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Methods of Experimental Particle Physics

Alexei Safonov

Lecture #11

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SEAN YEAGERResearch Topic Assignment

Multiple Scattering Charged particle passing through a

material Coulomb scattering Hadronic projectiles will also scatter

strongly Distribution described by Moliere We use θ0 as a measure of how big each

deflection is

Defining θ0

β is the particle's velocity p is the particle's momentum z is the particle's charge x/X0 is the thickness of the material in

terms of scattering lengths Roughly Gaussian for small θ0

Rutherford scattering for large θ0 (bigger tails correspond to backscattering)

Visual Representation

θ is the exit angle ψ is the line from entry to exit y ~ xψ s is perpendicular to ψ

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ALEXX PERLOFFResearch Topic Assignment

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MAIN LECTURE STARTS HERE

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Today• We mostly finished basics of particle interactions

• So far we cared about what happens with the particle (energy losses, stopping power etc.)

• This time: • Talk about effects on the media from passing particle

• Cherenkov radiation• Scintillation• Transitional radiation

• Reminders of basics:• Measurement of the momentum for a particle in magnetic field

• Next time we will start talking about actual detectors• Types, characteristics etc

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Charged Particle in Magnetic Field• A charged particle

with momentum p moves in a helix

• is the part of p transverse to B

• B is in Tesla• R is in meters• z is +1 or -1 charge

• If you can map particle’s trajectory, you can solve for p

• It’s not velocity• To measure velocity,

one needs to know the mass of the particle

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Charged Particle Tracking• If you have a detector that can find

positions of the particle’s trajectory moving in magnetic field at several points: • “Reconstruct” R and use B to get momentum p • Often more convenient: curvature k=1/R

• Resolution if you • For p=1 TeV: k=0.3B/103

• To have dp=10%: • dk=0.1k=0.3x10-4B

• For L~1m, N=10, e=1mm:• 0.3x10-4B = 10-3/1 x 7

• Need B=10 x7 /0.3= 200 T • 200 T is insane! Then one need either much larger L or

much better e• Typical resolutions are some tens to hundreds of microns (need

only 2T for 10 micron resolution)

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Scintillation• When a charged particle passes

through media, it excites molecules• Some materials will emit a small

fraction of this energy as optical photons• Various plastic scintillators

(polystyrene) are frequently used in particle physics

• If the media is optically transparent, you get light propagating inside the material• What if you can detect the light? Then you can tell there was a

particle passing through it• But photons can get re-absorbed

• Attenuation length – how much a photon will travel before it is reabsorbed

• Want to pick materials which don’t have absorption wave lengths close to that of the emitted photons

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Scintillation• The problem is that most good

scintillators are not “transparent enough”• Small attenuation lengths

• Solution: add “waveshifters” into the mix • Scintillators are usually composites

• Waveshifter is material that absorbs primary photons and re-emits photons• Often more than one waveshifter

• Goal: get lots of light that can propagate far• Larger detectors are cheaper than small ones

• Mix wave-shifters to make a composite material to optimize how much light you get (you want more), photons wave length (depends on how you detect it) and the attenuation length (want large enough so it propagates far enough to get collected)

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Scintillators• Scintillator crystals (inorganic scintillators):

• They are more dense which can come handy • Have dopants, e.g. NaI crystal with thallium dopant (Tl)

• A simple detector to detect charged particles

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Scintillator Detectors

• Single charged particle detector

• Coincidence• To reduce noise

• Or even “sandwiches”:• Calorimeter

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Photomultipliers (PMT)• One of the oldest detectors of photons:

• Something you would need if you wanted to collect the light from a scintillator

• Details matter (material for the window: quartz, glass, what kind, amplification, operating voltage)

• Now solid state PMTs are becoming more and more used • Silicon based (SPMT)

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Cherenkov Radiation• A particle moving in media with the speed higher

than the speed of light in the media• Media: electrically polarizable dielectric

• Similar to the supersonic “boom”

• As the charged particle passes, it polarizes media

• It gets back by emitting a photon• Usually collective effects are

random and no light• But if the wave of emissions

happens faster than the speed of light, can get a collective effect of coherent constructive interference

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Cherenkov Radiation• The emitted spectrum:

• Mostly in UV part• Visible part appears to

be blue• Important as e.g. glass

window of a PMT will kill most of the signal

• Reacts to velocity (not momentum like tracking) • Can be used as a single

threshold detector

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Imaging Detectors• As Cherenkov light

comes in cones, can arrange a detector in which you will see rings• Ring Imaging

Cherenkov (RICH) Detector

• Angle depends on the velocity of the particle• Can distinguish

particles and measure their velocities

• LHCb detector has RICH

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Transition Radiation• When a relativistic charged particle crosses a boundary

of two media with different refractive indices• Number of photons when a particle crosses a border of

vacuum and media:

• The number is typically not very large (but grows with gamma) • Solution is to use multiple surfaces• Many hundreds of very thin foils

• e.g. polypropylene to not reabsorb photons• X-rays can be detected and will signify that there was indeed a

particle

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What Else?• A number of technologies exploring what we have

already learnt about ionization• Semiconductor detectors – essentially a diod which you use as

an ionization chamber• Ionization creates currents

• Gazeous detectors – chambers with little material where ionization electrons/ions are made to form avalanches and create currents

• Neutrons/protons detected via nuclear interactions (create showers including charged particles which we know how to register)• Talked about it last time

• A number of early detectors relying on ionizatoin no longer used• Cloud/bubble etc. chambers - slow and difficult to maintain and

operate