eddie from quanta to quarks.docx

8/14/2019 Eddie From Quanta to Quarks.docx

http://slidepdf.com/reader/full/eddie-from-quanta-to-quarksdocx 1/19

From Quanta to Quarks

Problems with the Rutherford model of the atom led to the search for a model

that would better explain the observed phenomena

Discuss the structure of the Rutherford model of the atom, the existence of the

nucleus and electron orbits

Geiger and Marsden investigated the scattering of alpha particles on very thin metal foils to

confirm Thomson’s “plum pudding” model

whereby negatively charged electrons were

considered to be distributed throughout sphere of

positive charge

Predicted only small scattering because atom had

no large concentrations of charge or mass to deflectmassive and fast moving alpha particles

They were shocked to find approx. one alpha

particle in every 8000 was deflected by the thin gold foil

through angles greater than 90 degrees

Rutherford model of the atom:

Hypothesised that for alpha

particles to be deflected, massive

but tiny, positively “charge centre”

must exist within a set of orbiting

electrons (like planets orbing theSun)

Predicted then the tiny, positive

charge in the centre contained

99.9% of mass and atom is mostly

empty space

Rutherford proposed electrical

attraction provides centripetal

force to keep electrons in orbit and

the electrons could be anywhere (like planets around the Sun)

o However, orbiting electrons are accelerating, which should radiate away energy

(predicted by Maxwell’s classical EM theory), and spiral into nucleus

Analyse the significance of the hydrogen spectrum in the development of Bohr’s model

of the atom

Bohr’s model of the atom was quite similar to Rutherford’s but with two important differences:

o There are assigned positions to the electrons

o Electron energy levels are quantised

The Balmer Series helped Niels Bohr in proposing a model of the atom. Bohr understood that

atoms produce the radiation, which formed the characteristic spectrum for each element. After

being introduced to the Balmer series and the Balmer equation, Bohr connected the energy

emitted by the hydrogen atom (when excited electrons fall back to the second lowest energy

state (n=2)) to the emitted spectral lines. He used the Balmer series to explain that emission linesare due to electrons in excited states falling back to their original energy levels.



Discuss Planck’s contribution to the concept of quantised energy

Max Planck in 1901 proposed a theory to model the

spectrum of a black body

Planck was the architect of the concept of

quantisation, that is, that energy can be emitted or

absorbed only in discrete packets of energy(quanta) (determined by the equation E=hf)

Bohr needed to explain why electrons in atomic

orbitals did not continually radiate energy (and

spiral into the nucleus) and why there were distinct

spectral lines present in atomic spectra. He used

Planck’s ideas in what we today refer as Bohr’s

postulates:

o Electrons move in metastable orbits

without radiating energy. Quantisation prohibits continuous radiation

o An electron may move from a lower energy orbital to a higher energy orbital by

absorbing a quantum of energy (E=hf) and from a higher energy orbital to a lower energyorbital by releasing a quantum of energy

Define Bohr’s postulates

Electrons in an atom exist in “stationary states” in which they possess an unexplainable stability

Electrons absorb or emit specific quanta of energy when they transition between stationary

states (orbits)

o In contradiction to the classical electromagnetic theory

Angular momentum of electrons is quantised

o Electron in a stationary state, has a quantised angular momentum that can take nay

values of

where is the principle quantum number

Describe how Bohr’s postulates led to the development of a mathematical model to

account for the existence of the hydrogen spectrum:

Balmer series: visible emission spectrum of hydrogen emitted when an electron in energy level

( ) falls to level 2

Balmer originally devised the Rydberg equation empirically by examining the lines in the

hydrogen spectrum and creating a formula to fit them

Rydberg used Bohr’s postulates and manipulated them to create the same formula

o Major accomplishment and provided strong support for the Rutherford – Bohr atomic

model of the hydrogen atom

Discuss the limitations of the Bohr model of the hydrogen atom

Bohr model works well with one electron in outer shell but does not work for any others

o Because atoms larger than hydrogen have more than one electron which interact in a

complex manner

Examination of spectra showed that the spectral lines were not of equal intensity

o Bohr model does not explain why some electron transitions would be favoured by others

Perform a first – hand investigation to observe the visible components of the hydrogen

spectrumMethod



Glass tube containing hydrogen gas (hydrogen spectral tube) and two electrodes were attached

to an induction coil in a darkened room

o Induction coil was attached to a power supply and the tube began to glow

A hand spectroscope was used to examine the light from the glow and four lines in the visible

spectrum was observed

Apparatus Hydrogen spectral tube

Power supply

Hand spectroscope

Risk

Induction coil produces harmful X – rays. This was

addressed by standing at least 3 – 4 metres away

from the experiment

Process and present diagrammatic information

to illustrate Bohr’s explanation of the Balmer

series According to Bohr, Balmer series was caused by

electrons changing energy levels

When an excited electron in a stationary state of

jumps down to the stationary state

, a photon is emitted

o Energy of this photon is equal to energy

difference between these two

stationary states

Balmer series is formed by successive transitions

to the 2nd shello Transitions to other shells produce additional lines, named after their discoverers

The diagram shows the energy of the stationary states in which an electron can exist in an atomo The lower the horizontal line, the lower the energy of that state



Lower the energy gap between horizontal line, lower the energy emitted when

electrons transition between those two states

o Length of vertical arrow gives the energy of photon emitted when an electron falls from

an initial state () to a final state ()

o n is the principle quantum number

Analyse secondary information to identify the difficulties with the Rutherford – Bohr

model, including its inability to completely explain:

- the spectra of larger atoms

- the relative intensity of spectral lines

- the existence of hyperfine spectral lines

- the Zeeman effect

Not possible to calculate wavelengths of spectral lines of all other atoms

The spectra of larger atoms

Bohr and his colleagues were unable to develop an arrangement of stationary states that

matched the experimental observations of spectral patterns

Atoms larger than hydrogen have more than one electron and these interact with each other in a

complex manner

o Bohr’s simple quantised planetary model explained only the hydrogen atom, helium ion

( and the lithium ion ( – all have single electrons orbiting the nucleus

The relative intensity of spectral lines

Spectra of hydrogen atom and larger atoms all displayed three identifiable types of spectral lines

based upon their width categorised as:

o Sharp (s) lines

o Primary (p) lines

o Diffuse (d) lines

Intensity of individual lines varied also

The model and Bohr’s postulates provided no explanation for these observations

The existence of fine and hyperfine spectral lines

Using interferometry, Michelson observed that some even finer spectral lines (hyperfine) existed

o Bohr’s model did not provide any explanation for this observed phenomena

The Zeeman effect

When a gas is excited while in a magnetic field, emission spectrum produced shows a splitting of

spectral lines (Zeeman Effect)

o Bohr model could not account for this



The limitations of classical physics gave birth to quantum physics

Describe the impact of de Broglie’s proposal that any kind of particle has both wave

and particle properties

Louis de Broglie equated Einstein’s energy – mass relationship with Planck – Einstein equation

o ,

and then

o showed light had properties of particles and

showed light had properties

of waves

Led de Broglie in 1924 to make the bold proposal that all particles must have a

wave nature as well as a particle nature

At this time, particles of matter such as electrons, atoms and alpha particles were known to

possess properties of mass and charge but no experimental evidence to indicate particles

exhibited wave phenomena

Through the use of mathematics, (using and

) he worked out that any moving

object should have a wavelength of:

o

Impact:

Significant impact in the scientific community as it provided an alternative way of describing the

nature of matter

Initiated the revolution in which Heisenberg, Schrodinger, Dirac, Born, Pauli and others

developed a detailed theory called quantum mechanics

Led to physical understanding of stationary states in Bohr’s model of the atom and gave model

credibility it required to be accepted and developed upon by the scientific community

Define diffraction and identify that interference occurs between waves that have been

diffracted Diffraction –bending of waves around obstructions such as an edge, narrow opening or slit

o Diffraction of light occurs when light passes through a very finely ruled grating

o Diffraction occurs when waves meet an opening/edge which acts as a point source of

coherent circular waves

Interference – interaction between two or more waves travelling through the same medium

o Interference occurs between waves that have been diffracted

o Constructive interference - when crest of one wave meets the crest of another (or

trough – trough), they combined to give an even greater crest (or greater trough)

o Destructive interference – when a crest meets a trough, they cancel each other



Describe the confirmation of de Broglie’s proposal by Davisson and Germer

De Broglie proposed mathematically that all particles have wave and particle properties

o Made prediction using theoretical work, having no experimental evidence

Clinton Davisson and Lester Germer studied surface of nickel by examining scattering of

electrons

o Expected that even the smoothest surface would still appear “rough” for electrons Accident occurred and air entered vaccum chamber and oxide film formed on metal surface

o To remove oxide film, they heated the metal to temp below its melting point

Had effect of annealing the metal – recrystallised surface

Crystals that formed had greater width than their electron beam

Experiment was performed again

o Diffraction patterns were observed

Since diffraction is a property of waves which particles do not possess, they established electron

had both wave and particle nature

o De Broglie’s proposal was confirmed

Explain the stability of the electron orbits in the Bohr atom using de Broglie’shypothesis

De Broglie proposed that the electrons in atoms behave like standing waves, which wrap around

the nucleus in an integral number of wavelengths – electron waves

o Standing waves –waves that do not propagate but vibrate between two boundaries:

points that they do not vibrate are called nodes and points that vibrate between max

and min positions are known as anti nodes

Electron wave model of the model explained the stability of electron orbits in the Bohr atom

o Bohr’s first postulate can be explained

as electrons are now standing waves, they are no longer moving charges and

hence will not emit radiation

standing waves do not propagate, and therefore are stable and will not lose anyenergy

De Broglie’s hypothesis enables mathematical derivation for Bohr’s third postulate –

quantisation of angular momentum

o Circumference of electron orbit is and there has to be a number fitting in the

circumference:

Correlation between de Broglie’s matter wave equations with Bohr’s third postulate in relation

to the quantisation of angular momentum reinforces the suitability of the matter wave concept

to explain the Bohr model

If non – integer values for the standing waves occurred (e.g. a Bohr radii of for a

quantum shell), then the vibrations would be forced and attenuate rapidly, that is, not be

sustainable. Only when , does this stability function=



Gather, process, analyse and present information and use available evidence to assess

the contributions made by Heisenberg and Pauli to the development of atomic theory

Werner Heisenberg

Heisenberg developed matrix mechanics in 1925 which successfully explained and predicted

electron energy levels in the Bohr atomo Purely mathematical

In 1927, he proposed the uncertainty principle that states there are pair of quantities that

cannot be determined simultaneously

o The pair he identified was position and momentum – it is impossible to know both these

quantities simultaneously

– calculated that product of uncertainty in position and

uncertainty in momentum is never less than amount involving

Uncertainty principle arises due to impossibility of measuring the position of a

particle without causing a disturbance in the moment (vice versa)

Contributions:

Added further dimension to electrons inside the atom

His uncertainty principle has changed the way science view atomic structure and is now regarded

as one of the most important and central principles of quantum mechanics

Brought change in which scientists approach quantum physics

Wolfgang Pauli

Applied Heisenberg’s quantum mechanics and applied it to the hydrogen atom – was able to

derive Balmer’s equation and Rydberg’s constant

o Able to do this solely with quantum mechanics and not as a mix of classical and quantum

physics as Bohr had done previously

Validated quantum mechanics, the new atomic theory

The first three quantum numbers are the principle quantum number (), angular momentum

quantum number () and the magnetic quantum number ()

o Pauli introduced the fourth quantum number – the magnetic spin quantum number ()

Pauli proposed his exclusion principle that states no two electrons in the same atom can have all

four quantum numbers the same

o Introduction of fourth quantum number and his exclusion principle showed there were

maximum combinations of quantum numbers for electrons in each shell – used this to

explain the max number of electrons that could exist in each shell

Explained position of first 20 elements in periodic table

Contributions:

Explained the maximum number of electrons in each shell

o Provided a quantum explanation for the structure of the periodic table

Most electron behaviours can now potentially be explained by Pauli’s exclusion principle and

quantum numbers

Proposed existence of the neutrino, a significant subatomic particle

Therefore contributed significantly to development of atomic theory



The work of Chadwick and Fermi in producing artificial transmutations

led to practical applications of nuclear physics

Define the components of the nucleus (protons and neutrons) as nucleons and

contrast their properties

Protons and neutrons are both nucleons – particles found in the nucleus and are slightly

different

o Both have masses on the same order (measured in amu)

o Neutron is slightly heavier than proton

o In terms of charge, proton has same charge as electron but positive while neutron has

no charge at all

Protons affected by magnetic and electric fields / neutrons are not

Therefore neutron is harder to detect

Discuss the importance of conservation laws to Chadwick’s discovery of the neutron

In 1920, Rutherford proposed that a neutral particle, with mass comparable to that of a proton

must be another constituent of the nucleus

o Successfully explained why atoms had a larger mass number compared to actual number

of protons

o Explained beta emission

Was incorrectly believed that electrons were inside the nucleus

Bothe and Becker in 1930 fired alpha particles at beryllium and found that a high penetrating

radiation was produced

o Believed it was gamma rays, however it was much more highly penetrating

Joliot and Curie allowed the mysterious radiation to fall onto paraffin – hydrogen carbon very

rich in hydrogen atoms and found that it knocked protons (hydrogen nuclei)

o If gamma rays, high penetrating power would have resulted in fewer interactions with

protons

o When conservation of energy and momentum was applied to collision with gamma rays

and a proton, there must have been an tenfold increase in the interaction

Did not obey the conservation of energy and momentum

Chadwick set up two experiments where he fired alpha particles at a beryllium target. He

allowed unknown radiation in first experiment to pass through paraffin blocks and in the second

experiment to pass through nitrogen gas

Chadwick applied the conservation of energy and conservation of momentum to the interaction

of a neutral particle with a proton. He made measurements of the recoil of nuclei of hydrogen

after interactions with his proposed neutron



o Measurements were difficult but led to the mass of a neutron being calculated to be

1.15 times that of a proton

o After a month, submitted for publication paper entitled “possible existence of a

neutron”

o Chadwick was able to produce extremely convincing evidence, not only of the neutron,

but also for the idea of protons and neutrons making up the nucleus of atoms, with mass

–energy being involved in the binding of the particles into a nucleus

o

Define the term “transmutation”

Transmutation are nuclear reactions where one element is transformed into another because

the number of protons in the nucleus change

o Can occur due to alpha or beta decay

In 1919, Rutherford fired alpha particles into nitrogen gas and detected a highly energetic

particle that he identified to be a proton

o First artificially induced transmutation as alpha particle had collided with nitrogen

nucleus to produce an oxygen nucleus and a highly energetic proton

Describe the nuclear transmutations due to natural radioactivity

Nuclei with more than 83 protons ( ) or atomic mass numbers greater than 209 ( )

are unstable and decay

o Repulsive electrical forces between protons overcome the strong nuclear force

Odd number pairing of protons and neutrons make nuclei exist outside zone of stability and are

inherently unstable

Alpha decay:

Usually occurs with large unstable nuclei

o Remember: helium has no electrons and is therefore positively charged

Beta decay:

Beta – minus decay occurs when ratio of neutrons to protons is too high

o Transformation of a neutron into a proton, an electron an antineutrino

Beta – plus particle is identical to an anti – electron, commonly called a positron

o Transformation of a proton into a neutron, positron and a neutrino

Electron capture – process in which an electron from an inner shell is captured by the nucleus

o

Gamma radiation:

After emission of alpha or beta particle, daughter nucleus sometimes left in excited state

o Change in energy from excited energy state to ground state result in emission of a very

high energy photon called a gamma ray

o Gamma radiation does not change the atomic number or mass, therefore not an

example of transmutation



Discuss Pauli’s suggestion of the existence of neutrino and relate it to the need to

account for the energy distribution of electrons emitted in - decay

In alpha decay – energies of ejected alpha particles have well

defined values of kinetic energy whereas in beta – particle decay,

ejected beta particles have a broad spectrum of kinetic energies

By 1930s, experiments clearly showed spectra of kinetic energiesof ejected beta particles when graphed were a smooth and

continuous curve ranging from just above zero to a max. value

dictated by parent nuclei

o slow beta particles were missing kinetic energy – violating

conservation of energy

o sum of momentums before and after beta decay was not

equal

Wolfgang Pauli in 1930 proposed that a new particle was also emitted during the beta decay

process

o New particle had no charge and would only very weakly interact with matter

o Energy released during a decay could be shared between beta particle and neutrino inany ratio

Allows distribution of energies to be explained

Fermi formally proposed a theory for beta decay whereby a neutron in the nucleus of an atom

was transformed into a proton, an electron and an antineutrino

o Theory also proposed fourth force of nature – weak nuclear force

Detection of neutrinos can be extremely difficult:

o Neutrinos have no charge and do not cause ionisation – cannot be detected by

conventional detectors (cloud chambers)

o Neutrinos have a mass of virtually zero

o Neutrinos are invisible

Properties of neutrinos:o Neutral

o Extremely small mass

o Travel at speed of light

o Possess both momentum and energy

o Have an intrinsic spin

Evaluate the relative contributions of electrostatic and gravitational forces between

nucleons

Both the gravitational and electrostatic forces are inverse square forceso Both should become large at small separation of nucleons in a nucleus

Force of gravity will provide an attractive force between proton – proton, proton – neutron and

neutron – neutron, but there must be electrostatic repulsion between pairs of protons

Account for the need for the strong nuclear force and describe its properties

New model of nucleus posed a problem – what forces held the protons and neutrons together to

form a stable nucleus?

o Electrostatic repulsion is far stronger than gravitational force, therefore third

fundamental force of nature was required to explain stability of nucleus

Strong nuclear force include:

o An independence of charge – all nucleons bind together with the same force



o Acts over short distances of about ( ), the diameter of a nucleus and within

this range, force is much stronger than electrostatic forces

o Strong nuclear force between nucleons acts only between adjacent nucleons, not on

entire nucleus

At extreme short distances, strong nuclear force becomes repulsive, then it becomes attractive

as distance increases, then increasingly weaker at large distances

Explain the concept of a mass defect using Einstein’s equivalence between mass and

energy

Mass defect – difference between mass of a nucleus and the total mass of its constituent

nucleons

o Total mass of a stable nucleus is less than total sum of mass of protons and neutrons

When nucleus forms, some energy is radiated away and since , loss of

energy reduces mass of nucleus

o Unstable nucleus of a large atom splits – the combined mass of daughter nuclei is less

than the mass of parent nucleus

Loss of mass is caused by some of original parent mass being transformed into

kinetic energy of the fragments Binding energy – how much energy needed to separate the nucleus of the atom back into

separate protons and neutrons

Stability of nucleus is indicated by average binding energy per nucleon – measure of how

strongly an average nucleon is bound to a particular nucleus

o Fusing light nuclei together will increase binding energy per nucleon, thus energy would

be realised – fusion

o If nucleus of a heavy atom splits, binding energy per nucleon will increase – fission



Describe Fermi’s initial experimental observation of nuclear fission

1934 – 1938: Enrico Fermi and fellow researched fired neutrons at a variety of target elements

and produced many new unstable radioactive nuclei

o In most cases, nuclei in targe would absorb neutron and emit a beta particle

Fermi tried to cause uranium to undergo beta decay to produce transuranic elements heavier

than uraniumo Found that slow electrons were far more effective than fast neutrons because they had a

greater chance of being captured by the nuclei

Slower neutrons have a much longer wavelength, increasing the likelihood of the

neutrons interacting with nearby nuclei and hence increasing likelihood of it

being absorbed/captured

o He observed when he bombarded uranium with neutrons, instead of producing a single

heavy radioisotope, separate products were identified including barium, radium and

lanthanum

First observation of fission, although he did not realise what was happening in

his experiment

Describe Fermi’s demonstration of a controlled nuclear chain reaction in 1942

Fermi realised that since fission of a uranium atom released 3 neutrons, and that since 1 neutron

is required to cause fission in a uranium nucleus, a chain reaction of nuclear fission could be

produced that would release a great deal of energy

o Fermi saw that if neutrons were absorbed such that not all of them produced additional

fission, a controlled chain reaction could be produced

IN 1942 Fermi and his team built a fission reactor in a squash court at Chicago University

o 40 000 graphite bricks, weighing 350 tonnes – used as a moderator

o Control rods made from sheets of cadmium was used – prevent reaction from going out

of control

His reaction was successful and was able to generate 0.5 watts in a self sustaining reaction

Compare requirements for controlled and uncontrolled nuclear chain reactions

Requirements for uncontrolled nuclear chain reaction:

Mass of fissionable material such as Uranium – 235

Supercritical mass of fissionable material

o critical mass - amount of fuel that produces sufficient neutrons to cause new fission

A means of slowing neutrons down

Example: large lump of fissionable material will meet all 3 criteria

Requirements for controlled nuclear chain reaction: Mass of fissionable material

Super- critical mass of fissionable materials

Mechanism to capture neutrons so that overall number of neutrons that cause fission is constant

o E.g. uranium releases 3 neutrons – control rods in a reactor captures two of the neutrons

so that only one goes on to cause another fission reaction

If all 3 neutrons caused fission, each stage of fission would have triple the

number of reactions, resulting in exponential growth of energy release and an

uncontrolled reaction

o By absorbing excess neutrons, reaction is controlled and doesn’t spiral into explosion

Moderator to slow fast neutrons in the reactor

o Fast neutrons travel past nuclei rapidly and have a low chance of being absorbed



Perform a first hand investigation or gather secondary information to observe

radiation emitted from a nucleus using Wilson Cloud Chamber or similar detection

device

An understanding of the nucleus has led to large science projects and

many applications

Explain the basic principles of a fission reactor

Fission reactor uses a nuclear reaction to generate electricity

o Heat from nuclear reaction is used to produce steam which turns turbine

The reactor must meet several requirements for a controlled reaction. Firstly it must ensure a

reaction takes place and secondly that the reaction doesn’t go out of control

There are several components of a fission reactor

o Core: houses the fuel rods, control rods, a coolant system and moderator materialo Fuel rods: Tubes filled with enriched uranium to provide the critical mass required

o Control rods: usually made from cadmium or boron are located between the fuel rods.

They absorb neutrons and are adjusted so that the chain reaction proceeds at a constant

rate (can be moved in and out to control reaction rate)

o Moderators: Entire reactor is immersed or surrounded by a moderator to slow down

neutrons, improving chance of neutron capture by a nucleus. Commonly moderators

consist of either heavy water, graphite or other organic compounds

o Coolants: required to extract heat from the reaction and prevent reactor from melting.

IT flows through the reactor then out into a heat exchange that takes heat extracted

from coolant and uses it to boil water

o Radiation shielding: Reactor core emit large quantities of gamma radiation and neutrons.Lead and graphite are used to absorb and reflect radiation, which protects the

containment walls of the reactor vessels

Designed to protect people and environment and prolong working life of the

reactor facility

Describe some medical and industrial applications of radio – isotopes

Radioisotopes are increasingly being used in medicine, scientific research and industry as they

provide a unique tool that is often cheaper and more effective than alternative techniques and

processes

In medical fields, radioisotopes are used for imaging/diagnosis and for treatment

o In imaging, transmission of radiation through the body and degree to which radiation isabsorbed can be used to remotely examine the body – often used to examine brain

activity (using Positron Emission Tomography)

o By injecting radioisotopes and examining where they end up, circulatory system can be

investigated

o Radioisotopes are also frequently used to kill cancer cells, the radiation destroying them

In industry

o Used to examine stress fractures in metals such as in aircraft wings

o Detecting leaks in pipes that may be otherwise difficult to find

Since radiation will escape from a leaking pipe

o To irradiate medical supplies and food to kill bacteria

o Monitor and control thickness of sheet metals, foils and paper



Describe how neutron scattering is used as a probe by referring to the properties of

neutrons

Neutrons has several advantages as a probe

o Neutron scattering is used to probe materials

Neutrons, being neutral are not affected by electric fields and unlike X – rays are able to deeply

penetrate matter, through electron clouds, to then scatter and form interference patterns thatcan reveal important structural detail

o Interference patterns can be used to determine the arrangement of atoms in molecules

and have proved valuable in determining the structure of various organic molecules and

the structure of viruses

o Energies of neutrons scattered from various materials can be measured using sensitive

detectors

Information can provide very important detail on chemical composition and

structural characteristics of complex molecules

Neutron scattering is particularly useful in identifying protons and small nuclei which is very

difficult with other techniques

o Have a very short de Broglie wavelength making them able to resolve very smallobjects, and reveal detail regarding the location and structure of nuclei

Magnetic moment of the neutron makes it useful for studying magnetic materials

o E.g. superconductors

Neutrons have an energy similar to the vibrational energy of atoms in solids and liquids which

enable neutrons to be used to study the motion of atoms in molecules in detail

Neutrons interact strongly with nuclei as they both have comparable size

o Strength of interaction varies for different nuclei which makes it possible to study

isotopes of light elements

Disadvantages:

Nuclear reactor is required to produce the neutrons which presents problems associated with

wastes and social issues

Identify ways by which physicists continue to develop their understanding of matter,

using accelerators as a probe to investigate the structure of matterLinear accelerator

Charged particles are fired through a long evacuated tube (can reach a few kilometres)

Charged particles pass through one cylindrical electrode and are accelerated by an electric field

as they pass through a gap before encountering another electrode

o Process repeated and particles increase their energy

As velocity of particle increase, the alternating accelerating potential has to keep in step with the

particles and this requires the cylindrical electrodes to become longer and longer



Cyclotron

Cyclotron, a particle accelerator gives a charged particle many “kicks” as it passes through the

electric fields between the “dees” of the cyclotron

Whole apparatus lies between the poles of a large magnet. Therefore the particles experience

magnetic force while inside the hollow Dee and move in circular paths, with radii of circular path

increasing each time the particle gains energy as it passes through the gap between the dees

Developing the Standard Model

Particle accelerators has been instrumental in providing experimental evidence that led to the

development and then support for, the “Standard Model” of matter

By smashing particles into targets or other particles travelling in opposite directions, particles

formed could be studied (properties and nature) by using known physical laws to analyse tracks

and trailed collected by detectors/sensors e.g. bubble chamber

Discuss the key features and components of the standard model of matter, including

quarks and leptons

In 1930s, list of identified sub – nuclear particles comprised the electron, the proton and the

neutron; the positron and neutrino had been proposed by Pauli

Quarks:

1964 – Murray Gell-Mann and George Zweig proposed existence of quarks which was later

recognised as fundamental particles – smallest particles that could not be broken down further

o Carries the properties of mass, charge and colour

There are six varieties (flavours) of quarks

o Anti – quarks also exist

Quarks possess charges that are either

or

of the charge of an electron

All quarks act through strong nuclear force



Hadrons:

Quarks do not usually exist by themselves as they are unstable

o They exist in more stable forms by combining with one or two other quarks

Combination of quarks is known as hadrons

o There are two types of hadrons: baryons (three – quark combinations) and mesons (two

– quark combinations)

Baryons:

o Make up everyday matter as they form the nucleons

Protons consist of two up quarks and one down quark

Neutrons consist of one up quark and two down quarks

o Have half integer spin

Mesons:

o Meson consists of a quark and anti – quark

o Have zero or integer spins

Leptons:

Another type of fundamental particle that have either very little or no mass

o Single elementary particles that exist by themselves and are not affected by the strong

nuclear force

Most commonly known lepton is the electron

These leptons has an associated neutrino that is named accordingly

Every lepton has a corresponding anti – lepton

All leptons interact through weak nuclear force and charged leptons interact through

electromagnetic force

Bosons:

There are four fundamental forces in the Universe and using the standard model of matter,

these four forces are thought to act through the exchange of force particles – bosons

o Forces are mediated by bosons Bosons are force – carrying particles



o Electromagnetic force acts through photons

o Strong nuclear force acts through gluons

o Weak nuclear force acts through weakons

o Gravity acts through gravitons

Has not been proved. Included in the model for sake of completeness

Attraction forces is by having matter pulling on the force particles as they are exchanged

whereas repulsion forces are conveyed by having force particles being pushed away as they are

exchanged

Generation:

As generation number increases, mass of particles increase

Second generation particles are less stable and quickly decay to form the first generation

particles and the third generation particles are even less stable and decay rapidly to form the

second generation particles

Conclusion:

Scientists have come a long way from the most “primitive” model of the atom suggested byThompson, to the idea of the quant and Bohr’s and eventually to the more sophisticated

quantum mechanics, based upon which de Broglie, Wolfgang and Heisenberg further advanced

the model of the atom

Gather, process and analyse information to assess the significance of the Manhattan

Project to society

The Manhattan Project was the code name used by the US army to describe the project to

develop atomic bombs during World War II

The impact of the production of the atomic bomb has been immense and far reaching and still

impacts on our society today



The dropping of two atomic bombs led to deaths of many Japanese people in Hiroshima and

Nagasaki, however this brung a swift end to the war, avoiding a long process of invasion with

even more casualties.

It set up a political cold war that maintained for some forty years, however USA and Russia never

broke out into conflict as either side was concerned that aggressive action would result in

nuclear warfare

o Peace was maintained due to the Manhattan Project and its development of nuclear

weaponry

In modern times however, nuclear power is proving to be a dangerous bargaining chip for rogue

states such as North Korea and Iran which are using nuclear weapons as leverage in negotiations

with the Western world

o They provide a constant threat to global security

However the project accelerated the development of nuclear technologies, leading to nuclear

medical applications in the treatment of such conditions as cancer and other malignant tumours.

It has also offered an alternative way to produce power, decreasing fossil fuel consumption

Assessment:Although the Manhattan Project led to the deaths of many people and resulted in a build up of nuclear

arsenals across many nations, in the end, it allowed a nuclear stalemate between the USSR and USA

preventing several wars and averting possible deaths. The Manhattan project has also led to the

acceleration of nuclear medicine technologies that are benefiting those with conditions that can now be

treated. Therefore, the Manhattan Project has had an overall positive impact on society.

Identify data sources, and gather, process and analyse information to describe the use

of:

- a named isotope in medicine

- a named isotope in agriculture

- a named isotope in engineering

Medicine: Technetium – 99m

This radioisotope is incorporated into some blood serum and injected into a patient’s bloodstream. It is a

gamma emitter and therefore the distribution throughout the body can be measured with a scintillation

counter and used to detect blood clots, constrictions and other circulation disorders. It can also be

attached to biological carriers to study different areas of the body (e.g. combined with tin compound, it

can become attached to blood cells which can be traced to detect abnormalities in the heart) Properties

of technetium – 99m that make it useful for medical diagnosis are:

Its short life of 6 hours – long enough for medical investigations and short enough to minimise

the patient’s exposure to radiation

It is readily excreted (minimum exposure)

Ability to be attached to a range of biological carriers and thus can concentrate in a number of

different types of tissues and organs for diagnostic test to be performed

Is a gamma emitter – penetrative enough so that it can be detected outside the body and also

causes least amount of ionisation comparable to beta and alpha emitters, making it safer to use

To produce Technetium – 99m:

Neutron bombardment of Mo – 98

o

Above process generates Mo- 99(half life 67 hours)

Mo -99 decays to form Tc 99m (Mo – 99 sent to hospital by a radioisotope generation kit)



Tc – 99m can then be separated from the Mo -99 with saline solution at the hospital

Engineering: Strontium – 90

A radioactive source and a detector can be used to monitor and control the thickness of materials such

as steel sheets, paper, aluminium foil and plastic film as it emerges from the rollers in factories. The

amount of radiation passing through the material depends on its thickness; therefore variations in the

intensity of radiation detected when passed through the material indicate problems with the thickness.

For e.g. thicker material absorbs more radiation and so the signal drops. For this purpose, radioisotopes

such as strontium – 90 are used. These radioisotopes have fairly low energy emission and a long half-life

(28 years). A long half-life is ideal so that the radioisotope does not have to be replaced too frequently.

Lower emission intensity also results in a larger proportion of the radiation being absorbed by the sheet,

making any changes in absorption more noticeable, thus increasing the sensitivity of the device. It also

means that safety precautions are minimal.

Agriculture: Phosphorous – 32

Used as a biological tracer to study natural processes such as nutritional uptake by plants in the

natural environment and in agricultural settings

Phosphorous – 32 can be introduced into plants or crops as radioactive phosphate ions

o Phosphate is an essential nutrient for the growth of plants and crops

o The plants or crops process the radioactive phosphate in the same way as they handle

normal phosphate

By tracing the radiation (beta emissions) of the phosphorous, biochemical processes such as

nutritional uptake, transportation and storage can be studied – leading to a better

understanding of favourable conditions for plant growth, maximising yield and increasing

efficiency of the farming process

Properties that make it useful are:

o Can be easily introduced into biological systems and can be traced

o Compounds formed by phosphorous – 32 have same chemical properties compared to

those formed by non – radioactive phosphorous (phosphorous – 31) so they are

indistinguishable by plants

eddie from quanta to quarks.docx

Documents