particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...

and

nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies the ...

, the branching fraction (or branching ratio) for a decay is the fraction of particles which decay by an individual decay mode or with respect to the total number of particles which decay. It applies to either the radioactive decay of

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, ...

s or the decay of

elementary particles In particle physics, an elementary particle or fundamental particle is a subatomic particle that is not composed of other particles. Particles currently thought to be elementary include electrons, the fundamental fermions (quarks, leptons, anti ...

. It is equal to the ratio of the partial decay constant to the overall decay constant. 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 Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable at ...

. The partial half-life is merely an alternate way to specify the partial decay constant , the two being related through: :

t_ = \frac.

For example, for spontaneous decays of ¹³² Cs, 98.1% are ε (

electron capture Electron capture (K-electron capture, also K-capture, or L-electron capture, L-capture) is a process in which the proton-rich nucleus of an electrically neutral atom absorbs an inner atomic electron, usually from the K or L electron shells. Thi ...

) or β⁺ ( positron) decays, and 1.9% are β⁻ (

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

) decays. The partial decay constants can be calculated from the branching fraction and the half-life of ¹³²Cs (6.479 d), they are: 0.10 d⁻¹ (ε + β⁺) and 0.0020 d⁻¹ (β⁻). The partial half-lives are 6.60 d (ε + β⁺) and 341 d (β⁻). Here the problem with the term partial half-life is evident: after (341+6.60) days almost all the nuclei will have decayed, not only half as one may initially think. Isotopes with significant branching of decay modes include

copper-64 Copper-64 (64Cu) is a positron and beta emitting isotope of copper, with applications for molecular radiotherapy and positron emission tomography. Its unusually long half-life (12.7-hours) for a positron-emitting isotope makes it increasingly us ...

, arsenic-74, rhodium-102, indium-112, iodine-126 and holmium-164.

References

External links

LBNL Isotopes Project

Particle Data Group
(listings for particle physics) * '
Nuclear Structure and Decay Data - IAEA
'' for nuclear decays Particle physics Nuclear physics Radioactivity Ratios {{particle-stub