Radioactivity - The Becquerel (Bq)

Source: Ionactive Radiation Protection Resource

The SI unit of radioactivity is the Becquerel (Bq). One Bq is a tiny 'amount' of radioactivity and can be defined as a decay rate of 1 disintegration per second (1 DPS). This means that every second, on average, one radioactive nucleus decays, and in doing so is converted into a new substance (which may or may not be radioactive), with the release of ionising radiation (energy).

The word 'amount' is in quotes as the becquerel is really a rate rather than a specific quantity. For a further study of this statement you need to consider specific activity which we will deal with in other resource.

During the decay one or more ionising radiations may be emitted including alpha particles, beta particles, gamma rays etc. What is actually emitted, and the probability of emission, is related to the nuclear physics (proton:neutron ratio) of the process for a given radioactive substance (radionuclide). This will not be explored further here since we want to keep this resource practical and unit based.

Radioactivity and the number system

As already noted, 1 Bq is a small activity and so we will tend to apply unit multipliers in every day use. Examples of this are as follows.

1Bq = 1 DPS

1kBq = 1000 DPS

1MBq = 1,000,000 DPS

1GBq = 1,000,000,000 DPS

1TBq = 1,000,000,000,000 DPS

From this basic data we can do conversions such as

1MBq = 1000 kBq

1GBq = 1000 MBq

1TBq = 1000 GBq = 1,000,000 MBq (and so on)

Note: DPS does not imply that only one emission event occurs (such as the emission of an alpha particle). In any one disintegration (decay), one or more particles / emissions may take place (and a probability of emission per disintegration may also exist). Some of this has been explored in other Ionactive resource such as Formula for calculating dose rates from gamma emitting radioactive materials.

Non SI unit of radioactivity - the curie (Ci)

The US widely still use the curie (Ci), as do some other users (see later for some specific examples).

This unit was defined in 1910, before the becquerel (Bq) was adopted by the SI system. History shows that the unit was formally defined as the activity of 1g of radium, however precisely, the curie (Ci) is defined as follows:

1Ci = 3.7 x 1010 DPS. A closer inspection of this shows that:

1Ci = 37 GBq (if this is not obvious then look again at the previous section). From this we can state some common conversions that are often used in the UK (but also around the world in certain circumstances).

10 Ci = 370 GBq (or 0.37 TBq)

20Ci = 740GBq (or 0.74 TBq)

1kCi = 1000 Ci = 37 TBq

37MBq = 1 mCi

9.25MBq = 250µCi

18.5MBq = 500µCi (or 0.5 mCi)

37kBq = 1µCi

Some example values of radioactivity

Here we present some examples of radioactivity (in no particular order!)

  • Activity of a domestic smoke detector using Am-214 will be 37 kBq (1µCi)
  • Average banana will contain about 15 Bq of naturally occurring radioactive k-40
  • The human body will contain about 4kBq of K-40 at any one time (controlled by metabolic processes)
  • About 1.3 mBq (yes, 0.0013 Bq) of Po-210 is inhaled by smoking a typical cigarette
  • About 300 MBq (0.3 GBq) of F-18 is used in a whole body PET/CT scan (medical exposure)
  • Radon activity of 1000 Bq/m3 will yield a dose rate of 4.7 µSv/h (this is an illustrative example only, and the dose rate is the committed effective dose expressed as a whole body dose for radon lung irradiation in a building)
  • Radioactivity concentration of about 40 kBq/kg of K-40 will be present in typical tinned tuna (naturally occurring)
  • Around 40 kBq of Ra-226 will be found in a typical antique watch (where Ra-226 is used in the luminescence process - glow in the dark)
  • An activity of 400 GBq Ir-192 would be used in typical brachytherapy procedures (i.e. medical prostate cancer treatment)
  • Typically 740 GBq or I-192 would be used in industrial radiography applications (i.e. looking for patent defect in pipe welds)
  • Around 5MCi (note the units - see below) / 185000 TBq or 185 PBq of Co-60 is typically used in the industrial sterilisation of products (e.g. medical supplies, hip replacements, spices etc)

Some interesting facts

In the UK (and elsewhere), the industrial radiography industry still often uses the curie (Ci) despite it not being an SI unit. Why ?

The reason is that traditionally radioactive sources of Ir-192 (or Se-75, Co-60 etc) were supplied in curie (Ci) amounts - e.g. 10, 20, and 50 Ci. Even with the transfer to SI units, the activity values have stayed the same. So the conversions are:

10Ci = 370GBq

20Ci = 740GBq

50Ci = 1.85TBq

The above examples are regularly used and often radiographers will still talk in curie terms, however their records will all be in SI units (this being a legal requirement in the UK).

In a similar fashion, the industrial sterilisation industry still still use the curie (Ci) quantity in local record keeping. Like industrial radiography, this is due to the long term historic supply of radioactive sources in curies.

An industrial sterilisation plant might be loaded with 1MCi of Co-60. This is used for routine plant record keeping. however the activity recorded formally will be 37 PBq. Plant loading may be even higher than this, perhaps up to 5MCi Co60.

Individual sources may range from 10kCi to 25 kCi of Co-60 (each!) which is 370TBq and 925TBq respectively.

The sources described so far are massive activities, but similar matters occur at the lower end of the scale, particularly in research and universities.

Way back, researchers might order packs of radioactive P-32 in the following example amounts - 1mCi, 500µCi or 250µCi. Even in 2022 most suppliers will offer packs of 37MBq, 18.5MBq and 9.25MBq. Indeed, many tried and tested assay protocols over the years were derived from mCi activities and so what worked then, works now (despite better counting equipment). As for larger industry, whilst the researcher might 'think' in curies, all record keeping (certainly in the UK) should be in SI units (i.e. Bq etc).

Physics is, hopefully, simple. Physicists are not

– Edward Teller -