconstruct the 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)
“Ideal detector” measures all produced particles their energy, momentum type (mass, charge, life time, Spin, decays)
Measured quantitiesMeasured quantities
The creation/passage of a particle ( --> type)Electronic equipment
eg. Geiger counter
Its four-momentum Energymomentum in x-dirmomentum in y-dirmomentum in z-dir
E
p=
Its velocity = v/c
px
p = py
pz
Derived propertiesDerived properties
Mass in principle, if E and p measured: E2 = m2 c4 + p2c2
if v and p measured: p = m v / (1 - 2)
from E and p of decay products: m2 c4 = (E1+E2)2 - (cp1+ cp2)2
m
E1,p1
E2,p2
Further properties...Further properties...
The charge (at least the sign…) from curvature in a magnetic field
The lifetime from flight path before decay
Magnetic field, pointingout of the plane
Negative charge
positive charge
length
So, how measure the four-momentum?So, how measure the four-momentum?
Energy : from “calorimetercalorimeter” (see later)
Momentum : from “magnetic spectrometer+tracking detectormagnetic spectrometer+tracking detector”
Magnetic field, pointingout of the plane
Negative charge
positive chargeR1R2
p2
p1
p1<p2 R1 < R2 p1<p2 R1 < R2
q v B = m v2/R
q B R = m v = p
Lorentz-force
velocity : time of flight time of flight oror Cherenkov radiation Cherenkov radiation (see later)(see later)
Lv t = L
Principles of a measurementPrinciples of a measurementMeasurement occurs via the interaction (again…)
of a particle with the detector(material) creation of a measureable signal
IonisationIonisation
Excitation/ScintillationExcitation/Scintillation
Change of the particle trajectoryChange of the particle trajectory• curving in a magnetic field, energy loss• scattering, change of direction, absorption
p
e-
p
e-
pp
Detected ParticlesDetected Particles
Charged particles e-, e+, p (protons), , K (mesons), (muons)
Neutral particles (photons), n (neutrons), K0 (mesons), 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 conceptTypical detector concept
Combine different detector types/technologies into one large detector system
Interaction point
Precision vertex detector
trackingdetector
Magneticspectrometer
Electro
mag
netic calo
rime
ter
Had
ron
ic calo
rimeter
Mu
on
detecto
rs
Trac
king
sys
tem
Ele
ctro
mag
netic
calo
rim
eter
Had
roni
c ca
lori
met
er
Muo
n de
tect
or
syst
em
Electron e-
Photon
Hadron, eg.
proton p
Muon -
Meson K0
Tracking DetectorsTracking Detectors
Basic goalBasic goal: make the passage of particles through
matter visible --> measure the tracks
ReconstructReconstruct from the measured space points the flight path
Extract information on the momentummomentum (see previous transparencies)
NOTE: the particle should not be too much affected by the detector: No dense materials No dense materials !
This is achieved byThis is achieved by
Detectors where Ionisation signals are recorded
Geiger-Müller counterMWPC (Multi-Wire Proportional Chambers)TPC (Time Projection Chamber)silicon detectors
Bubble chambers (see separate lecture)
Scintillation light is producedeg. scintillating fibers
Principle of gaseous countersPrinciple of gaseous counters
+ HV
signal
cathode
Anode Wire
Gas-filled tubeGas-filled tube
---
--
+++
++ t = 0
- ---
-
+ +++
+
t = t1
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
Principle of gaseous counters...Principle of gaseous counters...
gas filling
Now : TrackingNow : Tracking
Basic idea : put many counters close to each other
Realization:Realization:wire chamberwire chamber
(MWPC)(MWPC)Nobel prize: G.Charpak, 1992Nobel prize: G.Charpak, 1992
Anode wiresAnode wires
Cathode: pads or wiresCathode: pads or wires
x
y
Tracking: MWPCTracking: MWPC
ITC (ALEPH)Inner Tracking Chamber
Further development:Further development:Time Projection Chambers (TPC)Time Projection Chambers (TPC)
Gas-filled cylinderGas-filled cylinder
Anode Wires
MWPC
gives r,
MWPC
gives r,
E
B
- -- - - - -- -
--
--
++
+
+
++
z = vdrift tz = vdrift t
TPCTPC
LimitationsLimitations Precision limited by wire distance
Error on space point d cannot be reduced arbitrarily!
Uncertainties on space points Uncertainties on track origin andmomentum
Step forward:Step forward:Silicon Microstrip DetectorsSilicon Microstrip Detectors
Now precision limited by strip distance 10 - 100 m
Now precision limited by strip distance 10 - 100 m
Creation of electron-hole pairs by ionising particle
Creation of electron-hole pairs by ionising particle
Same principle as gas counters
Silicon wafers, doped
0.2 - 0.3 mm
Silicon microstrip detectors...Silicon microstrip detectors...
Silicon Microstrip detectors...Silicon Microstrip detectors...
ALEPH VDET
OPAL VDET
Future ATLAS tracking detector
Increase in precisionIncrease in precision
0 1cmx
=Beam crossing point
Mean Lifetime of tau =290 x 10-15 sec !! --> c = 87 m !?
Scintillating fibersScintillating fibers Certain materials emit scintillation light after particle
passage (plastic scintillators, aromatic polymers, silicate glass hosts….)
Photomultiplier: converts light into electronic signal
Scintillatingmaterial
Scintillatingmaterial
PM
Total reflection
Put many fibers close to each other--> make track visible
Scintillating fibers...Scintillating fibers...
CalorimetryCalorimetry
Basic principle: In the interaction of a particle with dense
material all/most of its energy is converted into secondary particles and/or heatsecondary particles and/or heat.
These secondary particles are recordedeg. Number, energy, density of secondariesthis is proportional toproportional to the initial energy
NOTE: last year calorimetry was discussed in detail in talks prepared by teachers
NOTE: last year calorimetry was discussed in detail in talks prepared by teachers
Electromagnetic showersElectromagnetic showers
Interactions of electrons and photons with matter:
Matterblock, eg.
lead
Lead atom
Shower partially or completely absorbed
How to measure the secondaries?How to measure the secondaries?
1. With sampling calorimeterssampling calorimeters:
Dense blocks, such as leadDetectors, such as wire chambers,
or scintillators
Sandwich structure !
Total amount of signalsregistered is proportionalto incident energy.
But has to be calibrated with beams of known energy!
Sandwich structure !
Total amount of signalsregistered is proportionalto incident energy.
But has to be calibrated with beams of known energy!
Sampling CalorimetersSampling Calorimeters
e+
e-
ALEPH ECAL
pions electron
muonsphotons
How to measure the secondaries?How to measure the secondaries?
2. With homogenous calorimetershomogenous calorimeters, such as, such as crystal crystal calorimeterscalorimeters:
signal
photons
Note : these crystals are also used in other fields (eg. Medical imaging, PET)Note : these crystals are also used in other fields (eg. Medical imaging, PET)
Photo diode
Crystal (BGO, PbWO4,…)
CMSCMS
L3L3
Hadronic calorimetersHadronic 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
Dense blocks, such as iron, uraniumDetectors, such as wire chambers,
or scintillators
Sandwich structure !
Total amount of signalsregistered is proportionalto incident energy. Same energy lost in nuclear excitations!
Has to be calibrated with beams of known energy!
Sandwich structure !
Total amount of signalsregistered is proportionalto incident energy. Same energy lost in nuclear excitations!
Has to be calibrated with beams of known energy!
ALEPH ALEPH
iron
Particle IdentificationParticle 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 (!)independently (!) the momentum
Observables sensitive to velocity are
mean energy loss
Cherenkov radiation
Mean energy lossMean energy loss Particles which traverse a gas loose energy, eg. by by
ionizationionization
Elost / path length = func( particle-velocity v/c )
Elost / path length = func( particle-velocity v/c ) Bethe-Bloch formula
Elost amount of ionization size of signals on wires
Note : if plotted as a functionof v and not p all the bands would lie on top of each other!
Note : if plotted as a functionof v and not p all the bands would lie on top of each other!
Cherenkov radiationCherenkov radiation
Particles which in a medium travel faster than the faster than the speed of light in that mediumspeed of light in that medium emit a radiation
--> Cherenkov radiationCherenkov radiation
v
1
nvsin 0cc
v
1
nvsin 0cc
c0 = speed of light in vacuum
Cherenkovlight
wavefront
Compare : shock wave of supersonic airplanes
See http://webphysics.davidson.edu/applets/applets.html for a nice illustration
Large detector systemsLarge detector systems
All these concepts have been put together and realized in large detector systems
Examples at LEP ALEPH , OPAL , L3 , DELPHIALEPH , OPAL , L3 , DELPHI
Fixed Target NA48NA48
Future experiments at LHC ATLAS, CMS, LHCb, ALICEATLAS, CMS, LHCb, ALICE
SummarySummary I have tried to explain
what what are the things we want to measure in HEP experiments
how 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
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