Giant resonance effects in the angular distributions of inelastic proton scattering on 24Mg

Abstract

Differential cross sections of proton scattering were measured for the six lowest levels of 24Mg at E p = 22.50, 23.25, 23.50, 24.00, 24.25, 24.75, 25.00, 25.25, 25.50, 25.75, 27.00 and 28.50 MeV in the angular range between 20° and 160°, generally in 5° steps. The elastic scattering has been analysed by means of the optical model. An anomalous variation of the OM parameters was obtained around 25.50 MeV. The method of the coupled channels was applied for the analysis of the inelastic data. These calculations give a fairly good description of the scattering results for the J π = 0 1 +, 2 1 +, 4 1 + and 2 2 + levels. However, the differences between the calculated and observed cross sections for the J π = 3 2 + and 4 2 + levels amount to more than one order of magnitude. It is suggested that the very strong transition to these channels can be accounted for by assuming that multipole giant resonances play an important role as intermediate states of a “two-step” inelastic process. The non-normal-parity state ( Jπ = 3 2 +) at E x = 5.23 MeV is particularly sensitive to such an excitation of giant resonances since its “one-step” excitation may happen only by spin-flip. Using the excited-core model in which the resonating exchange amplitude is the dominating contribution a good fit was obtained for the 0 + → 3 + angular distributions. Angular distributions of the 0 + → 3 + inelastic transition were analyzed as a function of incident energy by means of this model. A rather strong El contribution and evidence for quadrupole giant resonance strength were found. The E2 strength is concentrated mainly around E p = 24 MeV corresponding to the upper part of the GQR.