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The half-lives of nine isomeric states in the rare earth region were measured using magnetic beta-ray spectrometers and an associated fast-slow coincidence system utilizing a time-to-pulse-height converter. The half-lives of the 8. 40-, 118- and 139-keV levels in Tm1 69 were determined to be (4. 13 ± 0. 12) nsec, (63 ± 7) psec and (289 ± 24) psec, respectively. The half-life of the 8. 40-keV level was determined using both Erl69 and Ybl69 decay. Several different methods were used to measure the 8. 40-keV level lifetime including applying various accelerating voltages to the radioactive sources. Comparisons are made with theory for the K = 1/2 rotational band in Tm169. The half-life obtained for the 8. 40-keV level in Tm169 corresponds to an energy level width of (1. 10 ± 0. 03) x l0-7 eV. If there were no broadening of the individual line profiles in the Mossbauer effect pattern, the width at half-maximum of the resolved Mossbauer lines would be (2. 20 ± 0. 06) x 10-7 eV compared to the narrowest experimental Mossbauer gross line width of 5. 04 x 10-7 eV. The half-lives of the 87- and 105-keV levels in Gd155 were also determined and were found to be (6. 27 ± 0. 35) nsec and (1. 20 ± 0. 04) nsec, respectively. Comparisons were made with the theoretical predictions of Nilsson for these two levels. The theoretical-experimental comparison would assign a spin of 5/2 to the 1 05-keV level and a spin of 3/2 to the 87-keV level if the comparison could be taken at face value. The half-lives of the 122-keV level in Sml52, 89-keV level in Gdl56, 396-keV level in Lu175 and 80-keV level in Prl44 were also determined. The half-lives of these four levels were found to be (1. 36 ± 0. 06) nsec, (2. 22 ± 0. 08) nsec, (3. 31 ± 0. 10) nsec and (143 ± 11) psec, respectively. These four half-life values agree well with values obtained by other investigators.

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