Radiation Protection Glossary
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In general terms, LD-50 is used as an index to describe effectiveness of some entity producing a response in a subject, where that response occurs in 50% of the subjects. In Radiation Protection
it is commonly used to describe the value of Absorbed Dose
in humans which would lead to Deterministic Effects
and therefore radiation sickness (leading to death without medical intervention). The quantity varies depending on the literature but is likely to be between 3 and 5 Gray (Gy)
(with some variance related to medical treatments).
With respect to Radiation Protection
, lead together with perhaps concrete, is the most likely shielding material for attenuating X-Rays
and Gamma Rays
. It has a density approximately 11 times greater than that of water and is easily formed into sheets and interlocking bricks. For example, 4 cm lead will attenuate Co-60 gamma rays to 1/10th of the unshielded value (ignoring geometric and beam effects).
LET is short for Linear Energy Transfer. This quantity actually reflects the linear rate of energy absorption, by the absorbing medium, as the Ionisation
event traverses the medium. In simple terms low LET radiation (e.g. Beta Particles
) transfer less energy per unit path length than high LET radiation (e.g. Alpha Particles
) to absorbing mediums. LET has most use in radiobiology and has the units of KeV/micron.
It is generally thought that Leukaemia is a likely form of malignancy which can result from whole body exposure to Ionising Radiation
, the likelyhood being Probabilistic
in nature and increasing linearly with dose without threshold (although there does remain a controversy around the non-threshold hypothesis).
Limitation is the last of the three principles of the ICRP
system of protection. Limitation, together with Justification
, are used as the basis of Radiation Protection
internationally. Limitation is defined in ICRP 60 as '..effective dose to individuals shall not exceed the dose limits recommended..
'. ICRP requires that Deterministic Effects
are avoided and that Probabilistic
effects are as low as reasonably achievable (ALARA
With respect to Ionising Radiation
, a line source describes a Radioactive
source which can be represented by a line between points A-B in any chosen plane. Calculations of Dose Rates
from line sources are more complicated than those from Point Sources
and in many cases direct measurements are going to always be preferable. Such calculations usually involve knowing (or estimating) the Activity
per unit length of the source, then calculating the dose rate at a chosen point by integrating contributions from integral points along the line. Commercial programs such as Microshield can undertake this process automatically.
Linear Attenuation Coefficient
The Linear Attenuation Coefficient is used in the description and calculation of exponential absorption of Gamma Rays
. The quantity normally has the dimensions cm-1 and describes the fraction of gamma rays that are attenuated per unit thickness of absorber.
Linear Dose Response
Linear Dose Response in Radiation Protection
relates to the zero-threshold model
which predicts that every small addition of radiation exposure contributes to an increment in the probability of a Probabilistic
effect. The response relies on the assumption that even one Photon
has the ability to cause an Ionisation
event in DNA
which may initiate cancer (or other genetic effect). In adopting this model one has to remember that there is no certainty that an ionising radiation event actually leads to biological damage (and thus cancer etc), rather, it it the likelihood of induction which increases.
Low Level Waste (LLW)
Low Level Waste (LLW) consists mainly of items such as protective clothing, laboratory equipment, packing materials, containers and site equipment which have come into contact with Radioactive
material and are no longer useful. In the UK the definition of LLW is waste with a radioactive content not exceeding 4 GBq/tonne of Alpha
activity or 12 GBq/tonne of Beta
activity. See Radioactive Waste
for more information.