The 48Ti(α, α′) reaction and systematics of octupole states in the Ti isotopes

Abstract

The 48Ti(α, α′) reaction has been studied at 31 MeV with 100 keV resolution and high statistics. Spin and parity assignments have been made on the basis of the systematic shapes of the differential cross sections and by comparison with distorted wave Born approximation calculations. These assignments have been made up to an excitation energy of 6.4 MeV. The magnitudes of the cross sections have been related to electromagnetic transition rates by use of the vibrational model. Based on the assumption of pure 1 f 7 2 configurations, the lowest 4 + and the second 2 + states should be excited by two-step processes only. The differential cross sections for these states do not support this hypothesis. The most interesting feature of the results is the identification of seven 3 − states in 48Ti ranging in excitation energy from 3.36 to 6.33 MeV which vary in strength from 1 to 3 Weisskopf units. Three 3 − states of comparable strength have been previously located in 46Ti and five 3 − states in 50Ti. The strongest of these states has 3.5 Weisskopf units in 46Ti and 7 Weisskopf units in 50Ti. The weakness of the octupole states in this region compared to the 26 Weisskopf units found for the lowest 3 − states in 40Ca is surprising and may be difficult to reconcile with our understanding of current microscopic descriptions of this mode.