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Paper 4 Particle Physics
Alpha Particle Scattering was an experiment in order to investigate the strucuture of the atom. An alpha particle source was aimed at an extremely thin gold foil. Three observations were made:
1. Most alpha particles passed straight through with little/no deflection. This indicated that most of the atom is empty space.
2. Some alpha particles deflected through a large angle (1090°). The atoms positive charge is concentrated in one place and deflects the positive charge of the Alpha particles.
3. A few alpha particles bounced back to the source side of the foil. This indicated that most of the mass, and all positive charge is in a tiny central nucleus.
A nucleus is made up from nucleons, these include neutrons and protons. The proton number is the number of protons in the nucleus, and determines the element of the atom.
Above atomic number 20, more neutrons are required than protons in order to have a stale nucleus. The neutrons help bind the nucleus together, and exert the strong nuclear force, and act as a buffer between
positively repelling protons. The strong nuclear force only acts on a very close range, and energy is required to bring nucleons close together.
Electrons in a metal can escape from the surface when the metal is heated. This is thermionic emmission. Free electrons in an electric field will gain kinetic energy by:
Ek = qV
Using thermionic emmisssion (in the form of a cathode) and accelerating anodes, a beam of fast movin electrons can be generated. A magnetic field can be used to deflect a beam of electrons.
The wavelength of a particle can be calculated using de Broglie's wave equation:
Where h is planck's constant, m is mass, and v is velocity.
In order to investigate the nature of particles, they are collided at high speeds and high energies. This is required because at low energies the particles just repel each other.
A Linac accelerator is linear. It consists of a long straight tube like electrodes. An alternating current is applied, so the charge of the electrodes switches. This is timed such that the particle is always attracted to the
next electrode, and repelled by the previous. As a particle's speed increases, longer electrodes are needed to create he same timing. The particle collides with a fixed target at the end of the tube.
A cyclotron is a circular accelerator. Two semicircular electrodes accelerate a particle across a gap inbetween. Again, the charge of them is alternated using an alternating current. A magnetic field is used to keep
particles in a circular motion.
In a syncrotron, particles are accelerate in both directions of a circle and collided (both with velocity). It is like a linac bent into a circle. Switching the charge of electrodes is timed to be in sync with the movement
of the particles. Magnets are used to keep the particles moving in a circle, and to focus the beam.
In a magenetic field, the force on a particle F = Bqv . To move in a circle, the force F = mvr . Combining these gives a formula for the radius of a particle in a magnetic field.
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This shows that in a magnetic field, the radius of movement is proportional to momentum. This is useful when identifying particles from their tracks.
Frequency f = 1 2r
T , where T = v . Given the formula for r, the frequency of a particle moving in a circle is
f = 2 m
The GM tube works on the principle of ionising radiation creating ions which creates a current in the tube. This current can be detected to count radioactivity.…read more