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# Option J - Particle Physics

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IBD HL Physics Option J

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### Option J - Particle Physics

1. 1. Option J
2. 2.  Don’t break down  Don’t show any internal structure  No measurable size
3. 3.  Antimatter was first postulated by the English physicist; ◦ Paul A. M. Dirac, ◦ in 1928.  His postulate was verified by Anderson in 1932; ◦ who experimentally discovered the positron, ◦ in a cloud chamber.
4. 4.  It has been shown that a very high energy photon can actually produce matter.  The most common process; ◦ is the production of an electron and positron.  A positron has the same mass as an electron, ◦ but the opposite charge.
5. 5.  The positron is said to be the antiparticle of the electron.  This is called pair production; ◦ the photon disappears in the process.
6. 6.  This is an example of rest mass being produced from pure energy; ◦ as suggested by the formula, ◦ E = mc2.
7. 7.  The photon cannot just produce an electron; ◦ the Law of Conservation of Charge will be broken, ◦ and so a positron is produced. ◦  - + + (NB occurs in presence of a nucleus) ◦ 1.02 MeV  0.511 MeV each
8. 8.  The reverse can also occur.  The electron and positron can annihilate, ◦ and their energy create a photon of very high energy ◦ a gamma ray  Because of this process, ◦ positrons do not last long in nature.
9. 9.  The energy produced is given by E = mc2  E = (2 x 9.11 x 10-31) x (3 x 108)2  E = 1.64 x 10-13 J  E = (1.64 x 10-13)/(1.6 x 10-19)  E = 1.03 x 106 eV  E = 1.03 MeV
10. 10.  The photon produced has a frequency of:  E = hf  f = E/h  f = (1.64 x 10-13)/(6.63 x 10-34)  f = 2.47 x 1020 Hz
11. 11.  Pair production cannot, ◦ occur in empty space.  In the above example, ◦ energy is conserved but, ◦ the pair has no momentum to carry away, ◦ the initial momentum of the photon.
12. 12.  No matter what energy, a massive object such as a nucleus must be involved, ◦ to conserve the momentum.  We have already seen that a positron can be produced in + decay; ◦ along with a neutrino.
13. 13.  Other anti particles have also been produced.  In 1955 the antiproton; the bar representing an antiparticle. __ p
14. 14.  was discovered by Emilio Segré, ◦ and Owen Chamberlain ◦ at the University of California in Berkeley.
15. 15.  Soon afterwards, ◦ the antineutron was also discovered.  There are some particles, ◦ such as the photon, ◦ which do not have antiparticles.  They are considered to be their own antiparticles.
16. 16.  Through the study of collisions in particle accelerators, ◦ we have discovered a number of antiparticles.  It appears, ◦ some reactions do not take place.  One example is: __ pppnp
17. 17.  This is even though charge and energy are conserved.  To explain this, ◦ a new conservation law was hypothesised, ◦ the conservation of Baryon Number.
18. 18.  The Baryon number is the same as nucleon number but; ◦ all nucleons have a number B = +1 while, ◦ all antinucleons have a number B = -1.  From the above example, if we add Baryon numbers:
19. 19.  +1+1 = +1+1-1  2 1  Another conservation law relates to other subatomic particles being discovered. __ pppnp
20. 20.  Another conservation law relates to other subatomic particles being discovered.  These particles include; ◦ Pions ( ), ◦ Kaons (K), ◦ Lambdas ( ).
21. 21.  It seemed that some of these particles reacted with high probability, ◦ while some other reactions did not, ◦ even though there were no obvious violations of the conservation laws.
22. 22.  These particles behaved strangely and so a new quantum number, ◦ strangeness,  and a new conservation law, ◦ the conservation of strangeness,  was introduced.
23. 23.  There are other particles that have been found; ◦ and other characteristics, ◦ that help distinguish between them.
24. 24.  They include: ◦ spin, ◦ charm, ◦ colour, ◦ flavour, ◦ bottomness, ◦ topness.
25. 25.  To identify these particles, ◦ a cloud chamber is used.  It was invented by a physicist; ◦ called Wilson in 1910.  A cloud chamber uses a gas; ◦ cooled to a temperature, ◦ slightly below its usual condensation point.
26. 26.  As the charged particle moves through the gas, ◦ the gas molecules condense on the particles, ◦ and leave a trail.
27. 27.  This shows an antiproton colliding with a proton  This produces a Xi-anti Xi pair  antiproton + proton - + +  They decay into other particles  Neutral particles are shown with dashed lines; ◦ as no bubbles are produced.
28. 28.  Two or more electrons cannot occupy the same quantum state at the same time.  Two electrons cannot occupy the same space at the same time  If in same orbital, they have the opposite spin  Applies to electrons, protons & neutrons (fermions) ◦ Not photons (bosons)
29. 29.  Explains why collisions in atoms can occur with no loss of energy  Low energy states fill up first  High energy states cannot drop down as lower energy states already occupied
30. 30.  In the mid 1930’s; ◦ atoms were considered to be made up of: ◦ protons, neutrons and electrons ◦ Called elementary/fundamental particles  Others were also known; ◦ positron, neutrino and photon  In the next few decades; ◦ hundreds of other particles were discovered
31. 31.  Many of these particles are now considered to be made up of; ◦ two or three quarks  Some particles discovered were considered fundamental
32. 32.  Elementary particle physics begun in 1935  Hideki Yukawa (1907 – 1981) ◦ A Japanese physicist  Predicted a new particle that; ◦ would somehow mediate the strong nuclear force
33. 33.  To explain his idea, consider the electromagnetic force.  Electric force acts over a distance without direct contact  The force one charged particle exerts on a second is due to; ◦ The electric field set up by the first
34. 34.  Similarly, the magnetic field can be said to carry the magnetic force  E/M fields can travel through space as a wave  E/M radiation can be considered a wave or particles (photons)  As there is the wave/particle duality the e/m force between particles is due to:
35. 35.  e/m field set up by one charged particle and felt by other  OR  The exchange of photons between them
36. 36.  How does this work?  An analogy is used  Imagine Harry & Julius throwing pillows at each other  Each catch results in the child being thrown backwards  A repulsive force
37. 37.  If they grab the pillow out of the other’s hand ◦ Exchange pillows  They are pulled towards each other  An attractive force
38. 38.  For the electromagnetic force between 2 charged particles  a photon is exchanged between 2 particles and  gives rise to the force
39. 39.  Can be shown using a Feynman diagram  Named after the inventor ◦ Richard Feynman (1918 – 1988) ◦ An American physicist  Based on quantum electrodynamics (QED)  Simplest example is two electrons approaching each other
40. 40.  It is a type of position vs time graph  Time is on the y axis
41. 41.  One electron emits a photon and recoils  The second electron absorbs the photon  In this collision ◦ Or interaction  energy and momentum is transferred
42. 42.  As the photon is absorbed; ◦ very shortly after being emitted by the first electron  The photon is not observable and so is called a; ◦ Virtual Photon  The photon mediates or carries the; ◦ Electromagnetic Force
43. 43.  Back to Hideki Yukawa’s prediction that  a new particle; ◦ would somehow mediate the strong nuclear force  If a photon can mediate the electromagnetic field ◦ or force  There should be a particle that mediates; ◦ the strong nuclear force
44. 44.  Yukawa predicted the new particle would have a mass between; ◦ An electron and a proton  Hence it was called a meson ◦ Meaning ‘in the middle’
45. 45.  If photons can be found as free particles  Why can’t Mesons?  A search began in the cosmic radiation from the Sun and other radiation sources  It was found in 1937  There were problems with the particle called a muon
46. 46.  It didn’t react strongly with matter  This meant it could not mediate the strong nuclear force  It can be found as a positive or negative particle  Seemed to be nothing more than a massive electron  It had a mass 207 x that of an electron
47. 47.  The Yukawa particle was finally found in 1947 ◦ By Cecil F Powell (1905 – 1969) ◦ And  It is called the pi meson ( ) ◦ Or pion  Comes in 3 charge states; ◦ +, - or 0
48. 48.  Mass of + and - = 139.6 MeV/c2 and;  o = 135.0 MeV/c2  All react strongly with matter  Reactions seen using particle accelerators include:  p + p p + p + o  p + p p + n + +
49. 49.  In 1949 Yukawa was awarded the Nobel Prize for his prediction  In 1950 Powell was awarded the Nobel Prize for discovering mesons
50. 50.  Others mesons were found over the following years  The recent theory of quantum chromodynamics; ◦ Has replaced mesons with gluons, ◦ as the basic carriers of the strong nuclear force ◦ More about quantum chromodynamics later
51. 51.  We have discussed particles that mediate e/m & the strong nuclear force  What about the other forces?
52. 52.  There is a weak nuclear force  We know of its existence only because; ◦ It shows itself in certain types of radioactive decay  Particles were found to mediate this force in 1983  They are called the exchange bosons; ◦ W+, W- and the Zo
53. 53.  They were discovered by:  Carlo Rubbia (1934 - ) ◦ An Italian  Simon Van Der Meer (1924 - ) ◦ Dutch  They shared the Nobel Prize in 1984 for their discovery
54. 54.  The electromagnetic and weak nuclear forces are; ◦ instances of a single electroweak force with, ◦ two different types of exchange particles
55. 55.  The carrier for the gravitation force has not be found  It has been given a name, however;  Gravitron
56. 56.  The Four Forces in Nature by relative strength Type Relative Strength (2 p in nucleus) Field Particle Strong nuclear 1 Gluons (was mesons) Electromagn etic 10-2 Photon Weak nuclear 10-6 W and Zo Gravitational 10-38 Graviton (?)
57. 57.  Since the discovery of the meson in the late 1940’s  Many other sub nuclear particles have been discovered ◦ Through the use of particle accelerators  They have been arranged in groups ◦ Based on their interactions  All have antiparticles
58. 58.  GAUGE (exchange) BOSONS  Named after the theory that describes them ◦ Gauge Theory  Include those particles which carry the e/m and weak interactions  Photons, W & Z
59. 59.  LEPTONS  Some of these particles have charge  And interact by the e/m force  Electron, muon ( ) & tau ( ) ◦ was discovered in 1976 ◦ 3000 x heavier than an electron
60. 60.  Particles which interact via the weak nuclear force ◦ And gravitation force  Neutrino  3 types ◦ Electron neutrino ( e) ◦ Muon neutrino ( ) ◦ Tau neutrino ( )
61. 61.  HADRONS  Interact via the strong nuclear force ◦ Can interact by other forces but the strong force dominates at short distances
62. 62.  Divided into two groups  Baryons ◦ Particles with Baryon number of +1  Or –1 for antiparticles ◦ Protons, Neutrons, Lamdas ( ), Sigmas( ), Xis ( ) and Omegas ( ) and others
63. 63.  Mesons  Baryon number of 0 ◦ Pions ( ), Kaons ( ) and Eta ( )  A selected summary of the particles are shown on page 528 of your text book
64. 64.  In the early 1950s, the newly found particles ◦ , and ◦ behaved strangely in 2 ways  They were produced in pairs  - + p o + o ◦ Occurred with high probability  - + p o + n ◦ Never occurred
65. 65.  This was strange as it violated; ◦ no known conservation law ◦ and there was plenty of energy  Secondly; ◦ although they were clearly produced by the strong interaction, ◦ they did not decay at a rate characteristic of the strong interaction
66. 66.  To explain this, a new quantum number and conservation law was developed  Strangeness  The strangeness numbers were allocated as shown on p 528 (column S)  Antiparticles were assigned negative numbers
67. 67.  Strangeness is conserved; ◦ In strong interactions ◦ But not weak  This is the first example of a ‘partially conserved’ quantity
68. 68.  In the 1960’s physicists knew; ◦ hadrons interact via the strong interaction, ◦ while leptons do not  A problem they had was that there only 4 known leptons ◦ e-, -, e, :  the and were yet to be discovered  But there were hundreds of Hadrons
69. 69.  The leptons are considered to be truly elementary particles ◦ Don’t break down ◦ Don’t show any internal structure ◦ No measurable size (upper limit of 10-18 m)  Hadrons are more complex and so can’t be elementary
70. 70.  Murray Gell-Mann (1929 - ) and G Zweig in 1963; ◦ independently proposed no hadron was elementary  They proposed that hadrons are made up of; ◦ Combinations of 3 point like entities called quarks ◦ after the novel Finnegan’s Wake by James Joyce. ◦ The line is ‘Three quarks for Master Mark’ ◦ Quark should rhyme with Mark ◦ In German, quark means ‘slime’!
71. 71.  The first quark is called the ‘up’ quark. ◦ It has a positive charge of 2/3 of the electronic charge (+ 2/3e).  The second quark is the ‘down’ quark ◦ with a charge of -1/3 e.  The 3rd quark is the ‘strange’ quark; ◦ also with a charge of -1/3 e.
72. 72.  The strange quark also has the property; ◦ which helps elementary particles that contain it to live longer than it should.  Using this theory, a proton is made up of 2 up quarks and one down quark.
73. 73.  The neutron is made up of 2 downs and 1 up.
74. 74.  A meson consist of a quark, antiquark pair  + Q = 2/3 e + 1/3 e = +1e  - Q = -2/3 e + -1/3 e = -1e
75. 75.  Once proposed, physicists went looking for them  No direct detection has been possible  Perhaps they are so tightly bound; ◦ They might not exist in a free state
76. 76.  In 1964, several physicists proposed a 4th quark. ◦ As at this stage, there were 4 leptons and nature is symmetrical  This quark was said to be charmed ◦ As it solved some theoretical problems ◦ Has a new quantum number of charm  A charmed quark has a charge of +2/3 e
77. 77.  In 1974, the first charmed particle was discovered; ◦ J/ meson  Charm is conserved in strong interactions but not weak  Partial conservation law
78. 78.  In the 1970s, 2 other leptons were discovered.  This means there would be 6 leptons and only 4 quarks.  Two more quarks were postulated; ◦ Top and bottom quarks ◦ Some prefer the names Truth & Beauty
79. 79.  In 1977, using linear accelerators, they found the bottom (beauty) quark  Strong evidence for the top (truth) quark has only been found since 1995  It is hard to find as it has a high mass ◦ 200 GeV/c2 ◦ and requires huge energy to produce it
80. 80.  A summary of the quark properties can be found on page 531 of your text book.  The truly elementary particles are now considered to be:  6 quarks  6 leptons  Gauge Bosons
81. 81.  They can be arranged in 3 groups ◦ Or generations 1st Generation 2nd Generation 3rd Generation Quarks u, d s, c b, t Leptons E, e , , Bosons (photon) W , Zo gluons
82. 82.  The distinction between the 6 quarks was referred to as; ◦ Flavour  Soon after the quark theory was proposed, another property was suggested; ◦ Colour
83. 83.  Each flavour quark can have 1 of 3 colours ◦ Red, Green & Blue ◦ The primary colours  Antiquarks are coloured ◦ Antired, antigreen & antiblue  Colour has nothing to do with our senses
84. 84.  Baryons are made up of 3 colours ◦ One of each colour ◦ They are said to be white or colourless  The colours of quarks seem to neutralise each other ◦ Like electrons and protons form neutrons ◦ Allows protons to remain close to each other
85. 85.  Each quark carries a colour charge ◦ Like electric charge  The strong force between quarks is referred to; ◦ as the colour force  This new theory is called; ◦ Quantum Chromodynamics (QCD)
86. 86.  The strong nuclear force arises out of the colour force  This makes the colour force a fundamental force
87. 87.  The particles that transmit the colour force are called gluons
88. 88.  Particles have a wave nature ◦ Recall diffraction pattern of electrons through crystals  If ball hits stationary ball and we know velocity, we can calculate when they hit  If two electrons hit, we can’t exactly say when they will hit ◦ Can say when most likely to hit  Cannot say where electron is exactly is ◦ Can say where it is most likely to be
89. 89.  This means position and time are probability distribution  To know where a particle is, you must shine a light on it  Shine a light and you give it energy ◦ Change momentum of particle  Expressed mathematically ∆𝑝∆𝑥 ≥ ℎ 4𝜋
90. 90.  Can also be expressed in another form in terms of energy
91. 91.  This is not just about measurement  It is about how things are defined  Energy of a particle is not an exact value  The small the uncertainty in time ◦ greater the range of E  Does not match classical theory  Virtual photon can exist for short time without violating cons of E  Last for approx 10-20 s
92. 92.  Yet to be discovered  LHC aims to find it  When looking at collisions particles formed are based on conservation of charge, lepton number etc  Mass can change without following any rules ◦ Excepet cons of E & m
93. 93.  Can be explained if mass not part of particle but of space  Mass makes something difficult to accelerate.  Does this something have to be of the particle?
94. 94.  Analogy for Higgs Mechanism  Aaron Russo enters crowded party  Immediately surrounded by girlfriends making it difficult to move.  OK if stationary but needs to get a drink.  Finds it hard to move.
95. 95.  Josh G arrives and tells a group of friends some gossip about Owen  Gossip makes it around room  People cluster to hear it  Cluster makes it hard to move across the room  Cluster can exist even if no-one wants to move across room
96. 96.  For mechanism to work  All space must be covered by field ◦ Higgs Field  When particles move through field ◦ Exhibit properties we attribute to mass  e/m field has particle associated with it ◦ photon  Higgs field has particle ◦ Higgs Boson  In analogy, it is the cluster of people
97. 97.  The simplest example of a linear accelerator is ; ◦ the Van de Graaf, ◦ as the ions move in a linear path.  The T.V. is also a linear particle accelerator.
98. 98.  In a TV, electrons escape from a hot metal cathode; ◦ same way as water molecules escape from the surface of a hot pot of water.  The electrons are accelerated to a small speed by the effect of an electric field, ◦ due to the potential difference between the cathode, ◦ and an anode a few centimetres in front of it.
99. 99.  They are then steered by magnetic or electric fields; ◦ transverse to their motion, ◦ to hit a particular spot on the screen.  The screen is covered by a material; ◦ which gives off light when struck by an electron, ◦ thus producing the picture.
100. 100.  Most TVs use electric potentials between parallel plates for steering, ◦ rather than magnetic fields from electromagnets.  Magnets are generally used in linear particle accelerators, ◦ to steer electron beams.
101. 101.  Generally, the name is used for more complex devices; ◦ that accelerate particles many times along a straight line.  There are other devices that accelerate particles; ◦ which move in a circular path.
102. 102.  The linear accelerator shown above works by passing ions through a series of tubes, ◦ each one a different polarity.  It is only when they arrive at a gap, ◦ they are accelerated.
103. 103.  This is because the tube ahead of will be of opposite charge to the particle.  The power supply is a.c. ◦ so that this occurs every time the particle moves from one tube to the next.
104. 104.  As the particle accelerates at each gap, ◦ their speed must increase, ◦ and cover more distance, ◦ in the same amount of time.  This means that each consecutive tube must be longer; ◦ if the a.c. current is to be kept in phase, ◦ with the particle movement.
105. 105.  The largest particle accelerator is 3 km long; ◦ can accelerate electrons to 50 GeV, ◦ located at Stanford  called the SLAC, Stanford Linear Accelerator Centre.  The way in which their accelerator works is described below.
106. 106.  To obtain a "polarised" electron beam a different electron source is used:  Polarised light from a laser is aimed; ◦ at a carefully-prepared gallium arsenide surface.
107. 107.  Electrons are knocked out of the surface by the laser light; ◦ collected into the accelerator, ◦ by accelerating them across a small potential difference.
108. 108.  Polarised electrons have their spins; ◦ internal angular momentum, ◦ aligned in a particular direction, ◦ in the same way that polarised light, ◦ corresponds to an alignment of the angular momentum of the photons, ◦ in the travelling electromagnetic wave we call light.
109. 109.  Electric Fields Increase the Energy of the Electrons  The electrons at SLAC are produced in much the same way as in a TV.
110. 110.  After the first acceleration by the electric field; ◦ due to the potential difference between the cathode and the anode,  They enter a long copper pipe-like structure which is the accelerator itself.
111. 111.  This accelerator pipe is pumped out to produce a very good vacuum, ◦ so that there is nothing inside it, ◦ to deflect electrons travelling down the accelerator.  It contains discs every 3.6 cm with a small hole in the centre; ◦ 2 cm in diameter.
112. 112.  The beam travels through these holes.  The discs make the pipe into a series of resonant cavities, ◦ each about the size and shape of a baked bean can.
113. 113.  By feeding microwave power into this structure; ◦ we make pulses of electric field, ◦ which effectively travel along the structure, ◦ and provide further acceleration, ◦ for the electrons travelling with them.
114. 114.  The electrons ride the microwaves; ◦ like surfers on a water wave, ◦ gaining energy from the force on them due to the electric field.
115. 115.  The electron "beam" is actually a series of bunches of electrons; ◦ timed to enter each cavity just when the electric field in that cavity, ◦ is close to its maximum, ◦ and pointing in the right direction, ◦ to accelerate the electrons down the accelerator.
116. 116.  The bunches sit a little ahead of the peak of the wave.  Any electron that falls a bit behind the bunch; ◦ feels a greater accelerating field, ◦ catches up with the bunch.
117. 117.  Conversely any that get a bit ahead; ◦ feel a weaker field, ◦ are less accelerated, ◦ so the bunch catches up with them.
118. 118.  To describe the same thing in a slightly more technical fashion:
119. 119.  The cavities respond to the microwave input; ◦ by resonating in a particular mode of microwave oscillation, ◦ that has an electric field component along the axial direction  i.e. along the accelerator ◦ and a cylindrical magnetic field around the inside of the pipe.
120. 120.  This electric field is just what is needed; ◦ to accelerate electrons, ◦ or positrons down the accelerator.  The magnetic field provides an inward force towards the centre of the accelerator; ◦ that compensates for the electrical repulsion, ◦ among the many electrons in a bunch.
121. 121.  The relative phase of the wave is opposite in neighbouring cavities; ◦ but the spacing of the cavities and the frequency of their oscillation, ◦ is chosen so that the electrons travelling at close to the speed of light, ◦ are only in a cavity when the electric field, ◦ is pointing in the right direction, ◦ to accelerate them down the pipe.
122. 122.  Positrons can be accelerated too.  The positron is the antiparticle of the electron.  It has the same mass and the same magnitude of electric charge, ◦ but positive instead of negative charge.
123. 123.  When positrons are accelerated the positron bunches are timed to enter the cavities; ◦ when the electric and magnetic fields, ◦ are reversed compared to the electron case, ◦ so they too are accelerated down the pipe by the electric field, ◦ and bunched together by the magnetic field.
124. 124.  Extracting accelerated electrons from part way down the accelerator, ◦ and colliding them with a target, ◦ produce the positrons.
125. 125.  One of the possible processes that can occur; ◦ is that the incoming electron is deflected by collision with a target atom, ◦ and then radiates an energetic (virtual) photon, ◦ which produces an electron and a positron.
126. 126.  The positrons are collected using electric and magnetic fields to control their motion, ◦ and are sent back to the beginning of the accelerator.
127. 127.  Magnetic Fields are used to steer electrons.  At the end of the accelerator they use large magnets, ◦ to produce a magnetic field perpendicular to the direction of the beam.
128. 128.  An electron moving in a magnetic field is subject to a force; ◦ that is perpendicular to its motion, ◦ hence will be deflected.  The magnets are used to steer the electrons, ◦ to the various experimental areas.
129. 129.  For some types of experiments, electrons and positrons; ◦ are fed into storage rings, ◦ in which they circulate many times.
130. 130.  In the storage rings dipole magnets; ◦ i.e. magnets with the two opposite pole tips arranged above and below the beam,  Are placed periodically around the ring to produce a magnetic field perpendicular to the beam.
131. 131.  This gives the centripetal acceleration needed to make the electrons follow the curved path.  More complicated magnets, ◦ such as quadrupoles,  with four pole tips. ◦ Sextupoles,  with six  Are placed at regular intervals along the beam pipe.
132. 132.  These act on the beam like lenses in a beam of light, ◦ and are used to keep the bunches well focussed, ◦ and travelling in the centre of the storage ring structure as desired.
133. 133.  A cyclotron is a device used; ◦ to accelerate charged particles to high energies, ◦ generally so they may collide with atomic nuclei, ◦ and produce a nuclear reaction.
134. 134.  There are three main parts of a cyclotron: 1. Ion Source  A beam of protons; ◦ or sometimes deuteron,  which is heavy hydrogen.  Can be charged particle ◦ Positive Ion ◦ Negative Ion  How can you create these ions?  Produced in a cyclotron.
135. 135.  A small amount of hydrogen gas; ◦ is introduced into an evacuated chamber, ◦ which is then heated,  via an electric filament.
136. 136.  Electrons which have gained thermal energy; ◦ from the heating process.  Liberated from the filament; ◦ called thermionic electrons, ◦ collide with gas molecules.
137. 137.  Electrons are then ejected from the gas; ◦ molecules which are then ionised.  Originally they are H2 +; ◦ later become H+,  or protons.
138. 138.  Modern ion sources are generated from an electric arc; ◦ external to the cyclotron, ◦ vacuum is not compromised.
139. 139. 2. Semicircular Metal Containers (‘dees’)  Originally, two hollow copper electrodes; ◦ shaped like the letter ‘D’, ◦ their straight edges facing each other were used.
140. 140.  A large ac P.D. is applied between the dees.  The P.D. creates an electric field; ◦ in the gap between them, ◦ that is continuously changing.  As the dees are closed hollow metal conductors; ◦ they have no electric field inside of them.
141. 141.  The dees are in a magnetic field; ◦ produced by an electromagnet.  This means that there is a magnetic field; ◦ within the dees.  Within the gap there exists; ◦ an electric and magnetic field.
142. 142. 3. Evacuated Outer Chamber  The dees are placed; ◦ within an outer evacuated container.
143. 143.  As it is a vacuum; ◦ no energy loss will occur, ◦ due to collisions with gas molecules, ◦ they are not scattered away from their target.
144. 144.  If they were to collide with the walls of the dees; ◦ the dees may become radioactive.  This makes maintenance much more hazardous.
145. 145.  Consider the motion of a positive ion; ◦ inside the dees of a cyclotron.
146. 146.  The dees are placed between; ◦ poles of an electromagnet.  The ions are not shielded from; ◦ magnetic field, ◦ unlike the electric field.  This means the ions are affected; ◦ inside the dees, ◦ in the gap between them.
147. 147.  To make the ions move in circular path; ◦ uniform magnetic field is needed, ◦ perpendicular to the plane of the dees.  Polarity of the field is important; ◦ to make the ions move in the right direction.  The force is such that; ◦ always acting towards the centre of the circle, ◦ causing centripetal acceleration.
148. 148.  In the diagram below; ◦ at the particular instant shown, ◦ its velocity is down.  As the force must always act towards the centre of the circle; ◦ determine the direction of the magnetic field, ◦ using the right hand rule.
149. 149.  The field must be in a direction which is;  OUT of the page.
150. 150.  The function of the electric field; ◦ accelerate the ions to high energies.  The longer the ions is in the electric field; ◦ the higher the energy.
151. 151.  The function of the magnetic field;  Make the ions move in a circular path; ◦ it repeatedly comes under the influence of the electric field, ◦ increases the energy level.
152. 152. 1. Radius of the circular arc described by an ion; ◦ is proportional to its speed, therefore:
153. 153. 2. The time for an ion to complete one semicircle; ◦ same irrespective of the speed of the ion.  The path followed by an ion is shown below.
154. 154.  Consider the ions in positions a, b and c; ◦ shown above and in more detail below.  At position a; ◦ +ive ion is affected by the electric field, ◦ experiences a force to the right, ◦ accelerates to the right.
155. 155.  Once it leaves the electric field; ◦ and enters the right hand dee, ◦ it experiences no force, ◦ therefore its speed is constant.  The magnetic field causes the ion to; ◦ move in a semicircular path.  At a time later, it reaches position b.
156. 156.  The frequency of the ac current is match so; ◦ that when the ion arrives at b, ◦ the E field has reversed.  During the time in the electric field; ◦ the speed increases further.
157. 157.  Now that the ion is in the left hand dee; ◦ the path followed is semicircular.  It now has a greater radius than previously; ◦ but it also has a higher speed.
158. 158.  This means that the time taken to return to the gap; ◦ will be exactly the same.
159. 159.  The a.c. current has; ◦ again reversed the electric field, ◦ by the time the ion gets to position c.  The acceleration process is repeated for many revolutions.
160. 160. Cyclotron
161. 161.  Summary of How a Cyclotron Works
162. 162.  As the ions pass through the gap; ◦ their speed increases, ◦ so must their kinetic energy.  This means work is done.  From previously; ◦ W = q V
163. 163.  Change in kinetic energy is also equal to the work done.  W = K
164. 164.  If we assume that the ions have very little kinetic energy to begin with; ◦ their kinetic energy will increase, ◦ by q V, ◦ for each pass of the gap.
165. 165.  As there are two passes of the gap per revolution; ◦ their kinetic energy per revolution, ◦ is 2q V.
166. 166.  Originally, cyclotrons were used as; ◦ research devices in nuclear reactions.  They are now used to produce; ◦ radioactive materials for a number of purposes.
167. 167.  Hospitals use them for producing pharmaceuticals; ◦ that undergo rapid radioactive decay.  The decay is so rapid; ◦ they must be produced on site, ◦ at the time of use.
168. 168.  The protons are used to bombard stable atoms; ◦ carbon, ◦ nitrogen, ◦ oxygen, ◦ Fluorine.  To produce radioactive forms of these elements.
169. 169.  They are then combine with glucose and are given to the patient.  The radioactivity can then be detected; ◦ bodily functions that use the above chemicals, ◦ can be monitored.  A medical diagnosis can then be made.
170. 170.  At higher doses, the radioactive elements can be used; ◦ as a treatment to kill cancers, ◦ in organs where the element, ◦ tends to concentrate.
171. 171.  Previously we stated:  The radius of the circular arc described by an ion is proportional to its speed.  We can apply Newton’s second law; ◦ F = ma,  as it applies to circular motion
172. 172.  And F = qvB  therefore, upon rearrangement: r mv F 2 r mv qvB 2 r mv qB
173. 173.  As the mass m; ◦ charge q, ◦ magnetic field B, ◦ are all constant,  r v qB mv r 
174. 174.  We also stated:  The time for the ion to complete one semicircle is the same irrespective of the speed of the ion.  From before, if the speed doubles; ◦ radius doubles.
175. 175.  This also doubles the; ◦ circumference (2 r).  Mathematically, this can also be shown to be true.  The velocity of an object undergoing circular motion is given by:
176. 176.  Rearranging for T  From  we can substitute for r. T r v 2 v r T 2 qB mv v T 2
177. 177.  This shows that the period is; ◦ independent of speed, ◦ or radius. qB m T 2 
178. 178.  Constant for ions of; ◦ a constant charge, ◦ Mass, ◦ in a given magnetic field.  Be careful.
179. 179.  The period refers to the time; ◦ for one complete revolution, ◦ not the time in a dee.  That is half a period.
180. 180.  Any nuclear reaction caused by a collision; ◦ requires a certain amount of energy.  It is critical to know the kinetic energy; ◦ with which the ions leave the cyclotron.
181. 181.  We previously stated that if we know the P.D; ◦ and the number of revolutions, ◦ we can calculate the kinetic energy.  While this is correct; ◦ the number of revolutions is not normally known.
182. 182.  An alternative method is required.  K = ½mv2  If we rearrange equation  m qBr v
183. 183.  Substituting into formula for K: 2 2 1 m qBr mK m rBq K 2 222
184. 184.  This indicates that the kinetic energy of an ion; ◦ of given charge, ◦ and mass.  Only depends on the radius; ◦ of the final circle, ◦ magnitude of the magnetic field.  This can be understood due to two points.
185. 185.  Point 1  If the magnetic field increases; ◦ the radii decreases, ◦ ions make more revolutions, ◦ more crossings of the gap between the dees.
186. 186.  At each crossing; ◦ they are accelerated, ◦ to higher kinetic energy.  Increasing the magnetic field results in; ◦ increase of the kinetic energy, ◦ of the emerging ions, ◦ at a given radius.
187. 187.  Point 2  If the P.D. is increased; ◦ the ions gain more speed with each crossing of the gap, ◦ and so make circles with larger radii, ◦ and make fewer revolutions.
188. 188.  This means that a larger P.D. does not result in; ◦ a larger kinetic energy, ◦ of the emerging ions at a given radius.
189. 189.  What is a Synchrotron?  A very large, circular, megavoltage machine about the size of a football field.  Inside there is a vast, circular network of interconnecting tunnels and high tech apparatus.
190. 190.  Uses electricity to produce intense beams a million times brighter than the sun.  Produced when high-energy electrons are forced to travel in a circular orbit inside the synchrotron tunnels by ‘synchronised’ application of strong magnetic fields.  Beams travel at just under c.  Light they produce is filtered and adjusted to travel into experimental workstations.
191. 191.  Provides information about the molecular structure of materials.  Used to research diseases, ◦ make plants more productive ◦ metals more resilient.
192. 192.  What is synchrotron light?  e/m radiation emitted when electrons, moving near c, are forced to change direction under the action of a magnetic field.  e/m radiation is emitted in a narrow cone in the forward direction, at a tangent to the electron's orbit.  Synchrotron light is unique in its intensity and brilliance and it can be generated across the range of the e/m spectrum.
193. 193.  Properties of synchrotron light  High brightness: synchrotron light is extremely intense (hundreds of thousands of times more intense than that from conventional x-ray tubes) and highly collimated.  Wide energy spectrum: synchrotron light is emitted with energies ranging from infrared light to hard x-rays.
194. 194.  Highly polarised: the synchrotron emits highly polarised radiation, which can be linear, circular or elliptical.  Emitted in very short pulses: pulses emitted are typically less than a nano-second (a billionth of a second), enabling time-resolved studies.
195. 195. Electrons are generated in the centre (electron gun) and accelerated to 99.9997% of c by the linear accelerator (linac). The electrons are then transferred to the booster ring, where they are increased in energy. They are then transferred to the outer storage ring.
196. 196.  The electrons are circulated around the storage ring by a series of magnets separated by straight sections. As the electrons are deflected through the magnetic field created by the magnets, they give off electromagnetic radiation, so that at each bending magnet a beam of synchrotron light is produced.
197. 197.  At each deflection of the electron path a beam of light is produced. The effect is similar to the sweeping of a search light. These beams can be captured and focussed to a specific wavelength appropriate for a particular technique.
198. 198.  Insertion Devices Intensity of light can be significantly increased by the use of 'insertion devices‘ ◦ in the straight sections of the ring.  There are two classes of insertion devices.
199. 199.  Multipole wiggler (MPW)  Cone of light is emitted at each bend in the 'wiggle' so ◦ cones of light superimpose on each other, ◦ intensity increasing with the number of bends.
200. 200.  At the peak of each wave a beam of light is emitted.  Beams reinforce each other and appear as a broad beam of incoherent synchrotron light when viewed in the horizontal plane ahead of the wiggler.
201. 201.  Second type of device is an undulator.  Uses less powerful magnets to produce ◦ gentler undulations of the beam.  Light cones just overlap and interfere with each other ◦ so that certain wavelengths of light are enhanced ◦ perhaps by 10,000 times.
202. 202.  Wavelengths can be changed by ◦ altering the gap between the component magnets ◦ so that the light is tunable to specific wavelengths. The poles produce less deflection of the electron beam. This results in a narrow beam of coherent synchrotron light, with certain frequencies amplified by up to 10,000 times.
203. 203.  Current ('third generation') designs aim to optimise the intensity that can be obtained from insertion devices.  Attention is given to the size and positioning of the straight sections that accommodate the insertion devices.
204. 204.  The Australian Synchrotron is an advanced 'third generation' design.  Uses the three different types of light sources ◦ bending magnets, ◦ multipole wigglers, ◦ undulators.
205. 205.  Bremsstrahlung  The charged particles travelling around the synchrotron accelerate each time their direction changes  When a charged particle accelerates, it emits e/m radiation.  When it emitted in the X-Ray part of spectrum, there is a loss of energy  The radiation is very intense, polarised and parallel.  Useful for X-Ray diffraction or other expts investigating properties of materials
206. 206.  Advantage of synchrotron  Can accelerate particles in opposite directions.  Energy is much higher than hitting a fixed target.  LHC does the same thing  On much larger scale and performs other expts.
207. 207.  Linac  High acceleration means radiation loss for small particles  As acceleration is a one off event, if collision does not take place, particles are lost
208. 208.  Synchrotron  Follows circular path therefore more chance of collision.
209. 209.  Cyclotron  Difficulties construction big Ds and magnets there high energies not reached.  Good for medical apps and is cheap to build
210. 210.  Head-on collisions  2 particles of equal mass ◦ Proton & antiproton  Energy available for new particle = ◦ Rest energy + Ek of both particles  Stationary target  Not all energy can be used to produce new particle ◦ Momentum must be conserved
211. 211.  Energy available  EA 2 = 2EMc2 + (Mc2)2 + (mc2)2  EA = energy available  M = rest mass of target  m = rest mass of accelerated particle  E = total energy of accelerated particle  Energy in eV
212. 212.  Bubble Chambers  Created by American Scientist Donald Glaser  Born 1926  Won 1960 Nobel Prize for invention of Bubble Chamber
213. 213.  Uses hydrogen in pressurised container  Temperature below boiling point  Can only form liquid around impurities ◦ None available  As ions move through, bubbles form ◦ Vapour trails seen and photographed  Placed in magnetic field ◦ Particles deflected and identified
214. 214.  Problems  Particles collide and produce other particles ◦ Need large cloud chamber to capture interactions ◦ Can’t be done  Once a particle triggers a vapour trail ◦ Cloud chamber must be reset
215. 215.  High energy collisions can produce γ – rays  Will not leave a track as a particle would
216. 216.  Single photon enters through window  Window coated & absorbs photon  Emits electron
217. 217.  Accelerated towards +ive dynodes  Each time collision with dynode ◦ More electrons liberated  Causes amplification  Signal then strong enough to measure change in potential
218. 218.  Problem  High energy γ – rays will not liberate electron directly  Some materials will emit visible light when they absorb a γ – ray.  Called scintillation.  If paced before window detection can be achieved
219. 219.  Ions conduct electricity  Concept used in G-M tube  Will detect particle but not direction  Use lots of G-M tube around room allows tracking of path  This is a wire chamber
220. 220.  GM tube consists of metal cathode surrounding a centre electrode  The front of the tube is a thin Mica window sealed to the metal cylinder. The thin mica window allows the passage and detection of the weak penetrating alpha particles. The GM tube is first evacuated then filled with Neon, Argon plus Halogen gas.
221. 221.  The GM tube is put into an initial state (ready to detect a radioactive particle), by applying + 500-volt potential to the anode (centre electrode) through a ten mega ohm current limiting resistor.
222. 222.  A 470K-ohm resistor is connected to the metal wall cathode of the tube and to ground. The top of the 470K resistor is where we see our pulse signal whenever a radioactive particle is detected.
223. 223.  In this initial state the GM tube has a very high resistance.  However, when a radioactive particle passes through the GM tube, it ionizes the gas molecules in its path and creates a momentary conductive path in the gas.  This is analogous to the vapour trail left in a cloud chamber by a particle
224. 224.  Electron liberated from the atom by the particle, and the positive ionized atom both move rapidly towards the high potential electrodes of the GM tube.  They collide with and ionize other gas atoms, creating a momentary avalanche of ionized gas molecules.
225. 225.  These ionized molecules create a small conduction path allowing a momentary pulse of electric current to pass through the tube allowing us to detect the particle.
226. 226.  This momentary pulse of current appears as a small voltage pulse across the 470 K ohm resistor.  The halogen gas quickly quenches the ionization and the GM tube returns to its high resistance state ready to detect more radioactivity.
227. 227.  Like a GM Tube except it has multiple wires  The data gives arrival time and the track of the particles
228. 228.  See p537 of text
229. 229.  See p537 of text
230. 230.  Standard model ◦ Breaks down matter to smallest number of fundamental units to construct all known particles  High energy particle collisions ◦ Support many predictions  Quark confinement means ◦ Not possible to detect free quarks  Using high energy electrons can show ◦ Hadrons & mesons are made of smaller particles
231. 231.  Scattering of high-energy particles ◦ By particles that make up nucleons.  Inelastic because ◦ Scattered particles lose energy to target  By analysing scattering angles can show: ◦ Nuclei are made of smaller charges knocked apart by collision
232. 232.  At higher energies ◦ Baryons scatter electrons in a way ◦ Consistent with being made of 3 point charges ◦ Mesons scatter in a way ◦ Consistent with being made of 2 point charges ◦ The size of charges is 1/3 e or 2/3e.  Total momentum of quarks ◦ much more than expected ◦ Implies other particles in nucleus - Gluons
233. 233.  Findings support quark model  No free quarks were found ◦ Due to confinement principle  Particles found are ◦ Consistent with standard model
234. 234.  Force holding quarks together is very strong  Caused by exchange of gluons  Problem  If force increased as distance between quarks decreased ◦ Matter would collapse to nothing  Believe at short range ◦ Force tends to zero  Quarks have asymptotic freedom
235. 235.  If true  Quarks which make up a hadron ◦ Won’t be held tightly by strong nuclear force ◦ Free to move about inside particles ◦ Like apples in a bag  Getting quarks out of bag takes lots of energy ◦ Not moving them around inside
236. 236.  Results of deep inelastic scattering ◦ Suggest small charges responsible for scattering ◦ Inside hadrons ◦ Are free to move around
237. 237.  Related to weak interaction ◦ Mediated by W+, W & Z+ bosons  Certain interactions can be predicted  An electron and a neutrino can collide  This is a weak interaction ◦ involves W exchange particle.  The resultant particles do not change charge ◦ called neutral currents  Difficult to observe ◦ First seen in 1973 at CERN bubble chamber.
238. 238.  Without neutral currents ◦ Holes would appear in standard model  Finding it validated the electroweak theory.
239. 239.  Big Bang  In the beginning (before 10-43s)…… ◦ No time ◦ No space  Just a singularity of very dense matter  What was there before this?  Nobel prize for first correct answer  At the beginning of time 4 forces were unified.  What was the temperature?
240. 240.  Can tell from energies required to produce various particles  Originally (after 10-43s) 1032 K  10-35 s – Grand Unification Era (GUT)  Cooled to 1027 K (1014 GeV).  Particles & antiparticles form radiation ◦ Photons produce particle – antiparticle pairs ◦ Particles – antiparticles can then decay  Result more particles than antiparticles
241. 241.  Particle Era  10-12 s  Temp now 1015 K (100 GeV) ◦ Particle accelerators can reach these energies  Quarks now form ◦ protons, neutrons ◦ Other baryons & mesons.
242. 242.  Nuclear Reaction Era  3 minutes  Temp now 109 K (0.1 MeV)  Protons & Neutrons combine to make nuclei  Universe now ◦ 90% Hydrogen ◦ 10% Helium
243. 243.  Recombination Era  105 years  Now below 4000 K (0.4 eV)  Electrons combine with nuclei to form atoms
244. 244.  Galaxy Era  109 years  Gravity pulls atoms together  Forms larger elements
245. 245.  Now  3 K (0.0003 eV)  Still hot places ◦ Stars  Most of universe is cold
246. 246.  Originally matter slightly dominated over antimatter  Once energies (temp) of photons was below that which could produce pair production ◦ Matter dominated
247. 247.  Have ignored gravity  Because when mass small ◦ Force is neglibgible  However, when distance is tiny ◦ force is very large  This doesn’t fit with matter ◦ being made of small particles  String Theory addresses problem
248. 248.  Instead of point masses  Particles are made of short strings ◦ 10-35 m  This is an alternative to quantum theory  Strings oscillate  Mode of vibration represents properties ◦ Such as mass  Only certain harmonics allowed ◦ Therefore only certain particles allowed  Need extra dimensions (difficult to visualise)
249. 249.  Elementary Particle Physics is opening up new areas.  One new theory is called the; ◦ Grand Unified Theory  A Nobel Prize beckons you if you wish to continue in this area