High-resolution (e, e') study of isovector M1 and M2 transitions in the oxygen isotopes: (III). 18O

Abstract

The excitation energy region in 18O from about E x = 11–27 MeV has been studied with low-momentum transfer, but high-resolution inelastic electron scattering. Two sharp lines are prominent in the spectra, corresponding to the excitation of T = 2 levels at 16.399 ± 0.005 MeV and 18.871 ± 0.005 MeV of J π = 2 − and 1 +, respectively. In contradiction to theoretical predictions no more strong M2 transitions could be found. Broad peaks were observed at 18.5, 19.7, 20.2, 22.5 and 23.8 MeV, the latter two are due to the giant dipole resonance as known from photonuclear reactions. The spectra show in addition considerable fine structure and the application of a cross correlation function technique for its analysis resulted in the location of twelve more low multipolarity weak transitions in the excitation energy range between 16 and 19 MeV. Tentative J π assignments are given for these levels. The spectra of isospin T = 2 states of A = 18 nuclei are discussed in view of the existing experimental and theoretical work. Finally, the pattern of the isovector M1 and M2 strength distributions of all the three oxygen isotopes 16,17,18O is discussed.