After readout , the TLD is heated again at a high temperature – a process called annealing - which clears any signal left on the TL material . This means that the TLD is reusable , and can be sent to another customer .
Other passive dosemeters used in the UK include Optically Stimulated Luminescence ( OSL ) and Direct Ion Storage ( DIS ) types . OSL devices are similar to TLD but use light , rather than heat , to stimulate the trapped electrons . Direct Ion Storage ( DIS ) dosemeters use ion chambers and an electronic element to detect radiation dose levels . However , unlike PEDs , they are passive devices and are not equipped with an alarm .
Internal dosimetry If a worker encounters radioactive contamination there is a possibility it may enter the body via inadvertent ingestion , inhalation , or through a wound in the skin . It may then be concentrated by metabolic processes into specific tissues or organs , leading to an internal radiation dose . Computational models developed by the International Commission on Radiation Protection ( ICRP ) allow calculation of the radiation dose from the amount of activity in the body or organs . Measurements are needed to calculate the amount of activity which is present , depending on the nature of the radiation . For gamma emitters a wholebody monitor can be used , whereas for alpha or beta radiation a bioassay sample is required for radiochemical analysis .
plastic blocks or larger liquid-filled sections sized to represent children or adults . Ultimately , phantoms are limited by the ability to accurately mimic an individual or activity in multiple organs or tissues , hence mathematical modelling has been developed to complement the physical calibrations .
Bioassay ( measurement of the concentration of a substance in living cells or tissues ) is used to complement whole body monitoring for gamma emitters , but is the only method capable of determining the activities of alpha and beta emitters which are present . Urine samples are the easiest to collect and work with , but faecal samples are required for some measurements . A range of analytical processes are used , and there is often a requirement to remove interfering nuclides ( e . g . removing uranium if looking for plutonium , as their spectra overlap ). These analytical techniques can be time consuming and expensive , requiring the use of specialist facilities , equipment and highly trained and experienced staff .
Whole body monitoring processes range from using a handheld instrument for triage up to the use of high-resolution detection systems housed in steel and lead enclosures for very accurate measurements . In all cases accurate calibration is required using physical phantoms ( artificial figures designed to simulate a body ) or validated mathematical modelling . Physical phantoms must provide either a reasonably homogeneous model of the whole body or anatomically accurate sections to simulate activity in a specific organ , such as the lungs or thyroid . Whole body phantoms are normally made from
Radiation Protection Today Spring 2024 11