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9.2 Space
9.3 Motors & Generators
9.4 Ideas-Implementation
9.7 Astrophysics
9.8 Quanta to Quarks

9.8 From Quanta to Quarks continued....

NOTE: This page is a continuation of the notes and worksheets for topic 9.8 From Quanta To Quarks.  Two separate pages were used for this topic because of the large volume of material in the topic.  This will keep download time within acceptable limits.

PREPARED NOTES

 

NUCLEAR FISSION

Nuclear fission is the name given to the process in which a heavy nucleus splits to form two lighter nuclei, each of which is more stable than the original nucleus.  The first artificially induced nuclear fission reaction was achieved by Enrico Fermi in 1934, although at the time he did not realise that fission had occurred.  Fermi bombarded uranium with neutrons and produced radioactive products that emitted b-particles.  Fermi assumed that he had produced a new isotope of uranium, U-239, and that this had undergone beta decay to form an isotope of the first transuranic element, atomic number 93, known today as neptunium-239.  Further transuranic elements could then be formed by further beta decays.

Two German chemists, Otto Hahn and Fritz Strassman, repeated Fermi’s experiments in 1938 and used careful isotopic half-life analysis to identify the products of the reaction.  To their surprise they found that not only was U-239 produced but also various lighter elements, such as Ba-141, Kr-92, Ba-144, Kr-89, La-148, Br-85, Xe-143 & Sr-90.  Hahn and Strassman suspected that these lighter elements were the products of the splitting of the uranium nucleus.  This suspicion was confirmed in 1939 by two Austrian physicists, Lise Meitner and Otto Frisch, who showed that when a U-235 nucleus absorbs a neutron, the nucleus splits into two smaller nuclei and emits one, two or three neutrons in the process.  Meitner & Frisch called the process nuclear fission

In 1940, when the Manhattan Project (to build an atomic bomb) was initiated in the USA, Fermi was placed in charge of the development of the first ever nuclear reactor (or pile).  Fermi determined theoretically that a fission chain reaction, that is a reaction where one reaction would lead to another and so on, could be achieved using naturally occurring uranium.  Fermi designed his reactor so that the uranium fuel was spread evenly throughout a pile of very high purity carbon blocks.  The carbon blocks were designed to slow (or moderate) the speed of neutrons ejected from uranium nuclei, so that they could then produce another fission reaction.  Cadmium rods were also inserted throughout the pile to capture neutrons and thereby control the reaction.  (Cadmium is a good neutron absorber.) 

Fermi’s reactor was built on the squash courts under the football stadium at the University of Chicago.  On December 2nd 1942, the cadmium control rods were slowly, partially withdrawn from the pile.  The amount of radiation produced and the rate and magnitude of temperature increase were in agreement with Fermi’s predictions.  The reactor ran at a steady rate, indicating that the control rods were absorbing sufficient neutrons to maintain a chain reaction.  Fermi had demonstrated the first artificially created, controlled, nuclear fission chain reaction. 

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ENERGY FROM FISSION

The binding energy curve shows that a heavy nucleus has a binding energy of about 7 MeV per nucleon, whereas nuclei of elements with roughly half the mass number have average binding energies of about 8 MeV per nucleon.  Thus, when the heavy nucleus splits to form two lighter nuclei, there is a release of about 1 MeV of energy per nucleon.  So, for a heavy nucleus of 200 nucleons, there would be a release of about 200 MeV of energy from each fission.  Clearly, tremendous amounts of energy can be produced from sustained fission reactions.  For example, the fission of 1 kg of uranium releases about 9 x 1010 kJ of energy.  Taking a typical energy value for coal of about 30 kJ/g means that the fission of 1 kg of uranium produces as much energy as the burning of about 3 million kg of coal.

An example of a typical fission reaction is:

 

           

We can calculate the energy released in this reaction in a couple of different ways.  We can either determine the difference between the binding energies of the products and reactants or we can find the difference between the masses of the products and reactants and then convert this mass difference into its energy equivalent using Einstein’s E = mc2 equation.

Note that as a general rule, energy is released from a nuclear reaction when the binding energy of the products is greater than that of the reactants.  Energy is released because some mass is converted to energy.  Cleary then, we can also say that energy is released from a nuclear reaction when the mass of the products is less than that of the reactants.

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CHAIN REACTIONS & CRITICALITY

On average, 2.4 neutrons are produced by every fission of U-235.  A fission chain reaction is one where the neutrons produced in one fission go on to produce another fission and so on.  In order for a fission chain reaction to occur, the sample of fissionable material must have a certain minimum size referred to as its critical mass.  Otherwise neutrons escape from the sample before they have an opportunity to strike a nucleus and cause fission.  The chain stops if enough neutrons are lost.  The reaction is then said to be subcritical.  As an example, critical mass for weapons grade plutonium-239 is about 4 to 6 kg depending on shape.  For weapons grade (highly enriched) U-235 it is even less.

If the mass is large enough to maintain the chain reaction with a constant rate of fission, the reaction is said to be criticalThis situation results if only one neutron from each fission is subsequently successful in producing another fission.  Such a reaction is controllable.  See diagram below.

           

If the mass is larger still, few of the neutrons produced are able to escape.  If one fission produces two neutrons, these two neutrons can cause two fissions.  The four neutrons thereby released produce four fissions and so on.  The number of fissions and their associated energies quickly increase and if unchecked the result is a violent explosion.  Such branching chain reactions are said to be supercritical and the reaction is uncontrollable.  See diagram below.

           

An excellent example of a controlled nuclear fission reaction occurs in a fission reactor.  We will examine the basic workings of fission reactors in the next section of this topic.  An excellent example of an uncontrolled nuclear fission reaction occurs in an atomic bomb.

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THE FISSION BOMB (ATOMIC BOMB)

In the U-235 atomic bomb, two subcritical masses of U-235 are held well apart at opposite ends of a tube (referred to as the gun barrel).  On detonation these two masses are imploded together by a conventional explosive charge (TNT), so that criticality is attained suddenly.  In this way the chain reaction spreads throughout the combined mass before it breaks up into subcritical fragments.  The result is an uncontrollable fission reaction.

As shown in the diagram below, the two subcritical masses of U-235 each have a hollow cut into their centres.  The hollows are lined with beryllium, a good source of neutrons.  When the two hemispheres come together, the hollows close around a ball of polonium, a good source of a-particles.  The a-particles hit the Be and produce a huge flux of neutrons, which then cause the supercritical fission reaction that leads to the explosion of the device.

     

The first such atomic bomb consisted of only a few kilograms of U-235, but had an explosive force of 20000 tons of TNT.  U-235 was used in the bomb dropped on Hiroshima (August 6 1945).  Pu-239 was used in that dropped on Nagasaki.  Together, these relatively small devices killed well in excess of 100 000 people and caused massive devastation to both cities.

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NUCLEAR FISSION REACTORS

The purpose of a nuclear fission reactor is to release nuclear energy at a controlled rate.  Fission reactors can be classified as either Thermal Reactors where the neutrons producing the fission have energies comparable to gas molecules at room temperatures (thermal neutrons, KE < 1 eV) or Fast Reactors where the neutrons producing the fission have high energies (fast neutrons, KE > 1 keV).  Most commercial reactors are Thermal Reactors.

In a Thermal Reactor, fuel (fissionable material) is bombarded by neutrons, which have been slowed down to thermal velocities by moderator material, and undergoes fission, which releases heat energyControl rods containing neutron-absorbing material are used to control the rate of reaction.  The heat produced is absorbed by a coolant material and can be transferred via a series of heat exchangers to boil water, to produce steam to drive turbines and produce electricity.  Let us now have a closer look at the basic components of a thermal fission reactor

FUEL: Thermal reactors are fuelled with natural uranium or more commonly with enriched fuel.  Enriched fuel is natural uranium, which has been processed by gaseous diffusion or centrifuge techniques to raise the percentage of fissile U-235 in it to between 3 and 7 percent, instead of the 0.7% in nature.  U-235 is fissionable with thermal neutrons, whereas U-238 and U-234, the other isotopic components of natural uranium, are not.  Pure U-235 is never used in a reactor. 

The fuel is converted to UO2 pellets and packed into zirconium or stainless steel tubes called fuel rods

MODERATORS: The reactor core is not just a mass of fuel.  The fuel rods are spaced out and surrounded by another material called the moderator.  Its purpose is to slow down the neutrons released by fission from high speeds to thermal speeds.  This is done for two reasons: (a) thermal neutrons are more efficient at fissioning U-235; and (b) fast neutrons are more likely to be captured by U-238 than to fission U-235. 

The moderator must contain light atoms so that when the fast neutrons collide with the moderator atoms, they move them and thus give away some of their kinetic energy.  If they collided with heavy atoms such as lead, they would simply bounce off with their original energies.  Commonly used moderator materials include ordinary water (in reactors using enriched fuel), heavy water (deuterium oxide D2O), and graphite.  Beyond carbon, the atoms are too heavy to do the job efficiently. 

CONTROL RODS: These are used to ensure the chain reaction does not accelerate into an uncontrollable state.  They are made of neutron absorbing material such as boron or cadmium encased in steel.  As soon as the temperature within the core rises above the acceptable value, the control rods are lowered into the spaces between the fuel rods.  There they absorb neutrons and slow down the chain reaction. 

COOLANT: Most of the energy released in a fission reaction is carried away as the kinetic energy of the fission products.  These products collide with other atoms in the vicinity and produce heat.  The heat from the reactor core is collected by the coolant.  The coolant may be ordinary water, heavy water, liquid sodium, gas (eg CO2 or air) or certain liquid organic compounds.  The coolant is in a closed system to lessen the risk of radiation leaks. 

RADIATION SHIELDS: There are usually two shields: (a) A shield to protect the walls of the reactor from radiation damage and at the same time reflect neutrons back into the core; and (b) A Biological Shield used to protect people and the environment.  It consists of many centimetres of very high density concrete.

The following diagram shows the basic structure of a thermal reactor.

 

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NEUTRON SCATTERING

One of the advantages of fission reactors is that they can be used to produce large numbers of neutrons for all sorts of research applications.  Neutrons are ideal for probing the inner structure of matter because of the following properties: 

u    They are neutral and can therefore penetrate deeply into matter.

u    The de Broglie wavelength of thermal neutrons is comparable to the spacing of the atoms in an atomic lattice.

u    The energy of thermal neutrons is similar to the energies of the lattice vibrations in solids.

u    They scatter well from protons, making them useful in determining the structure of solids containing hydrogen bonds (eg organic molecules).

Neutrons are directed from a reactor core onto a sample of material.  The neutrons collide with atomic nuclei and scatter in directions determined by the neutron’s wavelength and the structure of the material under study.  From the diffraction patterns obtained, physicists can deduce the internal structure of the material.

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APPLICATIONS OF RADIO-ISOTOPES

Another advantage of fission reactors is that they can be used to produce radioactive isotopes for a number of special applications.  The radioisotopes are produced by bombarding appropriate elements with neutrons in the reactor.  Alternatively, radioisotopes can be produced by bombarding appropriate elements with various sub-atomic particles in a particle accelerator (which we will discuss soon).

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MEDICAL APPLICATIONS

Radioisotopes are used in medicine in both diagnosis and therapy.  In diagnosis, the principle use is to locate abnormal tissue such as tumours.  In therapy, radioisotopes are used to destroy abnormal cells within the body. 

In diagnosis, a drug containing the radioisotope to be used is taken orally or intravenously.  The drug carries the radioisotope to the organ(s) under study.  Radiation detectors are then used to measure the concentration and distribution of the radioisotope and this can in turn result in the detection of abnormalities.  The radioisotopes used must be short lived to minimise harm to the body.  Most diagnostic radioisotopes are gamma emitters, since gamma radiation is the only natural radiation with sufficient penetrating power to escape from the body in detectable quantities. 

A typical example of a diagnostic radioisotope is technetium-99m, 99mTc.  This metastable (or excited) form of the Tc-99 isotope has a half-life of only a few hours and decays to stable Tc-99 via gamma ray emission.  Hospitals are sent Tc-99m generators, consisting of the molybdenum-99 isotope, which decays with a half-life of 67 hours to Tc-99m.

 

           

The Tc-99m so obtained is then injected into the body and used to scan for brain, bone,  liver, spleen, kidney or lung cancer, as well as for blood flow anomalies.  As the Tc-99m de-excites to Tc-99, the emitted gamma radiation is recorded and measured using a gamma ray camera.

 

           

In radiotherapy, high-energy radiation is used to cause localised radiation damage to cancerous cells and kill them.  A typical example is the use of cobalt-60, which kills cancer cells by the release of gamma radiation.  The cobalt-60 can be implanted in the tumour in the form of a thin wire so that it can release the gamma radiation over a period of time.  Alternatively, the gamma radiation from the Co-60 source can be directed onto the tumour from outside the body.

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INDUSTRIAL APPLICATIONS

There are many industrial applications of radioisotopes.  Three examples are given here.

Various sources of radiation (eg Co-60) are used to measure the thickness of metal, plastic, glass, paper and so on, during manufacture.  This is done by measuring the amount of radiation passing through the material, which is related to the thickness of the material.  If the material becomes too thick or thin, the detector senses the change in radiation and the machine’s control circuits can then adjust the machine’s settings to ensure the correct thickness.

Smoke detectors use americium-241, an alpha emitter.  The Am-241 ionises the air between two parallel plates and the flow of ions (current) between the plates is constantly monitored.  If there is smoke in the air, smoke particles are attracted to ions in the air, making them heavier.  This changes the flow of ions between the plates, which in turn sets off the alarm.

All radioisotopes generate heat as they decay.  Radioisotopic-powered thermoelectric generators (RTG's) use the heat from the radioisotope plutonium-238 to generate the electricity that runs certain types of  cardiac pacemakers.

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AGRICULTURAL APPLICATIONS

Phosphorus-32 is used in agriculture for tracking a plant's uptake of fertilizer from the roots to the leaves.  The phosphorus-32 is added to soil water.  As it has a half-life of 14.3 days and emits b-particles, its passage through the plant can be traced and the tagged fertilizer's uptake mapped.  b-particles have sufficient penetration power to emerge from root systems and from inside plant tissues.  They can be detected by Geiger-Muller tubes.  Only small, safe amounts need to be used in this process.

Try the sites below for more information on isotopes used in agriculture.

Food Irradiation in Australia & NZ

Phosphorus-32

Food Irradiation in USA

Agriculture & Food Irradiation in USA again

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WORKSHEET 1 ON NUCLEAR PHYSICS 

1.      In the deuterium nucleus, protons have a mass of 1.67 x 10-27 kg, a charge of 1.6 x 10-19 C and are separated by a distance of about 2.5 x 10-15 m.  Calculate the sizes of the electrostatic and gravitational forces between these protons.  Comment on the relative sizes of these forces.

2.      Use Einstein’s E = mc2 equation to calculate the energy equivalent of 1 atomic mass unit.  Give your answer in mega electron volts.  Take the speed of light as 2.997 925 x 108 m/s.  (1 u = 931.5 MeV)

3.      Determine the mass defect, the binding energy and the binding energy per nucleon of the He-4  (a-particle) nucleus.  The experimentally obtained mass of the alpha particle is 4.002604 u.  Ignore the masses of any electrons around the nucleus.

4.      In 1919 Rutherford bombarded N-14 with alpha particles and achieved the first artificial transmutation.  One of the two products of the reaction was a proton ejected from the nucleus.  Write a balanced nuclear equation to determine the other product.

5.      When a deuterium nucleus (deuteron) collides with a N-14 nucleus, the products are N-15 and a proton.  Write a balanced nuclear equation for this reaction and then calculate the mass difference* between the reactants and products and the total energy in MeV liberated in this reaction.  (Masses of nuclei: N-14 = 14.003074 u, H-2 = 2.014102 u, N-15 = 15.000108 u & proton = 1.007825 u.)

6.      For the following fission reaction, determine the mass difference* between the reactants and products and hence calculate the energy released in MeV.


          

(Masses: U-235 = 235.044 u, La-148 = 147.915 u, Br-85 = 84.911 u & neutron = 1.009 u.)

7.      Consider a fission reaction where U-235 fissions to produce Ba-141 & Kr-92.  Write a balanced equation to represent this reaction.  Calculate the energy released in this reaction.  (Masses: U-235 = 235.0439 u, Ba-141 = 140.9139 u, Kr-92 = 91.8973 u & neutron = 1.008665 u.)

 

*NOTE: The Syllabus uses the term “mass defect” to refer to the difference in mass between reactants & products in a nuclear reaction.  So be prepared to read questions asking you to “calculate the mass defect” in a particular reaction.  Strictly speaking the term “mass defect” refers ONLY to the mass equivalent of the binding energy of the nucleus – that is the difference between the measured mass of the nucleus and the theoretical mass determined by adding up the masses of all protons & neutrons in the nucleus.  It does not refer traditionally to the difference in mass between the reactants & products of a nuclear reaction.

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PARTICLE ACCELERATORS

Particle accelerators are used to accelerate sub-atomic particles to very high energies.  The two main uses of particle accelerators are: (a) to produce radioisotopes by bombarding elements with various sub-atomic particles; and (b) to produce beams of very high-energy particles that can be used to probe the structure of matter.  It is this second use that we will now study.

Beams of very high-energy particles are useful matter probes for two main reasons.  Firstly, the higher the energy (and therefore velocity) of a particle, the smaller the de Broglie wavelength.  The smaller the de Broglie wavelength, the smaller the detail the particle can “see” – ie the better the resolving power of a beam of such particles.  Secondly, the higher the energy of a probe particle colliding with a target particle, the more massive are the possible product particles, since in every reaction, some of the energy of the probe particle is converted into mass according to E = mc2.  This means effectively that physicists can re-create and study in the laboratory, conditions that may have existed in the early stages of the creation of the universe.

There are many different types of particle accelerator: the Van de Graaff Accelerator used to accelerate protons, deuterons & a-particles; the Cyclotron, Synchro-Cyclotron, Synchrotron and Linear Accelerator used to accelerate a variety of charged particles; and the Betatron used to accelerate electrons.  Two examples will be described here.

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The Synchrotron:

This consists of a single, circular, evacuated tube.  High-energy particles from another accelerator are injected into the synchrotron and are controlled by a large magnetic field as they move around the circular path.  The particles are accelerated by a high frequency electric field applied across gaps in metallic cavities inside the synchrotron tube.  The frequency is synchronised with the constant angular frequency of the particles in the accelerator.  The particles are accelerated while in the cavities.  As the particles gain velocity, the strength of the magnetic field is increased to counteract the relativistic increase in mass of the particles.  Both protons and electrons are commonly accelerated using the synchrotron.  Fermilab in Illinois, USA, has a synchrotron with a radius of 1 km, which can accelerate protons up to energies as high as 1000 GeV (1TeV).  See diagram below.

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The Linear Accelerator (Linac):

This is a particle accelerator in which electrons or protons are accelerated along a straight evacuated chamber by an electric field of constant radio frequency.  In older machines cylindrical electrodes called drift tubes are aligned coaxially with the chamber.  Keeping in phase with the radio frequency supply, the charged particles are accelerated in the gaps between the electrodes.  In other words, initially, the tube in front of the particles has an opposite charge to that of the particles, and hence attracts the particles.  Once in the tube, the polarity of the tube changes.  The particles are repelled from the tube and attracted to the next tube, and so on.  Since the frequency of the electric field is constant and the particles increase in speed, the tubes get progressively longer to ensure that the particles spend the same amount of time in each tube and therefore keep in phase with the electric field.  See diagram below.

 

 

Modern high-energy linacs are usually travelling wave accelerators in which particles are accelerated by the electric component of a travelling wave set up in a waveguide.  No drift tubes are used, the radio frequency being boosted at regular intervals along the chamber by means of klystrons (electron tubes used for the amplification or generation of high frequency waves).  Only a small magnetic field, supplied by magnetic lenses between the radio frequency cavities is required to focus the particles and keep them in a straight line.  Typical rates of energy gain in a linac are 7 MeV per metre for electrons and 1.5 MeV per metre for protons.  The linac at Stanford University in USA is over 3 km long and can accelerate electrons up to 99.99% of the speed of light.

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THE STANDARD MODEL OF MATTER

For more than two thousand years philosophers have asked the question: “What are things made of?”  As we have seen in this topic much progress has been made, especially in the 20th Century, towards answering this question.  Up until the 1960’s it was thought that the constituents of the nucleus were fundamental (indivisible) particles but experiments in which protons were collided with other protons or electrons at high speeds indicated that they were composed of smaller particles.  These particles were named quarks by the Caltech physicist Murray Gell-Mann, who won the 1969 Nobel Prize for his work on them. 

The currently accepted model of the structure of matter that has emerged over the last thirty years is called the Standard Model of Matter.  The standard model attempts to describe all interactions of subatomic particles, excluding those due to gravity.  The standard model uses a small number of fundamental particles and interactions to explain the existence of hundreds of particles and interactions.  The predictions of the standard model agree very closely with experimental evidence. 

The standard model asserts that matter can be grouped into three families: bosons, quarks and leptons.

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Bosons:

These are force-carrying particles.  Each of the fundamental forces in nature is carried between particles by a gauge boson, as described below.  Recall that there are believed to be four fundamental forces in nature

u    The gravitational force – a long-range force acting on all masses in the universe.  It is the weakest of all the forces.  It is believed to be carried by the graviton, which has not yet been observed experimentally.

u    The electromagnetic force – a long-range force that acts on all charges in the universe.  It holds atoms and molecules together.  It is carried by the photon.

u    The strong nuclear force – holds protons and neutrons together in the nucleus.  It is a short-range force operating at nuclear distances (10-15 m).  In the standard model, it also binds quarks together and is carried by the gluon.

u    The weak nuclear force – interacts with particles such as electrons to change them into other forms of particle.  It is short-ranged (10-17 m).  In the standard model it also transforms one quark type into another and is carried by the W and Z bosons.

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Quarks:

These are matter particles with charges that are sub-multiples of the electronic charge.  They are considered fundamental particles, since they have no known components.  The following table shows the flavours (types), symbols and charges associated with the quarks.           

QUARK FLAVOUR

SYMBOL

CHARGE

Up

u

(+2/3) e

Down

d

(-1/3) e

Strange

s

(-1/3) e

Charm

c

(+2/3) e

Bottom

b

(-1/3) e

Top

t

(+2/3) e

For every quark, there is a corresponding anti-quark, represented in the usual way with a bar above the symbol.  Quarks are never found in isolation, because the strong force that binds them together is such that it increases in strength with increasing distance.  So quarks act as the constituents of other particles.

A particle composed of quarks is called a hadron.  Hadrons can be divided into two groups: 

u    Baryons – 3 quark combinations.  The most well known, lightest and most stable baryons are the proton and neutron.  The proton is composed of two up quarks and one down quark (uud) and has a net charge of +1 e.  The neutron is composed of one up quark and two down quarks (udd) and is therefore neutral.  Many other baryons exist (lambda, sigma, xi, omega).  All baryons interact through the strong force.

u    Mesons – 2 quark combinations.  Mesons consist of a quark and an anti-quark.  They are unstable and decay in millionths of a second to produce other particles such as photons, electrons and neutrinos.  Examples of mesons are pions, kaons and eta-mesons.  For example a p+ meson is composed of an up quark and a down anti-quark ,

                                                 

giving a total charge of +1e.  All mesons interact through the strong force.

The existence of quarks has been well established by experimentation. When high-energy electron beams are used to probe the proton or neutron for instance, three distinct scattering centres are found inside each particle.

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Leptons:

These are matter particles with little or no mass.  They do not experience the strong force and interact through the weak force (and the electromagnetic force if they are charged).  Leptons are considered fundamental particles, since they have no known components.  The flavours (types), symbols and charges of the leptons are shown in the following table. 

           

LEPTON FLAVOUR

SYMBOL

CHARGE

Electron

e-  

-1 e

Electron-neutrino

ne  

0

Muon

m-  

-1 e

Muon-neutrino

nm  

0

Tau

t-  

-1 e

Tau-neutrino

nt  

0

As is the case with quarks, for every lepton there is a corresponding anti-lepton.  However, unlike quarks, individual leptons can be found in isolation.

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Success of the Standard Model:

It is a testament to the power of the Standard Model that all of the hundreds of subatomic particles so far discovered can be explained as combinations of these twelve fundamental particles (6 quarks & 6 leptons) and their anti-particles.  Everyday matter is composed of up and down quarks, the electron and the electron neutrino.  For this reason, these particles are called first generation particles. 

Second generation particles consist of charm and strange quarks, the muon and the muon-neutrino.  Third generation particles consist of top and bottom quarks, the tau meson and the tau-neutrino.  Second and third generation particles are unstable and decay into first generation particles. 

The standard model asserts that the forces governing the interaction of quarks and leptons can be understood by using the quantum mechanics of fields.

Quantum field theory suggests that forces are carried between particles by special force-carrying particles.  These are the gauge (or field) bosons mentioned earlier.  Two successful quantum field theories exist.  The first is called Electroweak Theory.  This successfully explains the source and operation of the Electromagnetic and Weak forces in terms of the photon and the W and Z bosons and shows that at high energies these two forces combine into a single electroweak force mediated by the W and Z bosons.

The second theory is called Quantum Chromodynamics (QCD) and successfully explains the source and operation of the strong force in terms of gluons.  QCD theory suggests that quarks do not only carry electrical charge but also possess another form of charge called colour.  This is the source of the powerful forces that bind quarks together and build up the baryons and mesons.  Whereas electrical charges are either positive or negative, there are three varieties of colour: positive or negative of either red, green or blue colour.  These have nothing to do with real colour, of course; colour is just a name used to distinguish them.  It is the quantum interactions between the various possible colours that determine which particles are formed.  It is the gluons that carry the colour force between quarks, just as it is the photon that carries the electromagnetic force between electrostatic charges. 

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PARTICLE PHYSICS & COSMOLOGY - Not Examinable

This section is no longer required by the Syllabus but has been left here as extension material.  It is well worth reading.

Cosmology is the study of the creation, nature and evolution of the universe.  Clearly, any theory that claims to satisfactorily explain the nature of the universe must be able to account for the existence of the fundamental forces and particles that are observed today.  Our currently accepted cosmological model, the Big Bang Theory meets this criterion and is based on a foundation of modern particle physics.

Big Bang theory suggests that for the first 10-43s (the Planck Time) after the big bang, all four fundamental forces were unified into one force.  In the brief period of time between the Planck Time and 10-12s after the big bang, the particle energy fell from 1019 GeV to 102 GeV and the four fundamental forces separated from one another in a series of symmetry breaking events.

As the four forces separated from each other they determined the nature of the particles that were present in the universe.  For example, once the strong force separated out, free quarks were forced to combine to form hadrons and antihadrons and eventually no free quarks existed.  Thus, Big Bang Theory suggests that the elementary particles that exist in the universe today were created as the four fundamental forces separated out from the initial unified force.  Clearly, the Big Bang Theory is a good example of the intimate connection between high energy particle physics and cosmology

Another example of the link between high energy particle physics and cosmology is seen in the attempt to understand the nature of dark matter.  Dark matter is a mysterious type of matter that emits no detectable EM energy and appears to make up most of the mass of the universe.  It is detectable due to its gravitational influence on observable masses, such as galaxies.  One theory is that dark matter consists of a new class of sub-atomic particle called weakly interacting massive particles (WIMPs).  Another theory suggests that dark matter consists of a mixture of dim stars and black holes referred to as massive compact halo objects (MACHOs) and a special kind of neutrino

Yet another example of the link between high energy particle physics and cosmology is provided by physicists studying the formation of black holes.  High energy particle physics theories must be used to describe the nature and behaviour of the fundamental forces and particles at the energies involved.  These energies are high enough to cause the fundamental forces to recombine. 

Indeed the link between high energy particle physics and cosmology is one of the reasons why physicists continue to develop more powerful particle accelerators.  The more energy provided by the particle accelerator, the better the replication of conditions in the very early universe.  The Relativistic Heavy Ion Collider (RHIC) at Brookhaven, USA, can accelerate gold ions to 99.995% of the speed of light.  Two beams of such ions are accelerated in opposite directions and are then deflected so that they collide.  Speculation has been made that it may be possible to produce a quark-gluon plasma with this collider.  The RHIC is now the second-highest-energy heavy-ion collider in the world. As of November 7, 2010, the Large Hadron Collider (LHC) at Europe's CERN laboratory has collided heavy ions of lead at higher energies than the RHIC.

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FURTHER THEORETICAL DEVELOPMENTS - Not Examinable

This section is not required by the Syllabus but has been left here as extension material.  It is well worth reading.

Although it is not included in the syllabus, it is worth mentioning that the Standard Model of Matter is not the end of the story.  Grand Unified Theories (GUTs) have been developed to explain the unification of the electroweak force and the strong nuclear force at very high energies.  The most successful of these theories, SU(5), predicts that at very high energies and very small distances, leptons and quarks become manifestations of the same fundamental particle. 

Another exciting development is Superstring Theory (Supersymmetric String Theory) in which the structureless point particles of conventional quantum field theory are replaced by one-dimensional “string-like” entities that can interact with each other and scatter according to a precise set of laws.  A single quantized string can be shown to be equivalent to an infinite set of “normal” elementary particles whose masses and spins are related in a special way.  For internal consistency Superstring Theory requires ten-dimensional space-time (the usual four dimensions of space and time, plus six extra spatial dimensions curled up in the string).  Superstring Theory automatically includes the gravitational force in the unification scheme and may eventually provide the first fully consistent quantum theory of gravity. 

Ultimately, physicists would like to develop a Theory of Everything (TOE).  Such a theory would have to successfully explain the unification of all four fundamental forces and all observed phenomena at both high and low energies.  Some physicists argue that Superstring Theory (specifically M-Theory - the unified framework incorporating all five of the existing Superstring Theories) may indeed become a TOE.  Many others, however, for reasons far too complicated to explain here, believe that we are still a long way off developing a TOE.  In any case, it is important to realize that a TOE, as powerful and useful as it would be, would still not be able to provide answers to all the questions that we might ask.  A TOE, by its very nature, must incorporate the fundamental indeterminism of the quantum mechanical world.  Likewise a TOE could not provide answers to questions such as “What existed before the big bang?” or “What caused the big bang?” – such things are fundamentally unknowable.  The phrase “before the big bang” has no meaning, since time did not exist until the big bang occurred.  Nor could a TOE provide an answer to a question such as “Why does the universe exist?” – this is a religious rather than a scientific question.  Indeed some physicists use the name Supergrand Unified Theory instead of TOE to remove the implication that a TOE would provide answers to all questions.

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WORKSHEET 2 ON NUCLEAR PHYSICS

1.      Perform a first-hand investigation or gather secondary information to observe radiation emitted from a nucleus using a Wilson Cloud Chamber or similar detection device.  (In Class)

2.      Gather, process and analyse information to assess the significance of the Manhatten Project to society.  (Homework)

3.      Identify data sources, and gather, process and analyse information to describe the use of a named isotope in: medicine, agriculture and engineering.  (Homework)

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