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2012-04-12 21:48:45

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  1. What are mesons?
    Hadrons that are composed of quark and anti-quark and are therefore very unstable. Mesons have zero or integer spin.
  2. What are baryons?
    3 quark combination e.g. protons and neutrons. Baryons have half-integer spin.
  3. Name the 6 flavours of quarks and their associated charge.
    • Up (2/3)
    • Down (-1/3)
    • Top (2/3)
    • Bottom (-1/3)
    • Charm (2/3)
    • Strange (-1/3)
  4. Name the 6 kinds of leptons and identify their electric charge.
    • Electron (e-) (-1), electron neutrino (0)
    • Muon (μ) (-1), muon neutrino (0)
    • Tau (τ) (-1), tau neutrino (0)
  5. What spin do leptons have?
    Half-integer spin
  6. Bosons - Standard Model of Matter - forces responsible
    • Gluons carry the strong force to bind quarks involved in these strong interactions. Residual strong interactions create the force responsible for binding nucleons together in a nucleus.
    • Radioactivity is explained by nuclear weak interactions.
    • Electromagnetic interactions are involved in binding the electron (a lepton) to the positive nucleus, with photons the boson, and thus resulting in the atoms that compose matter around us.
    • Weakest of four interactions is involved where particles with mass are attracted by gravity, a boson called graviton, that is still yet to be discovered.
  7. What are the 4 bosons and 1 other that is still yet to be discovered?
    Photon, W boson, Z boson, Gluon. The graviton is the final boson still yet to be discovered.
  8. What force is the W and Z boson responsible for?
    They create the weak nuclear foce in beta-decay to change nuclear particles into other particles.
  9. How many types of gluons are there?
  10. Which particles obey the Pauli Exclusion Principle and which don't?
    Fermions (Quarks and Leptons) obey, bosons do not obey.
  11. Describe Fermi's demonstration of a controlled nuclear chain reaction. How many tonnes of raw material, in how many slugs. How much graphite was used and how much power was generated.
    In 1942 in a squash court in Chicago at Stagg Field, Fermi took 50 tonnes of natural uranium in 20,000 slugs to demonstrate the first controlled fission chain reaction. His reactor contained 400 tonnes of graphite as a moderator. He also used cadmium rods to absorb excess neutrons and prevent the fission from going out of control. Fermi successfully generated 0.5 watts in his self-sustaining reaction.
  12. Describe how you constructed a cloud chamber.
    • Soak some filter paper with methylated spirits.
    • Place the soaked filter paper inside a transparent.
    • Cool the container by placing dry ice underneath the container.
  13. How is radiation detected using the cloud chamber constructed in your prac?
    Radiation source was placed next to the chamber and trails observed. Charged particles travel through, ionise the air which enable the vapour to condense onto it, creating a trail.
  14. How were alpha and beta particles identified in your cloud chamber prac? What about gamma rays?
    Alpha particles formed straight, short, strong trails. Beta particles formed relatively straight, longer, thinner trails. Gamma radiation do not create a stream of ions for condensation so gamma emissions were not detected.
  15. Why do alpha particles form strong, short, straight trails in the cloud chamber?
    Strong because they are more highly charged, so ionise air more than others, resulting in more condensation. Short because their strong charge attracts electrons rapidly, so before they travel a long distance they are converted into neutral helium and therefore can no longer ionise the air. Straight because of their large mass and so are not easily deflected by other particles.
  16. What indicates the stability of a nucleus?
    Average binding energy per nucleon - how strongly an anverage nucleon is bound to a particular nucleus.
  17. How does nuclear fusion work?
    The joining of light nuclei, creates a nuclei with a higher average binding energy per nucleon and hence energy is released.
  18. How does nuclear fission work?
    When a large mass number n ucleus is split in two, we produce two nuclei with higher average binding energy per nucleon than the original nucleus. So e nergy is released.
  19. Who discovered radioactivity?
    Henri Becquerel in 1896.
  20. What was observed in Rutherford's alpha particle scattering experiment?
    About 1 in 8000 alpha particles were deflected at an angle greater than 90 degrees.
  21. What was the relative size of the nucleus as proposed by Rutherford?
    Nucleus was about 10,000 times smaller than the radius of the atom
  22. What are the different types of hydrogen spectral lines?
    • Lyman series (ultraviolet lines) - transitions to ground state (n = 1).
    • Balmer series (visible region) - transitions to second lowest energy state, or first excited state (n = 2).
    • Paschen series (infrared lines) - transitions to third lowest energy state, or second excited state (n = 3).
    • Brackett series (infrared lines) - transitions to the third excited state (n = 4).
    • Pfund series (infrared lines) - transitions to the fourth excited state (n = 5)
  23. What are the limitations of Bohr's atomic model?
    • No explanation for the relative intensity of spectral lines
    • No explanation for the observed hyperfine splitting of spectral lines
    • Did not work for larger atoms
    • No explanation for the Zeeman effect
    • Fused classical and quantum physics without any real justification - Bohr said the angular momentum of electrons was quantised according to:
  24. What did Davisson and Germer study to confirm de Broglie's proposal?
    The surface of nickel using an electron beam.
  25. What accidentally occurred in Davisson and Germer's experiment?
    The nickel oxidised when exposed to air.
  26. How did Davisson and Germer resolve the problem in their experiment?
    They heated the nickel to near its melting point, resulting in the formation of crystals larger than the wide of their electron beam.
  27. What did Davisson and Germer find out from their experiment?
    When they fired the electron beam at the nickel and reflected it to a detector they observed that electron-scattering produced an interference pattern with distinct maxima and minima rather than being uniformly distributed, very similar to an x-ray diffraction pattern, confirming the wave nature of the electron and their wavelength as being very close to what de Broglie had mathematically predicted (2 x 10-12 m discrepancy), hence supporting his hypothesis.
  28. What explanation did de Broglie's hypothesis provide regarding Bohr's electron orbits?
    Bohr's stationary states existed and electrons did not fall into the nucleus because according to de Broglie their enegy produced a wavelength that resulted in its existence as a standing wave around the hydrogen nucleus. A 'quantised orbital' in Bohr's atom corresponded to an orbital in which the circumference equalled an integral number of the electron's wavelength.
  29. What was Fermi's 2 discoveres in his initial nuclear fission experiment?
    • In 1930s, many scientists, including Fermi, were interested in investigating the properties of heavy nuclei using neutron bombardment and the relationship between stability of nuclei and the number of particles they contained.
    • Otton Hahn & Lise Meitner's calculations - helped lead Fermi to try cause fission in U-235 by β-decay.
    • Slow neutrons slow down by paraffin wax were more effective in causing fission than fast neutrons.
    • He also found 4 separate products each with different half lives were formed rather than a single heavy radioisotope. Fermi did not realise what was happening in his experiment. What was eventually found was uranium was a mixture of isotopes U-235, U-238 and U-233. Here U-235 was reacting, absorbing the neutron and forming U-236 which then decayed into a Kr isotope, Ba isotope and 3 neutrons. Gamma rays were also emitted. The extra neutrons opens possibilitiy for a chain of n uclear fission to occur. Fermi did not realise this at the time.
  30. Why were slow neutrons more effective than fast neutrons for fission?
    Slow neutrons spend more time in the vicinity of the nucleus and hence have a better chance of being captured which is also due to their long de Broglie wavelength.
  31. What is a moderator and what substances are used?
    • Moderators are substances that slow down neutrons produced in the fission reaction to hehlp maintain the fision reaction at a desired rate.
    • The most effective moderator is one consisting of atoms of mass close as possible to that of the neutrons.
    • Ordinary water would be the best moderator where hydrogen atoms contain 1 proton & 1 electron.
    • However, ordinary water tend to absorb neutrons
    • Heavy water is used instead - hydrogen isotope with 1 proton, 1 neutron & 1 electron - slows down the neutrons but doesn't absorb them.
    • Other moderators include: graphite, or various other organic compounds. Moderators increase efficiency as slow neutrons are more effective in nuclear fission than fast neutrons.
  32. What are control rods made of?
    Cadmium or boron.
  33. Why is a coolant used in a fission reactor?
    It extracts heat from the reaction and prevents the fuel from melting. The coolant flows through the reactor then out into a heat exchanger that takes the heat extracted from the coolant and uses it to boil water.
  34. What are the 3 levels of shielding in the fission reactor?
    • Graphite shield reflects neutrons back into the core.
    • Thermal shield prevents unwanted heat loss from the core.
    • Biological shield of about 3 metres of concrete mixed with lead pellets, absorbs gamma rays and neutrons.
  35. How long is the linear accelerator?
    The linear accelerator is a 3 km evacuated tube.
  36. How does the linear accelerator work?
    A charge particle passes through one cylindrical electrode (drift tube) and are then accelerated by an electric field as they pass through a gap before encountering another eletrode. This process is repeated and the particles increase their energy.
  37. Why do the cylindrical electrodes get longer in the linear accelerator?
    The alternating accelerating potential connecting the electrodes has to keep in step with the particles increasing energy and hence speed.
  38. Describe the apparatus of the cyclotron.
    Between the 'dees' is an electric field which boosts the charge particle's energy. Whole apparatus lies between the poles of a large magnet.
  39. What happens to the path of the charged particle as it gains energy passing through the dees of a cyclotron?
    The radius of thehir path increases each time they gain energy. (r = mv/qB). When they reach the size limit of the magnetic field they are deflected into a target.
  40. What force is the photon responsible according to the Standard Model of Matter?
    The electromagnetic force which includes the electrostatic and magnetic forces. They bind charged particles, atoms and molecules together and act over long distances.
  41. Discuss the structure of the Rutherford model of the atom
    In 1909 Rutherford and his students scattered alpha particles (helium nuclei) off atoms in a thin sheet of gold foil. Rutherford's scattering experiments showed that atoms consist of a positively charged nucleus surrounded by a negatively charged electrons.

    Rutherford then envisioned the electrons orbiting the nucleus analogous to the planets orbiting the Sun, and believed to be held in orbits by a Coulomb force of attraction, which has the same mathematical form as Newton's law of gravity. There was also a lot of free space between the nucleus and the electrons.
  42. Whath are some problems with Rutherford's atomic model?
    Physicists at the time had problems with the solar system analogy of Rutherfrod's atomic structure. The positively charged nucleus should fly apart because of the electromagnetic repulsive forces between positive charges, as well as the lack of explanation for the atoms having stability without energy emission (in the form of EMR as predicted by Maxwell's equations for electromagnetic phenomena) from accelerating electrons. Because the electrons were suppose to lose energy, it would cause the electrons' orbits to decay until they crashed into the nucleus.

    Rutherford's model was also unable to explain atomic emission and absorption line spectra. Why do atoms such as hydrogen produce spectra at specific discrete wavelengths rather than the expected continuous wavelengths?
  43. What is a pion and what is it made out of?
    A pion (π+) is a meson (quark-antiquark combo) and is made up of an up quark and a down antiquark.
  44. What is an antipion and what is it made out of? What is its charge?
    An antipion (π-) or negative pion is the antiparticle of a pion (π+) and is made up of a down quark and an up antiquark. The charge is therefore -1/3 + (-2/3) = -1/3 - 2/3 = -1
  45. What generation is everyday matter composed of? What kind of particles are these?
    Flavours of quarks and leptopns can be paired in generations, where the particles of one generation are heavier than a previous generation. Generations II and III are t herefore unstable and decay into generation I particles, hence everyday matter is composed of generation I particles. Generation I particles are composed of up and down quarks to form protons and neutrons.
  46. What are the two categories that all matter can be divided into?
    • Matter particles - fundamental particles including quarks and leptons that have no known smaller components.
    • Force-carrier particles (strong nuclear, weak nuclear, electromagnetic, gravity). Standard model does not incorporate gravity as graviton has not yet been discovered.
  47. What are the limitations of the Standard Model?
    • Force of gravity is not incorporated.
    • No explanation for the masses that particles have e.g. why the top quark (t) is so massive (around 190 times the mass of a proton)
    • No explanation for the number of fundamental matter particles i.e. 6 quarks and 6 leptons.
    • No explanation as to whether there are additional quarks and leptons e.g. the question remains as to the existence of 'leptoquarks' which have fractional charges that form the integral charge of leptons.

    Hence, a grand unifying theory has not been achieved. Subsequently through research, scientists are continually developing the understanding of matter to make further developments to the standard model.
  48. What is Heisenberg's contribution to the development of quantum mechanics?
    • Explained some of the limitations of Bohr's model incl. presence of hyperfine lines and spectral lines of larger atoms other than hydrogen.
    • Heisenberg showed that uncertainty is an inherent property of quantum mechanics. He showed that there are pairs of quantities such as position and momentum that cannot be determined simultaneously with great accuracy - Uncertainty Principle.
    • This is a significant contribution - as Heisenberg devised matrix mechanics he emphasised that one can only describe the location and motion of the electrons in terms of quantum probabilities, rather than mixing classical and quantum theory as Bohr had done (stating angular momentum of electrons was quantised). Hence Heisenberg provided a mathematical understanding of the atom.
  49. What is Pauli's contribution to atomic theory?
    • Pauli developed the Exclusion principle - no two electrons can have the same set of 4 quantum numbers. This is a significant contribution to the development of atomic theory as:
    • it gives a better understanding of how electrons are arranged in an atom - more specific than the stationary states of the Rutherford-Bohr model because it considered the principal quantum number, n, from Bohr's model; angular momentum quantum number, l; magnetic quantum number, m; and the 4th quantum number associated with 'spin'.
    • it explains the max number of electrons that can fill each energy shell - explains octet rule for chemical stability in atoms, and hence can be used to explain the chemical reactivity of atoms. the max no. of electrons in each shell corresponded to the number of diff sets of quantum numbers available for each shell.
    • it explains the position of first 20 elements in the periodic table.
    • his work with quantum numbers explained the Zeeman effect
    • also proposed the EXISTENCE OF A NEUTRAL PARTICLE to account for variable kinetic energies during beta decay, another significant subatomic particle
  50. When a proton and a neutron undergo decay what is it called and what is produced?
    • For a proton: beta plus decay gives a neutron, electron and neutrino is formed.
    • For a neutron: beta minus decay gives a proton, electron and an antineutrino.
  51. What are the requirements for a controlled nuclear chain reaction?
    • Requires control rods & moderators that slow the rate of production of neutrons causing fission until process has a constant rate - number produced equals number causing fission.
    • Only one neutron from the splitting of the nucleus goes on to hit another nucleus to cause further fission. The other neutrons are absorbed.
    • Requires a critical mass of the fissile material that is distributed in fuel rods - so critical mass is less concentrated.
    • Energy is released steady from a controlled, self-sustained, chain reaction releasing steady amounts of heat to create steam for use in electricity generation.
  52. What are the requirements for a uncontrolled nuclear chain reaction?
    • Each neutron released by the splitting of the nucleus is allowed to hit another nucleus, causing further fission.
    • Requires a supercritical mass of fissionable material (U-235 or plutonium-239) to allow enough neutrons to trigger an uncontrolled nuclear chain reaction.
    • Energy is released at an increasing exponential rate because there is a rapid build up of nuclei undergoing fission.
    • Mass defect (Δm) resulting in the lighter mass of the produced form by fission - responsible for the energy released during the explosion. That is E = Δmc2.
  53. How is the leakage of neutrons addressed in a fission reactor?
    Neutrons that escape the fuel cannot be captured to cause fission of another nucleus. Therefore, the total amount of fuel in a reactor core should be at or greater than the critical mass.
  54. What kind of material is the coolant in a fission reactor?
    Usually water, heavy water or carbon dioxide.
  55. What is neutron scattering used for? What does it involve?
    To analyse the internal structure and properties of matter. It involves elastic and inelastic collisions with the molecules of interest, causing ne utrons to scatter in directions determined by the neutron's wavelength and the structure of the molecules. The interference patterns produced are used to deduce the internal structure and properties of materials.
  56. Relate the use of neutrons as a probe with its properties.
    • Thermal (hot) neutrons have a short de Broglie wavelength due to their higher momentum, meaning it can resolve very small features and easily pass through the inter-atomic spaces within the lattice giving much detail on atomic arrangement and composition.
    • Their neutral charge means they are not affected by the electric fields of electrons or nuclei - able to penetrate deeper into a sample than X-rays or electron beams and are also diffracted by the nucleus, giving a more precise measurement of the inter-atomic distances than if diffracted by electrons (as in the case of X-rays)
    • Neutrons have a mass comparable to protons - so probe the nuclei of light atoms such as hydrogen; also suitable for studying the structure of organic substances which usually contain a lot of hydrogen.
    • Neutrons have a magnetic moment make them an ideal too for studying atomic magnetism in materials.
    • Neutrons have an energy similar to the vibrational energy of atoms in liquids and solids - study motion of atoms in molecules in detail.
  57. What is the main way physicists develop their understanding of the fundamental constituents of matter?
    Particle accelerators - either beams of particles move at relativistic speeds and fired at a target, or two high energy beams move in opposite directions and brought close together.
  58. Impact of advances in the understanding of matter on the work of physicists (How has understanding matter helps society and the work of physicsts)
    • massive accelerators in high-energy phsycs - provides info about fundamental building blocks of matter e.g. leptons & quarks - provides society with technologies relevant to energy conservation, production & communication
    • development of computers many physicsts and society rely on - expands the kind of instruments availbale we can use - CCD chips value for astonomers & geophysicsts in ultra-violet imaging applications - provies high-resolution data
    • However understanding of matter has not always been a benefit - nuclear weapons & explosives used as a tool in countries with weaker conventional forces such as Iran to meet agreements with powerful countries such as America.
  59. How does accelerating particles help increase physicists' understanding of the particles and matter?
    • Particles with high enough energy may get close enough to the nucleus to experience the nuclear strong force. When scattered, details about the nuclear strong force may be determined e.g. its range.
    • Higher energies allow them to be absorbed and new radioisotope produced. These are then studied to determine their properties - increasing understanding of matter not found in nature e.g. Ununoctium produced using an accelerator.
    • At even higher energies, particles may shatter the nucleus and produce new particles - some extremely short lived, though their tracks may be studied using a bubble chamber where their properties may be deduced.
    • With high enough energy, its de Broglie wavelength is small enough to probe inside smaller particles such as neutrons and protons - allow further understanding of their structure.
  60. How did Bohr's postulates lead to a mathematical model to account for the existence of the hydrogen spectrum?
    • Bohr was able to derive an expression for the energy of the orbits by combining the expression for potential energy of the electron-nucleus system with its kinetic energy as well as using his 1st and 3rd postulates. He used En = 1/n2 x E1 where En is the energy in the nth orbit and E1 = the energy of the electron in the 1st level = -13.6 eV. Bohr subbed this into an equation representing his second postulate dE = Ei = Ef and ended up with an equation in the same form as Balmer's equation. If the value of -E1/hc is calculated, it agrees with Rydberg's constant in Balmer's equation.
    • Balmer's equation is an empirical equation. A theoretical equation derived from Bohr's model of the atom now agrees with the empirical equation. This is a major achievement and offers very strong support for the Bohr model.
  61. What is the main particle physics centre for studying matter using accelerators?
    • CERN is the largest particle physics centre in the world and includes the Large Hadron Collider (LHC). It employs over half the world's particle physicists. The particle research carried out at CERN has allowed for advances in cancer therapy, medical & industrial imaging, radiation processing and electronics.
    • It has also offered cosmologists an insight into some of the detail of the conditions present shortly after the Big Bang, and the opportunity to study the behaviour and interactions of the various matter and anti-matter particles.
    • Hence through the use of particle accelerators, scientists are able to develop their understanding of matter.
  62. What is a synchotron and what are some of its applications?
    • A synchotron is a large machine about the size of a football field. It accelerates electrons to almost the speed of light. As the electrons are deflected through magnetic fields they create extremely bright light. The light is channelled down beamlines to experimental workstations where it is used for research. Applications include:
    • Medical research (high resolution imaging and cancer radiation therapy)
    • Agriculture (animal and plant imaging, soil studies)
    • Mining industry (rapid analysis of drill core samples)
    • Engineering (high resolutiong imaging of cracks and defects in structures)
  63. Define diffraction.
    The bending of waves as they pass around the corner of a barrier or as they travel through obstacles such as a slit. Diffraction of light also occurs when it is reflected from a surface with fine lines ruled across it. Diffraction is solely a wave property.
  64. What determines the amount of diffraction?
    The amount of diffraction (the sharpness of the bending) increases with increasing wavelength and decreases with decreasing wavelength. In fact, when the wavelength of the waves is SMALLER than the obstacle, no noticeable diffraction occurs.
  65. How do you produce an interference pattern?
    Consider a wave passing through 2 slits that are adjacent to each other. The wave undergo diffraction. The diffracted waves will now make contact with each other - they will interact with each other to cause interference. If the two waves overlap in phase, constructive interference occurs resulting in a bigger crest or trough. When crest meets trough or the two overlapping waves are out of phase, they cancel each other out - destructive interference. Alternating constructive and destructive interference will occur throughout the region where the two waves are in contact. With waves bending around a single obstruction, the corners of the object acts as point sources and interference occurs between the original wave and the new point source waves. Overall the process of diffraction results in an interference pattern - at some points the waves interfere construtively and at others destructively.
  66. Who else confirmed de Broglie's proposal?
    G.P. Thomson passed an electron beam at a very thin celluloid film and a very thin metal foil and produced images on a photographic plate of concentric circular rings, showing the diffraction and interference of the electron beams. The measured result was in agreement with the wavelength according to de Broglie's hypothesis.
  67. What did Chadwick's discovery of the neutron involve?
    • 1930 - Bothe and Becker fired alpha particles at beryllium and found highly penetrating radium produced - was not detected in a cloud chamber and didn't seem to be a particle - initially thought to be gamma radiation. It was measured to have an energy about 10 MeV, much more highly penetrating than other gamma rays observed before.
    • Joliot and Curie studied the highly penetrating radiation on a block of paraffin - a hydrocarbon rich in hydrogen atoms. They found protons (hydrogen nuclei) was knocked out when radiation bombarded the paraffin, with the protons having about 5 MeV and many more were emitted than expected. Now that charged particles (protons) were involved easier to determine their properties as the neutral neutrons could not be detected. If gamma rays were involved, there would have been fewer interactions with the protons.
    • However if we apply the conservation of energy and momentum, we see that the high energy of the protons (5 MeV) was a problem because a collision beween a gamma ray and a proton required the incident gamma ray to have at least 50 MeV. This wasn't possible because the incident alpha particles only had 5 MeV. Hence there had to have been a tenfold increase in energy in the interaction!
    • Chadwick applied conservation of energy and momentum to the interaction of a neutral particle (of similar mass to a proton) with a proton. Chadwick made measurements of the recoil of nuclei of hydrogen and nitrogen after interactions with his proposed neutron. The measurements led to the mass of a neutron being calculated as 1.15 times that of a proton.
    • Chadwick used conservation of mass-energy to predict the kinetic energy of the neutron ejected from the beryllium. Chadwick used conservation of momentum to predict the collisions between the neutron and hydrogen n uclei, then measured the final energy of the protons received at the detector.
    • Chadwick was then able to confirm the existence of the neutron and determined its mass.
  68. Why are some atoms inherently unstable? What happens to these unstable atoms?
    • Their nuclei exist outside the zone of stability in terms of neutron: proton ratio
    • or because thtey have too many protons.
    • This causes them to undergo natural radioactive decay, resulting in nuclear transmutation
  69. What are the two forms of natural radioactive decay resultling in transmutations? Describe the processes.
    Alpha and beta decay.

    In alpha decay, nucleus emits an alpha particle consnisting of two protons and tow neutrons, so reduces mass by 4 and atomic number by 2. (e.g. alpha decay of uranium-238 to form thorium-234 and a helium nucleus)

    In betay decay, a neutron decays into a proton which remains in the nucleus, raising the atomic number by 1, an emitted electron or beta-particle, and an antineutrino. As an antineutrino is emitted, this form of beta decay is known as beta-minus.
  70. What is the other form of beta decay besides beta-minus?
    beta-plus - proton decays into a neutron, a neutrino, and a positron (antielectron)
  71. Why is there a need for the nuclear strong force?
    What distance does the strong nuclear force act over? Other properties?
    • Sinceit was known that protons in a nucleus form a stable nucleus (they didn't fly apart), there must be an extremely powerful force to overcome the huge repulsive coulombic force compared to the attractive gravitational force, in order to hold the protons bound within he stable nucleus. This very strong attractive force became known as the 'nuclear strong force'
    • It acts < 3 fm (3 x 10-15 m).

    At EXTREME short distances (much less than diameter of a nucleon), it changes from being attractive to repulsive, then it becomes attractive as distance increases, then increasingly WEAKER at larger distances but never becomes repulsive again.

    • independent of charge - acts between protons-protons, proton-neutrons & neutron-neutrons - only acts between nucleons - electrons unaffected
    • much stronger than the electrostatic repulsion caused by the repulsive coulombic force
    • favours binding of pairs of nucleons with opposite spins and pairs of pairs, with each pair having total spin of zero - accounts fo exceptional stability of two protons & two neutrons in an alpha particle
  72. Pauli & Fermi - proposing the neutrino (contributions of each?)
    • Initially scientists thought only beta particles were emitted during beta decay. They calculated a max. Ek that a beta particle should have had and hence should have been emitted with the associated velocity. However it was observed, the large majority of electrons had less than half the predicted maximum and basically none were emitted with the full amount of kinetic energy. This meant that beta particles were missing kinetic energy, leading to a violation of conservation of energy.
    • In order to resolve the 'lost-energy' issue, Pauli suggested a neutral particle was emitted along with the electron. He also suggested the total energy of the electron and the neutral particle would be equal to a constant and account for all conservation laws.
    • The properties of this proposal particle were that it was neutral, had a tiny mass, and obeyed Pauli's Exclusion Principle.
    • Thus, his proposal of the neutrino explained the variable kinetic energies of beta particles, and resolve conservation of energy and momentum.
    • early 1930sc
    • called Pauli's tiny neutral particle the 'neutrino'
    • explained β-decay in terms of Pauli's suggestions of the emission of a light weight neutral particle along with the electron and related it with Heisenberg's suggestiong that the nucleus contained only 'heavy' particles, protons & neutrons.
    • Pauli's created a mathematical explanation for β-decay
    • he proposed that the number of e- and neutrinos was not constant & could be created and dissapear like photons
    • produced a shape of the spectrum associated with β-decay - recognised neutrino mass of zero or very close to that
    • explained a neutron in a radioactive nucleus decayed to form a proton, electron & neutrino

    The neutrino was not detected until the 1950s with the use of more advanced techniques. It was also shown to be an antineutrino.
  73. Medical radioisotope
    • Technetium-99m used to conduct medical scans.
    • Half life: 6 hours.
    • Decays with a weak beta emission and gamma ray.
    • Gamma ray able to leave patient's body and detected.
    • Technetium-99m ideal as it can be easily attached to molecules to target particular parts of the body e.g. bones.
    • Decays relatively quickly to allow for the scan to be conducted but to minimise exposure to radiation by the patient.
    • Decays into stable product.
  74. Engineering radioisotope
    • Cobalt-60 - detect stress fractures in metals especially aircraft.
    • Cobalt-60 placed on one side of a metal, and a gamma detector on the other side, usually photographic film which turns varying shades of dark depending on its exposure to gamma emission as it passes through any cracks.
  75. Agriculture radioisotope
    • Phosphorus-32 used to study the uptake of fertilier by crops
    • Phosphorus is an essential nutrient for plants and phosphorus-32 can be added in phosphate fertiliser and the movement and uptake of the phosphorus can then be measured.
    • Areas in the plant where phosphorus is concentrated can be studied and may better scientists' understanding of favourable conditions for plant growth, thereby maximising yield and increasing efficiency in the farming process.
    • Emits beta rays and half life of just over 14 days.
    • Easily detected in the crop plants but decays relatively quickly from the environment to form stable sulfur nuclei.
  76. Manhattan project impacts
    • Consisted of American efforts to produce nuclear weaponry, which was successful and convinced Japan to surrender and end WW2. Project funded Fermi to develop the first nuclear power reactor, where he perfected a controlled fission reaction in 1942.
    • Although it led to end of war, created great stresses, political, economic, environmental.
    • 'Cold War' between USA and Societs led to development of even more powerful, fusion weapons. However with the knowledge of such destructive power caused by nuclear weapons, there was only the constant threat of a nuclear war or mutually assured destruction between the superpowers, but did result in great political tension.
    • In modern times, nuclear power proving to be a dangerous bargaining chip for rogue states such as North Korea and Iran - using nuclear weapons as leverage in negotiations with the Western world. It has led to a situation where small nations with comparatively weak conventional forces can use the threat of nuclear warfare to negotiate equally with large nations such as the USA. This has led to problems with power balance between nations. As a result of the nuclear threat, the UN and USA are focussing on a diplomatic, sanctions-based approach to resolving conflict rather than an aggressive military approach.
    • On a positive, applications to medical industry have arisen - cures for cancers and lives extended; agriculture - disease resistant plants, extending shelf life of food, cheaper produce; industry - easy detection of flaws and leaks.
    • Also nuclear power offers a solution to 'global warming' - large supply of energy while producing minimal greenhouse gases causing enhanced greenhouse effect.
    • Still many who oppose nuclear power especially since the disaster of Chernobyl in 1986 and problems with radioactive waste and how this can affect communities.
  77. Define mass defect. Relate to binding energy.
    • The different in mass between the nucleus and the sum of its constituent nucleons. It equals the binding energy used to bind the nucleons together in the nucleus and can be explained by Einstein's equivalence between mass and energy E = mc2.
    • Binding energy is the energy input required to restore the nucleons to their original state - energy required to break the bonds that hold them together in the nucleus.
  78. How did you observe the visible components of the hydrogen spectrum?
    • Discharge tube - vacuum tube with cathode and anode, powered by high-voltage inductionc oil
    • Low pressure hydrogen gas inside it.
    • High voltage current passed through the tube
    • Hydrogen fluoresced
    • Light was emitted visible in the darkened room
    • Observed the visible components of the spectrum using handheld spectrometers that used a diffraction grating to split the light
    • Observed the red and blue/violet hydrogen emission lines
    • Red line was very clear compared to others.Violet lines were hard to see.
    • Wavelength of each coloured line was noted using visible scale of the spectroscope. Position of each line was also recorded.
    • Results compared with references in textbook.
  79. The Balmer series & Rydberg's equation - how does the Balmer series provide strong experimental evidence for Bohr's hydrogen atom.
    • Balmer came up with an empirical equation - predict the visible wavelengths emitted from hydrogen.
    • Equation contained a constant, number 2, and another integer.
    • Balmer also believed, there was an infinite number of spectral lines that could be emitted by hydrogen which were soon found experimentally.

    • Rydberg modified Balmer's empirical equation - allowed the wavelength of all the hydrogen spectral series to be determined.

    • When Bohr came up with his postulates and applied them to hydrogen, he also produced an equation to determine the wavelength of light that was emitted when an electron in his proposed atom underwent a transition from one enery level to another.
    • Bohr's theoretical equation - identical in form to Rydberg's, but Rydberg's constant was now understood as the energy of the hydrogen electron in the ground state, divided by Planck's constant multiplied by speed of light.
    • So no Bohr's theoretical equation agrees with the empirical equation and the well-known Balmer series could now be explained as the transition of the electron from a higher energy state to the second of the possible stationary states existing for the hydrogon electron.