Radiation Fundamentals

Each atom contains a tiny central positively charged nucleus and a number of electrons. The electrons carry negative electric charge and move around the nucleus in shells with loosely defined boundaries.The nucleus of the atom contains protons, which carry a positive charge equal to the electron’s negative charge, and neutrons, which carry no charge at all.Atoms normally contain equal numbers of protons and electrons and are therefore electrically neutral.The illustration shows the oxygen atom planetary model with a nucleus of 8 protons, 8 neutrons and 8 orbital electrons.© IAEA

The number of electrons in the atom — and hence the number of protons in the nucleus, called the ATOMIC NUMBER — gives an element its unique characteristics. The atomic number of carbon is 6, for instance, whereas for lead it is 82.Because protons and neutrons, which have approximately the same mass, and are much heavier than electrons, most of an atom’s mass is concentrated in the nucleus. The total number of an atom's protons and neutrons is called MASS NUMBER.© IAEA

Since the number of electrons equals the number of protons in an electrically neutral atom, we can specify an ATOMIC SPECIES by the number of protons and neutrons it contains.Moreover, since the number of protons is unique to each element, we can simply use the name of the element together with the mass number to specify each SPECIES or NUCLIDE.So carbon-12 is a nuclide with six protons plus six neutrons. Lead-208, for comparison, is a nuclide with 82 protons and 126 neutrons.© IAEA

Nuclides of an element that have the same number of protons, but different numbers of neutrons, are called ISOTOPES of that element.Hydrogen, for instance, has three isotopes: <ul><li>hydrogen-1 (common hydrogen with a nucleus of only one proton);</li><li>hydrogen-2 called deuterium (one proton and one neutron);</li><li>and hydrogen-3 called tritium (one proton and two neutrons).</li></ul>Iron has ten isotopes from iron-52 to iron-61, all with the 26 protons that characterize the element, but with 26 to 35 neutrons.© IAEA

Atoms of the same or different elements can combine to form larger, electrically neutral entities called MOLECULES.Atoms combine to form molecules to gain stability and remain in the minimum state of energy.For example, two atoms of oxygen form one molecule of oxygen, and two atoms of hydrogen combine with one atom of oxygen to form one molecule of water.© IAEA

Alpha particles consist of 2 protons and 2 neutrons bound together — they are heavy and have 2 positive charges.Nuclei with too many neutrons tend to transform themselves into a more stable structure 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 convert the excess protons to neutrons. These transformations often leave the nucleus with excess energy that it loses as gamma rays.© IAEA

Larger stable nuclei have slightly more neutrons than protons.Some nuclei are unstable, meaning that they undergo some internal changes spontaneously. In this process, the nucleus gives off a subatomic particle, a burst of energy, or both.The act of transformation is termed decay and the nuclide that changes and emits radiation is called RADIONUCLIDE. Other naturally occurring radionuclides are formed in sequences or series of decays that originate from the elements uranium and thorium. Each of these series ends with a stable nuclide.The diagram shows the decay series from uranium-238 ending in the stable nuclide lead-206.© IAEA

An electron in the material may receive enough energy to escape from an atom, leaving the atom or molecule thus formed positively charged.The figure illustrates this process for a molecule of water. The molecule has ten protons and ten electrons altogether, but only nine atomic electrons remain after a charged particle passes by; the molecule as a whole is left with one excess positive charge.The process by which a neutral atom or molecule becomes charged is called ionization and the resulting entity an ion.Once removed from an atom, an electron may in turn ionize other atoms or molecules.© IAEA

RADIOACTIVE MATERIAL — Nuclides undergoing spontaneous random disintegration, usually accompanied by the emission of radiation.FISSILE MATERIAL — Nuclides capable of sustaining a chain reaction of nuclear fission. These include U-233, Pu-239.FERTILE MATERIAL — Nuclides that do not undego fission but from which fissile material can be generated by neutron absorption and nuclear conversion. These include Th-232, U-234, Pu-238.NUCLEAR MATERIAL is a term for nuclides used in nuclear technology systems.Diagram of uranium-235 fission© IAEA

During the past few decades, several hundred radioactive isotopes (radioisotopes) of natural elements have been produced artificially including, for example, strontium-90, caesium-137 and iodine-131.The heaviest element existing in nature is uranium. However, so-called trans-uranium elements have been produced by man.Several new radioactive elements have also been produced in quantity, for instance promethium and plutonium, although the latter does occur naturally in trace amounts in uranium ores.© PhotoDisk

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