The isoscalar strength distribution in 24, 26Mg, 28Si and 40Ca obtained from inelastic alpha scattering at 120 Mev

Abstract

The inelastic α-scattering reaction at E α = 120 MeV with an energy resolution of 90–150 keV has been used to investigate isoscalar strength distributions in 24, 26Mg, 28Si and 40Ca. For 24, 26Mg and 28Si the E2 strength between E x = 14 and 27 MeV is strongly fragmented. In 40Ca the E2 strength is mainly concentrated near E x ~ 65 A 1 3 MeV, although here the onset of fragmentation can be observed. The sum rule strength for the different multipolarities was obtained by applying for each nucleus an L-dependent normalization procedure. In this way we observed in total in 24, 26Mg, 28Si and 40Ca for excitation energies up to 27 MeV an amount of (61 +8 −6), (50 +9 −8), (38 +8 −6) and (94 ± 14)%, respectively, of the isoscalar E2 energy weighted sum rule (EWSR) of which (36 +7 −5), (28 +8 −7), (24 +7 −5) and (74 ± 12)% was found between E x = 14 and 27 MeV. In addition isoscalar E0, E3 and E4 strength was observed in this excitation energy region. A detailed comparison has been made between the isoscalar quadrupole strength distribution observed in the 24, 26Mg(α, α') reaction and the E2 strength excitation function obtained from radiative α-capture measurements. In the low excitation energy region coupled channel effects have been observed, especially for the excitation of the 3 + states. Moreover, a considerable percentage of the 1ħω isoscalar dipole and octupole strength has been observed for excitations below 14 MeV.