Radiation protection - the number system

Source: Ionactive Radiation Protection Resource

Familiar units and number scales

If numbers are not your thing, you may not realise you are already using numbers and units without thinking about it.

Take length. You are used to talking about meters (m), or perhaps feet (ft) if in the united states. For now we will consider SI units of length.

Consider 1m. Not only will you have used this from time to time but you can visualise in your head just how 'big' this is. You will also be familiar with 1mm and 1km, and again you can visualise in your head how small or big these values of length are. Often when considering radiation safety units, if you are unsure on the 'size' think back to length.

You will know that there are 1000mm in 1m, and 1000m in 1km.

You are also familiar with 1cm and will be able to visualise this in your head and also know instantly there are 100 cm in 1m. This is often useful to recall when using radiation safety units. For example, the unit of absorbed dose is the Gray (Gy). Often in the medical sector the unit cGy will be used. For some this causes a pause whilst trying to think about 'how big' (or small) it is. If this is you then think back to the cm and you will instantly know that 1cGy must be 0.01 Gy (i.e. 100 cGy = 1Gy).

Mass. Other units can be used - like mass (g). You will know for example that 1kg = 1000g and will be able to visualise this (e.g. 1g of sugar vs 1kg of apples). However, the cg whilst mathematically correct (0.01g), is not a common multiplier of this unit.

Likely number scales used in radiation protection

The following base units are used by way of example. Some of these will be highlighted further in later sections of this resource: m (meter), Sv (sievert - unit of effective dose), eV (electron volt - unit of energy used in radiation protection), Bq (becquerel - unit of radioactivity).

0.000001 or 10-6 or micro (µ) as in µm or µSv

0.001 or 10-3 or milli (m) as in mm or mSv

1 or 100 or unity (meaning 1) as in m or Sv

1000 or 103 or kilo (k) as in km or kSv

1000000 or 106 or Mega (M) as in MeV or MSv

1000000000 or 109 or Giga (G) as in GeV or GBq

1000000000000 or 1012 or Tera (T) as in TeV or TBq

The above are the most common number scales used in radiation protection. Occasionally you may also see the following:

0.000000001 or 10-9 or nano (n) as in nm or nSv

1000000000000000 or 1015 or Peta (P) as in PBq

Take care when using number prefixes

Be careful how you write number prefixes when considering the scale / magnitude of a number. Consider the following vales of effective dose (Sv):

1MSv = 1000000 Sv (a huge radiation dose - certainly a fatal exposure to a person).

1mSv = 0.001 Sv (about 1/3 of the exposure received by members of the public in the UK from background radiation each year).

Note the only difference is the number multiplier ('M' or 'm') - please make sure you use the right one!

If you start from nothing, it is very difficult to get anywhere

– Gerhard Herzberg -