Search

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 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.

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 4 x 10(-3) to 2 x 10(-2) and are the best direct limits for neutrinos (as opposed to antineutrinos) for the mass region of 0.7 to 3.5 MeV.

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.

Angular distributions of 12C(alpha,alpha)12C have been measured for E(alpha) = 2.6-8.2 MeV, at angles from 24 to 166, yielding 12 864 data points. R-matrix analysis of the ratios of elastic scattering yields a reduced width amplitude of gamma12 = 0.47 +/- 0.06 MeV(1/2) for the Ex = 6.917 MeV (2+) state in 16O(a = 5.5 fm). The dependence of the chi2 surface on the interaction radius a has been investigated and a deep minimum is found at a = 5.42(+0.16)(-0.27) fm. Using this value of gamma12, radiative alpha capture and 16N beta-delayed alpha-decay data, the S factor is calculated at E(c.m.) = 300 keV to be S(E2)(300) = 53(+13)(-18) keV b for destructive interference between the subthreshold resonance tail and the ground state E2 direct capture.