Positron Radioactivities ofO14,Ne18,Si26, andS30

Abstract

A Siegbahn-Sl\"atis intermediate-image spectrometer has been used to measure the positron spectra of the isotopes ${\mathrm{O}}^{14}$, ${\mathrm{Ne}}^{18}$, ${\mathrm{Si}}^{26}$, and ${\mathrm{S}}^{30}$ produced by a ${\mathrm{He}}^{3}$ beam from a 5.5-MeV Van de Graaff accelerator. ${\mathrm{O}}^{14}$ was formed in the ${\mathrm{C}}^{12}({\mathrm{He}}^{3}, n){\mathrm{O}}^{14}$ reaction and it emits a positron group of ${E}_{max}=(4.085\ifmmode\pm\else\textpm\fi{}0.030)$ MeV to the ground state of ${\mathrm{N}}^{14}$ and another group having ${E}_{max}=(1.821\ifmmode\pm\else\textpm\fi{}0.007)$ MeV to the 2.3-MeV level. The branching ratios are (0.65\ifmmode\pm\else\textpm\fi{}0.05)% ($logft=7.23\ifmmode\pm\else\textpm\fi{}0.04$) and (99.35\ifmmode\pm\else\textpm\fi{}0.05)% ($logft=3.49\ifmmode\pm\else\textpm\fi{}0.01$), respectively. The measured half-life of ${\mathrm{O}}^{14}$ is 71.3\ifmmode\pm\else\textpm\fi{}0.1 sec. ${\mathrm{Ne}}^{18}$ was formed in the ${\mathrm{O}}^{16}({\mathrm{He}}^{3}, n){\mathrm{Ne}}^{18}$ reaction and we find ${E}_{max}=(3.416\ifmmode\pm\else\textpm\fi{}0.009)$ MeV to the ground state of ${\mathrm{F}}^{18}$, and ${\mathrm{T}}_{\mathrm{\textonehalf{}}}$=1.47\ifmmode\pm\else\textpm\fi{}0.10 sec. From a study of the beta and gamma rays of ${\mathrm{Ne}}^{18}$ by means of scintillation detectors we find a second positron branch feeding the 1.04-MeV level of ${\mathrm{F}}^{18}$. The branching ratios obtained from these measuremtns are (91\ifmmode\pm\else\textpm\fi{}3)% ($logft=3.02\ifmmode\pm\else\textpm\fi{}0.02$) to the ground state and (9\ifmmode\pm\else\textpm\fi{}3)% ($logft=3.47\ifmmode\pm\else\textpm\fi{}0.13$) to the 1.04-MeV level. The isotope ${\mathrm{Si}}^{26}$, produced by the ${\mathrm{Mg}}^{24}({\mathrm{He}}^{3}, n){\mathrm{Si}}^{26}$ reaction, decays with two branches to ${\mathrm{Al}}^{26}$. The more energetic branch feeds the 0.229-MeV isomeric state of ${\mathrm{Al}}^{26}$ and has ${E}_{max}=3.828\ifmmode\pm\else\textpm\fi{}0.013$ MeV from a magnetic spectrometer measurement, while the second branch is masked by other positron activities. We find ${\mathrm{T}}_{\mathrm{\textonehalf{}}}$=2.1\ifmmode\pm\else\textpm\fi{}0.1 sec for ${\mathrm{Si}}^{26}$. The second position branch of ${\mathrm{Si}}^{26}$ has been confirmed by observation of (0.820\ifmmode\pm\else\textpm\fi{}0.010) MeV gamma rays in coincidence with positrons. Branching ratios of ${\mathrm{Si}}^{26}$, measured with scintillation detectors, are (${66}_{\ensuremath{-}4}^{+1}$)% ($logft={{3.52}_{\ensuremath{-}0.02}}^{+0.04}$) for the branch to the 0.229 MeV level of ${\mathrm{Al}}^{26}$ and (${34}_{\ensuremath{-}1}^{+4}$)% ($logft={{3.36}_{\ensuremath{-}0.05}}^{+0.02}$) for the branch to the 1.059-MeV level. ${\mathrm{S}}^{30}$ was produced by the ${\mathrm{Si}}^{28}({\mathrm{He}}^{3}, n){\mathrm{S}}^{30}$ reaction and it decays with a half-life of 1.4\ifmmode\pm\else\textpm\fi{}0.1 sec and with positron branches of ${E}_{max}=(5.085\ifmmode\pm\else\textpm\fi{}0.026)$ MeV to the ground state of ${\mathrm{P}}^{30}$ and ${E}_{max}=(4.422\ifmmode\pm\else\textpm\fi{}0.022)$ MeV to the 0.684-MeV level. The branching ratios, obtained by analyzing the positron spectrum, are (20\ifmmode\pm\else\textpm\fi{}1)% ($logft=4.39\ifmmode\pm\else\textpm\fi{}0.03$) and (${80}_{\ensuremath{-}15}^{+1}$)% ($logft={{3.49}_{\ensuremath{-}0.03}}^{+0.09}$) respectively. Positron-gamma coincidence measurements have confirmed the decay scheme. Some comments are made on the abnormal value of the matrix element in the decay of ${\mathrm{O}}^{14}$ to the ground state of ${\mathrm{N}}^{14}$ and the results on the pure Fermi transitions observed in each of the isotopes are discussed.