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Particles and Matter

Sarah EnoMD Quarknet9 July 2003Selection and Comments

Jim Linnemann MSU

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Detectors

Goal: produce some sort of detectable signal that depends on the things we want to measure (energy, position, particle type)

• current in a wire

• charge on a capacitor

• light (detectable with photomultiplier, for example)

The physics processes we care about will be the ones that lead to this kind of signal.

References: The Physics of Particle Detectors, Dan Green,Cambridge University Press (2000)Techniques for Nuclear and Particle Physics Experiments, William Leo, Springer-Verlag (1987)Radiation Detection and Measurement, Glenn Knoll, John Wiley and Sons (1979)

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A minor Miracle• Detectors must find 1 particle among 1023

• How???– The particle is very energetic

• So it behaves differently than the others

– The detector is in a special meta-stable state• So the particle disturbs it and causes a physical

change• This amplifies the effect• Then an electronic device can amplify it further

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ParticlesWhat particles do we detect?

But those that are stable on the time scale of a few microseconds when moving fast

Not the fundamental particles

Mostly… electrons, muons, neutrinos, photons (light), charged pions (bound state of ud quarks)

Also occasionally proton, neutron, alpha particle (ionized hydrogen), charged kaon (bound state of su quark)

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Bulk MatterWhat happens when a high speed particle passes through bulk matter?

Non-destructive

• elastic scattering (+ionization)

• scintillation

• Cherenkov radiation

• transition radiation

destructive

•bremsstrahlung

• pair-production

• Nuclear interactions

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Units

is the energy an electron gains when it is accelerated through 1 Volt. Light from the sun

keV → atomic energies X rays

MeV → nuclear energies Gamma Rays

GeV → high energy physics

1eV = 1.6x10-19 J

eV

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Cross section

Measure of the probability for an interaction to occur. Units of area.

1 barn = 10-28 m2

N L

L is the luminosity, which is a measure of the flux of the incident particles (number per area per time)

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Elastic energy When a charged particle passes through a bulk material (say gas, Silicon, etc), it will scatter elastically off the nuclei in the material (change direction) and inelastically off the electrons (lose energy) (we’ll discuss inelastic scattering of nucleii later)

transferred energy can do 2 things: atomic excitation or ionization. Atomic excitation can lead to the production of photons (talk about later), ionization of the bulk material -> freed electrons can be collected to give a current or charge

•tell us if the particle is charged or neutral

• energy loss/length depends on the particles velocity

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Bethe-Bloch Formula

Units

MeV-cm2/g

2 2 3

2 2 20.1535 ( )

is the orbital frequency of the bound electrons

dE MeV Z q mvLn

dx cm g A qe v

v

Because, for the same thickness, you’ll put more energy into a dense material than a non-dense one.

Multiply by the density to get MeV/length.

Lead: p=18 g/cm3

MeV/cm=36

Argon: p=0.0017 g/cm3

MeV/cm=0.0034

Tells you the total energy lost to both elastic and inelastic collisions

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Bethe-BlochWhy that shape?

At low energy-> slower. Spends more time near each nucleus -> more interactions. Higher energy… can’t go faster than c, so doesn’t keep getting smaller once near c

At high energy->relativity

Imagine a heavy particle with mass mp and momentum P incident on an electron at rest. Maximum kinetic energy that the electron receives is: 2

22 2

2 2( )e e

pp p

m P mKE m v

m m

If ignore , KE approaches a fixed value as v->c

This 2 factor accounts for the relativistic rise

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To keep in mind• The constancy of energy loss at high

speed• That means the total loss proportional to

the length of the path traversed

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Electron-Ion Pairs

When energy is transferred from the high energy particle to the bulk material, some of it causes excitations of the atom, and some causes ionization (delta rays).

Typically 1 ion-electron pair per 30 eV of energy lost.

Leo

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Uses

Most commonly used in “tracking” chambers. High energy particle going through gas or silicon. Freed electrons are collected to make a signal. Tells you where the particle is. Can tell you its velocity as well. And maybe radio signals?

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ScintillationAs we discussed, when a charged particle goes through bulk matter, it can excite the atoms of this matter.

Some materials, when they deexcite, emit photons with visible light wavelengths (usually blue, around 400 nm), and these (rather than electrons from ionization) can be collected as the signal, using a photomultiplier tube for example (which converts a photon to a current).

Fluorescent materials: emission of photons with a decay constant of 10-8 s

Leo

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ScintillatorsCommon

Plastics: Polyvinyltoluene, polyphenylbenzene, polystyrene

+ fast, cheap, flexible, easy to machine

- suffer radiation damanage

Inorganic Crystals: NaI, CsI

+ when need precision measurements

- expensive.

Other

Organic scintillator (aromatic hydrocarbon compounds containing linked or condensed benzene-ring structures. ) Organic Crystals (C14H10 (anthracene), C14H12(trans-stilbene), C10H8(naphthalene)) Organic Liquids (P-Terphenyl, PBD,PPO, and POPO, xylene, toluene, benzene, …)

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Total Internal Reflection

c

Particle

Quartz bar

Cherenkov light

Active Detector Surface

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Total Internal Reflection

1 1 2 2

1 1 2

sin( ) sin( )

sin( )

n n

n n

2

11

sin( )n

n

Note: can only happen when n1>n2!

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Scintillator

On display at MoMA in NY until Aug 31 (then on world world tour, “Signatures of the Invisible”, http://www.ps1.org). Scintillator from D0.

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Scintillator

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Uses

• Time of flight counters (measure particle speed)

• calorimeters (measure particle energy)

• D0 tracking system

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Cherenkov RadiationWhen a particle moves through bulk matter with a speed faster than the speed of light in that medium, it emits radiation (well, not really. Particles moving at constant speed don’t radiate. But, its field interacts with the

medium, which emits photons) in an electro-magnetic analog of the “sonic boom” that happens when a jet moves faster than the speed of sound or around the bow of a boat moving faster than the speed of sound in water. (happens for any kind of wave, not just sound!)

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Cherenkov Radiation

Radiation is emitted at a fixed angle to the particle. Cone shape. When photons hit flat surface, make circle. Size of circle depends on particle speed. However, can’t do a good measurement as ->1

/ 1cos

c n

v n

Hygen’s PrincipalGreen

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Cherenkov

1max

1/

cos (1/ ) 1

n

n

Minimum speed for emission

n-=1.33 (water)

>0.752

max=41 degrees

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Cherenkov

Energy loss for a solid around 10-3 MeV cm2 g-1. Photons typically in the visible light frequencies.

About 1 part per thousand of the ionization energy loss

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Cherenkov Radiation

BaBar

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Uses

Mostly used to get particle type (tell kaons from pions)

Cosmic Rays: look at extensive air showers directly

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Destructive Measurements

Typically happen with the aid of a heavy nucleus

Only way to measure neutral particles.

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Bremsstrahlung

e

Energy lost to this goes as 1/M4, so basically only happens to electrons (happens also for very very high energy muons (100 GeV))

When in the presence of a heavy nuclei, a particle can “bremsstrahlung” off a high energy photon (gamma ray). Because the electron undergoes a large acceleration due to nuclei’s field.

Mass of muon is 105 MeV, electron is 0.51 MeV.

How does the radiation loss for muons compare to electrons?

40,000

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Bremsstrahlung

The photon is not low energy! It is (almost) equally probable for the photon to have any fraction of the electrons energy, from 0% to 100%. On average, thus, will get about ½ the electron’s energy.

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Bremsstrahlung (“Brem”)

Happens more often in materials with heavy nucleii

Material distance an electron loses 63% of its energy to brem

Be 35 cm

C 19 cm

Al 9 cm

Pb 6 cm

U 0.3 cm

Air 30 m (at sea level)

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Pair Production

When in the presense of a heavy nucleii, a photon with energy above 1.022 MeV can turn into an electron-antielectron (positron) pair. On average, each will have ½ the photons energy.

Why 1.022 MeV?

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DestructiveCalorimeter: use these two processses to measure the energies of electrons and photons. More from Greg later this week.

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Nuclear InteractionsHard to predict the cross section, properties of these interactions from first principles, because they involve the strong force.

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Nuclear Interactions

Rarer than, for example, brem. For example, a high energy electron will lose 64% of its energy to brem every 6.4 g/cm2 in Pb, while a high energy pion will do the same via nuclear interactions only every 194 g/cm2.

A puzzle: nuclear interactions are due to the “strong” interaction.

So why are they rarer than the electromagnetic interactions?

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Nuclear Interactions

Messier than Brem.

Green

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Nuclear Interactions

Sometimes the interactions can produce neutral pions, which decay to two photons. These photons will then pair-produce, brem, pair-produce just like photons that came from the interaction point.

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Nuclear interactionsStudy empirically, using accelerator data

Green