Sunday, December 27, 2009

Nuclear Radioactivity

Although many nuclides are stable, most are not. Stability is determined mainly by the balance between the number of neutrons and protons a nuclide contains. Smaller stable nuclei have about equal numbers, larger stable nuclei have slightly more neutrons than protons.

Nuclei with too many neutrons such as 31H2 tend to transform themselves to a more stable structure 32He, by converting a neutron to a proton: this process, known as beta decay, results in the emission of a negatively charged electron called a beta particle.

Nuclei with too many protons such as 116C5 convert the excess protons to neutrons in a different form of beta decay: they lose positive charge through the emission of a positron, which is a positively charged electron.

These transformations often leave the nucleus with excess energy that it loses as gamma rays or high energy photons, which are discrete parcels of energy without mass or charge. The spontaneous transformation of a nucleus is called radioactivity, and the excess energy emitted is a form of (ionizing) radiation. The act of trans­formation is termed decay and the nuclide that changes and emits radiation is called a radionuclide.

Some heavy nuclei such as Am-241 decays by producing an alpha particle consisting of two protons and two neutrons. Identical with a nucleus of helium, the alpha particle is much heavier than the beta particle and carries two units of positive charge .

The rate at which spontaneous transformations occur in a given amount of a radioactive material is known as its activity. Activity is expressed in a unit called the becquerel, symbol Bq, where 1 Bq equals one transformation per second. The becquerel is named after the French physicist Henri Becquerel. As the unit is so small, multiples of the becquerel are frequently used, such as the megabecquerel, MBq, which is 1 million becquerels. One gram of radium-226,for instance, has an activity of approximately 37 000 MBq: it emits about 37 000 million alpha particles each second (an old unit of activity, the curie - named after the Polish-born French scientist Marie Curie - was originally defined as the activity of one gram of radium).

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