Edexcel A2 Physics Unit 4 : Chapter 3 : Particle Physics 3.1: Probing Matter Prepared By: Shakil Raiman

3.1.1: A Nuclear Atom

3.1.2: Alpha Particle Scattering:  Between 1909 and 1911, Geiger and Marsden, students of Lord Rutherford at Manchester University, undertook an experiment in which they aimed an alpha particle source at an extremely thin gold foil.

3.1.2: Alpha Particle Scattering:

3.1.2: Alpha Particle Scattering: Evidence and Conclusion

3.1.3: Chadwick’s Discovery of the Neutron:  Rutherford had determined that most of the atom’s mass and all the positive charge was held in a very small nucleus in the centre, and that electrons were held in a position at the edge of the atom. The difference between the nuclear mass and the known number of protons in it caused a problem though. Nuclei were too massive for the number of protons they contained. Rutherford suggested that additional proton-electron pairs, bound together, formed the extra mass in the nucleus.

3.1.3: Chadwick’s Discovery of the Neutron:  In 1930, Irene Joliot-Curie and her husband, Frederic, found that alpha particles striking beryllium caused it to give off an unknown radiation. Difficult to detect, this unknown, uncharged radiation could knock protons out of paraffin and these were detected by a Geiger-Muller tube.  The Joliot-Curies tried to explain the unknown radiation as gamma rays, but as these rays have no mass, this was a breach of the conservation of momentum.  James Chadwick repeated the experiments using other target materials as well as paraffin.

3.1.3: Chadwick’s Discovery of the Neutron:  By considering momentum transfer and conservation of kinetic energy in the collisions between the particles, Chadwick concluded that the beryllium radiation was a neutral particle which had a mass about 1% more than that of a proton. In 1932 he published his proposal for the existence of this new particle which he called a neutron, and in 1935 he was awarded the Nobel Prize for the discovery.

3.1.3: Chadwick’s Discovery of the Neutron:

3.1.4: Nuclear Structure

 Above atomic number 20, for the nucleus to be stable more neutrons than protons are generally needed. The neutrons held to bind the nucleus together as they exert a strong nuclear force on other nucleons, and they act as a space buffer between the mutually repelling positive charges of the protons. This buffering action means that as we progress through the periodic table to larger and larger nuclei, proportionately more and more neutrons are needed. By the time we reach the very biggest nuclei, there are as many as 50% more neutrons than protons.

3.1.4: Nuclear Structure

3.1.5: A Quantum Mechanical Atom

3.1.6: Problem

3.1.6: Answers

3.1.7: Electron Beam

3.1.8: An Electron Probe  Electron beams fired at a crystal will produce scattering patterns that can tell us about the structure of the crystal. In 1927, it was shown by Davisson and Germer that an electron beam produces a diffraction pattern. Louis de Broglie was bemused that light could be shown to behave as a wave in some situations and as a particle in other circumstances. The expression relating De Broglie wavelength and momentum is,

3.1.9: Problems

3.1.9: Answers

3.2: Particle Accelerator

3.2.1: Linear Accelerator

3.2.1: Linear Accelerator (Linac)  Linear accelerator is a series of electrodes to accelerate particles to a very high speeds in a straight line.  If the particles to be accelerated in the linear accelerator are electrons, they are generated by an electrostatic machine and introduced into the machine. The electrons are attracted towards tube A by making its metal cylinder positive. Once inside the cylinder, the electrons move in a straight line, as the electrode is equally attracting in all directions.

3.2.1: Linear Accelerator (Linac)  The alternating voltage supply is made to change as the electrons pass the middle of tube A, so it becomes negative. This repels the electrons out of the end of tube A and on towards tube B, which now has a positive potential. They accelerate towards it, and the whole process repeats as they pass through tube B and are then accelerated on towards tube C. This carries on until the electrons reach the end of the line, at which point they emerge to collide with a target.

3.2.1: Linear Accelerator (Linac)  In order to keep accelerating particles that are moving faster and faster, the acceleration tubes must be made longer and longer as the particles travel through each successive tube as a higher speed.  The time between potential difference flips is fixed as the alternating voltage has a uniform frequency of a few gigahertz.  The limit on the use of this kind of accelerator is how long you can afford to build it, remembering that the whole thing must be in a vacuum so that the particles do not collide with air atoms.

3.2.2: Was Einstein Right:

3.2.3: Accelerating Particles in Circles:

3.2.4: The Cyclotron:

3.2.5: The Cyclotron frequency:

3.2.6: The Synchrotron:

3.2.6: The Tevatron:

Thank You All  Wish you all very good luck.