Isovector giant E 2 resonance and overtone of the isovector giant E 1 resonance in photonucleon reactions
- PDF / 752,173 Bytes
- 14 Pages / 612 x 792 pts (letter) Page_size
- 37 Downloads / 158 Views
CLEI Theory
Isovector Giant E 2 Resonance and Overtone of the Isovector Giant E 1 Resonance in Photonucleon Reactions B. S. Ishkhanov and V. N. Orlin* Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119991 Russia Received November 16, 2011
Abstract—The effect of the isovector giant quadrupole resonance and an overtone of the isovector giant dipole resonance on the photodisintegration of medium-mass and heavy nuclei in the energy range of 20– 40 MeV is studied on the basis of a combined model of photonucleon reactions. Basic features of these resonances are evaluated within a semimicroscopic approach. Their deformation spitting is described on the basis of the model of polarization vibrations of a two-component nuclear liquid. DOI: 10.1134/S106377881212006X
1. INTRODUCTION
reaction on a natural mixture of lead and cadmium isotopes. It should be noted, however, that direct measurements of IVGQR features have not yet been performed. The ambiguity in the results of sorting the yields of different-multiplicity photoneutrons is not the least significant reason for this. The results obtained by measuring, by the induced-activity method, the yields of photonucleon reactions for heavy nuclei [3–5] also indicate that it is necessary to take into account high-energy giant resonances in the region around Eγ ∼ 20−40 MeV. In particular, the value obtained in those studies for the cross section for the (γ, p) reaction at Eγ ∼ 25 MeV proved to be overly large to be explained by the descending branch of IVGDR at high energies alone [5]. The probability for electromagnetic emission and absorption decreases fast with increasing angular momentum J. Moreover, electric absorption is approximately two orders of magnitude more probable than magnetic absorption at a fixed value of J [6]. Therefore, only E1, E2, and M 1 processes (an absorbed photon cannot have an angular momentum of J = 0 since its spin is equal to unity) may play a a significant role in photonuclear reactions. The set of giant resonances that affect the behavior of photonucleon reactions on medium-mass and heavy nuclei in the energy range of Eγ ∼ 20−40 MeV (where the contribution of IVGDR gradually dies out and where the quasideuteron mechanism of photodisintegration only begins to gain momentum) may be even narrower. Indeed, the isoscalar giant quadrupole resonance and the giant magnetic resonance lie much lower that this energy range; moreover, one can disregard all overtones of basic resonances, with the exception of secondary IVGDR (ISGDR2), since
In recent years, much attention has been given to experimental and theoretical investigations of various high-energy giant resonances in medium-mass and heavy nuclei at excitation energies in the region of E > 20 MeV. The majority of these resonances are overtones of basic giant resonances lying at lower energies. An overtone (also known as a secondary resonance) has the same quantum numbers—a spin– parity J π , an orbital angular momentum L, a spin S, and an isospin T and its projection Tz onto the z
Data Loading...
