The binding energy EB of a nucleus is the energy that must be supplied to separate a nucleus into individual protons and neutrons. Since the nucleus is a bound system, its total energy is less than the energy of Z protons and N neutrons that are far apart and at rest.
Each radioactive nuclide has a characteristic decay probability per unit time l. The activity R of a sample with N nuclei is
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Activity is commonly measured in becquerels (1 Bq = 1 decay per second) or curies (1 Ci = 3.7 × 1010 Bq).
The number of remaining nuclei N in radioactive decay (the number that have not decayed) is an exponential function:
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where the time constant is t = 1/l. The half-life is the time during which half of the nuclei decay:
A large nucleus can release energy by splitting into two smaller, more tightly bound nuclei in the process called fission. The energy released in a fission reaction is enormous—typically around 200 MeV for the split of a single nucleus.
Nuclear fusion combines two small nuclei to form a larger nucleus. Fusion typically releases significantly more energy per nucleon than fission.
