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Units for radiation measurement

The unit of activity of a radioactive source is the Becquerel (Bq). This has superseded the older and much larger unit the Curie

A source has an activity of 1 Bq if it undergoes one disintegration per second

and so a source with a strength of 1 Curie has an activity of 3.7 x 1010 Bq.

Radiation dose

The energy liberated by radiation within a material is known as the absorbed dose and is measured in units known as grays (Gy).

A gray is an energy liberation of 1 J kg-1 of the material.

Different radiations affect the body more than others so to allow for this a quantity known as the dose equivalent is defined. Each type of radiation has what is known as a relative biological effectiveness (RBE). Then

Dose equivalent (or effective dose) = radiation dose x RBE

The relative biological effectiveness of some radiations are given below.

Radiation Relative biological effectiveness (RBE)
beta, gamma, X rays 1
neutrons (slow) 10
neutrons (fast) 3
alpha 10-20
fission fragments 20

The unit for the dose equivalent is the sievert (Sv)

The dose equivalent in sievert (Sv) is when the absorbed radiation dose (measured in grays) multiplied by the relative biological effectiveness is 1 Jkg-1. The dose equivalent (or effective dose) is a measure of the likely damage to living tissue. A smaller unit is the millisievert (10-3 Sv).

Different parts of the body react differently to the same radiation dose – bone marrow and lungs being ten times more ‘sensitive’ to radiation damage than skin.

Our bodies can absorb up to about 0.25 Sv without immediate ill effects. However a dose of 1 Sv may produce radiation sickness, and more than 6 Sv causes death. Therefore for alpha particles aradiation dose of above about 0.4 grays is likely to be fatal while for beta radiation we could take up to 8 grays before fatality.

We are exposed all the time to background radiation. However because there is a considerable variation in the background radiation from place to place the effective dose is also different but a value of around 2.5 mSv/year is typical.

A unit previously used is the rem where 100 rem = 1 Sv or 100 mr = 1 mSv

Radioactive sources normally held in school Physics departments

Radium 185 kBq 5 m alpha, beta, gamma
Strontium 185 kBq 5 m beta
Plutonium 3.7 kBq 0.1 m alpha
Americium 3.7 kBq 0.1 m alpha

Doses for some occupations and treatments

Chernobly residents 430 mSv
Thyroid scan (I 131) 16 mSv
Full body CT scan 12 mSv
Australian uranium miner (per year) 10 mSv
Medical X ray technician (per year) 3.2 mSv
Airline flight crew (per year) 1.6 mSv
Barium meal 1.0 mSv
Dental X ray (to the teeth) 0.094 mSv
Chest X ray (received by the bone marrow) 0.1 mSv
Airline passenger (ten flights per year) 0.03 mSv
© Keith Gibbs 2011