Excitation of the isovector giant monopole resonances via the Pb-nat(He-3,tp) reaction

The Pb-nat(He-3,t) reaction and the subsequent decay of the excited nucleus by proton emission at backward angles were studied at E(He-3) = 177 MeV with the aim to separate the 2 (h) over bar omega isovector monopole strength in Bi isotopes from the continuum background. Since the continuum is largely due to quasifree processes associated with forward-peaked high-energy protons, the observed resonance-to-continuum ratio is drastically improved when coincidences between ejectile tritons and protons emitted at backward angles are required. Furthermore, the cross section for the monopole resonances excited via (He-3,t) is, unlike the cross sections for higher multipolarity excitations, strongly forward peaked. By comparing coincidence spectra at theta (t)approximate to0 degrees and at finite angles. monopole strength was found in the region -45