Search

We have studied the $\ensuremath{\beta}$-delayed particle decay of ${}^{17}$Ne to test the feasibility of determining both the $E1$ and $E2$ components of the ${}^{12}$C$(\ensuremath{\alpha},\ensuremath{\gamma}{)}^{16}$O cross section at energies relevant to helium burning in stars. In this context we have observed the breakup of the isobaric analog state in ${}^{17}$F at 11.193 MeV into three particles via three channels: proton decay to the 9.59 MeV state in ${}^{16}$O; and $\ensuremath{\alpha}$ decay to the 2.365 and 3.502/3.547 MeV states in ${}^{13}$N. This is the first reported observation of the decay of the IAS to the ${1}^{\ensuremath{-}}$ state in ${}^{16}$O at 9.59 MeV and the first reported $\ensuremath{\beta}$-delayed proton-$\ensuremath{\alpha}$ decay. With straightforward improvements to our detection apparatus to improve angular resolution, $\ensuremath{\beta}$ suppression, and solid angle coverage, we should be able to proceed to the measurement of the effect of the tail of the subthreshold state at 7.117 MeV in ${}^{16}$O on the $\ensuremath{\alpha}$ spectrum from the breakup of the 9.59 MeV state.

A magneto-optic trap (MOT) can provide a well-polarized, backing-free, localized source of radioactive atoms for $\ensuremath{\beta}$-decay experiments. We have trapped approximately 6000 atoms of ${}^{38}{\mathrm{K}}^{m}$ ( ${t}_{1/2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.925\mathrm{s}$) and 2000 atoms of ${}^{37}\mathrm{K}$ (1.226 s) produced at the TRIUMF on-line separator TISOL in a vapor-cell MOT. We have measured optical isotope shifts and deduced the nuclear charge radii, which show an unusual lack of change at the neutron number $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}20$ shell closure. Plans include a search for scalar contributions to the ${\ensuremath{\beta}}^{+}$- $\ensuremath{\nu}$ correlation in the ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ decay of ${}^{38}{\mathrm{K}}^{m}$.

The excitation energies of the proton unbound states in [sup 32]Cl have been measured in the [sup 32]S([sup 3]He,[ital t])[sup 32]Cl charge exchange reaction with high accuracy. The partial widths of the unbound levels have been calculated to derive the resonance strengths of these states in the [sup 31]S([ital p],[gamma])[sup 32]Cl reaction channel. The reaction rate for the [sup 31]S([ital p],[gamma])[sup 32]Cl reaction has been calculated and is compared with previous estimates. The role of this reaction for the closure of the SiP cycle is discussed in terms of the temperature and density conditions in hot stellar hydrogen burning.

The α spectrum following the β decay of 16N from the isotope separator TISOL has been measured by detecting 106 α particles in coincidence with 12C nuclei. These data, which show a low-energy interference anomaly accompanying the main α peak, permit a more precise determination of the p-wave amplitude of the astrophysically important reaction 12C(α,γ)16O. The α spectrum and previous γ-ray data have been fitted simultaneously by a K-matrix parametrization; a value of S(E=0.3 MeV)=57±13 keV b has been obtained for the E1 part of the 12C(α,γ)16O reaction.

We have set limits on contributions of scalar interactions to nuclear beta decay. A magneto-optical trap (MOT) provides a localized source of atoms suspended in space, so the low-energy recoiling nuclei can freely escape and be detected in coincidence with the beta. This allows reconstruction of the neutrino momentum, and the measurement of the beta-neutrino correlation, in a more direct fashion than previously possible. The beta-neutrino correlation parameter of the 0+ to 0+ pure Fermi decay of 38mK is $\tilde{a}$=0.9981+-0.0030+-0.0037, consistent with the standard model prediction a=1. Comment: 4 pages, 3 figures, accepted by Phys. Rev. Lett., centroid changed

A new technique, full neutrino momentum reconstruction, is used to set limits on the admixture of heavy neutrinos into the electron neutrino. We measure coincidences between nuclear recoils and positrons from the beta decay of trapped radioactive atoms and deduce the neutrino momentum. A search for peaks in the reconstructed recoil time-of-flight spectrum as a function of positron energy is performed. The admixture upper limits range from 4x10(-3) to 2x10(-2) and are the best direct limits for neutrinos (as opposed to antineutrinos) for the mass region of 0.7 to 3.5 MeV.

The \ensuremath{\beta} delayed deuteron decay of ${}^{6}\mathrm{He}$ has been measured at the TISOL facility at TRIUMF. For the \ensuremath{\beta}-particle measurement a silicon-germanium telescope array has been used. To separate deuterons from $\ensuremath{\alpha}$ particles and to obtain the spectral form of the deuteron decay, a $d\ensuremath{-}\ensuremath{\alpha}$ coincidence technique has been employed using two opposite silicon detectors. The \ensuremath{\beta} branch has been determined from the geometry of the experiment and also by comparison with the well-known $\ensuremath{\beta}\ensuremath{-}\ensuremath{\alpha}$ decay modes of ${}^{16}\mathrm{N}$ and ${}^{8}\mathrm{Li}.$ A branching ratio of $(1.8\ifmmode\pm\else\textpm\fi{}0.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ above the $\ensuremath{\beta}$ cutoff (350-keV deuteron laboratory energy) and $(2.6\ifmmode\pm\else\textpm\fi{}1.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ for the entire spectrum has been determined for this decay. In the course of the measurement the half-life of ${}^{6}\mathrm{He}$ has been remeasured to be ${T}_{1/2}=0.810\ifmmode\pm\else\textpm\fi{}0.008 \mathrm{s}.$ High statistical accuracy in the deuteron spectrum has been obtained that allowed a comparison of the shape of the deuteron spectrum to theoretical predictions.

Angular distributions of ${}^{12}\mathrm{C}(\ensuremath{\alpha},\ensuremath{\alpha}{)}^{12}\mathrm{C}$ have been measured for ${E}_{\ensuremath{\alpha}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.6--8.2\mathrm{MeV}$, at angles from $24\ifmmode^\circ\else\textdegree\fi{}$ to $166\ifmmode^\circ\else\textdegree\fi{}$, yielding 12 864 data points. R-matrix analysis of the ratios of elastic scattering yields a reduced width amplitude of ${\ensuremath{\gamma}}_{12}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.47\ifmmode\pm\else\textpm\fi{}0.06{\mathrm{MeV}}^{1/2}$ for the ${E}_{x}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}6.917\mathrm{MeV}({2}^{+})$ state in ${}^{16}\mathrm{O}(a\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5.5\mathrm{fm})$. The dependence of the ${\ensuremath{\chi}}^{2}$ surface on the interaction radius $a$ has been investigated and a deep minimum is found at ${a\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}5.42}_{\ensuremath{-}0.27}^{+0.16}\mathrm{fm}$. Using this value of ${\ensuremath{\gamma}}_{12}$, radiative $\ensuremath{\alpha}$ capture and ${}^{16}\mathrm{N}$ $\ensuremath{\beta}$-delayed $\ensuremath{\alpha}$-decay data, the S factor is calculated at ${E}_{\mathrm{c}.\mathrm{m}.}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}300\mathrm{keV}$ to be ${S}_{E2}(300){\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}53}_{\ensuremath{-}18}^{+13}\mathrm{keV}\mathrm{b}$ for destructive interference between the subthreshold resonance tail and the ground state $E2$ direct capture.

The strengths of low-energy resonances in ${}^{14}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma}{)}^{18}\mathrm{F}$ at 573 keV and 1136 keV have been measured using an activation method. In addition, their relative strength and the energy of the lower resonance have been determined in a prompt $\ensuremath{\gamma}$-ray experiment. The results of these measurements are used to reevaluate the stellar reaction rate of ${}^{14}\mathrm{N}(\ensuremath{\alpha},\ensuremath{\gamma}{)}^{18}\mathrm{F}.$ The present reaction rate at temperatures of astrophysical interest is a factor of 2 smaller than previously reported.

The $\ensuremath{\beta}$ decay of ${}^{9}\mathrm{C}$ ${(T}_{1/2}=126.5 \mathrm{ms})$ has been studied in two experiments observing about $15\ifmmode\times\else\texttimes\fi{}{10}^{7}$ and $8\ifmmode\times\else\texttimes\fi{}{10}^{7}$ decays, respectively, at the TISOL facility at TRIUMF; different detector configurations were employed in the two experiments. In this first of two papers, the two experimental setups are described, as well as data analysis and a phenomenological approach to deducing branching ratios to and from states in ${}^{9}\mathrm{B}.$ In the experiments single spectra, and double and triple coincidence spectra, were recorded. Several states in ${}^{9}\mathrm{B}$ were observed; $\ensuremath{\beta}$-branching ratios to these states, and particle decay channels from these states, are reported. In particular, secondary decays into the ${}^{5}\mathrm{Li}$ and ${}^{8}\mathrm{Be}$ ground states were observed. With the inclusion of a considerable continuum and additional states, fair agreement with the reported ${}^{9}\mathrm{Li}\mathrm{log}\mathrm{ft}$ values is found with a phenomenological approach for deducing the branching ratios. To extend the discussion, in a second, forthcoming paper, a multichannel, multistate R-matrix analysis of these data will be described.