Nuclear Physics A472 (1987) 12.5-138 North-Holland, Amsterdam THE ISOVECTOR QUADRUPOLE RESONANCE IN YTTRIUM EXCITED BY NEUTRON RADIATIVE CAPTURE R. ZORRO and I. BERGQVIST department of Physics, ~n~~rsity of Lund, Lund, Sweden S. CRONA, A. HAKANSSON, A. LIKAR”, A. LINDHQLM and L. NILSSON’ Institule of Radiation Sciences, Uppsala Universit)~, U~~sa~a, Sweden N. OLSSON Studsvik Neutron Research Laboratory, Nyksping, Sweden Received 15 April 1987 Abstract: In order to investigate the properties of the isovector giant quadrupole resonance (AT= 1, AS = 0) in the A = 90 mass region, gamma-ray spectra from the reaction *‘Y(n, Y)~“Y were recorded at several neutron energies in the energy range 12 to 27 MeV at 55”, 90” and 125”. The measured fore-aft asymmetry for the ground-state transition is very small in the low-energy region, but becomes appreciable above a neutron energy of I8 MeV. The observed asymmetry is attributed to interference between radiation from the isovector giant quadrupoie resonance and radiation of opposite parity (from the high-energy tail of the giant dipole resonance and direct El capture). The data obtained in the present work, interpreted in terms of the direct-semidirect capture model, indicate that the excitation energy of the isovector E2 resonance in yoY is 2611 MeV. The data are consistent with a resonance width of 10 i: 2 MeV and with complete exhaustion of the energy- weighted sum rule for the lower isospin component of the resonance, E NUCLEAR REACTIONS @Y(n, y), E = 12-27 MeV; measured y-spectra, asymmetry. 9oY deduced isovector quadrupole resonance, r, EWSR. Direct-semidirect model. 1. Introduction The investigation of giant multipole resonances has been one of the major subjects in nuclear physics research during the past fifteen years. Most of these studies have been concerned with isoscalar resonances, for which hadron and electron scattering have been found to be very efficient tools ‘). As far as isovector resonances are concerned, experimental studies are much more scarce (with the exception of the giant dipole resonance, the properties of which have been studied for several decades). The difficulties encountered in the study of isovector multipole resonances are connected with the relative strength of the isoscalar and isovector parts of the 1 Permanent address: Institute J. Stefan, University E. Kardelj, Jamova 39, PO Box 199, YU-61001 Ljubljana, Yugoslavia. a Address: The Svedberg Laboratory, University of Uppsala, PO Box 533, S-751 21 Uppsala, Sweden. 037%9474/87/$03.50 @ Elsevier Science Publishers B.V. (Noah-Holland Physics ~bi~sh~ng Division)