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Thermionic emission and Radiation and half-lives.

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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.

Two principles:

  1. Thermionic emission.  
  2. Radiation and half-lives.

Thermionic emission

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 different materials.

The process can occur at any temperature

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In medicine x-rays are an extremely important part of diagnosis as there is no need to perform any type of surgery in order to get an x-ray photograph, more importantly they are very reliable and deliver less radiation than other methods of diagnosis.

When an x-ray is taken however the patient does not receive all of the x-rays as some have no benefits and therefore just add to the amount of radiation received, these low energy x-rays are filtered out also collimators cut down the area of exposure to they desired area only, again to minimise exposure. The way the photograph is created is by the x-rays interfering with a phosphor screen, the more x-rays that hit a certain part of the screen the darker it becomes; this varies due to the differing densities within the body.

The main diagnostic uses of x-rays is to look for skeletal fractures, this is relatively simple as it requires no intake of various substances. Another use though is to search for problems such as tumours or perforations in organs. One problem is that it is very difficult to differentiate between different organs on an x-ray as they have very similar densities. This is countered by the intake of denser substances which can target different organs e.g. barium and iodine. This means the particular organ shows up far clearer than the surrounding organs, muscle and fat.

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The  aim of nuclear medicine is the development of receptor-specific carrier molecule, to which isotopes can be bound, which target specific organs or disease states, and carry technetium-99m or other radionuclides to the sites in the body that want imaging. Once this binding has occurred the substance is termed a radiopharmaceutical and can be administered in three different ways depending on where it must end up. It can be swallowed, injected or inhaled.

The picture is created by a tomography using gamma receptors (photomultipliers) which change the colour of their crystals depending upon the amount of gamma radiation it receives. After this the colour change occurs larger photomultipliers send a signal to a computer where the picture is built up in slices (cross-sectional of a horizontal body) the brighter areas are where there is a higher level of gamma radiation coming from.

...read more.

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