Nuclear and Particle Physics Franz Muheim 1 Accelerators and Accelerators and Detectors Detectors Accelerators Linear Accelerators Cyclotrons and Synchrotrons Storage Rings and Colliders Particle Physics Laboratories Interactions of Particles with Matter Charged Particles Neutral Particles, Photons Detectors in Particle Physics Position sensitive devices Calorimeters Particle Identification Experiments Mainly at colliders Outline SVT SVT 1.5-T Solenoid
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Accelerators and Detectors - School of Physics and …muheim/teaching/np3/lect-exp.pdfNuclear and Particle Physics Franz Muheim 1 Accelerators and Detectors Accelerators Linear Accelerators
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Nuclear and Particle Physics Franz Muheim 1
Accelerators and Accelerators and DetectorsDetectors
AcceleratorsLinear AcceleratorsCyclotrons and SynchrotronsStorage Rings and CollidersParticle Physics Laboratories
Interactions of Particles with MatterCharged ParticlesNeutral Particles, Photons
Detectors in Particle PhysicsPosition sensitive devicesCalorimetersParticle Identification
ExperimentsMainly at colliders
OutlineOutline
SVTSVT1.5-T Solenoid
Nuclear and Particle Physics Franz Muheim 2
Particle AcceleratorsParticle Accelerators
Accelerator PrincipleCharged particles are accelerated to high energies using electromagnetic fieldse-, e+, p, anti-p, ionised nuclei, muons
Why are Accelerators used?Higher energies or momentaallow to probe shorter distancesde Broglie wavelength
e.g. 20 GeV/c probes 0.010 fmCockroft-Walton Accelerator
High DC voltage accelerates particles through steps created by a voltage dividerLimited to ~ 1 MV
Fermilab InjectorIonised hydrogen, H2
- sourceaccelerated to 750 kV
Van de Graaff Acceleratorcharge transported by belt Limited to ~ 10 MV
[GeV/c] fmMeV 197
ppcc ⋅==
hλ
Nuclear and Particle Physics Franz Muheim 3
Linear Accelerators Linear Accelerators -- LinacLinac
Working Principle - LinacCharged particles in vacuum tubesaccelerated by Radio Frequency (RF) waves
RF tubes increase in length size aparticle speed increases (protons, for e- v≅c)
Radio Frequency AccelerationRadio Frequency fields O(few 100 MHz) Field strengths – few MV/m - klystronstransported by RF cavities Oscillating RF polarities producesuccessive accelerating kicksto charged particles when RF is deceleratingparticles shielded in RF tubesParticles in phase with RF
Fermilab Injector400 MeV protons, 150 m long
Stanford Linear Accelerator (SLAC)Largest Linac - 3 km long, 50 GeV e- and e+
Nuclear and Particle Physics Franz Muheim 4
Circular AcceleratorsCircular Accelerators
Cyclotron
Charged particles are deflected in magnetic field B Lorentz Force
radius of curvature ρ
Particle accelerated by RFin magnet with E perp. BProtons, limited to ~ 10 MeVrelativistic effects
SynchrotronB-field and RF synchronised SpS at CERNwith particle speedradius ρ stays constantSuperconducting dipole magnetsB-fields up to 8 TeslaQuadrupole magnets focus beamalternate focusing and defocusingin horizontal and vertical plane
Synchrotron RadiationAccelerated particles radiateEnergy loss per turnmost important for e-
CyclotronCyclotron
SynchrotronSynchrotron
BveFL
rrr∧=
[ ] [ ] [ ]mTBcGeVp ρ3.0/ =
ργβπ 432
34 eE =∆
Nuclear and Particle Physics Franz Muheim 5
Storage Ring Storage Ring -- CollidersColliders
Beams from synchrotron or linachave bunch structure
Secondary BeamsAccelerated beam from synchrotron or linacon target → e, µ, π, p, K, n, ν, ZAX beamsMany different types of experiments
Storage RingsParticle beams accelerated in synchrotron and stored for extended periods of time
CollidersTwo counter-rotating beams collide at several interaction points around a ring Luminosity
FermilabChicago, http://www.fnal.govTevatronCurrent highest ECoMenergy collider1 TeV p on 1 TeV anti-pmaximum 1012 anti-p
Ni # of particles in beam InB # of bunches/beamrx,y beam dimension in x,yf revolution frequency
N0 - Avogadro’s numberZ, A - atomic and mass number of medium x - path length in medium in g/cm2
x = ρt mass density ρ and thickness t in cmdE/dx measured in [MeV g-1 cm2]α - fine structure constant re = e2/4πε0mec2 = 2.82 fm (classical e radius)β, γ - speed and Lorentz boost of charged particle Maximum energy transfer TmaxMean excitation energy I
Valid for “heavy” particles (m≥mµ)
e- and e+ (mproj = mtarget) → BremsstrahlungdE/dx ~ 1/mtarget → scattering off nuclei very small
⎥⎦
⎤⎢⎣
⎡−−−=
22ln14 2
2
max222
21
2222 δβ
γββ
πI
TcmAZzcmrN
dxdE e
eeA
( ) eV10 with //21
2002
222max =≈
++= IZII
MmMmcmT
ee
e
γγβ
e-
θ
khh ,ω
0,mvr
Bethe-Bloch
Nuclear and Particle Physics Franz Muheim 10
BetheBethe--BlochBloch
⎥⎦
⎤⎢⎣
⎡−−−=
22ln14 2
2
max222
21
2222 δβ
γββ
πI
TcmAZzcmrN
dxdE e
eeA
dE/dx Energy Dependenceonly on βγ independent of mprojFor small β dE/dx ∝ 1/β2
Momentum MeasurementCharged particle trajectories are curvedin magnetic fieldsmeasure transverse momentum
Tracking Detectors (before 1970)mostly optical tracking devices - cloud chamber,bubble chamber, spark chamber, emulsionsSlow for data taking (triggering) and analysis
Geiger-Mueller CounterIonisation in gasO(100 e- ion-pairs/cm)Avalanche multiplicationnear wire with gain up to 106
Multi-Wire-Proportional Chamber MWPC
Many wires in a planeact as individual countersTypical dimensions: L = 5 mmd = 1 mm, a = 20 µmsignal ∝ ionisationFast, high rate capabilitySpatial resolution limitedby wire spacing
Energy measurementBetter resolution forelectromagnetic shower
BaBar experiment6580 CsI (Tl) crystals
2
2
2
2
0
22
31
183ln4
14mE
ZE
mcez
AZN
dxdE
A ∝⎟⎟⎠
⎞⎜⎜⎝
⎛=−
πεα
Nuclear and Particle Physics Franz Muheim 16
Particle IdentificationParticle Identification
How do we measure Particle type?Uniquely identified by its mass mParticles have different interactionsMomentum p = mγβc of charged particles measured with tracking detectors
Electrons, PhotonsElectromagnetic Calorimeter (crystal)Comparison with momentum (electron)Shower shape (electrons, photons)
Charged Particle IdentificationHave momentum p = mγβc γ2 =1/(1-β2)Need to measure particle velocity v = βcCharged particles radiate Cherenkov photons in medium with speed v larger than c/n (refr. index n)
Cherenkov anglemeasures v = βc
Ring Imaging Cherenkov Detector
MuonsMost penetrating particle little Bremsstrahlunga few metres of iron and only muons are left