5.11

THE ISOSPIN STRUCTURE OF THE GIANT DIPOLE RESONANCES IN_Ca ISOTOPES

L Nilsson and A Lindholm

Tandem Accelerator Laboratory, University of Uppsala,

S-751 21 Uppsala, Sweden

D M Drake

University of California, Los Alamos Scientific Laboratory,

Los Alamos, New Mexico 87545, USA

M Drosg

University of Vienna, Institut für Experimentalphysik,

A-1090 Vienna, Austria

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,γ_o)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,γ_o) and (p,γ_o) 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,Y_o)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, γ_o)⁴⁰Ca and ⁴¹K(p, γ_Q)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,γ_o)^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.

References

(1) P Paul, Proc Intern Conf on Photonuclear Reactions and Applications, Asilomar, 1973, ed B L Berman (US Atomic Energy Commission Office of Information Services, Oak Ridge, Tennessee, 1973) p 407

(2) E M Diener, J F Amann, P Paul and J D Vergados, Phys Rev C7 (1973) 705

(3) E M Diener, J F Amann and P Paul, Phys Rev C7 (1973) 695

(4) I Bergqvist, D M Drake and D K McDaniels, Nucl Phys A231 (1974) 29

S A Wender, N R Roberson, M Potokar, H R Weller and D R Tilley, Phys Rev Letters 41 (1978) 1217

A Lindholm, L Nilsson, M Ahmad, M Anwar and I Bergqvist, TLU 66/79, Tandem Laboratory Report, Uppsala 1979