Typical questions about dosimetry

What is dosimetry?back to top

Dosimetry is the science of tracking and monitoring exposure to ionising radiation. 

It is used to monitor and measure the radiation dose received by people who work with ionising radiation.

How are radiation doses measured?back to top

The build-up of radiation in the human body, or in parts of the human body, is measured with a small device called a dosemeter, usually worn on the outer clothing in the form of a badge or film strip. They contain highly sensitive elements that measure the total amount of radiation to which the dosemeter has been exposed.

Do dosemeters only measure quantity of radiation?back to top

Some dosemeters are more sophisticated. As well as measuring quantity of radiation, these use special filters to measure the type of radiation (i.e. beta or gamma), and whether the energy emitted is high or low.

What types of dosemeter does the RPII supply?back to top

 The RPII supplies and processes the results of three types of dosemeter 

• Panasonic 802AT Wholebody Thermoluminescent Dosemeters (whole body TLD)
• Panasonic 807 Extremity Thermoluminescent Dosemeters (extremity TLD)
• Neutron Monitoring (PADC) Dosemeters supplied by the UK Health Protection Agency

 

What are the differences between them?back to top

The wholebody TLD combines four elements and is used to measure not just the quantity, but also the type of radiation received. The extremity TLD consists of just one element and is specifically designed to monitor radiation to extremities, such as hands and feet. The PADC Dosemeter is insensitive to X-rays, gamma rays and beta radiation, and is used only to monitor radiation from neutrons.

How is the reading taken?back to top

The word ‘thermoluminescent’ derives from the ancient Greek words thermos and lumen, which mean heat and light. When crystalline materials are exposed to ionising radiation, then heated, they emit light. The basic element of a TLD comprises a sensitive crystal. When heated, the intensity of the light from the crystal reveals the quantity of energy which it has absorbed through exposure to radiation.

What are the legal dose limits?back to top

The dose limit for an exposed worker over 18 years old is 20 mSv in any 12-month period. For those aged 16 to 18, the limit is 6 mSv in any 12 month period. For a member of the public, the legal dose limit is 1 mSv over one year.

Which workers are legally obliged to be monitored for radiation?back to top

Under Irish law (Statutory Instrument 125 of 2000), all exposed workers and students must be monitored for radiation by an approved dosimetry service or, as appropriate, by an approved radon measurement laboratory.  Workers who are likely to receive an effective dose greater than 6 mSv in a period of 12 months are designated ‘category A workers’. For this category of employee, personal monitoring and medical surveillance are compulsory.