  Alpha from natural radioactivity may interact with a nucleus and emit a
neutron. The reaction introduces background to the liquid scintillator (LS)
based neutrino experiments. In the LS detector, alpha comes from 238U, 232Th
and 210Po decay chains. For Gadolinium-doped LS (Gd-LS) detector, alpha also
comes from 227Ac. The nucleus 13C is a natural component of Carbon which is
rich in the LS. The background rate and spectrum should be subtracted carefully
from the neutrino candidates. This paper describes the calculation of neutron
yield and spectrum with uncertainty estimated. The results are relevant for
many existing neutrino experiments and future LS or Gd-LS based experiments.
