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Radiation Protection Glossary

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Dead Time
Dead Time is used in counting / detector systems to describe the time between two recorded events (e.g. where an Ionisation event has been recorded), where either the detector, or its electronics, are unable to detect other incoming events. A counting system which has a large dead time will more than likely miss true events that occur at intervals less than the deadtime.
Decay
Also known as Radioactive Decay . Radioactive substances undergo radioactive decay, the rate of which is determined by the properties of the radionuclide. As decay proceeds the resulting activity of the parent Nuclide reduces and will eventually disappear. The daughter product may be stable (inactive) or may itself be Radioactive and undergo further decay (i.e. as part of a decay series). The rate of decay can be expressed in terms of its Half-Life (the time taken for the activity to reduce by half).
Decay Constant
The decay constant represents the probability of a Decay of a Radioactive material per unit time.
Decay Product
A radionuclide produced as a result of a parent radionuclide's decay. The decay products may be derived from their immediate predecessor, of through several other decays in a decay chain series. One example would be Radon Decay products where Ra-222 will decay to products including Polonium 218 (via alpha decay) and then Lead-214 (also alpha decay).
Decommissioning
A general term applied to situations were nuclear plant (or any other plant containing sources of Ionising Radiations ) comes to the end of its useful life. Decommissioning is then required in order to dismantle the plant and recover Radioactive materials for either reuse, recycling or disposal. Decommissioning uses an extremely controlled approach and on larger plants (e.g. nuclear power stations) will require full Safety Cases and regulator attention.
Decontamination
A general expression applied to situations where undesirable Contamination is removed from plant, fixtures, fittings or people. This process may simply be wiping up the contamination with a tissue, although could require applying an industrial striping technique. Two more specific subsets of this term are Environmental Decontamination and Personal Decontamination .
Dental x-ray
A diagnostic procedure undertaken by a Dental Surgeon / Nurse, usually involving patient exposure to x-rays where by the required image is captured on a film, which is then developed and used for assessment and treatment planning. The level of x-ray's are carefully controlled and usually amount to a small fraction of normal annual background radiation.
Depleted Uranium
A by-product of Enriched uranium production. Depleted Uranium can be used as Radiation Shielding) (approximately twice as effective as lead for gamma rays), battle armour and in conventional weapons (its high density and self-sharpening properties being more important than its modest Ionising Radiation proprieties).
Detector
A general term applied to parts of instruments which are designed to detect Ionising Radiation . Ionising radiations interact with the detector, the detector converts this interaction into a signal which can be amplified and interpreted by other devices. Detectors can be Passive Detectors (which require a subsequent process, e.g. chemical etching, to determine useful information), and Active Detectors (which enable real time information to be obtained, e.g. EPD ).
Deterministic Effect
A deterministic effect describes Ionising Radiation induced damage where a Dose threshold exists, and for which the severity of damage increases with increasing Dose above that threshold. Examples will include radiation burns (skin reddening), hair loss, cataracts and radiation sickness (nausea, vomiting and diarrhoea). All of these effects results from acute high doses of radiation to either a part of the body or the whole body. For whole body exposure it is generally thought that an absorbed dose of between 3-5 Gy will cause 50% of those exposed to die within 30 days if medical intervention is not given. This is known as the LD-50 dose.
Detriment (Radiation)
With respect to Radiation Protection , detriment is a term used to describe the 'total harm' experienced by exposing a population (and their descendants) to Internal Radiation . ICRP uses detriment to effectively sum all the Risks (probabilities) that exposure to ionising radiations might produce. For example it will include probability of fatal cancer induction, non-fatal cancer induction (and therefore years of life lost). It therefore as the dimensions of probability and thus can be expressed as a risk. In ICRP publication 60, radiation detriment is developed and used to derived Dose Limits .
Disposal
A general term applied to Radioactive Wastes which require disposal, for which there is no intention of recovery. In reality there may be circumstances where some capability for recovery is maintained (e.g. ILW wastes at UK nuclear power stations).
Distance
Distance, in the context of Radiation Protection , relates to one of the three key principles of protection against External Radiation hazards (i.e. Time , Distance & Shielding ). In simple terms, increasing the distance between a static source of Ionising Radiation and the absorbing medium (e.g. a person) will reduce the exposure to that person. For certain defined geometries, (e.g. Point Source ), the Inverse Square law can be applied which can be stated as: 'double the distance, quarter the dose'.
DNA
The full name for DNA is Deoxyribonucleic Acid. DNA is a substance found in the nucleus of living cells and is used to encode genetic information. Its use is to determine the structure, function and behaviour of all cells in a living entity. It is relevant to Radiation Protection since it is at the DNA level that concern is raised by the effects of Ionising Radiation , which may lead to cancer induction or genetic damage.
Dose
'Dose' is a general term applied to the quantity of Ionising Radiation received by a body which the radiation interacts with. Some degree of care is required when using the word 'dose' since it can mean a number of different quantities. Furthermore it is more often useful to express the dose in units which give the magnitude of damage sustained or perhaps the risk of cancer induction at some time after exposure. It is usually more useful to prefix (or postfix) terms which identify the quantity under consideration, e.g. Absorbed Dose , Effective dose , Dose Equivalent. At a fundamental level, 'Dose' may be best described as 'the amount of radiation absorbed per unit mass of material with which it interacts with'.
Dose Equivalent
Dose Equivalent (some refer to this as Equivalent Dose ) is a quantity which takes into account 'radiation quality' which relates to the degree in which a type of Ionising Radiation will produce biological damage. The Dose Equivalent is obtained by multiply the Absorbed Dose by a Quality Factor . The resulting quantity can then be expressed numerically in Rem (old units) or more commonly Sieverts (Sv). It is worth emphasizing that the quantity is independent of the absorbing material (i.e. tissue, water, air).
Dose Limits
With respect to Radiation Protection , dose limits are recommended by international bodies such as the ICRP , and then set as legal limits within individual counties legislation. For example, in the UK Dose Limits to employees of radiation employers, members of the public, females of reproductive capacity, pregnant employees etc are set out in the Ionising Radiations Regulations 1999. Whilst dose limits set a legal maximum, the practice of radiation protection requires that all doses are kept as low as reasonably practical (ALARP ) as required by the ICRP concepts of Optimisation and Limitation .
Dose Rate
Dose rate is a term applied for the rate at which Ionising Radiation is being absorbed by a medium (e.g. tissue). Whilst it is accurate to apply a prefix or postfix (e.g. 'absorbed dose rate'), the term is probably most often applied to Effective dose , which is a useful quality in expressing harm or overall risk of harm from whole body irradiation.
Dosimeter
A general term applied to devices designed to record Personal Exposure , as apposed to Environmental Exposure . The devices may be in the form of a Passive Dosimeter or an Active Dosimeter . Passive dosimeters will include Film Badges and TLD , whilst active dosimeters will include EPDs . The dosimeter is the standard way of measuring personal exposure for the purposes of complying with statutory Dose Limits .
Dosimetry
Dosimetry is a general term applied to the practice of measuring radiation exposure. Dosimetry is a very large area of Radiation Protection , see Dosimeter , Passive Dosimetry , Active Dosimetry and Biological Dosimetry for more specific information.