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This is a technical term in particle physics and nuclear physics. A branching ratio for a decay is the ratio between the decay rates of decay modes for a decay. These decay constants are called partial decay constants, and their sum is the decay constant for the reaction. Sometimes a partial half life is given, but this term is misleading; due to competing modes it is not true that half of the particles will decay through a particular decay mode after its partial half life. The partial half life is merely an alternate way to specify the partial decay constant λ, the two being related through:
<math>t_{1/2} = \frac{\ln 2}{\lambda}<math>
For example, of spontaneous decays of 132Cs, 98.1% are ε or β+ decays, and 1.9% are β− decays. Thus the branching ratio for 132Cs is 98.1 ε+β+ :: 1.9 β−. The partial decay constants can be calculated from the branching ratio and the half-life of 132Cs (6.479 d), they are: 0.10 d−1 (ε+β+) and .0020 d−1 (β−). The partial half lives are 6.60 d (ε+β+) and 346 d (β−). Here the problem with the term partial half life is evident: after 346 days almost all the nuclei will have decayed.
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