Giant resonances and bound collective states observed in the scattering of 92.5-MeV electrons from the closed-neutron-shell nucleusY89between excitation energies from 2.0 to 55 MeV

Abstract

92.5-MeV electrons were used to study in /sup 89/Y the excitation range between 2 and 55 MeV. Above neutron threshold, broad electric resonances have been seen at 14.0 (63A/sup -1/3/) MeV (E2, ..delta..T = 0) and 28 (125A/sup -1/3/) MeV (E2, ..delta..T = 1). The total width of the isoscalar E2 resonance is (4.5 +- 0.4) MeV and its strength (56 +- 6) % energy-weighted sum rule (E2, ..delta..T = 0). For the isovector E2 resonance only a minimal value of 7 MeV for the width can be given which is connected with (48 +- 5) % of the isovector sum rule. The strength of the E1 resonance (T/sub less-than/(104 +- 10) % of the Thomas-Reiche-Kuhn sum rule) agrees well with (..gamma..,n) measurements, thus giving a check for the accuracy of the evaluating methods employed. A structure around 19--20 MeV, believed to be the T/sub greater-than/part of the giant dipole resonance, carries (8 +- 3) % of the E1 sum rule. In addition to these generally well known states, clustering of E3 strength, (13 +- 2) % energy-weighted sum rule (E3, ..delta..T = 0), was found at 6.75 and 8.05 MeV; the enveloping line shape of these clusters was bestmore » described by a Breit-Wigner term. Other concentrations of strength include E3 at 2.6 MeV, (15 +- 3) % energy-weighted sum rule (E3, ..delta..T = 0), E2 at 4.0 MeV, (11 +- 3) % energy-weighted sum rule (E2, deltaT = 0), and E3 strength at 13.5 MeV. No resonance other than the E1 was found around 17 MeV, thus ruling out in /sup 89/Y the existence of a monopole state with 100% sum rule proposed for /sup 90/Zr from (..cap alpha..,..cap alpha..') and (e,e') measurements. In contrast to heavy nuclei, no resonant 3h..omega.. E3 strength could be located.« less