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Neutron capture is the process of one or more neutrons colliding with and merging with an atomic nucleus to create a heavier nucleus. Neutron capture is assumed to be a mechanism by which nuclei with odd atomic numbers are created (through subsequent beta decay after becoming neutron rich), the alpha process being associated with even atomic numbers.
The terms r-process and s-process refer to neutron capture within stars and supernovae, the two terms describing the rate at which it is occurring (rapid versus slow), which depends upon the rate at which free neutrons are being produced in the vicinity.
Capture must happen within the lifetime of a free neutron (i.e., a neutron not attached to other nucleons) which decays with a half-life of roughly 610 seconds, i.e., mean lifetime of roughly 880 seconds, the free neutron mean lifetime (often shortened to neutron lifetime in context). (This is one reason we are not bombarded by free neutrons and neutron capture does not occur from neutrons produced at astronomical distances.) Measuring the neutron lifetime has been a challenge and determinations are of current research interest because two types of experiments produce decidedly different results (though both are in the general vicinity of 880). The exact value has cosmological implications.