RADIOACTIVITY

The disintegration or decay of certain unstable nuclides with emission of radiation. The emission of alpha particles, beta particles, and gamma waves are the three most important forms of radiation that occur during decay.

Natural or artificial nuclear trans- formation; discovered by Becquerel in 1895. The energy of the process is emitted in the form of α-, β-, or γ-rays. Thus, radium-226 undergoes radioac- tive decay by the emission of an α-particle, and the new product is

radioactivity: The spontaneous disintegrationof certain atomic nucleiaccompanied by the emission ofalpha-particles (helium nuclei), betaparticles(electrons or positrons), orgamma radiation (short-wavelengthelectromagnetic waves).

In some elements, atoms are not stable, and they spontaneously emit radiation of various types - a phenomenon Henri Becquerel discovered in 1896. Marie Curie used the term 'radioactivity' to describe the emission of ionizing radiation by some of the heavier elements; it is measured in Becquerels (1 Bq = 1 disintegration/sec.)
Radiation causes damage to biological molecules, and is a health hazard. Radioactive substances emit three types of radiation: (a) Alpha radiation (a)composed of helium (He²⁺)ions, also called alpha particles, (b) Beta radiation (β) consisting of electrons/positrons emitted by a radioactive substance, also called B-particles, and (c) Gamma radiation (γ) consisting of highly energized, short wavelength electromagnetic waves resembling x-rays. The phenomenon of radioactivity is based on the principle that atoms contain still simpler parts and are not indestructible particles. Radioactivity can be induced in many nuclides by bombarding them with neutrons or other particles. The rates of decay of radioactive substances are unaffected by chemical changes, pressure, temperature or electromagnetic fields, and each nuclide has a decay constant or half-life. A radioactive nuclide decays to a further radioactive nuclide, followed by a series of transformations which end with the formation of a stable, daughter nucleus.
Radionuclides are unstable elements that disintegrate, emit radiation and form stable elements. There are two main sources of radioactive nuclides associated with fertilizers, which are (a) %, a naturally occurring isotope of potassium, and (b) uranium, thorium and their daughter elements present as constituents in phosphate rocks. Potassium contains 0.012% of the radioactive isotope potassium (⁴⁰K) that emits B and y radiations with a half-life of 1.8 x l09 years. Potassium is an essential element for all living beings; an adult body contains 0.2% potassium of its body weight. The amount of potassium in the topsoil varies between 0.2 and 3.3%, giving activity concentrations of about 50 to 1000 Bq/kg soil.
The main concerns related to radioactive nuclides in fertilizers are: (a) increased exposure of workers to the radiation, and (b) their accumulation in soil leading to an increased uptake in food crops.
The radioactivity level in some fertilizers ranges from less than 1,000 to 10,000 Bq/kg weight of fertilizer, depending on the source of the phosphate rock and the content of phosphorus and potassium in the fertilizer. Their accumulation in the top soil and uptake by plants depend upon (a) the input from fertilizers, (b) the duration of the fertilizer application, (c) soil conditions in terms of the pH, moisture, organic matter, clay content, etc., and (d) such factors as soil erosion, leaching, etc. Assuming an application of 30 kg phosphorus/ha mixed in the top 10 cm and non-removal of uranium from phosphorus fertilizers, uranium accumulates at about 0.25% of the soil, annually. However, if P is incorporated into the deeper soil layers or if uranium is removed at the process site, its accumulation will be low. Long-term data indicates that the risk of radioactivity accumulation in soil arising out of P application is low and does not pose any significant threat to human health.