A quantity of radioactive material is characterized by the number of nuclear disintegrations or transformations that occur within a specified time interval. The parameter is termed the disintegration rate, D, and the unit for this quantity is the becquerel (Bq), which is equal to one nuclear transformation per second. Another unit that is still in use is the Curie, equal to 3.7 x 1010 disintegrations per second (dps). Following on, the:

• Activity concentration, Ac, has the dimension dps per unit mass (or unit volume) of a sample (i. e. Bq/g or Bq/mL).

• Specific activity, As, has the dimension of dps per unit mass of the inactive counterpart of the tracer atom or molecule. For HTO, the dimension is Bq/g H2O or Bq/mL H2O.

For HTO in pure water, Ac = As. For HTO in aqueous mixtures with other water soluble components, Ac ф As.

When ionizing radiation from a radioactive material passes through a mass of some other material, the radiation will interact with the atoms of the material in different ways. The net result is that energy will be absorbed in the material. The quantity of energy that is absorbed per unit mass is termed the absorbed dose and is measured in gray (Gy), whereby 1 Gy = 1 joule/kg. The old unit RAD is still in use, whereby 1 RAD = 0.01 Gy.

The biological effect of the radiation does not depend solely on the absorbed dose, but also on the quality factor of the radiation. For small dose rates of beta and gamma radiations, this quality factor is set to 1 (for alpha particles, neutrons, etc., the quality factor is >1). The absorbed dose multiplied by the quality factor renders the equivalent dose, which is measured in sievert (Sv). The old unit REM is still in some use: 1 REM = 0.01 Sv.

Humans are continuously receiving radiation doses from ambient (natural and artificial) sources. For most people these doses are in the range 3-5 mSv per year. Approximately 60-70% of this dose is caused by inhalation of radon (222Rn) from materials in the surroundings which contain uranium (decay of 238U). For comparison, 5 mSv corresponds to the dose that a person receives from the intake of 300 MBq of HTO. For members of the public, the International Commission on Radiation Protection has recommended the use of 1 mSv/y, averaged over 5 years, as a limit for radiation doses caused by application of radioactive material or other types of ionizing radiation.

A material emitting ionizing radiation will create an absorbed dose rate that is usually measured in Gy/h. The corresponding equivalent dose rate is measured in Sv/h or the smaller units pSv/h, or mSv/h.

Injection of a radioactive tracer into a hydrocarbon reservoir at high pressure entails handling of materials with properties that are unfamiliar to most people. As radioactive materials in quantities used for injections are potentially dangerous, it is important that they are treated properly. In an injection project, safety consideration should already have been taken into account when the tracer is selected and the quantities fixed. During transport and storage at the well head site, the safety aspect should be borne in mind and safety precautions should be taken according to international rules and regulations.