High-Precision Branching Ratio Measurement for the Superallowed + Emitter 74Rb

A high-precision branching-ratio measurement for the superallowed β + decay of 74Rb was performed at the TRIUMF Isotope Separator and Accelerator (ISAC) radioactive ion-beam facility. The scintillating electronpositron tagging array (SCEPTAR), composed of 10 thin plastic scintillators, was used to detect the emitted β particles; the 8π spectrometer, an array of 20 Compton-suppressed HPGe detectors, was used for detecting γ rays that were emitted following Gamow-Teller and nonanalog Fermi β + decays of 74Rb; and the Pentagonal Array of Conversion Electron Spectrometers (PACES), an array of 5 Si(Li) detectors, was employed for measuring β-delayed conversion electrons. Twenty-three excited states were identified in 74Kr following 8.241(4) × 108 detected 74Rb β decays. A total of 58 γ -ray and electron transitions were placed in the decay scheme, allowing the superallowed branching ratio to be determined as B0 = 99.545(31)%. Combined with previous half-life and Q-value measurements, the superallowed branching ratio measured in this work leads to a superallowed f t value of 3082.8(65) s. Comparisons between this superallowed f t value and the world-average-corrected Ft value, as well as the nonanalog Fermi branching ratios determined in this work, provide guidance for theoretical models of the isospin-symmetry-breaking corrections in this mass region.IS

Reorientation-effect measurement of the first 2+ state in 12C : Confirmation of oblate deformation

A Coulomb-excitation reorientation-effect measurement using the TIGRESS γ−ray spectrometer at the TRIUMF/ISAC II facility has permitted the determination of the 〈21 +‖E2ˆ‖21 +〉 diagonal matrix element in 12C from particle−γ coincidence data and state-of-the-art no-core shell model calculations of the nuclear polarizability. The nuclear polarizability for the ground and first-excited (21 +) states in 12C have been calculated using chiral NN N4LO500 and NN+3NF350 interactions, which show convergence and agreement with photo-absorption cross-section data. Predictions show a change in the nuclear polarizability with a substantial increase between the ground state and first excited 21 + state at 4.439 MeV. The polarizability of the 21 + state is introduced into the current and previous Coulomb-excitation reorientation-effect analyses of 12C. Spectroscopic quadrupole moments of QS(21 +)=+0.053(44) eb and QS(21 +)=+0.08(3) eb are determined, respectively, yielding a weighted average of QS(21 +)=+0.071(25) eb, in agreement with recent ab initio calculations. The present measurement confirms that the 21 + state of 12C is oblate and emphasizes the important role played by the nuclear polarizability in Coulomb-excitation studies of light nuclei

High-Precision Half-life Measurements for the Superallowed β\beta+ Emitter 14^{14}O

The half-life of 14O, a superallowed Fermi emitter, has been determined via simultaneous and counting experiments at TRIUMF's Isotope Separator and Accelerator facility. Following the implantation of 14O samples at the center of the 8 spectrometer, a counting measurement was performed by detecting the 2313 keV rays emitted from the first excited state of the daughter 14N using 20 high-purity germanium (HPGe) detectors. A simultaneous counting experiment was performed using a fast plastic scintillator positioned directly behind the implantation site. The results, T1=2( ) = 70:632 0:094 s and T1=2( ) = 70:610 0:030 s, are consistent with one another and, together with eight previous measurements, establish a new average for the 14O half-life of T1=2 = 70:619 0:011 s with a reduced 2 of 0.99

High-precision half-life and branching-ratio measurements for superallowed Fermi + emitters at TRIUMF - ISAC

International audienceA program of high-precision half-life and branching-ratio measurements for superallowed Fermi emitters is being carried out at TRIUMF's Isotope Separator and Accelerator (ISAC) radioactive ion beam facility. Recent half-life measurements for the superallowed decays of 14O, 18Ne, and 26Alm, as well as branching-ratio measurements for 26Alm and 74Rb are reported. These results provide demanding tests of the Standard Model and the theoretical isospin symmetry breaking (ISB) corrections in superallowed Fermi decays

High-precision Half-life And Branching Ratio Measurements For Superallowed β+\beta^+ Emitters At TRIUMF-ISAC

International audienceHigh precision measurements of the ft values for superallowed Fermi β transitions between J π = 0 + isobaric analogue states allow for stringent tests of the electroweak interaction described by the Standard Model. These transitions provide an experimental probe of the ConservedVector-Current hypothesis, enable the most precise determination of the up-down (Vud) element of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, and allow one to set stringent limits on the existence of scalar currents in the weak interaction. An extensive program of superallowed branching-ratio and half-life measurements at TRIUMF’s Isotope Separator and Accelerator (ISAC) facility has covered the full range of superallowed emitters, from the lightest case, 10C, to the heaviest case for which precision data are currently available, 74Rb. These experiments have been performed using a 4π continuous-flow gas proportional β counter, the 8π γ-ray spectrometer, and, most recently, the new high-efficiency GRIFFIN γ-ray spectrometer. In this paper, recent highlights from the superallowed Fermi β decay program at TRIUMF will be summarized, including high-precision half-life measurements for all three of the lightest superallowed emitters, 10C, 14O, and 18Ne, with the greatest sensitivity to a potential weak scalar current contribution, as well as high-precision branching-ratio measurements for the heavy superallowed emitters 62Ga and 74Rb