James, of Christchurch, asks :-

What is radioactivity and how is it made?

Laura-Beth Crane, a scientist at the Ministry of Health's National Radiation Laboratory, responded.

Radioactivity is a process by which the nucleus of an atom spontaneously changes, with the emission of particles and rays. This is also known as radioactive decay. The decay products may be stable, or may themselves be radioactive. Radioactivity was discovered by Henri Becquerel in 1896 while conducting experiments with uranium, which indicated that the uranium salt emitted a type of radiation capable of penetrating through material opaque to ordinary light. The most common radiations are alpha particles, beta particles, gamma rays and neutrons. The radiation from unstable nuclei is important as it can affect other atoms and molecules, and may produce biological damage.

Radioactivity on earth occurs naturally in different ways. There are long lived isotopes of uranium and thorium that are still present in the earth's crust from the time of the formation of atoms in our galaxy and earth, and as these slowly decay, a series of other radioactive elements is created. Potassium also has a long lived radioactive isotope, which can be taken into the body in the form of food. Cosmic rays (high energy particles originating from outside the earth) interact with stable nuclides in our atmosphere to produce radioactive species such as carbon-14 and beryllium-7.

Artificial radioactivity can be created by simulating the cosmic ray processes that occur naturally, by bombarding stable nuclides with high energy particles and neutrons. This can be done using nuclear reactors, linear accelerators or cyclotrons. On the other hand, a patient undergoing an x-ray examination will not become radioactive, because the x- raying process does not involve high energy particles or neutrons. More information can be found at our web site: http://www.nrl.moh.govt.nz