Spreading widths of isobaric analog states and the isovector giant monopole resonance

Abstract

A global analysis of the experimental spreading widths F ~ of 65 isobaric analog states (IAS) in the range A = 110 to 238 has been performed assuming isospin mixing via the Coulomb force with T<= T0-1 doorway states mediated by coupling to the (To-D-component of the giant isovector monopole resonance (IVM). Excellent agreement has been achieved over the entire range, thus establishing general smooth trends for the parameters describing the resonance. The excitation energy of the resonance, E(IVM)= Vo/A 1/3-(To+ 1)Vt/A was found to reproduce the IAS data with V o in the range 155 to 170 MeV and V~ ~ 55 MeV. These values are taken from recent theoretical estimates of Auerbach and the extreme hydrodynamical estimate of Bohr, Damgaard and Mottelson. The charge-dependent matrix elements were found factors of two to three stronger than hydrodynamic or microscopic estimates, but in almost perfect agreement with an expression based on sum rules derived by Lane and Mekjian. A strength-function approach developed by MacDonald and Birse for the damping widths of isovector monopole resonances was successfully employed. However, it became necessary to introduce an explicit dependence on nuclear deformation to describe a splitting of the IVM strength due to coupling to the fl-vibration component of the isovector quadrupole resonance (IVQ). The postulated fl-dependence describes the strong increase with neutron excess of the spreading widths for several rare-earth nuclei, and an essentially one-parameter fit to all experimental spreading widths gives x2/f ~ 1.2. It is further concluded that the density of doorway states which are responsible for the isospin mixing is much lower than the density of all underlying states of lower isospin with the same spin and parity as the IAS, and the ratio decreases exponentially with increasing excitation energy.

Measurement of isospin mixing at a finite temperature in80Zr via giant dipole resonance decay

“…The evaluation of the isospin impurity provides an important correction to the Fermi-transition rates allowing the extraction, in a nucleusindependent way, of the up-down quark-mixing matrix element of the Cabibbo-Kobayashi-Maskawa matrix [1,2]. Concerning the IAS, they are known to have a narrow spreading width ↓ related to the isospin impurities [3,4] originating from the Coulomb interaction coupling them to states of different isospin.…”

Neutron decay from the isobaric analog state inSb120populated inSnRoberts1, Ashktorab2, Becchetti3 et al. 1995Phys. Rev. CSelf Cite5020View full textAdd to dashboardCite

“…It should also be noted that the calculations of Ref. [10] describe quite well the relatively small and constant values of I'" for the entire Sn region [23], and they describe the strong increase of I~with neutron excess for many deformed rareearth nuclei. It appears, though, that some disagreement may exist in the trends observed [24] for nuclei slightly lighter than Pb and the predictions from Ref.…”

“…These theoretical estimates [10] represent global trends, and they are not expected to give good agreement for individual nuclei. Nevertheless, the agreement for the IAS in ' Sb is very good.…”

“…[10]) were obtained indirectly by determining the total widths I of IAS's from their Lorentzian line shapes and measuring the branching ratios P~f or proton decay. The escape and spreading widths become I~= P~I. "…”

Observation of the Isovector Giant Monopole Resonance via theSi28(Be10,Scott1, Zegers2, Almus3 et al. 2017Phys. Rev. Lett.120110View full textAdd to dashboardCite

“…In a microscopic description, the IVGMR is an excitation of the nucleus that is a coherent superposition of one-particle one-hole (1p-1h) excitations across a major oscillator shell (2 ω), characterized by no change in the orbital angular momentum (∆L = 0), no change of spin ∆S = 0, and a change in isospin of one unit (∆T = 1) [1,5]. It has been postulated to mediate isospin-symmetry breaking and isospin mixing [6,[11][12][13]. More generally, measurements of the properties of the IVGMR serve as a stringent test of microscopic models of nuclei at high excitation energies [14].…”