Microscopic interpretation of the ( t , 3 He) reaction at 130 MeV on 90 Zr and isovector monopole strength
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NUCLEI Theory
Microscopic Interpretation of the (tt , 3 He) Reaction at 130 MeV on and Isovector Monopole Strength*
90
Zr
J. Guillot** Institut de Physique Nucleaire, ´ IN2P3-CNRS, Orsay, France Received October 31, 2006
Abstract—The 130-MeV primary tritium beam of the AGOR facility with an intensity of up to 108 pps and the Big Bite Spectrometer experimental setup have been used to study the (t, 3 He) reaction between 0◦ and 5◦ lab angles on 12 C and 90 Zr targets. The standard ray-tracing procedure has allowed us to obtain excitation-energy spectra up to 30 MeV in six angular bins for each residual nucleus, with an average energy resolution of 350 keV. We have used the DWBA reaction mechanism model to reproduce those spectra and their angular distributions. In this approximation, the form factor was treated as a folding of an effective projectile–nucleon interaction with a transition density. The effective projectile–nucleon interaction has been adjusted to reproduce the 0◦ cross section of the 1+ ground state of 12 B populated in the 12 C(t, 3 He) reaction. We have employed RPA wave functions of excited states to construct the form factors. This DWBA + RPA analysis is used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in the 90 Y nucleus. In this talk, we give some details on this analysis. We show that there are important contributions of L = 2 transitions in the observed cross sections for the 1+ final states that explain the previous difficulties in clearly identifying the monopole strength distributions. We then have a better indication of where the L = 0 part is located with this reaction and its microscopic analysis. PACS numbers: 24.10.Eq, 24.30.Cz, 25.55.Kr, 27.20.+n, 27.60.+j DOI: 10.1134/S1063778807080121
1. INTRODUCTION From a microscopic point of view, a giant resonance (GR) is a coherent superposition of oneparticle–one-hole (1p1h) hexcitations resulting from the operation on the ground state of a nucleus with a one-body operator. Among nuclear reactions which could probe such resonances, the most commonly used are the inelastic scattering and the chargeexchange reactions, which both lead to 1p1h excitations. Since the pioneering works, numerous GRs have been clearly observed. In the inelastic scattering channel, typical results have recently been obtained in the (α, α ) reaction at 240 MeV [1]. This reaction is particularly well suited for a multipole decomposition analysis owing to the fact that the zero spin of the α particle has the consequence that the spin and parity of the observed final states starting from a pair–pair target nucleus can be directly related to the L angular momentum transfer. In the charge-exchange reaction cases, when the spins of the projectile and ejectile are ∗ **
The text was submitted by the author in English. E-mail: [email protected]
different from zero, this is more complicated and spinflip as well as non-spin-flip transitions are mixed. While the observation of 0 ω L = 0 GRs in the τ− channel is eas
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