We have measured the absolute gamma-ray intensity, i.e., gamma's per 1
00 parent decays, of the 1266-keV line produced in Si-31 decay using n
eutron activation and delayed photon spectroscopy. Its value was deter
mined to be 0.050+/-0.004%. This result is lower than the only previou
sly reported value of 0.07, which was given without an uncertainty. In
a separate experiment to validate the method of measurement and analy
sis, the gamma-ray intensity of the 1779-keV line of Al-28 was determi
ned to be 98+/-8.8%, in good agreement with its adopted value of 100%.