Life Sciences Division

Research on the effects of low doses of radiation is a prerequisite for the rational assessment of exposure hazards. Physicians consider a ‘low dose' of radiation to be an exposure of less than about 100 millisieverts (mSv), and a ‘very low dose' to correspond to a few mSv, comparable to doses received by living organisms from natural background radiation. At these levels neither epidemiological nor experimental studies point to any harmful effect. However, there are still some uncertainties arising from the nature of the radiation and the exposure time. The new irradiation facility installed at CEA Fontenay-aux-Roses is a new addition to the technical resources available to researchers studying the effects of low doses and low dose rates. It is composed of two Alcyon cobalt 60 gamma-ray medical irradiators. These irradiators complement each other; the first is equipped with a 300 TBq source, which allows relatively brief irradiation, from a few seconds to 30 minutes. The second, with a 3.4 TBq source, offers the possibility of more prolonged irradiation of biological equipment (up to 48 hours). In this way researchers have at their disposal a broad range of dose rates (from 0.17 to 1600 mGy/min) of gamma radiation (see below).

The gamma or X-ray radiation serves as a reference in the analysis and understanding of the biological effects of ionising radiation. The cobalt 60 source permits a highly homogeneous irradiation of biological equipment. It will be possible to carry out irradiation for ranging period of time on several cell cultures simultaneously, or on laboratory animals when necessary for certain studies.

Besides helping to achieve the CEA's research objectives in radiobiology, the irradiation facility will be able to respond to the needs of pre-clinical studies conducted by the DSV or by organisations outside the CEA. It extends the choice of irradiation tools that are available to biologists, which range from the simplest IBL 637 (gamma irradiation) to the highly specific Ganil beamline (heavy ion irradiation.

Biological damage due to ionising radiation varies according to the quantity of energy deposited by the radiation in the cells of each organ. It also varies, for the same dose, according to the type of radiation, alpha, beta or gamma, the pattern of exposure, the organ irradiated and the age of the subject. The damage is characterised in terms of the quantity of energy absorbed by the irradiated material. This quantity is measured in grays. One gray corresponds to an energy of 1 joule received by 1 kg of matter. The dose rate depends on the duration of exposure. It corresponds to the dose divided by the duration: its approved unit is the gray per hour or gray per minute.
To allow for variations in biological efficiency in the same organ or tissue, according to the nature of the radiation, another unit, the sievert (Sv), was introduced. It expresses the amount of biological damage or equivalent dose, and is equal to the dose (Gy) multiplied by a weighting factor that takes into account the biological efficiency of the radiation. For X- and gamma rays it is equal to 1.