Table of Contents

Dose & Radiation Protection

What is Dose?

Dose is a measure of the amount of radiation a person is exposed to. There are three main ways in which dose can be calculated:

Absorbed Dose

Absorbed Dose = the energy deposited per unit mass.

Equivalent Dose

Equivalent Dose = Absorbed Dose x Radiation Weighting Factor (WR).

Radiation Type Energy Radiation Weighting Factor
Alpha (α)
All
20
Beta (β)
All
1
Gamma (γ)
All
1
Neutron (n)
Varies (keV - MeV)
5 - 20

Effective Dose

Effective Dose = Equivalent Dose x Tissue Weighting Factor (WT).

Tissue Type Tissue Weighting Factor
Genitals
0.20
Bone Marrow, Colon, Lung, Stomach
0.12
Bladder, Breast, Liver, Oesophagus, Thyroid
0.05
Bone, Skin
0.01

How is dose measured?

The measurement, calculation and assessment of radioactive dose is known as dosimetry.

Radiation Detectors are used to measure the amount of radioactivity in an area. There are three types of these:

Gas Filled Detectors

Ionisation chambers, proportional counters and Geiger-Müller tubes measure dose as electrical current: Incoming radiation ionises gas inside the detector, creating oppositely charged (+/-) ion pairs which are attracted by opposite voltages, generating a flow of ions and an electrical current that can be measured. These are used to measure alpha (α), beta (β) and gamma (γ) radiation in the air, for instance around nuclear plants and facilities.

Scintillation Detectors

Scintillation Detectors are mostly used to detect alpha (α) radiation, but can also be used to detect beta (β) and gamma (γ); they measure dose as electrical current. The detector has a foil-type surface in which incoming radiation can excite electrons to a higher energy state; upon de-excitation, light is emitted and converted into measurable electricity by a photomultiplier tube. Scintillation detectors can be used to check for contamination when radiation workers or objects exit radiation controlled areas.

Neutron Detectors

Due to their lack of charge and not being directly ionising, neutrons (n) can be challenging to detect; thus, specialist Neutron Detectors are required. When fast neutrons collide with nuclei, protons are ejected; these are charged and can be detected. Slow neutrons can be detected by activation: when they are absorbed into stable nuclei to generate an unstable nucleus that emits other forms of ionising radiation. Neutron detectors can be used to measure the power generated within nuclear reactors.

We also need to be able to measure the dose absorbed by any person working in a radiation-controlled area – this is called Personal Dosimetry and there are two types. Both are sensitive to beta (β) and gamma (γ) radiation; Passive Detectors also absorb x-ray (X) radiation.

Active Detectors

Active Detectors

Active Detectors, or Electronic Personal Dosimeters (EPDs) are electronic devices designed to give real-time information to the wearer. They can even be set to alarm when dose rate or total dose exceeds a pre-set limit. Active detectors are used to measure and thereby cap the dose received by radiation workers undertaking high-risk work.

Passive Detectors

Passive Detectors

Passive Detectors, such as ThermoLuminescent Detectors (TLDs – commonly known as dose badges) are simpler but do not give real-time information. Special materials absorb different types of radiation, and shielding can be used to make these sensitive to the different types of radiation. The materials are then analysed in a lab on a monthly or quarterly basis to calculate the wearer’s whole-body dose.

Radiation Protection

Radiation Protection aims to protect people from the harmful effects of exposure to ionising radiation. It is enshrined in law. These effects may be deterministic (i.e. they occur above a given threshold) or stochastic (i.e. a statistical probability that increases with dose).

There are three principles of radiation protection:

Justification

There should be a net benefit to receiving a radioactive dose.

Optimisation

Doses should be kept As Low As is Reasonably Practicable (ALARP), to reduce the probability of stochastic effects (probabilistic events that occur by chance, such as cancer).

Limitation

Exposure should be below dose limits, to prevent deterministic effects (which relate to the quantity of radioactive dose received).

The three principles of radiation protection are used throughout the entirety of the nuclear industry and are inherent in plant design. Such designs aim to reduce exposure to dose by considering:

Time
Reduce the time spent near the source, because Dose = Dose Rate x Time.
Distance
Increasing the distance between the person and the source, decreasing the dose received according to an inverse square law.
Shielding
Increase the shielding between the person and the source, because dose rate decreases with each half value layer of shielding added.
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