Nuclear Structure ofNa22. IV. Gamma-Ray Correlation Measurements

Abstract

$\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ angular correlations have been measured relative to the $\ensuremath{\alpha}$ beam in the ${\mathrm{F}}^{19}(\ensuremath{\alpha}, n\ensuremath{\gamma}\ensuremath{\gamma}){\mathrm{Na}}^{22}$ reaction using NaI(Tl) $\ensuremath{\gamma}$-ray detectors. An ancillary determination of $\ensuremath{\gamma}$ branching ratios for ${\mathrm{Na}}^{22}$ states of interest was made through $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence measurements utilizing Ge(Li) and NaI(Tl) detector combinations. From these measurements and previous information it was determined that the ${\mathrm{Na}}^{22}$ 1.95-, 2.97-, and 3.06-MeV levels have $J=2, 3, \mathrm{and} 2$, respectively. The quadrupole-dipole amplitude ratios of the 1.95\ensuremath{\rightarrow}0.58, 2.97\ensuremath{\rightarrow}1.95, and 3.06\ensuremath{\rightarrow}1.95 transitions were determined to be + (0.04\ifmmode\pm\else\textpm\fi{}0.06), + (0.017\ifmmode\pm\else\textpm\fi{}0.034), and - (0.05\ifmmode\pm\else\textpm\fi{}0.15), respectively; i.e., the results are consistent in each case with pure dipole radiation. From measurements of the angular correlation of the ${\mathrm{Na}}^{22}$ 1.528\ensuremath{\rightarrow}0.891\ensuremath{\rightarrow}0 $\ensuremath{\gamma}$-ray cascade, it was determined that the two members of this ${5}^{+}$ \ensuremath{\rightarrow} ${4}^{+}$ \ensuremath{\rightarrow} ${3}^{+}$ cascade have $\frac{E2}{M1}$ amplitude ratios of - (2.00\ifmmode\pm\else\textpm\fi{}0.15) and - (3.08\ifmmode\pm\else\textpm\fi{}0.32), respectively. From these mixing ratios and previously determined branching ratios and lifetimes for the ${\mathrm{Na}}^{22}$ 1.528- and 0.891-MeV levels the $E2$ strengths of the 1.528 \ensuremath{\rightarrow} 0.891 and 0.891 \ensuremath{\rightarrow} 0 transitions are 20.7\ifmmode\pm\else\textpm\fi{}7.0 and 29.5\ifmmode\pm\else\textpm\fi{}4.5 Weisskopf units, respectively. These and other properties of ${\mathrm{Na}}^{22}$ are in accord with a rotational-model interpretation of this nucleus.