THE ISOSPIN STRUCTURE OF THE GIANT DIPOLE RESONANCES IN_Ca ISOTOPES
L Nilsson and A Lindholm
Tandem Accelerator Laboratory,
D M Drake
Los Alamos, New Mexico 87545, USA
The isospin splitting of the giant dipole resonance (GDR) has been extensively studied (I). Nuclei with small neutron excess are convenient for these studies, because for such nuclei the two isospin components have about equal strength and the expected energy separation is a few MeV.
In the A=40 mass region, the gross structure of the giant dipole resonance in 42Ca has been studied (2) and compared with 40Ca (ref (3)). These studies involved proton capture in 41K and 39K, respectively, and excitation functions for the (p,γ0) reactions were recorded over the entire GDR region. The 41K(p,γo)42Ca excitation function could not be interpreted uniquely in terms of isospin slitting because the observed two components may as well be due to the K=0 and K=1 dipole oscillations in the deformed 42Ca ground state.
The ambiguities of K splitting can be eliminated by measuring the (n,γo) and (p,γo) cross sections to the same final nucleus. According to isospin selection rules only the T< isospin component is populated by neutron capture whereas both components are excited by proton capture.
In the present work differential 90° cross sections for the 40K(p,Yo)41Ca reaction have been measured over the entire giant dipole resonance region. The measurements were performed at the Los Alamos Scientific Laboratory tandem accelerator using time-of-flight techniques and an isotopically enriched 40K target. The gamma-ray spectrometer was an anti-coincidence arrangement consisting of a central and an annulus NaI scintillator. The results are compared with the data from the 39K(p, γo)40Ca and 41K(p, γQ)42Ca reactions and from neutron capture in 40Ca (ref (4)). The interpretation of the giant resonance shapes of the excitation functions is facilitated by direct-semidirect model calculations, performed with a complex particle-vibration interaction function. The energy shift between the resonances observed in %0K(p,γo)41Ca and 40Ca(n,γo)4lCa is well accounted for by isospin selection rules. The shapes of the excitation functions for proton capture in the potassium isotopes 39K, 40K and 41K are all well described by the direct-semidirect model. The agreement in magnitude, however, differs slightly In the three cases. The present work also illustrates that the magnitude of the calculated direct-semidirect cross sections depends on the optical model parameter set used in the calculations of the scattering wave functions.
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