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Register a new userHigh-Precision Branching Ratio Measurement and Spin Assignment Implications for $^{62}$Ga Superallowed $beta$ Decay
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A high-precision branching ratio measurement for the superallowed Fermi $beta^{+}$ emitter $^{62}$Ga was performed with the Gamma-Ray Infrastructure for Fundamental Investigations of Nuclei (GRIFFIN) spectrometer at the Isotope Separator and Accelerator (ISAC) radioactive ion beam facility at TRIUMF. The high efficiency of the GRIFFIN spectrometer allowed 63 $gamma$-ray transitions, with intensities down to $approx$1 part per million (ppm) per $^{62}$Ga $beta^{+}$ decay, to be placed in the level scheme of the daughter nucleus $^{62}$Zn, establishing the superallowed $beta$ branching ratio for $^{62}$Ga decay to be 99.8577$^{+0.0023}_{-0.0029}%$, a factor of 4 more precise than the previous world average. For several cascades, $gamma-gamma$ angular correlation measurements were performed to assign spins and/or determine the mixing ratios of transitions. In particular, the spin of the 2.342 MeV excited state in the daughter nucleus $^{62}$Zn was definitively assigned as $J = 0$. This assignment resolves a discrepancy between previous measurements and has important implications for the isospin symmetry breaking correction, $delta_{C1}$, in $^{62}$Ga superallowed Fermi $beta$ decay.
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Masses of the radioactive isotopes 62Ga, 62Zn and 62Cu have been measured at the JYFLTRAP facility with a relative precision of better than 18 ppb. A Q_EC value of (9181.07 +- 0.54) keV for the superallowed decay of 62Ga is obtained from the measured cyclotron frequency ratios of 62Ga-62Zn, 62Ga-62Ni and 62Zn-62Ni ions. The resulting Ft-value supports the validity of the conserved vector current hypothesis (CVC). The mass excess values measured were (-51986.5 +-1.0) keV for 62Ga, (-61167.9 +- 0.9) keV for 62Zn and (-62787.2 +- 0.9) keV for 62Cu.
The beta-decay half-life of 62Ga has been studied with high precision using on-line mass separated samples. The decay of 62Ga which is dominated by a 0+ to 0+ transition to the ground state of 62Zn yields a half-life of T_{1/2} = 116.19(4) ms. This result is more precise than any previous measurement by about a factor of four or more. The present value is in agreement with older literature values, but slightly disagrees with a recent measurement. We determine an error weighted average value of all experimental half-lives of 116.18(4) ms.
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A high-precision half-life measurement for the superallowed Fermi $beta^+$ emitter $^{22}$Mg was performed at the TRIUMF-ISAC facility using a 4$pi$ proportional gas counter. The result of $T_{1/2} = 3.87400 pm 0.00079$ s is a factor of 3 more precise than the previously adopted world average and resolves a discrepancy between the two previously published $^{22}$Mg half-life measurements.
A recent Penning-trap measurement of the masses of 46V and 46Ti leads to a Qec value that disagrees significantly with the previously accepted value, and destroys overall consistency among the nine most precisely characterized T=1 superallowed beta emitters. This raises the possibility of a systematic discrepancy between Penning-trap measurements and the reaction-based measurements upon which the Qec values depended in the past. We carefully re-analyze (n,gamma) and (p,gamma) reaction measurements in the 24 leq A leq 28 mass region, and compare the results to very precise Penning-trap measurements of the stable nuclei 24Mg, 26Mg and 28Si. We thus determine upper limits to possible systematic effects in the reaction results, and go on to establish limits for the mass of radioactive 26Al, to which future on-line Penning-trap measurements can be compared. We stress the urgency of identifying or ruling-out possible systematic effects.
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