Thermionic Emission and Radiation and Half-Lifes
For the write-up section of my coursework I went on a trip to the Kent
and Canterbury Hospital in order to look at the application of physics
within the field of medicine. I intend to spend time explaining two
physics principles which have uses in this scientific field, and
illustrate how they are used, either within the diagnosis or treatment
of a patient.
1. Thermionic emission.
2. Radiation and half-lives.
Thermionic emission is a phenomenon by which electrons are emitted
from the surface of a metal or metal oxide. The flow of these
electrons can only occur when the thermal vibrational energy of the
matrix overcomes the electrostatic forces preventing the electrons
leaving the surface.
Thermionic emission is entirely reliant upon the 'sea of free
electrons'. This is the collective name given to the one or two
electrons per atom, in any metal, which are free to move around and
are not bound to the atom. The velocities of these free electrons
follow a statistical distribution, and occasionally an electron will
have enough velocity to overcome the electrostatic retaining forces.
The minimum amount of energy required for one of these electrons to
escape is called the work function. The work function varies within
The process can occur at any temperature above absolute zero but it is
extremely inefficient, therefore when ever thermionic emission is
being used or tested the material is heated to a very high
temperature. Another method used in increasing the amount of
thermionic emission taking place is to coat the surface of the metal
with a metal oxide due to the fact that oxides lower the work
function; this is simply because the electrostatic retaining forces
within oxides are far lower than those in metals.
Thermionic emission is an incredibly important principle within
medical science because it can be used to create x-rays, for use in
x-ray machines. An x-ray is created when a fast moving electron, such
as those produced in thermionic emission, loses a considerable amount
of energy instantaneously.
In x-ray machines, thermionic emission occurs in vacuum tubes. There
is a cathode which is also a heating filament so as to allow high
temperatures but also to provide a large area from which thermionic
emission can occur. The filament emits electrons very effectively when
it is red-hot therefore it is supplied with a huge voltage (70KV) it
is this potential difference which causes the huge velocitieof the
electrons. Directly opposite this filament is an angled tungsten anode
which attracts the electrons and causes huge velocity, this anode then
instantaneously slows them down which as I stated above causes the
production of x-rays. The...