Gamma radiation is NOT made of particles but is an electromagnetic radiation; that is, it is energy travelling by a wave motion. This means that it is similar to light but has a very short wavelength and a very high frequency.
The wavelengths of gamma rays can be from about 5 x 10{U-11} to about 1 x 10{U-15} metres or even less. They overlap in wavelength size with X-rays but because they originate from the nucleus (unlike X-rays, which originate from energy emitted by orbital electrons) they are distinguishable from X-rays.
Like all electromagnetic waves they have a transverse wave motion, travel at 3 x 10{U8} metres per second - the speed of light in a vacuum - obey the wave equation (speed = frequency x wavelength) and under certain circumstances can be reflected, refracted, diffracted, absorbed and display interference.
Mass:
Gamma radiation has no particles and therefore has no mass.
Charge:
Gamma radiation has no particles and therefore has no charge.
Range:
Gamma radiation has a very long range in air, as do most electromagnetic radiations. Its intensity reduces with distance from the source. The intensity reduces according to an 'inverse square law', which means that if the distance from the source is doubled (i.e. 2x) then the intensity is reduced by 2{U2} (i.e. a quarter of the original intensity). Similarly, if the distance from the source is trebled (i.e. 3x) then the intensity is reduced by 3{U2} (i.e. a ninth of the original intensity).
Behaviour:
Gamma radiation is very penetrating and can pass through many centimetres of lead or several metres of concrete.
As it possesss no electric charge gamma radiation is not deflected by magnetic or electric fields.
(2) Detection:
Gamma radiation can be detected in a variety of ways.
Scintillations: if gamma rays strike a surface coated with certain zinc or barium compounds then fluorescence (glowing) occurs at the point at which the rays hit. This method was employed during the early experiments with gamma rays and X-rays. The radiation can therefore be detected by observation, but it is also possible for the quantity of light energy given off to be measured, thus providing an accurate means of measuring the radiation.
Cloud chambers: gamma radiation can cause ionisation of gases, that is, electrons can be removed from atoms. Ions (electrically-charged particles) are therefore formed. If these ions are produced in an atmosphere rich in water vapour then droplets of water appear where the ions are produced. The path of the gamma rays therefore becomes visible.
Geiger-Muller tube and ratemeter: a suitable G-M tube can be used to detect gamma rays. Gamma rays entering the tube cause particles of the gas inside to become ionised, that is, they become electrically charged. The charged particles are then detected inside the G-M tube, amplified and sent to a suitable ratemeter which displays the number of ionisations occurring as a measure of the intensity of the radiation.
Photographic film is affected by gamma radiation. Upon development, the film would appear darker; the intensity of the 'darkness' depends upon the intensity of the radiation striking it. A version of this type of detector was used for personal-dose measurement in the nuclear industry.
(3) Discovery:
Gamma rays were named by Ernest Rutherford in 1903 when he was able to show that radiations which had been assumed to be beta particles were actually composed of two types of radiation. Rutherford managed to separate the two radiations and show their distinct differences.
(4) Biological effects:
The biological effects of radiations can be considered in two groups: short term (immediate) effects on the individual; and long term effects, including genetic effects.
Short term:
Gamma radiation is extremely penetrating but the damage done depends upon the intensity and duration of the exposure as well as the area covered and the organs concerned. A sufficiently large dose will kill any organism.
Radiations can damage individual cells which will then divide and reproduce the defect. Almost any cell can be affected but the most common are those of the skin, the blood-forming tissues and the male reproductive system.
In addition to damaging cells, the rate of cell-division may be slowed down or stopped. The consequences of this can be deformed development of children or a variety of failures in adults, depending upon the area of the body affected.
The embryo and foetus of a developing child are particularly susceptible to excessive doses of radiation, with links to leukaemia, tumours, physical underdevelopment and mental retardation.
An adult exposed to excessive radiation will experience loss of appetite, vomiting, diarrhoea and fever. Rapid hair loss will occur. A reduction in white blood cell production will increase the chances of subsidiary infections. The blood then starts failing to clot, leading to internal bleeding. Death can be as a result of any of the above problems. The intensity of each of the conditions depends upon the level of exposure. For lower levels of exposure recovery rather than death is probable. Generally, excessive exposure to radiation shortens the lifespan.
Long Term:
The major long-term effect of exposure to radiation is in the reproductive organs, particularly of the male. Depending upon the level of exposure varying degrees of sterility can be produced. In addition, both chromosomes and genes can be changed (mutated), which will then pass on to any offspring. The results of the mutation are many and varied depending upon the particular change. In general terms any children produced will suffer physical and/or mental defects.
(5) Applications:
Discovery of cracks and other flaws in metal structures.
Sterilising medical equipment.
Irradiation of food to prolong life by destroying organisms (such as fungal spores) that cause deterioration.
