Gamow-Teller strength in the continuum and quenching problem

Abstract

Differential cross sections at θ lab = 0.0°–12.3° and a complete set of polarization transfer observables at θ lab = 0.0° are measured for the 90Zr( p, n) reaction at T p = 295 MeV. A multipole decomposition (MD) technique is applied to the cross sections to extract partial cross sections σ L with L=0, 1, 2, and 3 contributions. A significant amount of σ L=0 is found in the highly excited continuum region. After subtracting the estimated iso-vector spin-monopole strength from σ L=0 , the Gamow-Teller (GT) strength B(GT) is deduced to be S β − = Σ B(GT) = 28 ± 1.6 ± 5.4. The total spin transfer Σ(0°) which is independent of the nuclear reaction mechanisms such as the effective nucleon-nucleon interaction or distortions is derived from the complete set of polarization transfer observables. It is close to unity up to 50 MeV excitation indicating the strong spin-flip character of the continuum. By using the relation between the polarization transfer observables the fractions of natural-parity and unnatural-parity transitions are also obtained and the dominance of unnatural-parity transitions in the continuum is found. These results are consistent with those derived from the MD analysis. The S β − − S β + value is 27.1 and this is 90% of Ikeda's sum rule value of 3( N - Z)=30. Consequently it is concluded that the quenching of the GT strength is small and the quenching mechanism due to the ΔN − admixture seems to play a minor role.