RADIATION MONITORING
objectives
¢Requirement for Personnel Monitoring
¢Purpose of Personnel Dosimeter
¢Characteristics of personnel dosimeters
¢Type of personnel monitoring
üFilm badge.
üThermoluminescent Dosimeter (TLD).
ü Extremity dosimeter (TLD ring badge).
üOptically Stimulated Luminescence dosimeter(OSL).
¢What is personnel radiation monitoring and why do some people who work with ionising radiation need to wear dosimeters?
¢Personnel radiation monitoring devices or dosimeters (PMDs) are badges that detect various forms of radiation a worker may be exposed to.
¢The dosimeter or badge detects the exposure of a person to x-rays, gamma radiation, neutron and beta particles.
¢Workers are required to wear the dosimeters for periods of up to three months.
¢The accumulated dose from the various types of radiation is measured by the dosimetry service provider and reported to the employer
Requirement for Personnel Monitoring
¢A personal monitoring dosimeter must be issued to persons working with the following uses of radiation:
Øradiotherapy
Øindustrial radiography
Ønuclear medicine
¢ monitoring of radiation exposure to any person occupationally exposed regularly to ionizing radiation is recommended.
¢Exposure monitoring of personnel is required whenever radiation workers are likely to receive 10% or more of the annual occupational effective dose limit of 50 mSv in any single year.
Purpose of Personnel Dosimeter
¢The personnel dosimeter provide an indication of the working habits and working conditions of diagnostic imaging personnel.
¢It determines occupational exposure by detecting and measuring the quantity ionizing radiation to which the dosimeter has been exposed over a period of time.
¢This instrument, however, does not protect the wearer from exposure.
CHARACTERISTICS OF PERSONNEL DOSIMETERS
¢A personnel dosimeter must be lightweight and easy to carry
¢It should made of materials durable enough to tolerate normal daily use.
¢The dosimeter must be able to detect and record both small and large exposure in a consistent and reliable manner.
¢Outside influences such as very warm weather, humidity, should not affect performance of the instrument.
Type of personnel monitoring
¢Four types of personnel dosimeters are used to measure individual exposure of the body to ionizing radiation:
üFilm badge.
üThermoluminescent Dosimeter (TLD).
ü Extremity dosimeter (TLD ring badge).
üOptically Stimulated Luminescence dosimeter(OSL).
film badges
¢Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles.
¢film badges record whole-body radiation exposure accumulated at a low rate over a long period of time.
¢The film badge is composed of three parts:
Ø durable, lightweight plastic holder.
ØMetal filters.
ØFilm packet.
¢Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles.
¢film badges record whole-body radiation exposure accumulated at a low rate over a long period of time.
¢The film badge is composed of three parts:
Ø durable, lightweight plastic holder.
ØMetal filters.
ØFilm packet.
¢The film holder should be made of a plastic material of a low atomic number to filter low-energy x-radiation, gamma radiation, and beta radiation.
¢Inside the plastic holder are metal filters of aluminum or copper that are secured in a permanent position.
¢These filters allow the measurement of the approximate energy of the radiation reaching the dosimeter.
¢Penetrating radiations cast a faint shadow of the of the filters on the processed dosimetry film, whereas soft radiations cast a more pronounced image of the filters.
¢The density of the image cast by the filters permits estimation of the energy of the radiation.
¢From these data the radiation dose can be evaluated as deep (penetrating) or shallow (non penetrating).
¢In addition, the direction from which the radiation reached the film (front to back or back to front) can be estimated from the appearance of the filter shadows imaged on the processed dosimetry film.
¢The filter images may also be used to determine whether the exposure was the result of excessive amounts of scattered radiation or a single exposure from a primary beam.
¢The radiation dosimetry film contained
in the radiographic film packet is similar
to dental film.
¢This film is sensitive to doses ranging
from as low as 0.1 mSv to as high as 5000 mSv.
¢The outside of the film packet forms a light-free
envelope for the dosimeter film.
¢Inside the envelope, a sheet of lead foil backs
the film to absorb scatter radiation coming from
behind the dosimeter.
¢Radiation interacting with the film in the badge causes
the film to darken.
¢After processing, the density of blackening, of the image of the filters recorded on the dosimeter film is proportional to the amount of radiation received and the energy of the radiation.
¢An instrument called a densitometer is used to measure this density.
¢What are the advantages and disadvantages of film badges dosimeter?
Thermoluminescent Dosimeter (TLD).
¢When ionizing radiation interacts with any material, such as crystals, the radiation deposits either all or part of the initial energy in that material. Some of the atoms in the material that absorbs that energy become ionized, producing free electrons and holes.
¢Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light. The number of photons counted is proportional to the amount of energy deposited in the crystal. This technique of measuring radiation dose by reading photons emitted from crystals as it is heated is called thermoluminescent dosimetry (TLD).
¢Thermoluminescent materials such as lithium fluoride (LiF) provide a simple inexpensive method of measuring radiation dose over an extended period of time. Normally when energy is applied to an electron, it will move up to a higher energy state (orbital), then drop back to its ground state releasing the excess energy.
¢Crystals of LiF have met a stable energy state in which excited electrons may be trapped for periods up to 80 years. Heat applied to the crystal will raise the electron out of the meta-stable trap, allowing it to revert back to it ground state by emitting the excess energy as photons.
¢Construction of a TLD consists of two Lithium Fluoride crystals of different thickness.
These crystals are shielded by either a metal foil or an aluminum planchet. The difference in the shielding and thickness of the crystals allows the differentiation between whole body dose and skin
¢TLD badges should be worn at the chest position (if worn at waist bench tops will shield the badge from radiation giving low results)
¢The TLD badge should be stored away from light, radiation and dust when it is not used.
¢TLDs are changed every three
months except for badges worn
by pregnant women.
These badges change every two weeks.
Extremity Dosimeter (TLD ring badge).
¢Ring badges consist of a single Lithium Fluoride crystal inside a plastic holder. This type of badge is used to measure extremity dose.
¢It is recommended that extremity dosimeter, or TLD ring badge, be worn by an imaging professional as a second monitor when performing radiographic procedures that require the hands to be near the primary X-ray beam.
¢The monitor measures the approximate equivalent dose to the hands of the wearer of the dosimeter.
¢Te badge cover contains information such as the account number, participant’s name and number, wear date, indication of hand (right or left), size, and reference number of TLD ring dosimeter.
¢Ring badges should be worn on the dominant hand inside the glove to prevent contamination.
¢The Ring badge should be stored away from light, radiation and dust when it is used.
¢Ring badges are changed every three months.
¢It is imperative that Neutron,Ring and TLD badges be stored away from sources of radiation, dust and light to ensure the results reflect the actual dose recieved by the worker.
¢What are the advantages and disadvantages of TLD?
Optically stimulated luminescence dosimeter(OSL).
OSL dosimeters are plastic disks infused with aluminum oxide doped with carbon Al2O3:C
¢What are the advantages and disadvantages of OSL?
please have a look the following website:
http://www.osldosimetry.com/introduction/technology.asp
please have a look the following website:
http://www.osldosimetry.com/introduction/technology.asp
