section_path: Characteristics/Isotopes
section_name: Isotopes
paragraph_text: Isotopes
  
  Naturally occurring silicon is composed of three stable isotopes, Si (92.23%), Si (4.67%), and Si (3.10%). Out of these, only Si is of use in NMR and EPR spectroscopy, as it is the only one with a nuclear spin (I = 1 / 2). All three are produced in Type Ia supernovae through the oxygen-burning process, with Si being made as part of the alpha process and hence the most abundant. The fusion of Si with alpha particles by photodisintegration rearrangement in stars is known as the silicon-burning process; it is the last stage of stellar nucleosynthesis before the rapid collapse and violent explosion of the star in question in a type II supernova.
  
  Twenty radioisotopes have been characterized, the two stablest being Si with a half-life of about 150 years, and Si with a half-life of 2.62 hours. All the remaining radioactive isotopes have half-lives that are less than seven seconds, and the majority of these have half-lives that are less than one-tenth of a second. Silicon has one known nuclear isomer, Si, with a half-life less than 210 nanoseconds. Si undergoes low-energy beta decay to P and then stable S. Si may be produced by the neutron activation of natural silicon and is thus useful for quantitative analysis; it can be easily detected by its characteristic beta decay to stable P, in which the emitted electron carries up to 1.48 MeV of energy.
  
  The known isotopes of silicon range in mass number from 22 to 44. The most common decay mode of the isotopes with mass numbers lower than the three stable isotopes is inverse beta decay, primarily forming aluminium isotopes (13 protons) as decay products. The most common decay mode for the heavier unstable isotopes is beta decay, primarily forming phosphorus isotopes (15 protons) as decay products.
  
  Silicon can enter the oceans through groundwater and riverine transport. Large fluxes of groundwater input have an isotopic composition which is distinct from riverine silicon inputs. Isotopic variations in groundwater and riverine transports contribute to variations in oceanic Si values. Currently, there are substantial differences in the isotopic values of deep water in the world's ocean basins. Between the Atlantic and Pacific oceans, there is a deep water Si gradient of greater than 0.3 parts per thousand. Si is most commonly associated with productivity in the oceans.