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When energy is transferred from one particle of a given medium to another particle in a series, disturbing the atoms but essentially not moving the molecules of the medium, this is known as a wave. Waves yield two measurable dimensions: wavelength (often notated by the Greek letter Î») which is the distance between two 'peaks' of the waveform and amplitude which is the height of the wave. The main difference between light waves and sound waves is the wavelength, where light has a very short length, between around 0.00004 and 0.00007 centimetres (Netting, 2007), compared to sound waves which are usually much bigger at approximately 17mm to 17m (Van de Water & Staecker, 2006). Light waves travel in straight lines and cast sharp shadows, where sound waves can bend and bounce off surfaces and can easily be stopped by a surface which can absorb the energy of the wave. These behavioural differences can be explained by the waveform's wavelength; waves can only curve around an obstruction which is not much larger than the wavelength (Asimov, 1987). This means for a human interpreting these waves, that a sound wave can become easily distorted by almost any obstruction between the emitter and the ear, where only very small objects such as sub-microscopic particles can allow light to pass around them. Another example of the distortion of light is when a light wave enters a transparent substance, through which its speed is slowed, at an angle to the vertical it is always refracted into a path that forms a smaller angle to the vertical and under the right conditions a light wave can effectively be split into its component wavelengths by such a transparent object and so a spectrum (from the Latin word for 'ghost') of the light colours red through to violet can clearly be seen on a plain surface.
It is generally recognised that the human ear can detect sound waves which fall into the frequency (wavelength) range of 20Hz to 20kHz and is particularly sensitive to the 1kHz to 4kHz range (Smith, 1997). Therefore, in order to test the sensitivity of the human ear to high frequency sound waves, the amplitude of the wave must be increased as the wavelength is shortened. Furthermore, the intervals of tones generated for testing must be at a greater ratio than 3% in order for the listener to be able to distinguish a difference in pitch - smaller intervals would present as an un-noticeable ramp in pitch and the listener would fail to report any change in sound.
The human eye is naturally shaped to direct light waves hitting the cornea through the eye's lens where it is refracted and focussed into a single point at the retina (fig. 1.1). Common sight defects such as myopia (near-sightedness) and hyperopia (far-sightedness) occur in the eye where the cornea is more or less curved respectively or where the distance between the lens at the front of the eye and the retina at the back of the eye is too long or short. The rays of light entering a myopic eye come to a focal point in front of the retina (fig. 1.2); this has the effect of maintaining reasonable eyesight when looking at close objects but presents a blurred image when attempting to focus on far away objects. Conversely the rays of light entering a hyperopic eye would converge at a focal point behind the retina (fig. 1.3) and so the sufferer finds close objects more difficult to distinguish than objects further away.
Myopia can therefore be corrected using concave lenses which are calibrated to increase the distance of the focal point so that it hits the retina (fig. 1.4); while convex lenses, which shorten the distance to the focal point, are used to correct hyperopia (fig. 1.5). The powers of these lenses can be calculated to equal a value sufficient to negate the refractive error of the eye.
Radiation is the process by which energetic particles emanate from a source and travel through a medium, such as air, in straight trajectories in all directions. The term can describe any particles traveling in such a way, for instance visible light or radio waves, but it is most commonly used with reference to high energy radiation which can ionise atoms. Ionisation occurs when an electron is detached from an atom by the radiating particles, leaving the atom with a positive charge; this form of radiation is the most harmful to organisms and in extreme cases can cause mutations to cells and DNA and is a known catalyst for cancerous mutations (Stabin, 2007).
The quantity of ionising radiation received by a human is measured in terms of energy absorbed into the body tissue and uses the unit gray (Gy). One gray is one joule absorbed per kilogram of mass. The effective absorbed dosage of radiation into biological mass is measured using a relative unit: the Sievert (Sv); Sieverts are so large however that more commonly mSv are used when referring to human dosage. Background radiation is all around us - it is present majorly in the form of radon gasses emanating from the ground, this makes up around half the range of background radiation; the other half of the range is then made up of roughly equal parts of food and drink sources, buildings and the ground, cosmic rays from outside the Earth's atmosphere and artificial sources such as nuclear power and weapons and medical radiation (WNA, 2011). The following table (fig. 2) gives an indication of the likely effects of a range of whole body radiation doses and dose rates to individuals:
10,000 mSv(10 sieverts) as a short-term and whole-body dose would cause immediate illness, such as nausea and decreased white blood cell count, and subsequent death within a few weeks.
Between 2 and 10 sieverts in a short-term dose would cause severe radiation sickness with increasing likelihood that this would be fatal.
1,000 mSv(1 sievert) in a short-term dose is about the threshold for causing immediate radiation sickness in a person of average physical attributes, but would be unlikely to cause death. Above 1000 mSv, severity of illness increases with dose.
If doses greater than 1000 mSv occur over a long period they are less likely to have early health effects, but they create a definite risk that cancer will develop many years later.
Above about 100 mSv, the probability of cancer (rather than the severity of illness) increases with dose. The estimated risk of fatal cancer is 5 of every 100 persons exposed to a dose of 1000 mSv (ie. if the normal incidence of fatal cancer were 25%, this dose would increase it to 30%).
50 mSvis, conservatively, the lowest dose at which there is any evidence of cancer being caused in adults. It is also the highest dose which is allowed by regulation in any one year of occupational exposure. Dose rates greater than 50 mSv/yr arise from natural background levels in several parts of the world but do not cause any discernible harm to local populations.
20 mSv/yraveraged over 5 years is the limit for radiological personnel such as employees in the nuclear industry, uranium or mineral sands miners and hospital workers (who are all closely monitored).
10 mSv/yris the maximum actual dose rate received by any Australian uranium miner.
3-5 mSv/yris the typical dose rate (above background) received by uranium miners in Australia and Canada.
3 mSv/yr(approx) is the typical background radiation from natural sources in North America, including an average of almost 2 mSv/yr from radon in air.
2 mSv/yr(approx) is the typical background radiation from natural sources, including an average of 0.7 mSv/yr from radon in air. This is close to the minimum dose received by all humans anywhere on Earth.
0.3-0.6 mSv/yris a typical range of dose rates from artificial sources of radiation, mostly medical.
0.05 mSv/yr, a very small fraction of natural background radiation, is the design target for maximum radiation at the perimeter fence of a nuclear electricity generating station. In practice the actual dose is less.
Fig. 2: Effects of radiation doses. Source: (WNA, 2011)
When ionising radiation is emitted, the particles given off are unstable as their natural atomic arrangement has been altered by the energy produced by the reaction and as a result the atoms being radiated will start to decay - they do this on an entirely random and individual basis. There are three main types of radioactive decay: Alpha, Beta and Gamma. Alpha decay occurs because the nuclei of the emanating atoms have too many protons which results in a repulsion reaction inside the atom whereupon a helium nucleus (alpha particle) is emitted. There are three ways in which beta radiation can decay: when the neutron to proton ratio is too great the nucleus becomes unstable, a neutron is turned into a proton and an electron and the electron is emitted as a beta particle; when the neutron to proton ratio is too small a proton is transformed into a neutron and a positron (a positively charged electron) and the positron is emitted; under the same circumstance, where the neutron to proton ratio is too low, the atom can attract an electron which has the effect of turning a proton into a neutron. Finally, gamma radiation occurs when the nucleus is at an energy level which is too high; here the nucleus regresses to a lower energy state and in the process emits a high energy photon (gamma particle). When an atom of a certain element emits an alpha particle it becomes a negatively charged isotope or nuclide and when it emits a beta particle it becomes positively charged. Although the change in atomic mass should make the element change fundamentally, the neutral atom and its isotope are, chemically speaking, identical and indeed the word isotope (coined by Frederick Soddy) is made from Greek words meaning "same position". The rate of decay of radioactive isotopes and nuclides is measured in Becquerels (Bq) in terms of the half-life, which is the time required for the number of parent nuclei to fall to fifty per cent.
In order to demonstrate the relative penetrating powers of alpha, beta and gamma rays and also demonstrate the uses of different absorbers of radiation a lab experiment may be set up. It is very important to note some vital safety precautions:
Never handle sources without tools - any skin contact with radioactive substances can lead to irradiation of the body's cells.
Do not point sources at any person - any direct contact with radioactive rays can also lead to irradiation of the body's cells.
Replace sources into lead-lined containers as soon as possible - this is to protect the source's decay as well as to protect the environment the source is stored in from exposure to radiation.
Wash hands thoroughly after handling - if any skin contact has been made this will prevent any further absorption of radiation and also prevent the spread of contamination to others through contact.
The most important safety consideration is to make sure to avoid all un-necessary exposure to ionising radiation. In the case of all deliberate exposure, the benefit must outweigh the risk.
Method and Apparatus
Closely observing the above safety precautions the source, which should be capable of emitting alpha, beta and gamma particles, is placed in a stable holder such that its exposed face is directed toward a radiation detector which has already been activated so as to take a reading of the level of background radiation present before the source was exposed. A control value can now be taken for the radiation given off by the source at very close proximity to the detector.
Alpha particles are fast moving helium atoms with high energy but a large mass which means they are stopped by just a few centimetres of air or a standard piece of paper. In order to test these rays a sample square of regular copier paper can be inserted and held (by tool not by hand) in place between the source and the detector, while the two are still in close proximity. A significant reduction in the reported radiation should immediately be evident. Removing the paper and carefully increasing the distance between the source and the detector should result in a similar reduction from the control level.
Next, to test the beta particles, fast moving electrons with less energy typically than the alpha helium atoms but which are less dense and so able to penetrate further, through a much larger expanse of air, or several millimetres of plastic or thin, light wood. With the source and detector set to their control positions a thin sheet of plastic or wood can be held in place between them and the difference in measured radiation can be observed. At this point, increasing the distance between the source and the detector may or may not yield similar results, dependant on how much air can be placed between the apparatus.
The gamma particles which are photons, like light except much higher energy, can now be tested. Depending on their energy, these particles are able to penetrate materials in the range of thin aluminium foil through to several inches of lead and in every case are easily able to penetrate skin and do harmful damage to organic tissue. Different thicknesses of different types of metals can now be tested in between the source and the detector until the radiation levels detected settle upon or around the initial background radiation level recorded at the start of the experiment. Once testing is complete, observing the safety precautions requires the source to be safely re-housed in its lead lined container and hands and other exposed skin to be thoroughly cleansed to avoid unknowing contamination.