First Forbidden Nonunique Beta Transitions inRe186

The beta spectrum of the main inner group of 89.0-hr ${\mathrm{Re}}^{186}$ ($E_0=934.3\mathrm{}\pm 1.3$ kev; 24% of total betas; $logft=8.0\mathrm{}$) has been magnetically analyzed in coincidence with the 137.2-kev gamma. The observed spectrum does not have the allowed shape nor the unique ($\Delta I=2\mathrm{}$, yes) shape; its spectrum shape correction factor increases about 18% from 150 kev to 900 kev. Subtraction of this spectrum, weighted with its measured relative abundance, from the composite spectrum, produces the spectrum of the ground state transition ($E_0=1071.5\mathrm{}\pm 1.3$ kev; 76% of total betas; $logft=7.68\mathrm{}$). Its similarly nonallowed, nonunique shape factor increases by about 22% from 150 kev to 1050 kev. Both transitions are interpreted as first forbidden ($\Delta I=1\mathrm{}$, yes); ${\mathrm{Re}}^{186}$ is assigned a ground state spin and parity 1(-). A theoretical parametric fitting of these spectra with the combination of scalar and tensor beta interactions is demonstrated; the limited ranges of the fitting parameters, which are functions of the nuclear matrix elements, are obtained.