5.3 Nuclear physicsG485 Fields, Particles, Frontiers of

Physics

KS5 OCR PHYSICS H158/H558Mr Powell 2012

Index

5.3.1 The Nuclear Atom

5.3.2 Fundamental

Particles

5.3.3 Radioactivity

5.3.4 Fission and Fusion

Mr Powell 2014

5.3.2 Fundamental Particles

Assessable learning outcomes..

(a) explain that since protons and neutrons contain charged constituents called quarks they are, therefore, not fundamental particles; (b) describe a simple quark model of hadrons in terms of up, down and strange quarks and their respective antiquarks, taking into account their charge, baryon number and strangeness; (c) describe how the quark model may be extended to include the properties of charm, topness and bottomness; (d) describe the properties of neutrons and protons in terms of a simple quark model; (e) describe how there is a weak interaction between quarks and that this is responsible for β decay; (f) state that there are two types of β decay; (g) describe the two types of β decay in terms of a simple quark model; (h) state that (electron) neutrinos and (electron) antineutrinos are produced during β+ and β- decays, respectively; (i) state that a β- particle is an electron and a β+ particle is a positron; (j) state that electrons and neutrinos are members of the lepton family.

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Particle Model...

Copy out this flow chart on A3 paper and add any information you can to the bubbles explained why they are separated as such....

Particles

Leptons: fundamentalParticles e.g. electron,

neutrino

Hadrons: not fundamental,

made from quarks

Baryons: made up ofThree quarks

Mesons: made up of two

quarksGauge Bosons:

Fundamental particles,Force carriers e.g.

photon

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(a) explain that since protons and neutrons contain charged constituents called quarks they are, therefore, not fundamental particles;

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Hadrons

Hadrons are unstable with the exception being the proton-the only stable Hadron.

Hadrons are composed of smaller fundamental particles called Quarks.

Meson have 2 Quarks and Baryons 3. Hence mesons don’t decay to protons or neutrons.

They all have masses much larger than that of leptons.

Some carry charge i.e. (p, K , Kˉ +)

Some have no charge i.e. (n, Ko)

Meson

q

q

Quarks

Baryon

q

q q

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(b) describe a simple quark model of hadrons in terms of up, down and strange quarks and their respective antiquarks, taking into account their charge, baryon number and strangeness;

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(c) describe how the quark model may be extended to include the properties of charm, topness and bottomness;

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Quarks....

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Scale of Quarks

While an atom is tiny, the nucleus is ten thousand times smaller than the atom and the quarks and electrons are at least ten thousand times smaller than that.

We don't know exactly how small quarks and electrons are; they are definitely smaller than 10-18 meters, and they might literally be points, but we do not know.

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Quarks close up...

There are six quarks, but physicists usually talk about them in terms of three pairs: up/down, charm/strange, and top/bottom.

(Also, for each of these quarks, there is a corresponding antiquark.)

Quarks have the unusual characteristic of having a fractional electric charge, unlike the proton and electron, which have integer charges of +1 and -1 respectively.

Quarks also carry another type of charge called color charge. (Not required)

The most elusive quark, the top quark, was discovered in 1995 after its existence had been theorised for 20 years.

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Naming of Quarks..

There are six flavours of quarks. "Flavours" just means different kinds. The two lightest are called up and down. The third quark is called strange. It was named after the "strangely" long lifetime of the K particle, the first composite particle found to contain this quark. The fourth quark type, the charm quark, was named on a whim. It was discovered in 1974 almost simultaneously at both the Stanford Linear Accelerator Centre (SLAC) and at Brookhaven National Laboratory. The bottom quark was first discovered at Fermi National Lab (Fermilab) in 1977, in a composite particle called Upsilon. The top quark was discovered last, also at Fermilab, in 1995. It is the most massive quark. It had been predicted for a long time but had never been observed successfully until then.

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Four main quark configurations

The Proton

Made up of two ‘up’ quarks and a ‘down’ quark.

u

du

The Pion (π+)

Made up of an ‘up’ quark and an ’anti-down’ quark.

u

d

The Neutron

Made up of 2 ‘down’ quarks and an ‘up’ quark.

u

d

d

The Kaon (K+)

Made up of an ‘up’ quark and an ‘anti-strange’ quark.

u

s

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Extension on Quarks (not required AS)

In fact the model predicted all sorts of strange particles some have been discovered and some have not!

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(c) describe how the quark model may be extended to include the properties of charm, topness and bottomness;

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(d) describe the properties of neutrons and protons in terms of a simple quark model;

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(e) describe how there is a weak interaction between quarks and that this is responsible for β decay;

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Neutrino (wider reading)

Neutrinos are elementary particles that travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos have a minuscule, but nonzero mass. They are usually denoted by the Greek letter (nu)

Created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms.

There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos (not needed for AQA; each type also has an antimatter partner, called an antineutrino.

Are generated whenever neutrons change into protons or vice versa, the two forms of beta decay. Interactions involving neutrinos are generally mediated by the weak force (rad decay)

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Neutrino (wider reading)

Neutrinos are elementary particles that travel close to the speed of light, lack an electric charge, are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos have a minuscule, but nonzero mass. They are usually denoted by the Greek letter (nu)

Created as a result of certain types of radioactive decay or nuclear reactions such as those that take place in the Sun, in nuclear reactors, or when cosmic rays hit atoms.

There are three types, or "flavors", of neutrinos: electron neutrinos, muon neutrinos and tau neutrinos (not needed for AQA; each type also has an antimatter partner, called an antineutrino.

Are generated whenever neutrons change into protons or vice versa, the two forms of beta decay. Interactions involving neutrinos are generally mediated by the weak force (rad decay)

Neutrino

Fundamental

No charge

Created in Sun or Collisions

Low cross section

Produced in Weak n->p

Three Flavours

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(f) state that there are two types of β decay;

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(h) state that (electron) neutrinos and (electron) antineutrinos are produced during β+ and β- decays, respectively;

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The Neutrino..

These lines represent the energy levels each beta radiation was giving out

The gap between the energy of the beta radiation and the constant energy value is the amount of energy the neutrino must take up to agree with the ‘conservation of energy’ laws.

How where they first predicted?

Wolfgang Pauli saw that beta radiation did not give off any fixed energy value, he suggested the Neutrino was emitted with the high energy electron to keep the energy level in Beta decay as shown below;

e

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Energy Level ideas..

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Energy Level ideas..

For each emission the energy is constant overall but the share changes for each emission.

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(g) describe the two types of β decay in terms of a simple quark model;

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(j) state that electrons and neutrinos are members of the lepton family.

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(j) state that electrons and neutrinos are members of the lepton family.

5.3.2 Fundamental particles

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Matter & Antimatter Basics

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What is Antimatter?

Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge.

The laws of nature are very nearly symmetrical with respect to particles and antiparticles. For example, an antiproton and a positron can form an antihydrogen atom, which has almost exactly the same properties as a hydrogen atom.

Particle-antiparticle pairs can annihilate each other, producing photons; since the charges of the particle and antiparticle are opposite, charge is conserved. For example, the antielectrons produced in natural radioactive decay quickly annihilate themselves with electrons, producing pairs of gamma rays.

Although particles and their antiparticles have opposite charges, electrically neutral particles need not be identical to their antiparticles. The neutron, for example, is made out of quarks, the antineutron from antiquarks, and they are distinguishable from one another because neutrons and antineutrons annihilate each other upon contact.

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Antimatter SummaryFor each particle of matter there is an equivalent antiparticle. A few particles (e.g. photons) are their own antiparticles.

Antimatter consists of antiparticles. An antiparticle and a particle pair can be produced from a photon of high-energy radiation, which ceases to exist as a result.

An antiparticle has:

1. equal but opposite spin to its particle counterpart (not req AS)

2. equal but opposite charge to its particle counterpart if its particle counterpart is charged;

3. a mass (rest energy) equal to the mass of its particle counterpart.

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AnnihilationIn which a particle and a corresponding antiparticle collide and annihilate each other, producing two photons of total momentum and total energy equal to the initial momentum and energy of the particle and antiparticle, including their combined rest energy 2mc2.

protonantiproton

photon

photonWe can use the proton as an example of this....

If we take mass of a proton 1u = 1.66 x 10-27kg.

Then we can say that the energy of the proton (at rest) and antiproton is found as;

E = mc2 = 1.67 x 10-27kg x (3.00 x 108 ms-1)2= 1.503 x 10-10J

E= 1.503 x 10-10J/ 1.6 x 10-19 JeV-1 = 939MeV or 0.939GeV so Total = 1878MeV.

This energy will then be split between the two. The energy contained in the two photons must be double this or 2mc2 = 2 x 939MeV photons in opposite directions

NB: properties such as charge, spin, and lepton or baryon number are equal but opposite for particles and their antiparticles.

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Electron- Positron AnnihilationConserved quantities in electron–positron annihilation

Energy is conserved

= kinetic energy of particles+ rest energy of particles

total energy before

minimum value of energybefore is rest energy:

= 2 mc2 = 2 0.511 MeV

total energy after

energy after isenergy of gammaphotons

= 2 0.511 MeV

=

Momentum is conserved

=total linear momentum before total linear momentum after

Charge is conserved

e– e+

same mass; equal andopposite velocities

energy E,momentum p = E/c

photons identical,momenta opposite

total momentum before = 0 total momentum = 0

=total charge before total charge after

charge

(–e) + (+e) = 0

charge

0 + 0 = 0

e–Simplify:assume head-oncollision withequal speeds

e+

Conserved quantities in electron–positron annihilation

Energy is conserved

= kinetic energy of particles+ rest energy of particles

total energy before

minimum value of energybefore is rest energy:

= 2 mc2 = 2 0.511 MeV

total energy after

energy after isenergy of gammaphotons

= 2 0.511 MeV

=

Momentum is conserved

=total linear momentum before total linear momentum after

Charge is conserved

e– e+

same mass; equal andopposite velocities

energy E,momentum p = E/c

photons identical,momenta opposite

total momentum before = 0 total momentum = 0

=total charge before total charge after

charge

(–e) + (+e) = 0

charge

0 + 0 = 0

e–Simplify:assume head-oncollision withequal speeds

e+

NB: Assume equal collision speeds

Mass proton 1u = 1.66 x 10-27kg.

Mass of electron = (1/1840)u

What is the rest energy in Joules and MeV for an electron?

E=8.12 x 10-14JE = 0.507MeV

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Pair Annihilation & Creation

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Pair Production

In which a high-energy photon produces a particle and its antiparticle.

This can only occur if the photon energy E= hf = hc/ is greater than or equal to 2mc2,

where m is the mass of the particle, with rest energy mc2 for each particle of the pair produced.

More generally, particles are always created in particle–antiparticle pairs. The masses of particles and their antiparticles are identical.

All other properties, such as charge, spin, lepton or baryon number, are equal but opposite in sign.

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Pair Production

In which a high-energy photon produces a particle and its antiparticle. This can only occur if the photon energy hf is greater than or equal to 2mc2, where m is the mass of the particle produced, with rest energy mc2 for each particle of the pair.

Using the diagram above as an example if we take mass of an electron to be 1u/1840 = 9.11 x 10-31kg

Then we can say that the energy to produce an electron (at rest) is found as;

E = mc2 = 9.11 x 10-31kg x (3.00 x 108 ms-1)2= 8.199 x 10-14J

E= 1.503 x 10-10J/ 1.6 x 10-19 JeV-1 = 0.51MeV

The energy contained in the particle and antiparticle must be double this or

2mc2 = 2x 0.51MeV= 1.02MeV.

This energy will then be split between the particles. Hence the gamma ray photon must have at least this energy to produce these particles.

positron e+

electron e-

Gamma ray

nucleus

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Carl Anderson – Evidence of Positron?

Positron enters and is slowed by lead plate. Then curvature increases.

Beta particle would curve in the other direction.

Lead platePositron e+ arrives

e+ loses energy in lead plate, slows, curves more.

Magnetic Field into the page makes particle follow a curved path

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Bubble Chambers.. (Wider Reading)

The development of bubble chambers in the 1950s allowed particle physicists to ‘see’ particle interactions more easily and more rapidly than earlier work which used cloud chambers or photographic emulsions.

Many bubble chambers consisted of liquid hydrogen which is held at its boiling point. When the pressure is reduced the liquid becomes ‘superheated’ (a strange concept at –253 °C!) and bubbles will form on any ions in the liquid.

The passage of a charged particle through the chamber produces ions in the liquid and the bubbles formed on the ions trace its track.

These chambers were used as the targets for beams of particles – with the interactions triggered when the incoming particles collide with (or pass close to) a hydrogen nucleus (which is simply a proton).

Before they became obsolete with the advent of electronic detectors and massive computing power, bubble chambers provided much of the evidence which led to the Standard Model of particle physics.

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Pair Production.

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Particle Interactions….

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Bubble Chamber....

Basically an electron and a positron (an anti-electron) are drawn together due to their opposite charges.

When they inevitably collide their material existence comes to an end and they are turned into gamma ray photons.

Then two gamma ray photons can be converted into an electron-positron pair, bringing forth matter from whence none existed.

Gamma ray photons

Electron

Positron

A more energetic pair

Scattered atomic electron

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Proton–antiproton annihilation

Here an antiproton (coming in from the bottom left) strikes a proton.

Mutual annihilation leads to four pairs of + and – These curve in opposite directions in the magnetic field.

To think about:

the antiproton is being deflected slightly to the right. So can you identify the + and – particle tracks?

The magnetic field is directed “into the page” (NB anti-proton has a negative charge). It is a rotation of previous slide.

The + and – particle tracks are red and green respectively (the – will deflect the same way as the anti-proton)

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Another view!

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The proton enters from the bottom and strikes a proton in the liquid hydrogen bubble chamber.

The collision produces a spray of negative and positive particles as well as an unseen neutral lambda particle.

The unseen lambda decays into a further pair of positive and negative particles slightly further up from the collision point.

What produces the spiral track shown at the bottom of the picture?

ProtonElectron

Neutral Particle

NegPos

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Alpha & Protons

This picture shows the tracks produced by an alpha particle and a proton in a strong magnetic field.

Which track was made by the proton and which by the alpha particle? (Think about the charge to mass ratios of the two particles.)

Alpha - > +2/4 = +0.5Proton -> +1

Alpha

Proton

Electron

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Can you explain this idea?

5.3.2 Fundamental Particles Covered in my lesson

Revised/ Made my own notes or reviewed at

home

Attempted Exam or Revision Questions

(a) explain that since protons and neutrons contain charged constituents called quarks they are, therefore, not fundamental particles; (b) describe a simple quark model of hadrons in terms of up, down and strange quarks and their respective antiquarks, taking into account their charge, baryon number and strangeness;

(c) describe how the quark model may be extended to include the properties of charm, topness and bottomness;

(d) describe the properties of neutrons and protons in terms of a simple quark model; (e) describe how there is a weak interaction between quarks and that this is responsible for β decay; (f) state that there are two types of β decay;

(g) describe the two types of β decay in terms of a simple quark model; (h) state that (electron) neutrinos and (electron) antineutrinos are produced during β+ and β- decays, respectively; (i) state that a β- particle is an electron and a β+ particle is a positron;

Next Steps for me? / (what do I need to ask for help on)