C. Djalali

201 MeV proton excitation of giant resonances in 208Pb: Macroscopic and microscopic analysis

Abstract The region of the giant resonances in 208 Pb has been investigated by inelastic scattering of 201 MeV protons. To test the analysis, angular distributions were measured for the low-lying 3 − , 5 − , 2 + and 4 + collective states. The giant isoscalar quadrupole resonance (ISGQR) is split into two structures, one at 9.0 MeV with a full width at half-maximum Γ = 1.0 MeV, the other one at 10.6 MeV ( Γ = 2.0 MeV), with fine structures at 8.9, 9.3, 10.1, 10.6 and 11 MeV. A macroscopic analysis using the distorted-wave Born approximation (DWBA) leads for the low-lying collective levels, as well as for the ISGQR, to transition probabilities too small by a factor of two, compared with those obtained in other reactions. Microscopic analysis using the distorted-wave impulse approximation (DWIA), with three different sets of random phase approximation (RPA) transition densities, is in very good agreement with the data. At forward angles, in the 12 to 16 MeV excitation energy region, a strong resonance at 13.5 MeV ( Γ = 3.6 MeV) is accounted for by the Coulomb excitation of the isovector giant dipole resonance (IVGDR); at larger angles the results are compatible with the excitation of the isoscalar monopole resonance (ISGMR) located at 13.9 MeV ( Γ = 2.6 MeV). A resonance located at 21.5 MeV ( Γ = 5.7 MeV) appears as the superposition of an isovector quadrupole resonance (IVGQR) excited by Coulomb interaction and a resonance of multipolarity L = 1 ΔT = 0 (ISGDR “squeezing mode”).

Systematics of the excitation of M1 resonances in medium heavy nuclei by 200 MeV proton inelastic scattering

Abstract In a series of seventeen nuclei ranging from 51 V to 14 Ce, broad resonance structures are observed at energies between 8 and 10 MeV, nearly mass independent. These resonances have very forward peaked angular distributions which imply that they are populated by an angular momentum transfer of zero. This together with the observed excitation energies suggests an M1 character for these resonances. In 51 V, 58 Ni, 60 Ni, 62 Ni, a sharp peak located at an excitation energy above the threshold for neutron emission is interpreted as a part of the T 0 + 1 component of the M1 resonance. Cross sections are given for all the M1 resonances. For 58 Ni, 90 Zr, 92 Mo, 120 Sn and 140 Ce, an “attenuation” factor for the cross sections is extracted in a DWIA calculation assuming simple shell-model structures for these resonances.

On the nature of low-lying collective 1+ states in the heavy deformed nuclei 154Sm, 156Gd and 164Dy and in the f72 shell nucleus 46Ti

Abstract High resolution inelastic proton scattering at 201 MeV on 154Sm, 156Gd and 164Dy is used to clarify the nature of low-lying magnetic dipole states previously observed in inelastic electron scattering. An upper limit on the spin contribution to these collective magnetic dipole excitations is presented. The roles of spin and convection currents are also studied by inelastic proton and electron scattering to the low-lying 1+ state at 4.319 MeV in the f 7 2 - shell nucleus 46Ti. A substantial orbital contribution is observed in all cases.

POLDER: a tensor polarimeter for intermediate energy deuterons

A new deuteron tensor polarimeter (POLDER), based upon the charge exchange reaction 1H(→d, 2p)n, has been constructed and calibrated in the energy range 175–500 MeV for all polarisation signals. The large figures of merit obtained make POLDER a unique instrument in this energy domain. The operation of this polarimeter has been checked in practice in a test experiment on 40Ca(→d, →d′)40Ca elastic scattering performed at Ed = 380 MeV.

Observation of l=0, spin-flip transitions in 48Ca☆

Abstract Observations down to 2° in the 48 Ca(p,p′) reaction at 201 MeV reveal 0 + →1 + strength with certainly only at the previously known excitation of 10.22 MeV. Compared to DWBA calculations, the strength of this state is only ∼ 30%.

Spin-flip transitions in 46Ti, 48Ti and 50Cr excited by inelastic proton scattering

Abstract orward-angle cross sections for 1 + states have been measured in the non-closed-shell nuclei 46 Ti, 48 Ti, 50 Cr by 201 MeV proton inelastic scattering. The total measured 1 + strength is compared with microscopic distorted-wave impulse approximation calculations using large-scale shell-model wave functions. The quenching for the 1 + strength ranges from 0.3 to 0.5. For the low-energy isovector 1 + states the ratio of the orbital to the spin excitation is extracted.

The nuclear spin response to intermediate energy protons

Abstract Measurements of the spin-flip probability S nn for inclusive inelastic proton scattering around 300 MeV from nuclei between 12 C and 90 Zr show that an enhanced spin response near 40 MeV excitation at q ∼ 100 MeV/ c is a general feature of nuclear structure. Data for 40 Ca at 800 MeV confirm that the enhancement is not a peculiarity of 300 MeV scattering. In addition, measurements in 44 Ca up to 75 MeV show that the enhancement cannot be attributed solely to a relatively narrow resonance. Continuum RPA calculations suggest that the enhancement is due to the exhaustion of most S = 0 strength at lower energy and a shift of S = 1 strength to higher energy.

Excitation of 1+ AND 2− states in the neon isotopes

Abstract High resolution inelastic proton scattering is used to study the excitation of 1 + states in 20 Ne and 22 Ne. By comparing these results with those from gamma-ray fluorescence measurements, the strong role of the orbital contribution in the electromagnetic transitions, predicted by the theory, is confirmed. Shell model predictions of the 1 + strength are compared to the measured strength in 20 Ne and 22 Ne. The (p, p′) excitation of 2 − states in 20 Ne is compared to (π − , γ) and (e, e′) results.

Calibration of the polarimeter POMME with polarized deuterons at 1.8 GeV

Abstract We report here the results of the calibration of the polarimeter POMME for vector polarized deuterons at an energy of 1.8 GeV. The results show that inclusive deuteron-carbon scattering has substantial vector analyzing power even at this high energy. The results obtained on two analyzers, carbon, which is generally used and a lighter material, paraffin, are found to be similar.

Medium modifications of light vector mesons in photoproduction reactions at JLab

Theoretical calculations predict the modification of properties of vector mesons, such as a shift in their masses and/or broadening of their widths in dense nuclear matter. These effects can be related to partial restoration of chiral symmetry at high density or temperature. Photoproduction of vector mesons off nuclei were performed at Jefferson Lab using the CEBAF Large Acceptance Spectrometer. The data were taken with a beam of tagged photons with energies up to 4 GeV on various nuclear targets. The properties of the ρ vector mesons were investigated via their rare leptonic decay to e+e−. This decay channel is preferred over hadronic modes in order to eliminate final state interactions in the nuclear matter. The combinatorial background in the mass spectrum was removed by a self-normalizing mixed-event technique. The ρ meson mass distributions were extracted for each of the targets. Statistically significant results regarding medium modification of the ρ meson in the nuclear medium rule out large medium effects. Transparency studies of the ω and vector mesons allow a determination of their widths in the medium.