Gamow-Teller and dipole strength in the 208Pb(n,p) reaction

Earlier data on the ²⁰⁸Pb(n,p)²⁰⁸Ti reaction at 198 MeV has been analysed using the multipole decomposition technique, and the contributions to the spectra from L = 0 and 1 transitions determined. The L = 0 cross section has been found to be smaller than values given by the calculations of MacFarlane [3] for Gamow-Teller transitions, using three different forms for the population of single particle states about the Fermi energy. Comparison is made with GT-strength values obtained from a more recent mean field calculation of these populations, and with an older nuclear structure calculation of properties of the lead nuclei. The L = 1 strength distribution is also reported.     

Measurement of the exclusive 3He(e,e'p) reaction below the quasielastic peak

New, high-precision measurements of the 3He(e,e(')p) reaction using the A1 Collaboration spectrometers at the Mainz microtron MAMI are presented. These were performed in antiparallel kinematics at energy transfers below the quasielastic peak, and at a central momentum transfer of 685 MeV/c. Cross sections and distorted momentum distributions were extracted and compared to theoretical predictions and existing data. The longitudinal and transverse behavior of the cross section was also studied. Sizable differences in the cross-section behavior from theoretical predictions based on the plane wave impulse approximation were observed in both the two- and three-body breakup channels. Full Faddeev-type calculations account for some of the observed excess cross-section, but significant differences remain.     

The 208Pb(α, 3He)209Pb reaction at 58 MeV

The ²⁰⁸Pb(α, ³He)²⁰⁹Pb reaction at 58 MeV has been used to search for high-spin states in ²⁰⁹Pb. Only three levels are excited with appreciable intensity: the ground state $\text{(2}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}\text{)}$ and levels at $\text{0.781 (}\text{li}{}_{\mathrm{<mtext>11</mtext><mtext>2</mtext>}}\text{)}\text{and}\text{1.426}\text{MeV}\text{(}\text{lj}{}_{\mathrm{<mtext>15</mtext><mtext>2</mtext>}}\text{)}$. The angular distributions for these levels have been measured and analyzed using standard DWBA calculations to obtain spectroscopic strengths. The ²⁰⁸Pb(α, α) elastic scattering was measured and optical parameters deduced from the data. A normalization value N = 50 yields spectroscopic values which are close to the values measured in the (d, p) reaction. The (α, ³He) reaction should easily pick out any appreciable components of the $\text{j}{}_{\mathrm{<mtext>15</mtext><mtext>2</mtext>}}$ shell model state, which weak-coupling calculations predict should be fragmented. However only three weak transitions previously seen in a (d, p) experiment are observed.     

One- and two-step processes in natural- and unnatural-parity 208Pb(p,t)206Pb reaction at Ep = 22 MeV

Reaction mechanism and nuclear-structure dependence of the natural- and unnatural-parity ²⁰⁸Pb(p, t)²⁰⁶Pb(2⁺, 3⁺, 4⁺, 5⁻ and 7⁻) reaction are studied by using a 22 MeV polarized proton beam. Angular distribution of the analyzing power for the second 3⁺(3₂⁺) transition has an opposite sign to that for the 3₁⁺ one and the difference is explained in terms of the j-dependence in the sequential two-step (p, d)(d, t) process. The observed analyzing powers and cross sections for the two 3⁺ transitions are reproduced by the finite-range first- and second-order DWBA calculation. Two-step analysis is necessary to explain the variety of angular distributions of the analyzing powers for the natural-parity transitions. Microscopic analysis in terms of the one- and two-step (p, t) calculation is made for the transitions to the 2_1–5⁺, 4_1–3⁺, 5_{1, 2}⁻ and 7₁⁻ states in ²⁰⁶Pb. The calculation reproduces the absolute values of the observed cross sections within a factor of 2.     

Interpretation of the processes 3He(e,e'p)2H and 3He(e,e'p)(pn) at high missing momenta

Using realistic three-body wave functions corresponding to the AV18 interaction, it is shown that the effects of the final state interaction in the exclusive processes 3He(e,e'p)2H and 3He(e,e'p)(pn), can be successfully treated in terms of a generalized eikonal approximation based upon the direct calculation of the Feynman diagrams describing the rescattering of the struck nucleon. The relevant role played by the double rescattering contribution at high values of the missing momentum is illustrated.     

Precise extraction of the induced polarization in the 4He(e,e'p)3H reaction

We measured with unprecedented precision the induced polarization P(y) in (4)He(e,e'p)(3)H at Q(2)=0.8 and 1.3 (GeV/c)(2). The induced polarization is indicative of reaction-mechanism effects beyond the impulse approximation. Our results are in agreement with a relativistic distorted-wave impulse approximation calculation but are overestimated by a calculation with strong charge-exchange effects. Our data are used to constrain the strength of the spin-independent charge-exchange term in the latter calculation.     

Measurement of the 3He(e,e'p)pn reaction at high missing energies and momenta

Results of the Jefferson Lab Hall A quasielastic 3He(e,e'p)pn measurements are presented. These measurements were performed at fixed transferred momentum and energy, q=1502 MeV/c and omega=840 MeV, respectively, for missing momenta p(m) up to 1 GeV/c and missing energies in the continuum region, up to pion threshold; this kinematic coverage is much more extensive than that of any previous experiment. The cross section data are presented along with the effective momentum density distribution and compared to theoretical models.     

Contribution of the Δ(1232) to the (e, e'p) reaction along with Coulomb distortion effects

The Δ-resonance contribution has been included in the (e, e'p) reaction along with Coulomb distortion effects. We treat the resonance via a non-relativistic Δ current operator and use a Dirac Hartree single-particle model for the ground-state single-particle wave function and a relativistic optical model for the knocked-out proton wave function. It is assumed that the π-meson created by the virtual photon is absorbed in the target nucleus following the production of a Δ-resonance. Our DWBA calculation shows that the Δ-resonance contribution to the (e, e'p) reaction cross-section is 10-15% for an energy of 250 MeV transfered to the proton knocked out of the s-shell of ⁴⁰Ca, in the parallel and perpendicular kinematics. Received: 21 July 2001 / Accepted: 21 March 2002     

Final state three-body Coulomb effects in the 208Pb(8B,7Bep)208Pb Coulomb breakup reaction

We present results of the first calculation of the double differential cross section for the 208Pb(8B,(7)Bep)208Pb Coulomb breakup reaction which treats the postdecay acceleration of the ejectiles within a genuine three-body approach. From this we conclude that, in order to minimize postdecay Coulomb acceleration effects, experiments should be performed at as small as possible scattering angles, not too low 7Be-p relative energies, and high incident energy.