F.C. Barker

Nuclear levels near thresholds

Abstract Cross sections for reactions proceeding through unstable states of nuclei may be expressed in terms of a density-of-states function. A widely-used form ϱ (1) of this function is compared with a recently suggested form ϱ (2) and it is shown that cross sections involving levels of the nuclei Be 8 , Be 9 and B 9 which have been interpreted in terms of ϱ (2) can be satisfactorily explained on the basis of ϱ (1) . Certain inconsistencies in the use of ϱ (2) are pointed out, in addition to a property which makes it unsuitable for general use. From the form of ϱ (1) it is concluded that an anomalous peak, or “ghost”, may be associated with an isolated level sufficiently close to a threshold, and that such a ghost could be mistakenly identified as a separate level. A discussion of ghosts is given, and related to levels in some light nuclei.

The low-energy 7Be(p, γ)8B cross section from an R-matrix approach

Abstract Calculation of the low-energy 7 Be(p, γ) 8 B cross section, using R-matrix formulae with parameter values chosen to fit experimental data for the mirror reaction 7 Li(n, γ) 8 Li, leads to a ‘low’ value of the S factor, S 17 (0) = 17 ± 3 eV · b. This value agrees with those obtained from potential-model calculations, from calculations of nuclear-vertex constants, and from a Coulomb-dissociation experiment, but is less than the values obtained from cluster-model calculations.

Electron screening in reactions between light nuclei

Abstract Values of the electron-screening potential derived from previous fits to measurements of low-energy cross sections of nuclear reactions have generally been much larger than the values estimated from model calculations. Here we refit the data using somewhat different procedures and show that better agreement is possible in some cases.

The beta-decay of 8He

Abstract A many-level, many-channel R -matrix formula is used to fit the measured 8 He β-decay data, including the half-life, branching ratios and delayed neutron and triton energy spectra. Satisfactory fits are found with contributions from four 1 + levels of 8 Li, at 0.98, 3.08, 5.15 and 9.06 MeV. The 32 reduced width amplitudes are taken from a shell-model calculation, with only two requiring adjustment. Gamow-Teller matrix elements obtained from the fits are in reasonable agreement with shell-model values.

Determination of |Vud| from Fermi decays and the unitarity of the KM-mixing matrix

We re-evaluate the ƒt-values and their radiative corrections for the eight superallowed β+ decays for which very accurate data are available and also include the results for 10C, for which the measurements are approaching a similar accuracy. The main new feature is a recalculation of the nuclear-structure dependent part CNS of the O(α) radiative correction using wave functions more realistic than those used previously. This determines the value of the element Vud of the quark mixing matrix. Moreover, we recalculate Vus in the framework of a quark model. The resulting values of these two matrix elements indicate a violation of unitarity of the mixing matrix. We discuss implications of this regarding possible extensions of the minimal version of the standard model of electroweek interactions.

Cross section of the reaction 6Li(p, γ)7Be

Abstract The cross section of the reaction 6 Li(p, γ ) 7 Be has been measured using Ge(Li) γ-ray spectrometers for proton bombarding energies E p from 200 keV to 1200 keV. At E p = 800 keV, the total (p, γ) integrated cross section is found to be 3.1 ± 0.4 μ b. The cross section adopted from consideration of this and previous measurements is in good agreement with that predicted from the known thermal neutron cross section for 6 Li(n, γ ) 7 Li on the assumption that properties of mirror direct capture reactions can be well described by optical potentials that use the same parmeter values for the two reactions.

Calculated values of the nuclear-mismatch correction for superallowed 0+→0+ beta decays

Abstract Numerical calculations of the nuclear-mismatch correction δ C for superallowed 0 + →0 + beta decays are made from formulae previously developed using the formalism of R-matrix theory. The present values of δ C are considerably smaller than those obtained in previous calculations, and lead to ft -values that are not consistent with either the CVC hypothesis or unitarity of the KM mixing matrix.

On the nuclear-mismatch correction for superallowed 0+ → 0+ beta decays

Abstract A method based on the formalism of R -matrix theory is proposed for calculating the nuclear-mismatch correction for superallowed 0 + → 0 + beta decays. The resulting formula includes contributions not contained in previous calculations.

The 8He delayed triton spectrum and β-decay to unstable levels

Abstract Two previous fits to 8 He β-decay data led to significantly different values of log ft and B (GT) for the strong decay to the 1 + level of 8 Li at E x ≈ 9 MeV. It is suggested that this difference is due largely to errors in the evaluation of f for this broad level.

Levels of N13 near 7 MeV excitation (II)

Abstract Excitation functions and angular distributions of protons inelastically scattered by carbon have been measured for beam energies between 5.2 and 6.6 MeV in order to give information on the levels of N13 near 7 MeV excitation. An analysis is made of these data, together with previously measured angular distributions of the 4.43 MeV γ-rays emitted following the inelastic scattering. Good agreement with these results can be obtained by assuming contributions from a small number of interfering levels of N13. Levels at 7.16 and 7.38 MeV are found to have spins and parities 7 2 + and 5 2 − or 7 2 − , respectively, which differ from assignments previously accepted. Partial widths are obtained for these levels and for the 3 2 + level at 6.88 MeV. The 5 2 + level at 6.38 MeV gives important contributions to the inelastic scattering cross section in the region studied, and small but significant contributions can come from the 1 2 − ground state and the 3 2 − level at 3.51 MeV.