RUTHERFORD AND BOHR

THE RUTHERFORD MODEL OF THE ATOM

J. J. Thompson had previously identified the electron as a component of the atom in 1897.

The model of the atom changed from the small indestructible sphere of Dalton to the plumb pudding model of Thompson. Negatively charged electrons were considered to be distributed through a sphere of positive charge.

Classical physics was used up to the beginning of the 20th century, and deals with matter on a large scale, revolving around the physics of Newtons laws and Maxwells equations.

Quantum physics began to be developed in the early 20th century, and focuses on matter at atomic and subatomic scales, revolving around quantum theory, that all matter and energy is quantised.

Limitations of the classical theory:

Assumed that the transfer of energy was continuous

Cannot explain speeds close to the speed of light, c

Cannot explain matter at the subatomic level

Cannot explain the photoelectric effect, black body radiation and line spectra.

THE FIRST ALPHA PARTICLE SCATTERING EXPERIMENT

Rutherford showed that alpha particles were deflected by magnetic fields.

Becquerel studies the passage of alpha particles through magnetic fields and believed (incorrectly) that he had observed that the radius of curvature of alpha particles increased as they moved greater distances through the magnetic field.

He believed that alpha particles increased in mass when passing through air and that the increase in mass was responsible for the increase in radius.

Rutherford demonstrated that alpha particles slow down when they collide with air molecules.

He repeated Becquerels experiment with alpha particles passing through a magnetic field in air and also in a vaccuum. He found that the beam of alpha particles was wider in air than in a vaccuum.

GEIGER AND MARSDEN ALPHA PARTICLE SCATTERING EXPERIMENTThey used a simple apparatus with a source of alpha particles that fired them at a sheet of gold foil. The alpha particles were then deflected onto the fluorescent screen that surrounded the gold foil. Geiger and Marsden confirmed that each scintillation was produced by an alpha particle, and that all of the alpha particles produced a scintillation.

SCINTILLATION: A flash of light on a scintillation/fluorescent screen. OBSERVATIONS

Only 1in 8000 alpha particles were deflected, whereas it was expected that most of the particles would deflect as significant angles.

As the thickness of the gold foil increased, the number of particles that were scattered increased initially, but then remained constant.

Most particles passed through the gold foil unaffected.

INTERPRETATIONS: THE NUCLEAR ATOMThe deflections of more than 90 degrees showed that the deflections must be due to the interaction between an alpha particle and a single atom.

Because most alpha particles remained undeflected, Rutherford concluded that the change that caused the deflection is concentrated in a region about 10000 times smaller than the radius of the atom.

Most of the atom is therefore empty space.

Geigers results confirmed that the scattering of the alpha particles was caused by an inverse square electrostatic force.

The largest angle of scattering of the alpha particles arose from the closest interactions with the nucleus (concentrated positive charge).

CONCLUSION: An atom with a small nucleus of concentrated positive charge, with electrons in orbit around it.

PROBLEMS OF THE RUTHERFORD MODEL

The electrons could not be at rest, since they would be attracted into the nucleus.

Orbiting electrons would, according to classical electromagnetic theory, emit electromagnetic radiation. Thus they should continually lose energy and spiral into the nucleus.

Line spectra and the fact that the atoms are obviously stable showed that electrons had none of the properties of accelerated electric charges existing outside of the atom. This was in disagreement with Maxwells theories.

The electrostatic repulsion of positive charges in the nucleus should cause the nucleus to fly apart.

PLANCKS CONTRIBUTION

Planck found an equation that solved the problem of the ultraviolet catastrophe that presented a problem to the theory of black body radiation.

Planck found that the atomic oscillators that produced the radiation in black bodies could only vibrate with certain discrete amounts of energy, called quanta.

QUANTA: The smallest amount of energy possible for a given situation. Plancks atomic oscillators could vibrate only with precise amounts of energy.

The energy of each quantum can be given by E = hf.

In proposing this theory he founded the branch of physics that is now known as quantum theory.

BOHR USES QUANTISED THEORY FOR THE HYDROGEN SPECTRUM

Bohr new that, somehow, atoms must produce radiation that formed a characteristic spectrum for each element.

Bohr realised that the atomic oscillators of Planck were probably electrons in the atom.

The Rutherford model failed to provide any information about the radius of the atom or the orbital frequencies of the electrons.

After seeing Balmers equation (below), Bohr realised how electrons were arranged in the hydrogen atom, and also how quantum ideas could be introduced to the atom.

THE SPECTRA OF GASES

There are three types of emission spectra: continuous spectra, bright-line spectra and band spectra.

Continuous spectra are produced by incandescent objects.

Line spectra are produced by excited gases.

Band spectra are produced by excited molecules.

Spectral lines are produced as images of the slit that is an essential component of any spectroscope.

After passing through the slit, the different wavelengths of light are diffracted by different amounts by a grating or dispersed by a prism by different amounts.

The images of the slit corresponding to the different wavelengths are separated.

When the slit is very narrow, closely spaced lines can be separated/resolved.

If the slit is wider, more light is admitted but at the expense of the resolution.

EMISSION SPECTRUM

A series of brightly coloured lines on a dark background that is produced when light from an excited gas is viewed through a spectroscope.

A gas can be excited by heating it or passing an electric discharge through it.

Each element has its own characteristic spectrum that can be used to identify the gas.

ABSORPTION SPECTRUM

A series of dark lines on a coloured background that is produced when white light is passed through a cool gas and viewed through a spectroscope.

The atoms in the gas absorb energy from the white light, and then re-emit the energy that was absorbed. The energy will be emitted as light, in random directions.

Therefore, the transmitted beam of light will be deficient in wavelengths that were absorbed.

BOHRS POSTULATESWhile Bohr believed that he knew the arrangement of electrons, he could not explain why the electrons were arranged in this way.

He published three papers: his Great Trilogy.

He started with the problem of electrons in the Rutherford model and pointed out that the accelerating electrons must lose energy by radiation and collapse into the nucleus. He then applied quantum theory to the atom, and generally assumed that the electrons were in circular orbits.

Bohrs Three Postulates:

1. Electrons in an atom exist in stationary states, in which they possess an unexplained stability. They orbit the nucleus without emitting any electromagnetic radiation. Any permanent change in their motion must consist of a complete transition from one stationary state to another.

2. No radiation is emitted from an atom in a stationary state, in contradiction to the classic electromagnetic theory. A transition between two stationary states will be accompanied by the emission or absorption of electromagnetic radiation (a photon).

The frequency of this photon is given by the equation:

hf = E1 E2, where E is two values of the energy of two stationary states: the initial and final states of the atom.

An electron moving to a higher energy level/state may do so only if the electron is able to absorb a photon with a frequency above the threshold frequency.

3. Also called his quantization condition. An electron in a stationary state has angular momentum, the rotational equivalent to linear momentum.

In his first postulate, Bohr proposed a radical theory by predicting that electrons exist in states in which they do not radiate energy.

The second postulate involves the quantum of energy being absorbed or emitted when an electron jumps from one stationary state to another, and explains the origin of the spectral lines.

The quantization condition is an intuitive guess.

Using these postulates and the energy of the electrons, calculated from classical physics, it is possible to derive a theoretical equation for the wavelengths of the spectral lines of hydrogen. It is a great success of the Bohr model that this theoretical equation is the same as the empirical equation of Balmer. NOTE: This equation only applies for the hydrogen atom.

THE HYDROGEN ATOM EXPLAINED

The wavelengths of the spectra lines of hydrogen can now be calculated.

BALMER SERIES

Contains the four spectral lines in the visible region of the spectrum. These lines correspond to electron jumps/transitions to the second lowest energy state; the first excited state, n = 2. LYMAN SERIES

Ultraviolet lines with transitions to the ground state, n = 1. An electron has the lowest possible amount of energy when it is in the ground state: the lowest possible energy state of an electron.

PASCHEN SERIES

Infra-red lines with transitions to the second excited state, n = 3.

BRACKETT SERIES

Infra-red lines with transitions to the third excited state, n = 4.

PFUND SERIES

Infra-red lines with transitions to the fourth excited state, n = 5. If an electron exists in a stationary state in which it has more energy, it is said to be in an excited state.

LIMITATIONS AND PROBLEMS OF THE BOHR MODEL OF THE ATOM

The model predicted spectral lines accurately for the hydrogen atom.

It is not possible to calculate the wavelengths of the spectral lines of all other atoms.

The model is not accurate for larger atoms. The Bohr model works reasonably well for atoms with one electron in their outer shell, but does not work for any others.

Examination of spectra shows that the spectral lines are not of equal intensity but the Bohr model does not explain why some electron transitions are favoured over others. The transitions that are favoured show spectral lines with a higher intensity (more photons of that particular wavelength are emitted). The Bohr model does not explain the presence of hyperfine lines, found with careful observations using better equipment. There must be some splitting of the energy levels of the Bohr atom, but the Bohr model cannot account for this.

When a gas is excited while in a magnetic field, the emission spectrum produced shows a splitting of the spectral lines, but the Bohr model cannot account for this. This is called the Zeeman effect.

The Bohr model is a mixture of classical physics and quantum physics, which is a problem in itself.

DEVELOPMENT OF QUANTUM MECHANICS

DIFFRACTION: THE SPREADING OF WAVES THROUGH A SLIT, HOLE OR AROUND A BOUNDARY.

Interference is caused by waves overlapping with each other, causing a cancellation of the wave where troughs coincide and amplification of the wave where crests coincide.INTERACTION: THE INTERACTION OF TWO OR MORE WAVES PRODUCING REGIONS OF MAXIMUM AMPLITUDE (CONSTRUCTIVE) AND ZERO AMPLITUDE (DESTRUCTIVE). STEPS TOWARDS A COMPLETE QUANTUM THEORY MODEL OF THE ATOMDe Broglie argued that the fact that nobody had managed to perform an experiment that concluded whether light was a wave or a particle was because the two kinds of behaviour were inextricably linked.

E = hf and p = hf/c have quantities that are properties of particles on LHS and waves on RHS. WAVE-PARTICLE DUALITY.

De Broglie made the proposal that all particles must have a wave nature as well as a particle nature. Decided that electrons must also have wave characteristics.

De Broglies work was not just speculation. His great achievement was to take this idea and develop it mathematically. He described how matter waves should behave and suggested ways that they could be observed. Broglies wavelength of a particle: = h/mv.

De Broglie suggested that it should be possible to observe the wave nature of a beam of electrons diffracted from the surface of a crystal.

He made a prediction based on his theoretical work and suggested the observations that would support his theory, instead of the other way around.

SIGNIFICANCE: These ideas presented by De Broglie resulted in the creation of a new branch of physics; quantum mechanics.

Quantum mechanics is a complete theory; there are no Classical physics ideas remaining.

In quantum mechanics, particles have both a wave and a particle nature.

DAVISSON AND GERMER: CONFIRMATION

Davisson and Germer studied the surface of a piece of nickel by examining the scattering of electrons from it. During the course of their experiment, air accidentally entered the vaccuum chamber. AN oxide film formed on the metal surface. In an attempt to remove the oxide film, they heated the metal to a temperature just below its melting point, which had the effect of annealing the metal. Large single crystal regions, larger than the width of their electron beam, were produced.

As a result of the change, they now observed the diffraction of the electrons. As diffraction is a property of waves and not particles, they established that electrons had a wave nature as well as a particle nature.

STABILITY OF ELECTRON ORBITS: DE BROGLIES HYPOTHESISBohrs electron orbits explained.

When De Broglie developed the idea of matter waves, he had believed that the orbits of the electrons in the hydrogen atom were something like standing waves.

Therefore, if we consider an electron as setting up a standing wave pattern as it orbits around a nucleus, there must be an integral number of wavelengths in the pattern.

This is similar to the formation of standing waves on a string, where the length of the string must be an integral number of half wavelengths.

Circumference of electron orbits: The De Broglie wavelength is = h/mv.

To form a standing wave there has to be an integral number, n, of wavelengths in the circumference, therefore n = .

Combining the equations: This is Bohrs quantization condition, that angular momentum can exist only in integer multiples of .

The quantised electron orbits of Bohr can be explained by De Broglies proposal that particles have a wave nature, = h/mv.

CONTRIBUTIONS OF HEISENBERG AND PAULI

Heisenberg was able to develop a completely mathematical model of the atom, rejecting the conventional mechanical model. He developed this mathematical model using mattrices of numbers.

Pauli was then able to apply Heisenbergs theory of quantum mechanics to the hydrogen atom. Using quantum mechanics, he derived Balmers equation and Rydbergs constant.

Pauli then introduced the idea of a fourth quantum number to explain the maximum number of electrons in each orbital shell, and developed his exclusion principle, stating that no two electrons can occupy the same energy state; have the same set of quantum numbers.

Heisenberg then developed his uncertainty principle, in which there are pairs of quantum numbers which cannot be determined simultaneously.

Paulis and Heisenbergs contributions to quantum mechanics are significant, because it has allowed for an understanding of different materials, and without their contributions no further steps could be made in quantum mechanics. Because of their work, the development of devices such as transistors and technology such as genetic engineering is possible.

Their mathematical model of quantum mechanics made it possible to explain the behaviour of atoms without the need to visualise it.

Heisenberg has given us maths that can be applied for tiny particles, and can be applied for many uses.

Pauli demonstrated that Heisenbergs maths worked, and so validated it.

PROBING THE NUCLEUSComponents of the nucleus: nucleons.

TRANSMUTATION: THE EMISSION OF RADIATION FROM A NUCLEUS, RESULTING IN THE PRODUCTION OF A NEW DAUGHTER ELEMENT FROM THE PARENT ELEMENT.

NOTE: Gamma radiation does not cause a transmutation.

CHADWICKS DISCOVERY OF THE NEUTRON: CONSERVATION LAWS

It was observed that firing alpha particles at a sheet of beryllium (Be) caused the emission of an unknown radiation type. When this unknown radiation was fired at a block of paraffin, it was found that it caused the emission of hydrogen nuclei (protons).

Because the other characteristics of protons were known, it was possible to calculate the kinetic energy that they were emitted with.

Too many protons were emitted for the unknown radiation to be gamma rays (gamma rays are too highly penetrating).

The emitted protons were found to have 5 MeV of energy, which was a problem, because if the radiation was gamma rays, applying the conservation of momentum laws to the collision between a gamma ray and a proton gave the gamma rays 50 MeV, but the energy of the incident particles was only 5 MeV, violating the Law of Conservation, as energy was added to the system.

HSC ANSWER:

It was observed that when a sheet of beryllium was bombarded with alpha particles, highly penetrating radiation was emitted. Chadwick proposed that this radiation was a stream of neutral particles.

Chadwick was able to apply the laws of conservation of energy and momentum to the collisions between a supposed neutral particle with the protons/hydrogen nuclei in a block of paraffin.

Chadwick was able to find the energy of the emitted protons and using the laws of conservation of energy and momentum, he found that the unknown radiation had too much energy for it to be gamma rays. By using the conservation laws, he was able to find the mass of the neutron to be 1.15 times the mass of a proton.

Chadwick was able to confirm the neutrons existence by showing that energy had been conserved, but hisChadwicks identification of the neutron relied on the laws of conservation holding true for quantum physics.

DISCOVERY OF THE NEUTRINO

PROBLEMS OF BETA DECAY

Beta decay is the emission of an electron from a nucleus, but electrons cannot be confined to the nucleus because the masses of atoms could not be explained in terms of the numbers of protons and electrons.

The De Broglie wavelength of an electron also meant that an electron could not be confined to a region with a radius smaller than its wavelength.

Beta particles were emitted from nuclei with a range of energies. This was a problem, because beta decay from one nucleus should be emitted with the same energy as beta decay from another similar nucleus, as both decays produce the same new nucleus.

Chadwick detected that when the emitted beta particles were deflected in a magnetic field, they had a continuous range of radii, showing that they have been emitted with a continuous wide spread of kinetic energy.

At this point, beta decay did not appear to follow conservation laws, because beta particles were emitted from similar situations with different amounts of energy, indicating that energy had either been created or lost.

Pauli predicted that there must be another sub-atomic particle, and that the conservation laws are still valid, because the emission of beta particles is accompanied by the emission of small, penetrating neutral particles which had not yet been detected.

Thus, the total kinetic energy of the beta particle and the neutrino is always the same for any situation.

This particle was later named the NEUTRINO.

The neutrinos account for the missing energy of beta decay.

FERMI EXPLAINS BETA DECAY

Fermi explained beta decay in terms of Paulis suggestion of a small, neutral particle; the neutrino, along with the electron and the theory that the nucleus contained only heavy particles: protons and neutrons.

PROPERTIES OF NEUTRINOS

Neutral/no charge

Zero or extremely small mass

Travel at the speed of light

Possess momentum and energy (and carry away from a beta decay the momentum and energy that was previously seen to be missing)

They have an intrinsic spin

High penetrating power

Rarely interact with matter

Pauli predicted that this particle would be very difficult to detect because it has a very small mass and is neutral.

THE STRONG NUCLEAR FORCE

ELECTROSTATIC AND GRAVITATIONAL FORCES BETWEEN NUCLEONS

The gravitational force provides a force of attraction between neutrons of Gm1m2/r2. The electrostatic force provides a force of repulsion between the protons in a nucleus.

The ratio of the gravitational force to the electrostatic force between two protons in a nucleus shows that the gravitational force is smaller than the electrostatic force by a factor of 8.1 x 10-37.

The gravitational force is insignificant, compared to the electrostatic force between nucleons, so there must be another force present in the nucleus to hold the nucleons together: the STRONG NUCLEAR FORCE.

NEED FOR A STRONG NUCLEAR FORCE

Because the attractive force of gravitation is insignificant compared to the electrostatic force between two protons in the nucleus, there must be another force of attraction present to hold the nucleons together, to overcome the electrostatic force of repulsion.

This strong nuclear force:

Is independent of the charge of the nucleon

Is a very strong attractive force

Is stronger than the electrostatic force of repulsion between two protons

Acts only over very short distances. The force exists only between neighbouring nucleons

Has a favouring of the binding of nucleon pairs with opposite spin

Is carried by pi mesons/pions; the exchange particles.

MASS DEFECT AND BINDING ENERGY OF THE NUCLEUS

MASS DEFECT: The difference between the mass of the constituent nucleons and the mass of the nucleus.

The key to the large energy involved in nuclear reactions is the fact that mass and energy are equivalent and are linked by the relationship, E = mc2.

If more nucleons could be added to build bigger nuclei, energy would be released and the total mass defect would increase.

BINDING ENERGY

If we tried to split a deuterium nucleus into an isolated proton and neutron, we would find that it is not possible. There would not be sufficient mass for an isolated proton and neutron to exist.

To accomplish the separation, the missing mass must be provided. Energy must be supplied to the deuterium nucleus and that energy must be converted into mass. BINDING ENERGY: The energy equivalent of the mass defect of the nucleus. The energy that would have to be provided and converted to mass to enable all the nucleons in a nucleus to be separated from each other.

As the number of nucleons in a nucleus increases, so does the mass defect. This means that the total binding energy of the nucleus must also have increased.

The stability of the nucleus is indicated by the average binding energy per nucleon. AVERAGE BINDING ENERGY PER NUCLEON: The total binding energy of a nucleus divided by the number of nucleons in the nucleus. It is a measure of the stability of the nucleus.

The average binding energy gives an indication of how strongly an average nucleon is bound to a particular nucleus.

If we were able to join together light nuclei, we would produce nuclei with a higher average binding energy per nucleon and, hence, energy would be released. This is the process of nuclear fusion.

If we were able to take a heavy nucleus and split it in two, we would produce two new nuclei with higher average binding energy per nucleon than that of the original nucleus. Again energy would be released. This is the process of nuclear fission.

NUCLEAR FISSION

DISCOVERY OF NUCLEAR FISSION: FERMI

After the discovery of the neutron Fermi set out to study the neutron bombardment of as many elements as possible. In the majority of cases, heavy isotopes of the same element were formed. In some cases, these heavier isotopes were unstable, and would undergo radioactive (beta) decay to form new transuranic elements in artificial transmutations.

TRANSURANIC ELEMENTS: an artificially produced element with an atomic number higher than 92, which does not occur naturally. Fermi also found that as well as the anticipated transuranic elements, there were also many other isotopes produced.

Lise Meitner and Otto Frisch explained this observation by stating that the neutron bombardment led to nuclear fission, whereby the atom was broken into two smaller nuclei of roughly equal size. Fermi showed that at different speeds the neutron reacted with target nuclei differently.

At high speed a neutron could collide with a nucleus, knock a proton out and decrease the atomic number by one.

At a slower speed a neutron spent more time in collision with a nucleus and could be absorbed into it. In these cases, the new nucleus was usually unstable and split into two smaller nuclei in the process of nuclear fission.

REQUIREMENTS FOR CONTROLLED AND UNCONTROLLED REACTIONS

Both controlled and uncontrolled fission reactions require fuel and a moderator.

To initiate a self-sustaining chain reaction a critical mass of fissile material is needed. If the fuel mass is too small, there will be insufficient neutrons to continue the reaction.

FISSILE: A nucleus that may undergo fission.

CRITICAL MASS: The smallest amount of fuel necessary to sustain a chain reaction. The amount of fissionable material (fuel) needs to sufficiently large (critical mass) so that a chain reaction can be sustained because:

There is a possibility that neutrons can be captured by the fuel rods without causing fission.

There is a possibility that neutrons will be captured by other non-fissionable elements mixed within the fuel.

There is a possibility that the neutrons will escape without being captured.

The moderator is required in both controlled and uncontrolled reactions to sufficiently slow down a neutron, because slow neutrons are more efficient in causing nuclear reactions than fast neutrons.

DIFFERENCES; CONTROLLED REACTIONS ALSO REQUIRE:

Control rods

Coolant

Radiation shield/Casing

In an atom bomb the release of energy has to occur in a very short time. All of the fissile nuclei must capture neutrons and undergo fission. As many as possible of the neutrons produced in each fission must produce further fissions.

Because of the very short time involved, slow neutrons are useless in an atomic bomb. The fuel would be blown apart before the neutrons could be slowed or captured.

THE DEVELOPMENT OF THE ATOM BOMBTHE FIRST NUCLEAR REACTOR: FERMIFermi and a group of scientists built a nuclear reactor in a basement squash court to see if a chain reaction was possible. The aim was to see if it was possible to obtain a neutron multiplication factor greater than one: a chain reaction would occur.

The question was whether sufficient neutrons would produce fissions to enable a chain reaction to occur.

Fermis atomic pile contained 50 tonnes of Uranium, which was dispersed throughout 400 tonnes of graphite: the moderator. Cadmium control rods were placed between the blocks of uranium fuel to absorb excess neutrons.

They started slowly, withdrawing the cadmium control rods, while Fermi would measure the radiation count.

The reaction eventually became self-sustaining, once enough control rods were removed (less electrons were absorbed by the control rods; more were free to cause a fission reaction).

THE CAPTURE OF NEUTRONS

Slow neutrons have a larger de Broglie wavelength, and may interact with a nucleus, even if it passes at a large distance from it. The faster the neutron, the smaller the wavelength, the less the probability of this happening.

The slower the neutron, the longer it is in the vicinity of the nucleus and have a better chance of being captured.

NUCLEAR FISSION REACTORS

In a nuclear reactor that has reached its desired power level, one neutron from each fission must produce another fission to maintain the reaction at a steady rate.

Neutrons are highly penetrative and may travel a large distance through matter before being captured. If the size of the fuel is increased, the number of neutrons that escape will decrease, and when the critical mass is reached the problem of leakage is overcome.

FEATURES OF A FISSION REACTORFUEL RODS: Fission occurs in the fuel rods, producing a neutron which is then slowed whilst in the moderator, before reentering another fuel rod where another fission may occur. The rods can be inserted or removed to help control the rate of reaction. The fuel is usually Uranium-235 and/or plutonium.

MODERATOR: Neutrons produced during fission have too much energy to cause another fission reaction. The moderator slows the neutrons to enhance the probability of the neutrons being captured by another nucleus. (The slower the neutron, the more likely it is that it will interact with a nucleus). The moderator fills the space between the fuel rods.

CONTROL RODS: Control rods absorb excess neutrons to control the rate of fission reactions, to increase or decrease the rate of reactions, or to shut down the reactor.

COOLANT: The coolant is used to extract the heat energy. It then transfers the heat to another coolant, which carries thermal energy to a boiler to heat water, which is used to produce steam, to drive the turbine. Two coolants are used because the first coolant will be radioactive, and it is unsafe to use it to heat the water.

RADIATION SHIELD/CASING: Reactors are lined with graphite and lead to reflect neutrons back into the core, to prevent them from escaping from the reactor. The reactor is also shielded by thick walls of concrete.

TURBINE: Experiences a torque due to the steam, causing it to turn, transforming heat energy into kinetic energy.

GENERATOR: Electricity is generated due to the torque on the turbine, as it rotates in an external magnetic field. PRODUCING ELECTRICITY

The coolant that passes through the reactor core passes through a heat exchange unit where it heats coolant from another circuit.

As a safety precaution, the coolant from the core would usually not pass outside the main reactor building, because it is radioactive.

The coolant from the second circuit carries the thermal energy to a boiler, where it heats water to produce steam to drive a turbine to produce electricity.

APPLICATIONS OF RADIOISOTOPESMEDICALPOSITRON EMISSION TOMOGRAPHY (PET)Radioisotopes such as carbon-11, nitrogen-13 and oxygen-15 are used in Positron Emission Tomography (PET). It is a non-invasive means of producing diagnostic images. The patient is injected with a metabolically active tracer; a molecule that will be used by the body that contains the positron-emitting isotope.

After a positron is emitted, it will combine with and annihilate an electron, usually after travelling less than a millimetre. This produces two gamma rays that travel in opposite directions. These gamma rays are then simultaneously detected by detectors on opposite sides of the patient, to produce a tomographic reconstruction.

RADIATION THERAPY

Beta radiation from Cobalt-60 is used to destroy malignant tumours, because when living tissue is exposed to high levels of radiation, the cells may be damaged or destroyed in a way that stops them from reproducing.

INDUSTRIAL/ENGINEERINGRADIOISOTOPES IN GAUGES

Beta radiation from iridium-192 or cobalt-60 is used to monitor and control the thicknesses of sheet metals, textiles, paper and metal foils.

The radiation passes through the material, and is then detected by a sensor on the other side.

The amount of radiation passing through the material depends on its thickness and density. The amount of radiation detected indicates whether or not the material is of the correct thickness.

SMOKE DETECTORS

Americium-241 is used in smoke detectors. It emits alpha particles that ionise the air between two parallel plates with a potential difference between them.

If there is smoke in the air, some is attracted to the ionised particles and changes the current flow between the plates, setting off the alarm.

AGRICULTURAL

FOOD STERILISATION

Gamma rays from cobalt-60 are used to sterilize food products and seeds, extending their shelf life. The radiation kills bacteria and viruses that would otherwise cause earlier decomposition of the substances.

Many people are opposed to this because of the belief that there may be undetected genetic modifications in the food or seed that could have long-term harmful effects.

RADIOACTIVE TRACERS

Radioisotopes such as Carbon-14, Phosphorous-32 and Nitrogen-15 are tagged into compounds used by plants or animals. Their path is then followed through the organism because the beta radiation that is emitted is detected by sensors outside the organism.

With this increased knowledge of how organisms function, scientists can manufacture more efficient fertilizers and determine the most ideal growing conditions for plants.

NEUTRON SCATTERING

Diffractometers and spectrometers are used to detect scattered neutrons.

Neutron scattering has been used for research in fields such as geology, environmental science, biology and engineering.

Neutrons are useful because:

The de Broglie wavelength of the neutron is comparable to the spacing between atoms in molecules. Neutrons scattered from an atomic lattice will therefore produce interference patterns, providing information about the crystal structure of the material.

Neutrons have an energy similar to the vibrational energy of atoms in solids and liquids, meaning they can be used to study the motion of atoms in molecules in detail.

Since neutrons have no charge, they are not repelled by interactions with nuclei, so any deviation in their path must be due to either a physical collision with other nuclei or because of diffraction, due to their wave nature. This makes neutrons an ideal probe to investigate the structure of matter.

The relatively large mass of neutrons means that they can penetrate further into a lattice than x-rays, so more detailed structural information can be gained. The fact that they are not charged also means they can pass through electron shells unaffected.

Neutrons have a magnetic moment, making them an ideal tool for studying magnetic structures and minerals.

Neutrons interact strongly with nuclei. The strength of the interaction varies for different nuclei, which makes it possible to study isotopes of light elements.

The disadvantage of neutron scattering is that a nuclear reactor is required to produce the neutrons.

PARTICLE ACCELERATORS

The quest for higher energy particles saw the development of a variety of particle accelerators. The higher energy particles from the particle accelerators were used to bombard nuclei and produce a wide variety of new particles.

THE FIRST PARTICLE ACCELERATORSThe first particle accelerators were positively charged plates which attracted electrons to them from an electron gun in a cathode ray tube.

Van de Graaff generators followed, but these were only able to give charged particles a single boost.

As knowledge increased, scientists began to realise that atoms could be split, and more energetic particles were needed.

LINEAR ACCELERATORS

Charged particles are accelerated through the gaps between successively longer cylindrical electrodes. Linear accelerators are up to 5 km long.

The charged particles pass through one cylindrical electrode and are then accelerated by an electric field as they pass through a gap, before passing through another longer electrode.

The alternating potential difference has to be synchronized with the particles, requiring to cylindrical electrodes to become longer and longer.

CYCLOTRONS

Charged particles travel in an evacuated shape between two D-shaped magnets that exert a force on them, causing a circular path. They are then accelerated by an electric field across the space of the two dees.

As the particles accelerate, they gain energy and the radii of their paths increases each time the particles pass through the gap. When the particles reach the limit of the magnetic field they are deflected into a target.

In a cyclotron, charged particles are emitted continuously from the source and accelerated to hit various targets using various magnetic fields.

SYNCHROTRONS

The main particle accelerators today are synchrotrons.

Synchrotrons keep the particles in a path of constant radius. As the particles gain energy, the magnetic field is increased to maintain the same path.

The particles move through a small-diameter, evacuated tube that forms a large-diameter ring. Once the particles accelerate to the required speed, they are deflected to the target.

The disadvantage of the synchrotron is that it can only accelerate one packet of charged particles at a time, which must be removed before another package can be started.

THE TERATONA synchrotron capable of accelerating particles to at least 1 TeV is being built by CERN underground. It will have a 27 km circumference and use 1232 superconducting magnets.

By colliding matter particles with antimatter particles accelerating in opposite directions, extremely high energy collisions are possible.

The higher the energy available, the better scientists can study the particles that make up atoms. COMMON FEATURES

They can use any type of charged particle as a projectile and provide those particles with large amounts of kinetic energy. These particles can then be aimed at target atoms. They can provide these particles at great rates in a beam.They can focus these particle beams to increase the probability of collisions and interactions with specific targets.

IMPACTS

The development of these particle accelerators have led to an increased understanding of the nature of matter.

The collisions of very high energy particles travelling relativistic speeds have allowed scientists to discover a huge array of subatomic particles.

Known physical laws could be used to analyse the tracks and trails collected by sensors to identify the properties and nature of the particles produced in the collisions.

Although initially confusing, the results have led to the formulation of the Standard Model for atoms.

The particle accelerators have allowed for a greater understanding of the nature of the interactions between various quarks, leptons and gluons.

The increasing energy and improved detectors in accelerators have led to increased support for the Standard Model.

THE STANDARD MODEL

The discovery of natural radioactivity led to the discovery of artificial transmutations and nuclear fission. Observations made were confusing. The energies with which some particles were released were wrong. Some had too much, and the amounts were too varied. Unidentified particles were predicted to exist to account for this.

As particle accelerators were built more particles started being detected. The idea that the nucleus contains only protons and neutrons was starting to look dubious. Today it is accepted that there are six flavours/types of quarks and six flavours/types of leptons.

Quarks and leptons can be divided into groups called generations. All the visible matter in the universe is composed of first-generation quarks and leptons; the up and down quarks, and electrons.

QUARKS

There are six different flavours of quarks; up, u, down, d, strange, s, charm, c, top, t and bottom, b.

Quarks possess charges that are either +2/3 or -1/3 of the charge of an electron. Only up and down quarks exist in matter as we know it. The other four exist only at high temperatures in very energetic nuclear reactions. Up quark: +2/3

Down quark: -1/3

A proton is composed of two up quarks and one down quark, giving the proton a charge of +1.

A neutron is composed of one up quark and two down quarks, making it neutral.

LEPTONS

There are six different flavours of leptons: electron, electron neutrino, muon, muon neutrino, tau and tau neutrino.

Only the electron and the electron neutrino exist in matter as we know it. The other four exist only at high temperatures in very energetic nuclear reactions.

BOSONS

The Standard Model also proposes another group of particles called bosons which are responsible for the four forces that interact in matter.

BosonForce responsible forWhat it does

PhotonElectromagnetic forceBinds charged particles, atoms and molecules together; acts over long distances; includes electrostatic and magnetic forces.

Intermediate vector bosonWeak nuclear forceInteracts with nuclear particles to change them into other particles. Acts over 10-17 m.

GluonStrong nuclear forceBinds quarks together in hadrons, binds neutrons and protons together to form nuclei. Acts over 10-15 m.

GravitonGravityDraws masses together, acts over very long distances.

NOTE: The existence of each of the different bosons except for the graviton has been confirmed.

HADRONS: Particles that experience the strong nuclear force, such as mesons and baryons.

BARYONS: Hadrons that have half-integer spin, such as the proton and neutron.

MESONS: Hadrons that have zero or integer spins.

LEPTONS: Leptons are particles which do not experience the strong nuclear force, such as the electron.

FERMIONS: Particles that have half-integer spins, such as hadrons. They obey the Pauli Exclusion Principle.

BOSONS: Particles that have either integer or zero spin. They do not obey the Pauli Exclusion Principle. Bosons are force-carrying particles.

The fundamental forces interact with the 12 basic subatomic particles.

There are also 12 subatomic anti-particles, because every particle has an antiparticle equivalent in mass, but opposite in charge (not really necessary).

The leptons interact by the electromagnetic and weak nuclear forces.

The hadrons interact by the strong nuclear force. They include particles from the groups known as baryons and mesons.

SIGNIFICANCE OF THE MANHATTAN PROJECT TO SOCIETY

One of its most significant impacts occurred when the two atom bombs were dropped on Hiroshima and Nagasaki. The use of the atom bombs brought a quick end to what could have been a prolonged conflict, but it also led to the deaths of millions of Japanese civilians in the two cities.

The Project also had a significant effect on the superpowers attitudes during the Cold War. Its development led to an arms race, as the USSR succeeded in acquiring their own atom bombs, causing tension and dividing the major powers into two distinct camps.

The money that was used to fund the research diverted financial aid from other areas, such as medical care for wounded soldiers and financial relief for the families of soldiers that were killed during the war.

The immediate result of the Project; the bombs in Japan, meant that every country saw the devastating potential of these weapons, which was one of the reasons why no atom bombs were used during the Cold War, because of past experience and the belief in mutually assured destruction. As such, the Manhattan Project led to the abandonment of use of the atom bombs in war since the end of World War II.

The Project is also significant to society because it has led to the development of nuclear fission reactors and nuclear power stations. This, in turn, has led to electrical energy becoming cheaper and potentially less damaging to the environment if viable methods are developed to safely dispose of the nuclear waste that is produced. The results of the research into nuclear fission have led to a controversial debate in many societies, because disasters such as Chernobyl and the nuclear waste problems have caused many to oppose the use of nuclear reactors. Others, however, support the use of nuclear reactors in power generation, because they produce only minimal amounts of greenhouse gases.

Overall, the Manhattan Project has had a huge significance on society, both due to its negative effects in the deaths of civilians and the fear and tension that it caused, and due to its positive impacts in the increased availability of cheaper electricity and also for its applications in agriculture, engineering, medicine and industry.