Excited States of Iodine-127

Abstract

The lifetime of the 59-keV first excited state of ${\mathrm{I}}^{127}$ has been measured using both xenon and tellurium parents. An average of the two results is 1.8\ifmmode\pm\else\textpm\fi{}0.3 nsec. In addition, angular correlation studies have shown the multipolarity of the 59-keV gamma ray to be predominantly magnetic dipole with an electric quadrupole admixture of 0.6\ifmmode\pm\else\textpm\fi{}0.6%. Angular correlation studies have also been carried out on two other cascades of ${\mathrm{I}}^{127}$. One of these, the 175-200-keV cascade, showed an anisotropy of 5\ifmmode\pm\else\textpm\fi{}3% when the ${\mathrm{Xe}}^{127}$ source was in the gaseous form; but when the source was adsorbed on charcoal, an anisotropy of 30% was found. The smearing of the angular correlation is attributed to the highly ionized states of the gamma emitters resulting from electron capture. The other, the 356-59-keV cascade, exhibited an anisotropy of 0.498\ifmmode\pm\else\textpm\fi{}0.07 after geometric corrections. This permits assignment of the value $\frac{5}{2}$ for the spin of the 415-keV state, while the 356 gamma ray is either 9.3 or 85% electric quadrupole, depending on which of the two possible solutions is selected. The lifetime of the 59-keV state after correction for internal conversion is longer by a factor 67\ifmmode\pm\else\textpm\fi{}14 than predicted by the Weisskopf estimate for an allowed magnetic dipole transition. This is consistent with the values observed for other $l$-forbidden $M1$ transitions and with the most recent theoretical estimates.