Home
- Biology
- Business Management
- Chemistry
- English
- Gaelic
- Gàidhlig
- Geography
- History
- Maths
- Modern Studies
- Physical Education
- Physics
Games

Physics

Dosimetry and safety

Page:

Dosimetry

Activity

Some materials are radioactive because their nuclei are unstable. It is impossible to tell when a particular nucleus will break apart. What we can measure is the number of nuclei (N) in a quantity of a radioactive substance that will decay in a particular time (t).

The average activity (A) of a quantity of radioactive substance is given by

$A = \frac{N}{t}$

Activity is measured in becquerels (Bq) where one Bq is one nucleus decaying every second. Kilobecquerels (kBq) and Megabecquerels (MBq) are more usual units for activity.

Activity relates to a quantity of a radioactive substance. It is meaningless to refer to the 'activity of uranium oxide' for example, since the activity depends on how much of the substance is present.

Radioactive decay happens spontaneously. The number of nuclei in a quantity of radioactive substance still to decay depends on how many have already decayed. Because of these factors, activity is not constant over time.

Question

One litre of seawater has an activity of 10 Bq.

Approximately how many nuclei decay every day in this quantity of seawater?

Answer: $\eqalign{ A &=& {N \over t} \cr \therefore N &=& A\,t \cr &=& 10 \times \left( {60 \times 60 \times 24} \right) \cr &=& 864\;000{\rm{ nuclei}} \cr}$

Absorbed dose

Ionising radiation carries energy. This energy can be absorbed by tissue and possibly cause damage to the tissue.

Absorbed dose (D) is the energy (E) absorbed per unit mass (m) of the absorbing material. The absorbed dose can be calculated by using the following relationship.

$D = {E \over m}$

The unit of absorbed dose is the gray (Gy) where one gray is one joulejoule: The unit of energy or work, symbol 'J'. per kilogram (J kg^-1).

It is important to use the correct mass of tissue. If the energy is concentrated on a small mass of tissue, the absorbed dose is greater.

Question

A patient of mass 70 kg receives radiotherapy. During the treatment, a tumour of mass 250 g receives 20 J of energy.

Calculate the absorbed dose.

Answer: $\eqalign{ D &=& {E \over {m}}\cr\cr &=& {{20} \over {250 \times 10^{ - 3} }} \cr\cr &=& {80{ Gy}} \cr\cr}$

Equivalent dose

The risk of harm to biological tissue from an exposure to ionising radiation depends on three factors:

the absorbed dose, D
the type of radiation that is absorbed, for example alpha or beta particles, gamma rays or slow (thermal) neutrons
the type of body organs or tissue that is exposed to the radiation

To allow comparisons of the risk of harm due to different ionising radiations, each type of radiation is assigned a radiation weighting factor (W_R) as a measure of its biological effect.

The radiation weighting factor for some types of ionising radiation is given in the table.

Table about ionising radiation

Radiation	Radiation weighting factor (W_R)
alpha particles	20
beta particles	1
gamma rays	1
slow neutrons	3

Equivalent dose (H) measures the biological effects of ionising radiations. It takes account of

the type of radiation
the energy carried by the radiation
how much tissue absorbs the energy

Equivalent dose is the product of absorbed dose and radiation weighting factor. The equation can be written as

H = DW_R

Equivalent dose is measured in sieverts (Sv).

Since the radiation weighting factor has no unit, both the gray and the sievert are equal to one joule per kilogram.

Question

A worker in the nuclear industry receives an absorbed dose of 400µ Gy from slow neutrons and an absorbed dose of 2 mGy from gamma radiation.

Calculate the total equivalent dose received.

Answer

To calculate the total equivalent dose calculate the equivalent dose from each radiation separately and add all the equivalent doses together.

Equivalent dose from slow neutrons

H_neutrons= DW_R

= 400 × 10^-6 × 3

= 1.2mSv

Equivalent dose from gamma radiation

H_gamma = DW_R

= 2 × 10^-3 × 1

= 2.0mSv

Total equivalent dose

H = H_neutrons + H_gamma

= 1.2 + 2.0

= 3.2mSv

The time of exposure (t) to ionising radiation is also important. An equivalent dose of 100 mSv received in one day is more dangerous than the same equivalent dose received over the course of one year.

equivalent dose rate = $\.{H}\ = {H \over t}$

Equivalent dose rate can be quoted in a variety of units - sieverts/millisieverts/microsieverts per second/minute/hour. Make sure that the units you use in any problem are consistent.

Page:

Back to Radiation and Matter index

Dosimetry

Activity

Absorbed dose

Table about ionising radiation

bbc.co.uk navigation

BBC links

Dosimetry

Activity

Absorbed dose

Table about ionising radiation

BBC News - Science and Nature

On the web

bbc.co.uk navigation

BBC links