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Gadolinium isotopes were separated from a tantalum target which had been irradiated with 800-MeV protons. The concentrations of ' 'Gd in weighed aliquots of a solution of the isotopes were determined by isotope dilution mass spectrometry. Other aliquots were counted on a low geometry alpha counter to determine the disintegration rate of ' 'Gd per unit weight of solution. From the data obtained the half-life of the isotope was found to be 74.6+ 3.0 yr. [RADIOACTIVITY Gd measured T i~2.] In experiments designed to obtain the (n,2n) cross section of ' Gd, ' a measurement of considerable interest because of the neutron-deficiency of that alpha emitter, an accurate value of the half-life was needed. In this paper, we report the determination of such a value.A tantalum target was irradiated with 800-MeV protons for 56.3 mAh at the Los Alamos Meson Physics Facility (LAMPF). The target was dissolved in a mixture of concentrated nitric and hydrofluoric acids, and lanthanum carrier was added to precipitate lanthanide fluorides. Gadolinium was separated from the other lanthanides by selective elution with a-hydroxyisobutyrate through a cation exchange column. Prior to preparation of the final solution, ' 'Eu and " Eu, formed by the decay of gadolinium isotopes, were removed by an additional cation exchange column. The eluate was fumed in concentrated HC104 to destroy the ahydroxyisobutyric acid. A final gadoliniumcontaining sample was prepared by dissolving the residue in 0.2 M HC1 and carefully weighing the resultant solution. The concentration of each gadolinium isotope in the sample solution was determined by isotopic dilution spectrometry. Isotopic ratios were measured on weighed aliquots of spiked and unspiked sample solutions with the Los Alamos Radiochemistry Group's 6-inch solid source mass spectrometer. All mass spectrometric measurements were normalized to the ' Gd/ ' Gd ratio defined by Russ (' Gd/' Gd = 1.13583) to correct for mass fractionation in the instrument.The overall accuracy of the measurements is limited by the accuracy of the assumed value of the ratio.Two standard solutions of the spike were prepared from 99.999% pure, isotopically normal Gd203 (source: United Mineral and Chemical Corporation, batch No. R1469). The oxide was heated to constant mass in a platinum crucible, a weighed sample dissolved in about 50 ml of high-purity hydrochloric acid (source: NBS, batch No. 4028), and the solution made up to a weighed mass of about 1 kg with water from the Millipore Milli-Q water purification system. Each standard solution was cross calibrated by titration with standard ethylenediamine tetra-acetic acid (EDTA) solution.The results obtained by titration were biased by -0.4% and -0.2% relative to the gravimetrically determined values. Standard results obtained by titration were used for the final determination of isotopic concentrations.The relative gadolinium isotopic abundances in the standard and the sample solution are shown in TABLE I. Isotopic composition of gadolinium.