THE SPONTANEOUS EMISSION OF RADIATION

Radioactivity is the spontaneous emission of radiation from an unstable nucleus. Alpha (a) radiation consists of helium nuclei, small particles containing two protons and two neutrons (fHe). Beta (p) radiation consists of electrons ( e), and gamma (y) radiation consists of high-energy photons that have no mass. Positron emission is the conversion of a proton in the nucleus into a neutron plus an ejected positron, e or /3+, a particle that has the same mass as an electron but an opposite charge. Electron capture is the capture of an inner-shell electron by a proton in the nucleus. The process is accompanied by the emission of y rays and results in the conversion of a proton in the nucleus into a neutron. Every element in the periodic table has at least one radioactive isotope, or radioisotope. Radioactive decay is characterized kinetically by a first-order decay constant and by a half-life, h/2, the time required for the... 

Radioactivity The spontaneous emission of radiations by an element or its compound is called... 

If the spontaneous emission of radiation of the appropriate energy is the only pathway for a return to the initial state, the average statistical time that the molecule spends in the excited state is called the natural radiative lifetime. For an individual molecule the probability of emission is time-independent and the total intensity of emission depends on the number of molecules in the excited state. In a system with a large number of particles, the rate of decay follows a first-order rate law and can be expressed as... 

All the nuclear reactions that have been described thus far are examples of radioactive decay, where one element is converted into another element by the spontaneous emission of radiation. This conversion of an atom of one element to an atom of another element is called transmutation. Except for gamma emission, which does not alter an atom s atomic number, all nuclear reactions are transmutation reactions. Some unstable nuclei, such as the uranium salts used by Henri Becquerel, undergo transmutation naturally. However, transmutation may also be forced, or induced, by bombarding a stable nucleus with high-energy alpha, beta, or gamma radiation. 

Radioactivity involves the spontaneous emission of radiation by an unstable nucleus. 

Fluorescence6,30 is the spontaneous emission of radiation by an excited molecule, typically in the first excited singlet state Si, with retention of spin multiplicity. 

The term luminescence denotes the spontaneous emission of radiation from an electronically excited species and comprises fluorescence and phosphorescence, and also chemiluminescence (Section 5.6). 

The discovery and the history of radioactivity is closely connected to that of modern science. In 1896 Antoine Henri Becquerel observed and described the spontaneous emission of radiation by uranium and its compounds. Two years later, in 1898, the chemical research of Marie and Pierre Curie led to the discovery of polonium and radium. 

We considered some of the important experiments that led to the discovery and characterization of subatomic particles. Thomson s experiments on the behavior of cathode rays in magnetic and electric fields led to the discovery of the electron and allowed its charge-to-mass ratio to be measured. Millikan s oil-drop experiment determined the charge of the electron. Becquerel s discovery of radioactivity, the spontaneous emission of radiation by atoms, gave further evidence that the atom has a substructure. Rutherford s studies of how thin metal foils scatter a particles led to the nuclear model of the atom, showing that the atom has a dense, positively charged nucleus. 

The energy of the absorbed radiation corresponds to the energy of a transition from ground to an excited state. Decay of an excited state back to the ground state may take place by a radiative or non-radiative process. The spontaneous emission of radiation from an electronically excited species is called luminescence and this term covers both fluorescence and phosphorescence. A discussion of these phenomena requires an understanding of the electronic states of multi-electron systems, and we return to emission spectra in Section 20.8. 

Mathematically correct solutions also exist in which t,t are related by t = t + r - r /v. These are known as the advanced potentials, but since they appear to contradict the causality relation they seem to have no physical significance. Nevertheless they have been incorporated into a number of attempts to solve certain theoretical difficulties which arise in electrodynamics, one of which is briefly mentioned in section 4,4.1 in connection with the spontaneous emission of radiation. 

Thus the simple quantum theory is inadequate and to account for the spontaneous emission of radiation we adopt alternative techniques the first of these takes the classical result of section 4.2 and attempts to convert it into a quantum-mechanical form the second develops a complete quantum theory in which not only the atoms but also the radiation fields are properly quantized. We examine each of these methods in the following sections. 

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