How do we measure the radiation dose a person receives, and how is the biological harm taken into account?
Dosimetry: absorbed dose as energy per unit mass, equivalent dose using the radiation weighting factor, equivalent dose rate, the safe handling of sources, and background radiation.
An SQA National 5 Physics answer on dosimetry, covering absorbed dose as the energy absorbed per kilogram, equivalent dose found using the radiation weighting factor, equivalent dose rate, how to reduce the dose received, and the sources of background radiation.
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What this key area is asking
The SQA wants you to calculate absorbed dose as energy per unit mass, find the equivalent dose using the radiation weighting factor, calculate equivalent dose rate, describe how to reduce a radiation dose, and know the sources of background radiation.
Absorbed dose
The absorbed dose only counts the energy delivered; it does not, by itself, say how much biological harm is done, because some types of radiation are far more damaging than others for the same energy.
Equivalent dose and the weighting factor
Equivalent dose rate
The equivalent dose rate is how quickly an equivalent dose is received, that is the equivalent dose divided by the time:
Reducing the dose received
The dose a worker receives is reduced in three ways: limiting the time spent near a source, increasing the distance from it (the radiation spreads out and weakens), and using shielding (such as lead or concrete) between the worker and the source. People who work with radiation wear a film badge or dosimeter to monitor the dose they have received.
Background radiation
We are all exposed to background radiation all the time. Natural sources include radon gas from the ground, cosmic rays from space, rocks and soil, and even radioactive isotopes in food and in our own bodies. Artificial sources include medical X-rays and the small amounts released by the nuclear industry. Background radiation must be measured and subtracted (the background count) when measuring the activity of a source in an experiment.
Try this
Q1. State the relationship for absorbed dose and its unit. [2 marks]
- Cue. ; the unit is the gray (Gy).
Q2. Tissue absorbs of beta radiation (). Calculate the equivalent dose. [2 marks]
- Cue. .
Q3. State three ways to reduce the radiation dose a worker receives. [2 marks]
- Cue. Reduce the time, increase the distance, use shielding.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style3 marksA 60 kg person absorbs 0.030 J of energy from gamma radiation. Calculate the absorbed dose.Show worked answer →
Use the relationship for absorbed dose, which is the energy absorbed per unit mass.
Relationship: .
Substitution: .
Markers reward selecting , correct substitution, and a final answer in grays (), here or .
SQA N5 style4 marksTissue receives an absorbed dose of 0.0020 Gy of alpha radiation, which has a radiation weighting factor of 20. Calculate the equivalent dose.Show worked answer →
Use the relationship between equivalent dose, absorbed dose and the radiation weighting factor.
Relationship: .
Substitution: .
Markers reward selecting , correct substitution of the absorbed dose and weighting factor, and a final answer in sieverts (). The large weighting factor for alpha reflects how much more biological damage it does.
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Sources & how we know this
- SQA National 5 Physics Course Specification — SQA (2019)