Edexcel A2 Physics
Unit 4 : Chapter 3 : Particle Physics
3.1: Probing Matter
Prepared By: Shakil Raiman
3.1.1: A Nuclear Atom
3.1.1: 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
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: 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: Problems
3.1.9: Problems
3.1.9: Answers
3.2: Particle Accelerator
3.2.1: Linear 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:
3.2.2: Was Einstein Right:
3.2.2: Was Einstein Right:
3.2.3: Accelerating Particles in Circles:
3.2.3: Accelerating Particles in Circles:
3.2.3: Accelerating Particles in Circles:
3.2.4: The
Cyclotron:
3.2.4: The Cyclotron:
3.2.5: The Cyclotron frequency:
3.2.6: The Synchrotron:
3.2.6: The Synchrotron:
3.2.6: The Tevatron:
Thank You All
Wish you all very good luck.