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

. A program of high-precision half-life and branching-ratio measurements for superallowed Fermi β 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 14 O, 18 Ne, and 26 Al m , as well as branching-ratio measurements for 26 Al m and 74 Rb are reported. These results provide demanding tests of the Standard Model and the theoretical isospin symmetry breaking (ISB) corrections in superallowed Fermi β decays.


Introduction
High-precision measurements of the β decay f t values for superallowed Fermi transitions between nuclear analog states of spin J π = 0 + and isospin T = 1 provide demanding, and fundamental, tests of the minimal electroweak Standard Model, including the conserved vector current (CVC) hypothesis and the V − A form of the weak interaction [1,2]. The superallowed β emitters, in combination a e-mail: alaffole@uoguelph.ca with muon decay, currently provide the most precise determination of V ud , by far the most precisely measured element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, and, when combined with V us and V ub , confirm CKM unitarity to a precision of ±0.06% [1,2]. However, these fundamental tests require theoretical radiative and isospin symmetry breaking (ISB) corrections [3] to be applied to the experimentally measured f t values in order to obtain "corrected" f t values, denoted Ft; according to the CVC hypothesis, the Ft values should be constant for all superallowed decays involving states of a given isospin T . The ISB corrections, in particular, have been the focus of many recent studies employing a wide range of theoretical and semi-empirical approaches [2][3][4][5][6][7][8][9][10][11][12], and significant differences are found between the various models. Testing these ISB corrections through high-precision experimental measurements is thus crucial. A program at the Isotope Separator and Accelerator (ISAC) facility at TRIUMF, Canada's national laboratory for particle and nuclear physics, is in place to perform high-precision half-life, branchingratio, and Q value studies for these superallowed β emitters. These experiments are performed using TRIUMF's Ion Trap for Atomic and Nuclear Science (TITAN) [13]; a 4π gas proportional β counter; and the 8π γ-ray Spectrometer [14], a spherical array consisting of 20 Compton-suppressed high-purity germanium (HPGe) detectors, with ancillary detection systems including the Zero-Degree Scintillator (ZDS), the Scintillating Electron-Positron Tagging Array (SCEPTAR), and the Pentagonal Array for Conversion Electron Spectroscopy (PACES) [15]. With the capability of measuring all experimental quantities required for determining superallowed f t values, ISAC is providing crucial data to help distinguish between the theoretical models used for calculating ISB corrections.

Results and Discussion
The f t values for the lightest superallowed β emitters, 10 C and 14 O, are most sensitive to scalar interactions and are thus crucial for setting upper limits on the existence of fundamental or induced scalar currents in β decay [16]. In both cases, the half-life can be measured either by direct β counting or by measuring the γ activity since, with a branching-ratio of >99%, both decay to an excited state of their daughters, 10 B and 14 N, which then emit 718.4 and 2312.6 keV γ-rays, respectively [17,18]. An apparent systematic effect associated with the detection method used for previous half-life measurements calls into question the accuracy of the current average T 1/2 values for both 10 C and 14 O; resolving this issue motivated our 14 O half-life experiment that is described in Ref. [19]. A very recent half-life experiment for 10 C was completed at ISAC in fall 2013 and will further address the existing systematic discrepancy between the β and γ detection methods.
Among the 13 most precisely measured superallowed β decays, 26 Al m has one of the smallest and most precisely quoted theoretical ISB corrections [3]. This value does, however, exhibit one of the largest discrepancies between the Woods-Saxon and Hartree-Fock radial overlap corrections of Ref. [2], motivating improved experimental precision to test this model dependence. The precision of the 26 Al m superallowed f t value was limited by the ±0.03% precision of the world average half-life [2] prior to our recent studies at ISAC. Using a 4π gas-proportional β counter with a fast tape transport system and employing the dead-time correction and maximum likelihood fitting techniques described in Ref. [20], the 26 Al m half-life has now been measured with a statistical precision of ±0.007% [21]. A detailed investigation of potential systematic effects in this measurement led to a final result of T 1/2 = 6346.54(46) stat. (60) syst. ms, a factor of 2.5 times more precise than the previous world average. Additionally, a measurement of the 26 Al m superallowed branching-ratio with the 8π γ-ray spectrometer at ISAC led to an inclusive upper limit of 15 ppm at 90% confidence on the sum of all non-analog decay branches, resulting in a superallowed branching-ratio of 100.0000 +0 −0.0015 % [22]. These results lead to a superallowed f t value of 3037.53(61) s, the most precisely determined for any of the superallowed decays.

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As the ISB corrections are approximately proportional to Z 2 , the largest corrections occur in the heaviest superallowed emitters; among those with a high-precision f t value, 74 Rb is the heaviest and has an ISB correction of δ C = 1.63(31)% [3]. This decay has been a major focus of the superallowed program at ISAC, including a high-precision half-life measurement with a 4π gas-proportional β counter [23], a recent mass measurement using high-charge state radioactive ions with the TITAN Penning trap [24], and a charge radius measurement via co-linear laser spectroscopy [25]. Most recently, the superallowed branching-ratio for 74 Rb decay has been measured with the 8π γ-ray spectrometer. A total of 57 γ-ray transitions, as well as the E0 conversion electron decay of the low-lying 0 + state of 74 Kr at 509 keV were placed in the 74 Kr level scheme following 74 Rb decay [26]. These data, combined with the techniques developed in Refs. [27,28], allowed the determination of the superallowed branching-ratio to a precision of ±0.031% [26], a factor of 3 more precise than the previous measurement [29]. State-by-state comparisons [26] of the calculated isospin mixing corrections associated with the low-lying 0 + states in the daughter 74 Kr, however, suggest that the current shell-model calculations [30,31] have difficulty describing the 0 + states in this region of strong deformation, shape co-existence, and shape transitions [32][33][34][35].
Although the f t value for the superallowed decay of 18 Ne is not yet sufficiently precise to be included in the survey of the 13 highest precision cases [2], there is much interest in this case as the different theoretical descriptions of ISB for 18 Ne decay exhibit some of the largest differences for any of the superallowed emitters [36]. In anticipation of a high-precision branching-ratio measurement, a priority for several T z = −1 emitters [37], high-resolution β delayed γ-ray spectroscopy was performed following the decay of 18 Ne and the half-life was determined to be T 1/2 = 1.6648(11) s [36]. This result provides important input towards future experiments that aim to improve the precision of the 18 Ne f t value to the level of ±0.2%; the case of 18 Ne is particularly attractive as it potentially provides a means to investigate the role of nuclear deformation in the calculation of the ISB corrections. Very recently, a newly commissioned thick-tape transport system, used in conjunction with the 4π gas proportional counter at ISAC, has been designed and commissioned for the measurement of half-lives of (noble) gaseous samples. The tape is 1/2 inch thick with a layer of 2.51 mg/cm 2 thick aluminum on a mylar backing. The tape is controlled by a stepping motor and a magnetic brake to move samples from the implantation site to centre of the detector. A picture of the tape system is shown in Fig. 1. The tape system was commissioned with two experiments completed in the Fall of 2013 measuring the half-lives of the superallowed β emitters 18 Ne and 10 C using the methods described in Refs. [20,21,23]. The effect of diffusion from the tape was thoroughly investigated, and it was concluded that the gaseous samples were not susceptible to any effects on the time scale of the experiment.

Thick-Tape Transport System
The half-lives of two superallowed β emitters, 10 C and 18 Ne, have now been measured with this new experimental facility with statistical precisions better than 0.02% and 0.04%, respectively. These measurements, as well as simultaneous but independent measurements using the ZDS and 8π with comparable statistical precision, are more precise than any previous half-life experiments for these nuclei. Final analysis of systematic uncertainties in these measurements is currently in progress.

Summary
The superallowed branching-ratios for 26 Al m and 74 Rb decay, as well as the 14 O, 18 Ne, and 26 Al m halflives, have been measured to high precision at the TRIUMF-ISAC facility. The resulting Ft values for these decays not only remain in excellent agreement with those of the other precisely measured superallowed emitters when using the Woods-Saxon radial overlap corrections of Ref. [3], supporting the accuracy of these calculations, but provide a rigorous test of the CVC hypothesis. For 74 Rb, the non-analog Fermi β branches to excited 0 + states of the daughter nucleus [26] reinforce the need for expanded shell model spaces to accurately describe the 0 + wavefunctions in the A ≥ 62 region. The new high-precision experimental data for 26 Al m serve to highlight the large impact that the model dependence of the ISB corrections for this case has on the world-average corrected Ft value [21]. With an ongoing branching-ratio measurement and the high-precision half-life measurement performed at ISAC, 18 Ne may soon be included as one of the high-precision f t value cases and will help discriminate between theoretical models of ISB in superallowed decays. Finally, a high-precision half-life measurement for 10 C is being used to address a potential systematic discrepancy between β and γ-ray counting techniques. Further high-precision superallowed β-decay data, as well as additional independent, firstprinciples, approaches to the ISB effects in these decays remain a priority of the community.