Radiation is the general term given to the process by which energy is transmitted away from an energy source. The term can equally be applied to heat, light, sound, microwave, radio or atomic sources of energy. This guidance is only concerned with the radiation arising from atomic sources as these alone have the property of causing ionisation when they interact with other substances and are often referred to as ionising radiations.
Ionising radiation generally arises by one of two processes:
- Radioactivity: this is the phenomenon by which unstable isotopes [Isotopes are different forms of the same atom which are distinguished by having different numbers of neutrons in the nucleus but the same number of protons.] of some atoms break down to form a more stable isotope of a different atom by expelling a small amount of matter from the nucleus (centre) of the unstable atom. Although there are several ways in which this can occur, by far the most dominant are by alpha emissions or by beta emissions. Shortly after an alpha or beta emission has occurred it is usually, but not always, followed by a gamma emission. Radioactive materials continue to undergo this process, often many millions of times per second until all the original unstable atoms have changed into the new stable atoms; the radioactive material then ceases to exist. Radioactivity cannot be destroyed other than by being allowed to decay away. The time taken for a radioactive source to reduce to half its original quantity is known as the half life. If a radioactive material is burned in a fire, the equivalent amount of radioactivity will still exist in the smoke and the ash.
- X-ray emission: this occurs when electrons are accelerated by high voltages inside an evacuated tube and are allowed to collide with a target made from a heavy metal, usually tungsten. The energy associated with the colliding electron is transmitted to the tungsten target and stimulates the emission of an X-ray from the target metal. Since X-rays can only be created through the application of a very high voltage, as soon as the electrical power is switched off, all X-ray emission ceases.
Radiological emergency incidents differ to other hazardous materials incidents in the following ways:
- Firefighters generally have no experience of radiation emergencies as they are very rare
- Even very low levels of radiation, which pose no significant risk, can be detected rapidly with simple, commonly available instruments
- Radioactive materials can cause radiation exposure even when firefighters are not in contact with them
- The health effects resulting from radiation exposure may not appear for days, weeks or even years
- The public, media and firefighters often have an exaggerated fear of radiation
Characteristics and classification
When describing radioactive processes extremely large numbers and very small numbers are frequently discussed. It is therefore necessary to be able to use multiples and sub multiples of the units used.
As described above, there are in effect three types of radiation arising from radioactivity.
Alpha radiation has the greatest ionising potential of the three types. However, partly because of this, it has very poor penetrating power. Typically, alpha radiation can only travel about three centimetres in air and is completely absorbed by very thin layers of other materials (e.g. paper, layers of dead skin or water droplets).
Beta radiation has moderate ionising power. The penetrating range of beta radiation in air is approximately one metre. It is fully absorbed by relatively small thicknesses of metals and plastics (e.g. one-centimetre thick Perspex)
Gamma radiation has the lowest ionising potential of the three types but by far the greatest penetrating power. Gamma radiation will travel many hundreds or even thousands of metres in air. It is capable of passing through solid materials such as brick, concrete and metals although it will be attenuated as it does so. The more matter it passes through, the more its intensity is reduced. For this reason, dense metals such as lead or steel are the most efficient at absorbing gamma radiation.
Other nuclear processes give rise to radioactive emissions but these are much less common and are therefore beyond the scope of this guidance.
When measuring radiation (alpha beta, gamma or X-ray), two properties need to be classified; the activity (or strength) of the source and the dose (or amount) of ionising energy that is being absorbed by the body.
The modern unit of activity that has been adopted throughout Europe is the Becquerel (Bq). All radioactive sources found in the UK legally have to be measured in Becquerels. However, the Becquerel is an extremely small quantity (defined as one nuclear disintegration per second) and most sources will have activities of thousands, millions, billions or even trillions of Becquerels. This means that the usual SI multiples of kilo-, mega-, giga-, tera-, etc. are often encountered when recording the activity of a source. It should be noted that an older unit, the Curie, is sometimes encountered, particularly if the source originally came from the USA. Becquerels and Curies measure the same dimension in much the same way as centimetres and miles measure the same dimension but on a different scale and magnitude of measurement.
The modern unit that has been adopted throughout Europe for measuring the radiation dose received by a person is the Sievert (Sv). All personal dose (and dose rate) measurements must legally be expressed as Sieverts. In contrast to the Becquerel, the Sievert is a very large unit, and the common SI sub-multiples of milli- and micro- are commonly encountered.
Another unit of dose, the Gray, is also widely used throughout Europe. Under most circumstances the Sievert and the Gray are numerically identical. The differences between the two units are beyond the scope of this guidance and can be ignored.
Different units for dose measurement are used in the USA; the Rad (dimensionally equivalent to the Gray) and the Rem (dimensionally equivalent to the Sievert); one Rem is equal to 10 millisieverts (mSv).
Most hand-held monitoring equipment measures the dose rate, which is the speed at which dose is being accumulated, although only the total dose received is relevant as the cause of health problems.
A simple analogy that may assist firefighters in understanding the difference between dose and dose rate is a journey in a car:
- The dose a firefighter has received, usually measured in millisieverts (mSv), can be compared to the distance travelled as shown the car’s odometer (miles)
- The dose rate, usually measured in millisieverts per hour (mSv/hr), can be compared to how fast the car is travelling at any given moment, as shown on the car’s speedometer (miles per hour)
Two principal hazards arise from radioactivity, regardless of the type of radiation:
- Irradiation, which presents an external risk (i.e. from outside the body)
- Contamination,- which presents both an internal (i.e. inside the body) and possibly an external risk