Beta decay of C-16 and N-17

The delayed neutron emitters $^{16}\mathrm{C}$ and $^{17}\mathrm{N}$ have been studied with a $^{3}\mathrm{He}$ neutron detector and $\ensuremath{\gamma}$ radiations have been measured with a Ge(Li) detector. $^{16}\mathrm{C}$ was formed in the reaction $^{14}\mathrm{C}$($^{18}\mathrm{O}$, $^{16}\mathrm{O}$)$^{16}\mathrm{C}$ with 56-MeV $^{18}\mathrm{O}$ ions, and also in the reaction $^{11}\mathrm{B}(^{7}\mathrm{Li},2p)^{16}\mathrm{C}$ with 40-MeV $^{7}\mathrm{Li}$ ions. The thermalized neutrons following both reactions yield together a $^{16}\mathrm{C}$ half-life of 0.747 \ifmmode\pm\else\textpm\fi{} 0.008 sec with the well-known 4.17-sec $^{17}\mathrm{N}$ activity as an accompaniment formed in the $^{14}\mathrm{C}$($^{18}\mathrm{O}$, $^{17}\mathrm{N}$)$^{15}\mathrm{N}$ and $^{11}\mathrm{B}(^{7}\mathrm{Li},p)^{17}\mathrm{N}$ reactions. The neutron spectra in a bare $^{3}\mathrm{He}$ detector contain $^{16}\mathrm{C}$ lines of 0.79 \ifmmode\pm\else\textpm\fi{} 0.03 MeV and 1.72 \ifmmode\pm\else\textpm\fi{} 0.05 MeV with relative intensities $\frac{{I}_{0.79}}{{I}_{1.72}}=5.4\ifmmode\pm\else\textpm\fi{}0.7$ in addition to $^{17}\mathrm{N}$ peaks. The $^{16}\mathrm{C}$ neutron groups are assigned to the $\ensuremath{\beta}$ decay to the known 3.36- and 4.32-MeV ${1}^{+}$ states of $^{16}\mathrm{N}$ with branches of 84.4 \ifmmode\pm\else\textpm\fi{} 1.7% and 15.6 \ifmmode\pm\else\textpm\fi{} 1.7%, respectively.A search for $\ensuremath{\gamma}$ rays associated with $^{16}\mathrm{C}$ forbidden $\ensuremath{\beta}$ decay to the ${0}^{\ensuremath{-}}$, ${3}^{\ensuremath{-}}$, and ${1}^{\ensuremath{-}}$ levels of $^{16}\mathrm{N}$ at 0.121, 0.297, and 0.397 MeV, respectively, gave upper limits of 0.5%, 0.5%, and 0.7% for the respective branching ratios ($log\mathrm{ft}g6.85, g6.83, \mathrm{and} g6.64$, respectively). The $log\mathrm{ft}$ values for $\ensuremath{\beta}$ decay to the 3.36- and 4.32-MeV states, 3.551 \ifmmode\pm\else\textpm\fi{} 0.012 and 3.83 \ifmmode\pm\else\textpm\fi{} 0.05, respectively, compare with preliminary theoretical values of 6.49 and 3.94, respectively, due to Millener. Separate studies were made of $^{17}\mathrm{N}$ decay, formed in the $^{15}\mathrm{N}(t,p)^{17}\mathrm{N}$ reaction at ${E}_{t}=3.0$ MeV. By using an empirical neutron efficiency function, established for the $^{3}\mathrm{He}$ detector with the $^{3}\mathrm{H}(p,n)^{3}\mathrm{He}$ reaction, $\ensuremath{\beta}$-ray branches of $^{17}\mathrm{N}$ to the $^{17}\mathrm{O}$ neutron-emitting states at 4.55, 5.38, and 5.94 MeV were found to be 39.2 \ifmmode\pm\else\textpm\fi{} 2.0%, 48.0 \ifmmode\pm\else\textpm\fi{} 1.5%, and 7.9 \ifmmode\pm\else\textpm\fi{} 0.3%, respectively, in disagreement with two previously reported measurements. $\ensuremath{\gamma}$-ray measurements on $^{17}\mathrm{N}$ with a Ge(Li) detector yielded the following: ratio of $\ensuremath{\beta}$-ray branching intensities $\frac{{B}_{871}}{{B}_{3055}}=8.6\ifmmode\pm\else\textpm\fi{}0.4$; ${B}_{3841}l7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ corresponding to $log\mathrm{ft}g8.5$ for this $\ensuremath{\beta}$-ray branch, thereby more firmly fixing the $^{17}\mathrm{N}$ spin as ${J}^{\ensuremath{\pi}}={\frac{1}{2}}^{\ensuremath{-}}$; energy of $^{17}\mathrm{O}$ second-excited state 3055.2 \ifmmode\pm\else\textpm\fi{} 0.3 keV; and ground-state $\ensuremath{\gamma}$-ray branch from the 3055.2-keV level 1.5%. The mirror $\ensuremath{\beta}$ decays of the $A=17$ system are discussed. An incidental result is a half-life of 178.3 \ifmmode\pm\else\textpm\fi{} 0.4 msec for $^{9}\mathrm{Li}$ formed in the $t(^{7}\mathrm{Li},p)^{9}\mathrm{Li}$ reaction with 20-40-MeV $^{7}\mathrm{Li}$ ions.RADIOACTIVITY $^{16}\mathrm{C}$, $^{17}\mathrm{N}$; measured ${t}_{\frac{1}{2}}$, ${E}_{n}$, ${I}_{n}$, ${I}_{\ensuremath{\gamma}}$; deduced $log\mathrm{ft}$, decay schemes; compared with theory. $^{9}\mathrm{Li}$; measured ${t}_{\frac{1}{2}}$.