March 2011Particle and Nuclear Physics,1 Experimental tools accelerators particle interactions with matter detectors.

March 2011Particle and Nuclear Physics ,1

Experimental tools

• accelerators

• particle interactions with matter

• detectors


Accelerators

• Need high energy to: (1) create new particles; (2) explore structure of hadrons. For (2) need high momentum p=h/ to get to small .

• Till 1950 – cosmic rays the only source of high energy particles. Now – accelerators: (a) same projectile; (b) control their energy; produce monoenergetic beams to study energy dependence.

• Once have beams – need target: (a) fixed target experiments; (b) colliding beams.


Accelerators (2)

Principle of acceleration: use em forces to boost energy of stable charged particles (protons, electrons). Injection into machine from high intensity source of low energy particles – electron gun (filament) or proton ion source.

Electrostatic accelerator: Van de Graff. Limitation – up to 10 MeV (‘tandem’).

Much cheaper solution: Cockroft-Walton, up to 0.75 MeV, used as injectors to larger accelerators. Works on the principle of multistage voltage multiplier (used in all B&W TV sets and in the early color sets).


Cockroft-Walton


Accelerators (3)Two systems of acceleration: (a) linear accelerators – Linacs, (b) circular acceleratots – Synchrotrons.

Principle of Linac: many accelerations by linearly moving the beam along a line of acceleration sources.

Principle of Synchrotron: move beam (in a circle) many times through a given acceleration source.


LinacsIon injected into acceleration tube containing many drift-tubes. Accelerated in the gap between the drift-tubes – if voltage in proper phase. Length of tubes varies.

SLAC (Stanford, Cal.)– 2 mile linac

Stanford Linear Accelerator Center

80,000 drift tubes

Klystrons produce a higher energy version of microwaves, and these em waves push the electrons in the linac. Microwaves from the Klystrons are fed into accelerator via waveguides.


CyclotronCyclotron – repeated passage through same accelerating voltage. Fixed magnetic field guides particle in almost circular orbit

=p/(qB)=p[GeV]/(0.3q[e]B[T])

≤ 100 MeV


SynchrotronSynchrotron – fixed orbit and changing magnetic field. Change in B synchronized with increasing energy.

=p/(qB)=p[GeV]/(0.3q[e]B[T])

Conventional bending magnets: 1.5 T. Superconductive magnets – up to 5 T. Radius very large: Fermilab– 1km, HERA – 1km, LHC – 4.3km.

Fermilab (Batavia, Illinois): max energy – 1000 GeV = 1 TeV → Tevatron. 1000 superconductive magnets, cooled to 4.5° K, need 4000 l of He per hour.

Intensity of beam is about 1011 – 1013 particles/sec, limited by mutual repulsion. Particles make up to 105 revolutions before reaching max energy →need stability in orbit to remain in phase for acceleration → need focusing.


Circular accelerators (2)Focusing of beam (‘beam optics’) done by focussing (quadrupole) magnets:

Synchrotron radiation

Under circular acceleration, charged particles emit synchrotron radiation. The energy radiated per particle per turn 2 3 4

43qRE

since = E/m → 1/m4 → severe losses for electrons → circular electron machine ≤ 100 GeV (LEP, ≈ 0.2 GeV for E = 92 GeV.)

For protons – important only in TeV region. In LHC (p p, 7 TeV per beam) – power loss 4 kW per beam.


Secondary beams

In fixed target experiments, proton beam hits target nuclei – many particles are produced, mostly ’s but also K’s, p’s or ’s. Use collimators to select particles in given direction. Can also use magnets to select positive or negative particles with a given momentum (bending angle depends only on momentum), then use electrostatic fields + bending and focusing magnets to produce monoenergetic beam of secondary particles.


Fermilab


Neutral beams

Neutrino beams: decay in flight + → + + , will essentially follow direction of initial . Use very long absorber to stop ’s and ’s. Get a wide-band beam of .

Photon beams: (a) from electron beam – by bremsstrahlung (e p → e p ). Photon energy can be obtained by tagging the scattered electron.

(b) Backscattered Compton ( e + → + e): low energy photons from a laser (eV to keV) collide head-on with high energy electrons. The backscattered photons take most of the initial electron energy. They also retain their initial polarization.


Fixed target machines

After acceleration, beam is extracted on a stationary target – usually a solid or liquid. Large number of interactions produced. Produce secondary beams.

1 12 22 2( 2 ) (2 )cm B T T B T BE m m m E m E

Main disadvantage: center of mass energy is proportional to √Elab:


Colliding beamsFor fixed target machines Ecm ≈ (2 mT EB)½. For colliding two beams in opposite directions (neglecting masses):2 2

1 2 1 2 1 2 1 2

1 2

( ) 2 2( )

2cm

cm

E p p p p E E p p

if E E E E E

Colliders currently operational or under construction:


Colliders


Colliders (2)


Colliders (3)


Colliders (4)

Advantages of colliders: gets to much higher cms energies.

Disadvantages:

• can use only stable charged particles

• collision rate in the intersection region is low.

The reaction rate is given by R = L , is the interaction cross-section, L is the luminosity (in cm-2 s-1). For two oppositely directed beams of relativistic particles:

1 2N NL fn

A

N1 and N2 – number of particles in each bunch, n – number of bunches, f – revolution frequency, A – cross-sectional area of the beams. Typical values of L: 1031-1033 cm-2 s-1.

For fixed target: beam of 1013 protons s-1, hitting 1 m long liquid-hydrogen, luminosity is L ≈ 1038 cm-2 s-1.

March 2011Particle and Nuclear Physics,1 Experimental tools accelerators particle interactions with matter detectors.

Documents

nuclear physics

matter detectors particle

max energy

increasing energy

circular accelerators

acceleratorsneed high

principle of acceleration

study energy dependence