Positron Radioactivities of O14, Ne18, Si26, and S30

Abstract

A Siegbahn-Slätis 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 $E_{max}=(4.085\mathrm{}\pm 0.030)$ MeV to the ground state of ${\mathrm{N}}^{14}$ and another group having $E_{max}=(1.821\mathrm{}\pm 0.007)$ MeV to the 2.3-MeV level. The branching ratios are (0.65±0.05)% ($logft=7.23\mathrm{}\pm 0.04$) and (99.35±0.05)% ($logft=3.49\mathrm{}\pm 0.01$), respectively. The measured half-life of ${\mathrm{O}}^{14}$ is 71.3±0.1 sec. ${\mathrm{Ne}}^{18}$ was formed in the ${\mathrm{O}}^{16}({\mathrm{He}}^3, n){\mathrm{Ne}}^{18}$ reaction and we find $E_{max}=(3.416\mathrm{}\pm 0.009)$ MeV to the ground state of ${\mathrm{F}}^{18}$, and ${\mathrm{T}}_{\mathrm{½}}$=1.47±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±3)% ($logft=3.02\mathrm{}\pm 0.02$) to the ground state and (9±3)% ($logft=3.47\mathrm{}\pm 0.13$) 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 $E_{max}=3.828\mathrm{}\pm 0.013$ MeV from a magnetic spectrometer measurement, while the second branch is masked by other positron activities. We find ${\mathrm{T}}_{\mathrm{½}}$=2.1±0.1 sec for ${\mathrm{Si}}^{26}$. The second position branch of ${\mathrm{Si}}^{26}$ has been confirmed by observation of (0.820±0.010) MeV gamma rays in coincidence with positrons. Branching ratios of ${\mathrm{Si}}^{26}$, measured with scintillation detectors, are (${66}_{-4\mathrm{}}^{+1\mathrm{}}$)% ($logft={{3.52}_{-0.02\mathrm{}}}^{+0.04\mathrm{}}$) for the branch to the 0.229 MeV level of ${\mathrm{Al}}^{26}$ and (${34}_{-1\mathrm{}}^{+4\mathrm{}}$)% ($logft={{3.36}_{-0.05\mathrm{}}}^{+0.02\mathrm{}}$) 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±0.1 sec and with positron branches of $E_{max}=(5.085\mathrm{}\pm 0.026)$ MeV to the ground state of ${\mathrm{P}}^{30}$ and $E_{max}=(4.422\mathrm{}\pm 0.022)$ MeV to the 0.684-MeV level. The branching ratios, obtained by analyzing the positron spectrum, are (20±1)% ($logft=4.39\mathrm{}\pm 0.03$) and (${80}_{-15\mathrm{}}^{+1\mathrm{}}$)% ($logft={{3.49}_{-0.03\mathrm{}}}^{+0.09\mathrm{}}$) 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.