Overview

In Search of Giants Documentary

Splitting the Atom Cockcroft and Walton shared the Nobel Prize for their work in splitting the atom.

How it works The protons get accelerated across a potential difference of 800 kVolt. The protons collide with a lithium nucleus, and as a result two alpha particles are produced. The alpha particles move off in opposite directions at high speed. They then collide with a zinc sulphide screen, where they cause a flash and get detected by microscopes.

Transformers, rectifiers and capacitors were used to produce the necessary high d.c. voltage to accelerate the protons. Protons produced in a Hydrogen discharge tube were injected into the accelerating tube. Here they were accelerated by the high voltage. The protons struck a Lithium target placed at and angle of 45 degrees to the beam The products of the reaction were emitted at right angles to the proton beam and struck Zinc sulphide screens producing small flashes of light called scintillations which could be seen with a microscope It was shown that the products were Helium nuclei (i.e. alpha- particles), emitted in opposite directions with the same speed.

Left Hand Side: The total mass/energy in, consists of the proton, plus lithium plus kinetic energy of the proton. Right Hand Side: The total energy out consists of the two alpha particles, plus kinetic energy of the alpha particles. A transmutation is the changing of a nucleus of one atom into a nucleus with a different atomic number (i.e. the changing of an atom of one element into an atom of another element) ++ K.E

Using E = mc 2, the scientists could predict the kinetic energy of the alpha particles. Their predictions were consistent with the experiment results, thus confirming Einstein’s equation. Einstein explains his equation

Why was this experiment significant? First artificial splitting of the nucleus. First transmutation using artificially accelerated particles. First verification of Einstein’s E = mc 2 First Particle Accelerator

Maths Bit! Lets take a look at problem one and two on page 367 and 368. Now try Question 1 and 2 exercise 32.1

Converting other forms of energy into mass Nowadays the particle accelerators are much more powerful, and one of the more common experiments is to whack two protons off of each other. To do this they are sent in opposite directions around a circular particle accelerator (e.g. in CERN) Particle Accelerator Simulation

The kinetic energy gets transformed into new and exotic particles. p + p + kinetic energy = p + p + additional particles The larger the kinetic energy of the protons before collision, the greater will be the variety of new particles produced These particles make up what is known as ‘the particle zoo’. Large Hadron Rap

Anti-matter Each particle has its own anti- particle, which is identical in mass, but opposite in charge. The English physicist Paul Dirac predicted anti-matter mathematically before it was detected experimentally. Antimatter

The neutrino The neutrino was first predicted by the Italian physicist Pauli, to account for the apparent discrepancy between the momentum before and after a beta decay. The neutrino is extremely small, has almost no mass, and has zero charge (the term itself means ‘little neutral one’).

When a nucleus undergoes beta decay it appeared that momentum was not conserved. How did Fermi’s theory of radioactive decay resolve this? It proposed the existence of another particle, called the neutrino,, that had the missing momentum. The Neutrino

The existence of the Neutrino was proposed in 1930 but it was not detected until Give two reasons why it is difficult to detect a neutrino? The Neutrino: is uncharged has a very small mass interacts weakly with matter.

The Unified Atomic Mass Unit Most common way of expressing mass in particle physics is to use unified atomic mass units rather than kg. The symbol is u 1u = 1.66 x kg

Maths Again Look at problems 3 and 4 on page 370 Try questions 1 and 2 exercise 32.2

Antiparticles Same mass as their corresponding particles. If the particles is charged the antiparticle has an equal but opposite charge Symbol for antiparticle is the same as for the particle but with a bar on top (see board)

The Positron Discovered in 1932 by Carl David Anderson while doing an experiment to look at comic rays. Positrons have the same mass and charge as an electron, but the charge is of an opposite sign. The positron is an antiparticle of the electron

Pair Production Energy is converted into matter Photon (hf or gamma ray) loses energy when it collides with a nucleus Energy converted into the mass of the elcetron and positron the rest is the KE of electron and positron γ rays  e - + e + + K.E. hf  2mc 2 + E k1 + E k2.

Pair Production Note: Charge is conserved. Net charge before and after is zero. Momentum is conserved. You must use the phrase ‘gamma-ray photons’, and not just ‘photons’; the logic being that ‘gamma-ray’ implies a very high level of energy!

Pair Annihilation e- + e+  2γ + K.E. Conservation of Charge Again, it is straightforward to check that charge is conserved; net charge beforehand is zero, and the photons produced afterwards have no charge. Conservation of Momentum For momentum to be conserved, you must note that the electron and positron are moving directly towards each other beforehand and so have no (net) momentum. Therefore the two photons must fly off in opposite directions in order for there to be no (net) momentum after.

Quarks It turns out that many particles which we thought to be fundamental, are actually made up of more fundamental particles, called quarks

Where e is the charge of one electron, e.g. the Up quark has a charge of two thirds the charge of an electron.

Murray Gell-Mann The term quark was given to these particles by the American physicist Murray Gell-Mann who first predicted their existence in 1964 and won a Nobel Prize for his work in Particle Physics. He found the word in a book by James Joyce, called Finnegans Wake.

Anti-quark An anti-quark has the same mass as its quark counterpart, but opposite charge. e.g. an anti-up has a charge of – 2/3 e. If a particle is composed of three quarks it is called a baryon and if it is composed of two quarks it is called a meson (actually the quark will be composed of a quark and an antiquark).

Fundamental Forces of Nature Strong Binds nucleus together Short Weak Responsible for Beta decay Short Electro-Magnetic Force between charged particles Inverse square law Gravitational Force between planets Inverse square law

All particles can now be categorised as follows: Leptons: Indivisible point objects not subject to the Strong Force, e.g. positron, electron, neutrino. Hadrons: Feel all four forces. Mesons: Subject to all forces; mass between electron and proton. Baryons: Subject to all forces, e.g. protons and neutrons.