The half-life of ${}_{}{}^{241}{}_{}{}^{}\mathrm{Pu}$ was determined by periodically observing the decrease in isotope abundance of ${}_{}{}^{241}{}_{}{}^{}\mathrm{Pu}$ relative to other isotopes of plutonium with a thermal ionization mass spectrometer. A synthetic mixture of plutonium isotopes with initial values of ${}_{}{}^{241}{}_{}{}^{}\mathrm{Pu}$${/}^{239}$Pu, ${}_{}{}^{241}{}_{}{}^{}\mathrm{Pu}$${/}^{240}$Pu, and ${}_{}{}^{241}{}_{}{}^{}\mathrm{Pu}$${/}^{242}$Pu isotope ratios close to unity was prepared. This was done with the objective of achieving high precision (better than 0.1%) in the isotope ratio measurements. The decrease in the isotope ratio was measured at intervals of 6 months over a period of about 5 yr. A value of 14.38±0.02 yr was obtained using the double ratio method which eliminates variable systematic error due to isotope fractionation. The value obtained in the present work supports the results reported earlier by the daughter growth method from this laboratory and is in good agreement with the values reported recently by other laboratories during the course of our investigations.

• Received 5 April 1984

DOI:https://doi.org/10.1103/PhysRevC.31.1885