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Rays or particles carrying energy are known as radiation. Based on the effects of the radiation on matter, they are classified as ionising or non-ionising radiation. Ultraviolet, visible light, infrared, microwaves, and radio waves are examples of non-ionising radiation. Radioactive substances emit ionising radiation.
Some atoms – the building blocks of matter – are unstable (radioactive) and can decay into other types of atom. During this decay they radiate energy as particles or gamma (γ) rays. The amount of energy carried by the particles depends on their weight and speed. The gamma rays emitted are like light or radio waves, but are much more penetrating.
The most common particles or rays that are released from radioactive nuclei are alpha particles (α), beta particles (β), neutrons (n) and gamma (γ) rays. Alpha particles (helium nuclei, two neutrons and two protons) are heavy and slow and can be stopped by human skin. Beta particles (electrons) are fast, very light and more penetrating than alphas. Gamma rays are weightless electromagnetic radiation travelling at the speed of light, and are much more penetrating. Very strongly penetrating, neutrons are generally produced in nuclear reactors.
Mankind has always been subjected to radiation from outer space and from naturally occurring radioactive material in the Earth’s crust. Non-ionising radiation such as light and heat from the Sun are natural forms that are essential to human existence. Some examples of natural sources of ionising radiation are cosmic rays, radon and thoron decay products in the air, potassium and other radionuclides in foodstuffs, etc. Manmade sources include for example medical uses of radiation in diagnosis and therapy and the discharge of radio-nuclides to the environment.
When the nuclear (or ionising) radiation enters a detector or monitor, it interacts with the detector material (e.g. a gas) and releases a large number of low-energy electrons. These electrons are collected to form a voltage or current pulse that can be measured by the usual means and can be made to trigger an audio or another recording device. Geiger counters based on this principle are popular radiation-monitoring instruments.
Radiation can have adverse biological effects on living beings. At very high doses it can lead to death within relatively short periods. At low doses – typical of those encountered in the environment (from natural or artificial sources) or in medical diagnosis – it may result in an increased risk of malignant diseases in exposed people or inherited effects in their descendants. Radiation is, however, a relatively weak carcinogen and any increase in risk from exposure to low doses is, at most, small. Indeed, some research indicates that exposure to low doses has a beneficial effect in some circumstances but these findings need further investigation.
Alpha particles can scarcely penetrate the outer layer of the skin. The radio-nuclides that emit them are not hazardous unless they are taken into the body via inhalation or through the food chain. Beta particles may penetrate a centimetre or so into tissues so internal organs are not at risk unless they are also ingested. X-rays, gamma rays and neutrons can go right through the body and as a result radio-nuclides that emit them are hazardous anywhere.
Nevertheless, the use of artificial radiation has led to remarkable advances in medical diagnosis and treatment and it also has a wide range of applications in industry, agriculture and research. A careful balance needs to be made between the risks and benefits of ionising radiation in determining its use – indeed this is fundamental to the system of radiation protection which embodies the justification and optimisation of any use.
In the cell nucleus, there are large molecules of deoxyribonucleic acid (DNA) which control the structure and function of the cell, and replicate to produce copies of themselves. Under the effects of radiation, a DNA molecule may become ionised directly or it may be changed indirectly by free radicals in a water molecule of a cell that has been affected by radiation. This chemical change can manifest itself as a harmful biological effect leading to inheritable genetic defects or the development of cancer.
Radiation can be used for diagnostic purposes and therapeutically to kill cancerous cells.
Diagnostic Usage: X-rays are used most frequently for teeth, chest and limb examinations, for diagnosing injury or disease. These examinations involve relatively low doses. There are other techniques such as ‘body scanners’ which use tomographic techniques to reconstruct three-dimensional images of an organ or a part of the body. In such diagnostic examinations, the dose received is higher but is carefully monitored and controlled. In nuclear medicine, a drug incorporating a gamma-ray-emitting radio-nuclide is administered to the patient and the desired organ is imaged by tracing the distribution or flow of the radio-nuclide by scanning the patient with a gamma-ray camera.
The absorbed dose is expressed in a unit called gray (Gy). This is the quantity of energy imparted by ionising radiation to a unit mass of matter, such as tissue. One Gy is equal to one joule per kilogram.
All types of ionising radiation are not equal in terms of their potential for causing harm because an alpha particle, being slower and more heavily charged, loses its energy much more densely along its path through the tissue. A dose equivalent, sievert (Sv), has been devised, which is the absorbed dose multiplied by a radiation weighting factor. The radiation weighting factor for alpha particles is 20, whereas for beta, gamma and X-rays it is set at 1. The risk of fatal malignancy per Sv is not the same for the various tissues or organs of the body. It is lower, for example, for the thyroid than for the lung. This gives another weighting factor which when taken into account gives the ‘effective dose equivalent’.
The International Commission on Radiological Protection link 1 (ICRP), a non-governmental scientific organisation, regularly publishes and updates recommendations for protection against ionising radiation. Governments evaluate the recommendations and put them into practice in a manner appropriate to the countries concerned. The ICRP recommendations are based on the following principles: