Beta Spectrum ofW187

The $\beta$ spectrum of ${\mathrm{W}}^{187}$ has been analyzed with a thoroughly tested Siegbahn-Slatis $\beta$-ray spectrometer equipped with a plastic well-type detector, whose counting efficiency is unity down to 50 keV. The highest-energy group (${\frac{3}{2}}^{-}\to {\frac{5}{2}}^{+}$) is nonunique first-forbidden with a $log\mathrm{ft}$ value 8.0, and it deviates from the statistical shape as a result of the selection-rule effect. Its shape could be fitted with that of a modified $B_{\mathrm{ij}}$ approximation, namely, $C\left(W\right)=q^2+\frac{9L_1}{L_0}+D$ with the value of $D=23\mathrm{}\pm 3$. Though the calculations based on the Nilsson model and the $j-j$ coupling shell model predict a deviation from the $\xi$ approximation, only the former could give better agreement with experiment. A detailed analysis of the shape factor yields the end point of the outer group as 1314±2 keV. Subtraction of this outer group, corrected for its shape, revealed inner groups with maximum energies and intensities 625±5 (66%), 453±7 (3%), and 330±10 (5%) keV. Information is also obtained on a few conversion lines which are resolved by the spectrometer.