Determination of the half-life of HF-178m
Little, Donald Cubbison
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Coincidence counting techniques were used in the determination of the half-life of Hf-178m. The experimental sample, essentially a mixture of Hf-177 and Hf-178, was irradiated with thermal neutrons to produce Hf-178m and Hf-179m isomers. The isomeric transition of Hf-178m is followed by a cascade of gammas (.427, . 326, .214, and .093 mev). The isomeric transition of Hf-179m (approximately 18- second half-life) is followed by a . 215 mev gamma. The decay data of Hf-178m was effectively isolated from that of Hf-179m by counting coincidences between any two of three specific Hf-178m gammas (.427, .326, and. 214 mev). After being corrected for chance coincidence counts, this counting data was processed by a computer program (FRANTIC). By using the leastsquares techniques, the FRANTIC program fitted a calculated exponential decay curve to the counting data from a single experiment and also computed the half-life for that data. The average half-life of Hf-178m, derived from seven decay experiments, had a value of 3. 94 seconds and an uncertainty of 0. 5%. The additional uncertainties due to gain instability and background count-rate were estimated. The half-life of Hf-178m was found to be 3. 94 + 0. 04 seconds, all uncertainties included.
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