Scientific considerations — selected topics
Radioactivity can most easily be understood in terms of the simplified classical model of the atom described by the Danish physicist Neils Bohr in which the nucleus, containing approximately equal numbers of positively charged protons and uncharged neutrons, is surrounded by as many orbiting electrons as there are protons. Stable atoms have either equal numbers of protons and neutrons or a slight excess of neutrons. The number of protons determines the number of orbiting electrons and hence the element and its chemical properties. An element may have one or more stable configurations of neutrons (stable isotopes); for example iodine has only one stable isotope while tin has ten. Atoms which do not have a stable configuration of protons and neutrons are radioactive (radio-isotopes). Those with an excess of neutrons decay with the emission of a negatively charged beta (β-) particle identical to an electron in all respects except its source: β particles originate from the nucleus whereas electrons orbit around it. Atoms with a deficiency of neutrons emit positively charged particles of the same mass as an electron (positrons, β+) or reduce their positive charge by capturing an orbiting electron. Some radioactive isotopes of heavy elements (lead and above) emit alpha (α) particles, which resemble helium nuclei. The emission of a charged particle is commonly accompanied by the discharge of excess energy in the form of one or more gamma (γ)-rays, the energy of which is specific for each radio-isotope.
KeywordsSingle Photon Emission Compute Tomography Count Rate Gamma Camera Line Source High Count Rate
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