4.3.1: Energy Units in Nuclear Physics

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The relevant energies in nuclear physics, even the largest ones, are only tiny fractions of a Joule, the standard unit in the SI system. For instance, the products of the most energetic \(a, \beta\) and \(\gamma\) decays carry energies of the order of \(10^{-13} \mathrm{~J}\). The record-high energies, those observed in fission reactions, are still of the order of one hundredth of one billionth \(\left(10^{-11}\right)\) of one Joule. Working with such small numbers is certainly not too convenient. Therefore, nuclear physicists created a more "user-friendly" energy unit, the electronVolt, with a symbol "eV". By definition, it is the amount of energy gained (or lost) by the charge of a single electron moving across an electric potential difference of one volt: \(1 \mathrm{eV}=1.602^{*} 10^{-19}\).

Practical units based on the electron-Volt are: the kilo-electron-Volt, 1 \(\mathrm{keV}=\) one thousand eV; the mega-electron-Volt, \(1 \mathrm{MeV}=\) one million \(\mathrm{eV}\); as well as the milli-electronvolt, \(1 \mathrm{meV}=\) one thousandth of \(1 \mathrm{eV}\), and the micro-electron-Volt, \(1 \mu \mathrm{eV}=\) one millionth of \(1 \mathrm{eV}\).