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Register a new userPrecise branching ratio measurements in $^{19}$Ne beta decay and fundamental tests of the weak interaction
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We used the 8$pi$ $gamma$-ray spectrometer at the TRIUMF-ISAC radiocative ion beam facility to obtain high-precision branching ratios for $^{19}$Ne $beta^+$ decay to excited states in $^{19}$F. Together with other previous work, our measurements determine the superallowed $1/2^+ to 1/2^+$ beta branch to the ground state in $^{19}$F to be 99.9878(7)%, which is three times more precise than known previously. The implications of these measurements for testing a variety of weak interaction symmetries are discussed briefly.
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Measurements of the beta-neutrino correlation coefficient (a$_{beta u}$) in nuclear beta decay, together with the Fierz interference term (b$_F$), provide a robust test for the existence of exotic interactions beyond the Standard Model of Particle Physics. The extraction of these quantities from the recoil ion spectra in $beta$-decay requires accurate knowledge, decay branching ratios, and high-precision calculations of higher order nuclear effects. Here, we report on a new measurement of the $^{23}$Ne $beta$-decay branching ratio, which allows a reanalysis of existing high-precision measurements. Together with new theoretical calculations of nuclear structure effects, augmented with robust theoretical uncertainty, this measurement improves on the current knowledge of a$_{beta u}$ in $^{23}$Ne by an order of magnitude, and strongly constrains the Fierz term in beta decays, making this one of the first extractions to constrain both terms simultaneously. Together, these results place bounds on the existence of exotic tensor interactions and pave the way for new, even higher precision, experiments.
We present the full description of a measurement of the branching ratios for the beta-decay of 38Ca. This decay includes five allowed 0+ --> 1+ branches and a superallowed 0+ --> 0+ one. With our new result for the latter, we determine its ft value to be 3062.3(68) s, a result whose precision (0.2%) is comparable to the precision of the thirteen well known 0+ --> 0+ transitions used up till now for the determination of Vud, the up-down quark-mixing element of the CKM matrix. The 38Ca superallowed transition thus becomes the first addition to this set of transitions in nearly a decade and the first for which a precise mirror comparison is possible, thus enabling an improved test of the isospin-symmetry-breaking corrections required for the extraction of Vud.
In an experiment performed at the ISOLDE facility of CERN, the super-allowed beta-decay branching ratio of 10C was determined with a high-precision single-crystal germanium detector. In order to evaluate the contribution of the pile-up of two 511 keV gamma quanta to one of the gamma-ray peaks of interest at 1021.7 keV, data were not only taken with 10C, but also with a 19Ne beam. The final result for the super-allowed decay branch is 1.4638(50)%, in agreement with the average from literature.
Background: The $^{15}$O($alpha ,gamma$)$^{19}$Ne bottleneck reaction in Type I x-ray bursts is the most important thermonuclear reaction rate to constrain experimentally, in order to improve the accuracy of burst light-curve simulations. A proposed technique to determine the thermonuclear rate of this reaction employs the $^{20}$Mg($beta palpha$)$^{15}$O decay sequence. The key $^{15}$O($alpha ,gamma$)$^{19}$Ne resonance at an excitation of 4.03 MeV is now known to be fed in $^{20}$Mg($beta pgamma$)$^{19}$Ne; however, the energies of the protons feeding the 4.03 MeV state are unknown. Knowledge of the proton energies will facilitate future $^{20}$Mg($beta p alpha$)$^{15}$O measurements. Purpose: To determine the energy of the proton transition feeding the 4.03 MeV state in $^{19}$Ne. Method: A fast beam of $^{20}$Mg was implanted into a plastic scintillator, which was used to detect $beta$ particles. 16 high purity germanium detectors were used to detect $gamma$ rays emitted following $beta p$ decay. A Monte Carlo method was used to simulate the Doppler broadening of $^{19}$Ne $gamma$ rays and compare to the experimental data. Results: The center of mass energy between the proton and $^{19}$Ne, feeding the 4.03 MeV state, is measured to be 1.21${^{+0.25}_{-0.22}}$ MeV, corresponding to a $^{20}$Na excitation energy of 7.44${^{+0.25}_{-0.22}}$ MeV. Absolute feeding intensities and $gamma$-decay branching ratios of $^{19}$Ne states were determined including the 1615 keV state. A new $gamma$ decay branch from the 1536 keV state in $^{19}$Ne to the ground state is reported. The lifetime of the 1507 keV state in $^{19}$Ne is measured to be 4.3${^{+1.3}_{-1.1}}$ ps resolving discrepancies in the literature. Conflicting $^{20}$Mg($beta p$) decay schemes in published literature are clarified.
While the 12C(a,g)16O reaction plays a central role in nuclear astrophysics, the cross section at energies relevant to hydrostatic helium burning is too small to be directly measured in the laboratory. The beta-delayed alpha spectrum of 16N can be used to constrain the extrapolation of the E1 component of the S-factor; however, with this approach the resulting S-factor becomes strongly correlated with the assumed beta-alpha branching ratio. We have remeasured the beta-alpha branching ratio by implanting 16N ions in a segmented Si detector and counting the number of beta-alpha decays relative to the number of implantations. Our result, 1.49(5)e-5, represents a 24% increase compared to the accepted value and implies an increase of 14% in the extrapolated S-factor.
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