Ground state: due to non-central forces, the wave function for the J^π=0⁺ ground state of ⁴He can be a positive-parity mixture of three ¹s₀, six ³p₀, and five ⁵d₀ orthogonal states (1967Be74). The symmetric s-wave component is the dominant part of the wavefunction, with significant d-wave and almost negligible p-wave contributions. Since the d-state admixture can be inferred from measurements such as the tensor analysing powers for ²H(d,γ)⁴He, it has been the subject of much experimental and theoretical attention since the previous compilation (1973Fi04), despite confusion stemming from the fact that in some cases the results refer to only part of the full d-state probability as calculated in 1988Ca19, 1990Ch06 and 1991Ar01.

Recent variational and Green Function Monte Carlo (gfmc) calculations (1988Ca19, 1991Ca35) using realistic nucleon-nucleon potentials have been highly succesful in reproducing the ground-state properties of light nuclei. These calculations for ⁴He give d-state probabilities ranging from 15-17.5%, depending on the potential model (including 3-body forces) used, and p-wave probabilities that are much smaller (approx. 1%). Other theoretical and experimental estimates of the d-state percentage are considerably lower, but these inferences can be complicated by the presence of more than one multipole and other d-state effects. See further discussion in 1992Ti02.

Excited states: the unbound excited-state level structure presented here is based on the comprehensive, Coulomb-corrected, charge-independent R-matrix analysis discussed in 1992Ti02. This analysis takes its isospin-1 parameters from an analysis of p-³He scattering data, but with eigenenergies shifted by the internal Coulomb energy difference ΔE(C)=-0.64 MeV and the p-³h and n-³He reduced-width amplitudes scaled by the isospin Clebsch-Gordon coefficient 0.7071. The isospin-0 parameters are then varied to fit the experimental data for the reactions among the two-body channels p+³H, n+³He and d+²H, at energies corresponding to excitations in ⁴He below approximately 29 MeV. In this fit, the t=0 nucleon-trinucleon reduced width amplitudes are constrained and a small amount of internal Coulomb isospin mixing is introduced (see 1992Ti02) to reproduce the differences between the two branches of the d+d reaction. The Breit-Wigner resonance parameters at channel radii a(p-t)=a(n-³He)=4.9 fm and a(d-d)=7.0 fm are given. See further discussion in 1992Ti02.

Estimated uncertainties on the resonance parameters given for ⁴He are as follows: at excitation energies below 26 MeV, the positions are uncertain by 20 keV or less, except for the (1-, t=0) level at 24.25 MeV, which is uncertain by 150 keV. At excitation energies between 26 and 30 MeV, the uncertainties in the positions are generally less than 90 keV, with that of the (1-, t=0) level at 28.37 MeV level less than 10 keV. The widths of the levels (partial and total) are generally known to about 10%. See further discussion in 1992Ti02.

Q-value: Note: Current evaluation has used the following Q record -22.90×10³ 2120577.62 119813.85 1 1997Au07