In part one of this Ionactive Resource on NORM we will look at Cosmic (Cosmogenic Radiation) and Terrestrial (primordial) 'non series'. In Part two we will look at Terrestrial (primordial) 'series' (Uranium and Thorium).
Naturally Occurring Radioactive Material (or NORM) is present within the environment we all live in. It is accepted that most of the ionising radiation exposure you are likely to receive in any one year will be from NORM. The exact amount of radiation dose you might receive in a given year will depend where you live (since to a large extent the geological formations over which your home or office is built influence your potential exposure). The type of NORM present can be split into the following basic categories:
Cosmic (Cosmogenic Radiation)
Cosmic radiation from outer space, and its interaction with the upper atmosphere, can lead to the production of C-14. This is caused by interaction of a neutron and Nitrogen as shown below.
The C-14 will react with oxygen in the atmosphere to produce CO2 (or for this example we will write as '14-C'O2). This is a natural process that has been going on for years.
For every molecule of '14-C'O2 in the atmosphere there are 1 E-12 molecules of CO2 – this ratio is fixed by nature. The CO2 / '14-C'O2 in nature can be breathed in directly or can be incorporated into solid material as C / C-14. All living things will take in Carbon on a regular basis in this ratio.
When the living organism dies it stops taking in Carbon. Therefore, over time the C-14, which has a half-life of 5720 years, decays and so the ‘known’ ratio changes. This then can lead to the process of carbon dating.
Some data suggests that the total effective dose to the body from all Cosmogenic Radiation (of which 14C is the biggest contributor) is about 10 Micro Sv/year (not a lot!).
Of much more significance to natural background radiation and for occupational exposure if working with TENORM, is the terrestrial series of radioactive materials, found in the crust – many with half lives in line with the age of the earth i.e. 4.5 E9 years!
These can be split into two main groups: non-series radioactive materials, and the series radioactive materials.
The non-series radioactive material includes K-40 (potassium) and Rb-87. Of most significance is K-40 which is found in nature (rock) at a concentration of about 0.6 Bq/g (varies slightly around the world). K-40 can be found at enhanced concentrations as soon as Technical Enhancement (TENORM) is employed. K-40 is found within non-radioactive K at about 0.0118 %. It also has an extremely long half-life at 1.29×E9 years.
In terms of energy of emission K-40 is actually quite energetic with a beta emission at 1.33 MeV (for 89% of decays) and a gamma emission at 1.47 MeV (for the remaining 11% of the decays).
In nature K-40 is rarely problematic because the activity / activity concentration is so low.
The amount of K in the body is regulated by biological processes – an average male will have about 140g of K. This will have an activity of about 4.4KBq of radioactive K-40 – so we are all radioactive!
This contributes an annual effective dose of about 180 Micro Sv to the body of an average human being. Take a look at the pictures above - sodium reduced salt contains a small activity of K-40, as does a banana! Eating a single banana will provide a whole body effective radiation dose of about 0.1 micro Sv!