Superallowed Beta Research Articles

Radiative corrections ofO(α)for pion beta decay in the light-front quark model

If the CKM matrix element ${V}_{\mathrm{ud}}$ that can be derived from superallowed nuclear decays, neutron decay and pion beta decay is used for a precision test of the unitarity of the CKM matrix, the combination of the present world data seems to indicate a small violation of the unitarity condition for the first row. While an accurate calculation of the radiative corrections (RC) of $O(\ensuremath{\alpha})$ is crucial in order to determine the value of ${V}_{\mathrm{ud}}$ as precisely as possible, the theoretical analysis has been limited in the past by the rather crude estimate of the effect of the hadronic structure. Only the contribution due to the axial vector current depends on the hadronic environment. We develop a strategy to deal with the influence of the hadronic structure on the decay properties of the simplest hadron, the pion, and calculate the contribution of the axial vector current to the RC, using a light-front model for the pion. Its $q\overline{q}$ bound state structure is well described by two parameters, the constituent quark mass and confinement scale, that have been fixed by a comparison with the data. We take into consideration three different groups of two-loop diagrams, and derive their light-front representations. We discuss the associated zero-mode problem and show that the respective light-front amplitudes are free of spurious contributions. There is only a small model dependent uncertainty of the final result for the RC for pion beta decay.

The future radioactive ion beam research program at Oak Ridge

Early in 1995, the ORNL Holifield Facility is scheduled to return to operation as a dedicated radioactive ion beam (RIB) facility. This enhanced capability will open up new possibilities for research on the structure of and phenomena occurring in proton-rich nuclei. Among the opportunities will be the creation of heavier N≈Z nuclei, approaching 100Sn, and the study of exotic nuclear shapes, extensions of studies of the p-n interaction and super-allowed beta decay, and examination of nuclear structure near the proton drip line. In addition to new nuclear and atomic physics research opportunities, the beams from the Holifield RIB facility are expected to provide new capabilities for measurements important to nuclear astrophysics. To carry out this experimental program, various upgrades are planned to the existing experimental apparatus, and a new, third-generation, recoil-mass separator is being constructed.

Super-allowed beta decay of nuclei at the drip-line

We have investigated the continuous particle spectra following the beta decay of the neutron-rich nuclei that terminate the mass 6, 8, 9 and 11 isobaric chains. Strong beta transitions feeding the very top of the spectrum are found systematically. The favoured states are within a few MeV of the energy of the initial state. This phenomenon seems to be linked to the occurrence of a di-neutron structure or neutron halo in the drip-line nuclei.

Gv, KM and mt

Analysis of superallowed Fermi beta decay using a new approach to the nuclear mismatch factor and evaluation of the radiative corrections using renormalization group methods and including nuclear-structure-dependent terms in the “inner” radiative correction leads to | V ud| 2+| V us| 2+| V ub| 2 = 0.9999 ± 0.0012 for the first row of the Kobayashi-Maskawa matrix using a mass of 130 GeV for the top quark. We may, alternatively, assuming unitarity of the KM matrix, infer a top quark mass of (1.40±80) GeV from beta-decay data alone.

Limits on the presence of scalar and induced-scalar currents in superallowed beta decay.

Limits on the scalar and induced-scalar coupling constants are established from experimental ft values of superallowed Fermi transitions. The role of the uncertainties in the ft values is discussed, leading to minimal coupling strengths that can be extracted from ft values. The implications for the Kobayashi-Maskawa mixing angles are also discussed.

Discovery of Beta-Delayed Two-Proton Radioactivity:Al22

A proton-proton coincidence experiment has shown that, following the superallowed beta decay of the odd-odd, ${T}_{z}=\ensuremath{-}2$ nuclide, $^{22}\mathrm{Al}$, two protons are emitted with a summed energy spectrum (5.636, 4.139 MeV) characteristic of decay to the ground and first excited states of $^{20}\mathrm{Ne}$, respectively.

Charge-dependence in superallowed beta decay: A comment

Experimental ƒt-values are used to examine various theoretical treatments of analogue symmetry breaking in superallowed Fermi β-decay. None of the fits to experiment in more than barely acceptable and, contrary to a previous analysis, we find no clear preference for one type of treatment over another.

The threshold energy for the 34S(p, n) 34Cl reaction and the Q-value of the 34Cl superallowed beta decay

The threshold for the 34S(p, n) 34Cl reaction has been measured and the reaction Q-value determined to be −6274.27 ± 0.56 keV. From this the Q-value for the 34C1 superallowed β-decay has been deduced as 4469.8 ± 0.6 keV, which is in good agreement with other recent measurements. Using the presently accepted value for the 34C1 half-life, the Ft value has been calculated to be 3085.1 ± 3.8s.

The effect of excited 0 + states on the [formula omitted] values of superallowed beta decays

The effect of including an excited 0 + state in the sum over intermediate nuclear states which occurs in electromagnetic corrections to superallowed beta decays is investigated and found to be too small to a account for the variation in ƒt values.

Fermi functions for superallowed beta decays and the value of GV

To extract the polar vector beta-decay coupling constant G V from the data on 0 + → 0 + superallowed transitions, accurate values of the Fermi function are needed. We report the details and the results of a numerical evaluation of the relevant Fermi functions. Both the finite size charge distribution of the nucleus and the screening effect due to the atomic electrons are taken into account. The G V (or alternatively the ft values) calculated with these Fermi functions agree very well with earlier results for 10C, 14O, 26mAl and 34Cl but not for 42Sc, 46Vn, 50Mn and 54Co. The probable reason for the disagreement is given, and the implications of the results are discussed.

The beta decays of22Mg,30S and34Ar

The beta decays of 22Mg, 30S and 34Ar have been investigated with the following results. For 22Mg, τ1/2 = 3 99 ± 0 12 sec. The decay proceeds in (59 ± 2)% of the cases to the 656 kev 0+ level in 22Na, and in (41 ± 2)% of the cases to the 583 kev 1+ level in 22Na. The end point of the superallowed beta particles was determined using the 20Ne(3He, n)22Mg reaction to be 3 146 ± 0 035 MeV. For 30S, τ1/2 = 1 18 ± 0 04 sec. The decay proceeds in (20 ± 1)% of the cases to the ground state of 30P, and in (80 ± 1)% of the cases to the 676 keV 0+ level in 30P. Less than 1 6% of the decays proceed to the 707 keV 1+ level in 30P. For 34Ar, τ1/2 = 0 85 ± 0 10 sec. There is no decay that proceeds with more than 20% of the main decay which is to the ground state of 34Cl. fτ values are deduced and discussed.

Leptonic Decays of Hadrons

The experimental data on leptonic decays of baryons have been re-examined in the light of a two-angle Cabibbo theory, where the two angles, ${\ensuremath{\theta}}_{V}$ and ${\ensuremath{\theta}}_{A}$, are characteristic of the vector and axial-vector baryon currents, respectively. With certain assumptions about the energy dependence of the form factors in the vector (${K}_{e3}$) and axial-vector (${K}_{\ensuremath{\mu}2}, {\ensuremath{\pi}}_{\ensuremath{\mu}2}$) decays of mesons, it can be shown that the angles ${\ensuremath{\theta}}_{V}$ and ${\ensuremath{\theta}}_{A}$ derived from baryon decays are compatible with the corresponding angles derived from meson decays, thus justifying our use of a two-angle theory. There is no discrepancy between the information from hyperon and meson decays and the information from the superallowed nuclear beta decays (${\mathrm{O}}^{14}$, ${\mathrm{Cl}}^{34}$, etc.). Using the assumptions about the meson form factors, a fit of all data on leptonic decays of hadrons gives the values ${\ensuremath{\theta}}_{V}=0.212\ifmmode\pm\else\textpm\fi{}0.004$, ${\ensuremath{\theta}}_{A}=0.268\ifmmode\pm\else\textpm\fi{}0.001$, $\ensuremath{\alpha}=0.665\ifmmode\pm\else\textpm\fi{}0.018$, where the parameter $\ensuremath{\alpha}$ defines the relative content of $D$ coupling in the baryonic axial-vector current.

Phase-space integrals for beta decay and nuclear matrix elements

The use of phase-space integrals to determine nuclear matrix elements is discussed. The procedure for determining matrix elements is illustrated by analysing the allowed and super-allowed beta decays of isotopes with mass numbers in the range five to ten, utilizing the recent summary of the properties of nuclei in this region compiled by Ajzenberg-Selove and Lauritsen. Some f values for other beta decays of interest are also given.

Internal bremsstrahlung from 6He

The spectrum of internal bremsstrahlung from the superallowed beta emitter 6He has been measured using a NaI scintillation spectrometer. The shape of the experimental spectrum agrees with the theory of Knipp, Uhlenbeck and Bloch for energies greater than 0.3 MeV. In this decay, Coulomb corrections are of little significance; thus the results can be interpreted as confirming the fundamental concept of internal bremsstrahlung as a process in which the electron exists first in a virtual state and then goes to its final state by emitting a photon.