%'e measured the energies, widths, and cross sections of the isovector monopole and dipole resonances in various nuclei between Ca and osPb with the reactions (~-, m ). Both resonances exhaust approximately the same substantial fraction of the cross section calculated in a randomphase-approximationdistorted-wave-impulse-approximation model. The excitation energies and widths of the monopole and dipole are in good agreement with random-phase-approximation calculations and for the dipole they are also in agreement with other data. No isovector quadrupole resonance was observed, and the upper limits for the cross sections for the light elements are well below the sum rule strength for the isovector monopole and giant dipole resonance.

No abstract

Observation of the T +1 component of the isovector monopole and the giant dipole resonances in the reactions 90 Zr and 120 Sn(/r~,7r°) at T^-=165 MeV is reported. The isobaric analog state and T -1 component of the giant dipole resonance in the reaction 120 Sn(/r + , 7T°) were also observed. Excitation energies and widths of the monopole are given.PACS numbers: 24.30.Cz, 24.30.Eb, 25.80.Fm Macroscopic 1 as well as microscopic 2 theories of nuclear structure hypothesize the existence of a collective isovector monopole (IVM) excitation of the nucleus. The possibility of experimentally observing the IVM was discussed as early as 1958 by Danos, 3 who estimated the electroexcitation cross section. Isoscalar collective excitations with angular momentum / = 0, 2, and 3 have been observed and studied in hadron and electron scattering while the isovector dipole or giant dipole resonance (GDR) is a prominent feature of the scattering of electrons and photons from nuclei. Evidence has been given for the observation of the isovector quadrupole resonance. 4 However, at present there is no clear experimental evidence for the IVM. 5 Recently, arguments have been given that the (7r~, 77°) reaction at energies near the (3, 3) resonance might detect the IVM. 6 These arguments are supported by the previously reported observation 7 of the GDR in the (77*, TT°) reactions on 40 Ca. Pion charge-exchange excitation of the IVM is expected to have the following qualitative properties: (1) a zero-degree cross section of a few hundred microbarns per steradian; (2) a forward-peaked angular distribution; and (3) an excitation energy of approximately 170/A l/s MeV in the parent nucleus.Here we report the observation of the IVM in the (ir~9 TT°) reaction at pion kinetic energy 165 MeV on 90 Zr and 120 Sn. Results for 120 Sn(77 + , TT°) are also discussed. The experiments were carried out at the low-energy pion channel at the Clinton P. Anderson Meson Physics Facitity (LAMPF). The LAMPF TT° spectrometer 8 was used to measure the direction and energy of the outgoing 77°. Data were taken at spectrometer angles of 0° and 20° and sorted into several narrower ranges of scattering angle. The spectrometer acceptance and line shape were measured with the reaction n~p -n°n at 165 MeV. The ratio of calculated to measured acceptance was found to be independent of scattering angle to within the 4% statistical errors of the measurement. Figure 1 shows the double-differential cross sections for the reactions 120 Sn(7T + ,7r°) and 120 Sn(7r",7r°) as functions of ir° energy for scattering angles of 4.5°, 6.8% and 11,0°. The isobaric analog state (IAS) is clearly visible in the (fl" + , 77°) data at a 77° energy of 157 MeV. The cross section for excitation of this Z = 0 state is seen to decrease as the scattering angle increases. The IVM is seen in the (77", 77 0 ) data at a 77 0 energy of 150 MeV. The angular dependence is similar to that of the IAS as expected.To perform a quantitative analysis we make the assumption that the double-differential cross secti...

No abstract

No abstract