Dynamical $R$-matrix methodology is used to determine the properties of the recently proposed $J^{\pi }=2^{+}$, $\Gamma =3.5\mathrm{}$ MeV, $E_x\mathrm{}\left(\mathrm{res}\right)\mathrm{}=40.2\mathrm{}$ MeV resonance in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$. The calculations lead to a $J^{\pi }=2^{+}$ state with the following properties: $\Gamma =6.0\mathrm{}$ MeV, $E_x\mathrm{}\left(\mathrm{res}\right)\mathrm{}=40.3\mathrm{}$ MeV, and $T=0\mathrm{}$. The calculations indicate the importance of $4\hslash \omega$ components in the proper description of the 40 MeV state.

[NUCLEAR STRUCTURE Calculated via dynamical $R$-matrix methodology the following properties for the proposed 40 MeV resonance in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$: $J^{\pi }=2^{+}$, $T=0\mathrm{}$, $\Gamma =6.0\mathrm{}$ MeV, and $E_x\mathrm{}\left(\mathrm{res}\right)\mathrm{}=40.3\mathrm{}$ MeV.]

Exact shell model calculations for nuclei consisting of three nonidentical particles outside the ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ closed shell core have been performed using a basis that contains correlated pairs. Two kinds of effective interactions are tested and the results are compared with the experiment. The possibility of high spin isomeric states is suggested for nuclei studied.

[NUCLEAR STRUCTURE ${}_{}{}^{205}{}_{}{}^{}\mathrm{Hg}$, ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$, ${}_{}{}^{211}{}_{}{}^{}\mathrm{Po}$, ${}_{}{}^{211}{}_{}{}^{}\mathrm{Bi}$; calculated levels $J$, $\pi$ and spectroscopic factors, Kuo-Herling and Kim-Rasmussen interactions.]

The validity of including only the lowest order nuclear field theory diagrams is investigated in the case of four identical nucleons moving in any $j$ shell and interacting via arbitrary two body forces. It is shown that large discrepancies exist between the predicted and the exact (i.e., summing infinite order diagrams) results. In particular, incorrect normalizations for states are predicted, thereby rendering the usual criteria for removing the spurious states suspect. The treatment of diagonalization within the Rayleigh-Schrödinger perturbation scheme is also investigated in detail for this general example. Here again one finds disturbing consequences.

[NUCLEAR STRUCTURE Nuclear field theory, lowest order diagrams, higher order diagrams, single-$j$ shell, bosons, diagonalization treatment.]

The Gross approximation to the Bethe-Salpeter equation is investigated using a one-boson-exchange model for the nucleon-nucleon interaction. In particular, it is shown that the inclusion of corrections from the two-pion direct-box diagram to the driving force in general does not yield phase shifts closer to those obtained from the Bethe-Salpeter equation. Moreover, the cross-box contributions lead in the quasipotential approach to a non-Hermitian interaction at all energies. The electromagnetic form factors of the deuteron are calculated for various sets of potential parameters which give reasonable scattering parameters. A detailed analysis is presented of the relativistic and mesonic exchange current corrections to the nonrelativistic limit. As a result it is shown that the usual treatment of mesonic exchange currents for both pseudovector and pseudoscalar pion-nucleon interaction is not correct since other non-negligible contributions should be taken into account.

[NUCLEAR STRUCTURE $\mathrm{NN}$ system, quasipotential equations compared with Bethe-Salpeter equations. Effect of one-loop diagrams. Electromagnetic form factors. Comment on meson-exchange-current calculations.]

We report the first observation of the rare pion capture mode in beryllium consisting in the emission of an electron-position pair. Rate, angular distribution, and energy-spectrum are in quantitative agreement with the ones expected assuming internal pair-creation from single radiative capture.

[NUCLEAR REACTIONS (${\pi }^{-},ee$) on beryllium for stopped pions. Rate, angular correlation, energy spectra measured.]

The (${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$) reaction on even Ni and Zn isotopes has been investigated at 72 MeV bombarding energy. The mass excesses of ${}_{}{}^{62}{}_{}{}^{}\mathrm{Fe}$ and ${}_{}{}^{68}{}_{}{}^{}\mathrm{Ni}$ are determined to be -58.84±0.04 and -63.55±0.04 MeV, respectively. Previously unknown excited states are found. Spectroscopic factors for transitions to ground states and to the first excited states are shown to increase with the neutron number.

[NUCLEAR REACTIONS ${}_{}{}^{58,60,62,64}{}_{}{}^{}\mathrm{Ni}$(${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$) and ${}_{}{}^{64,66,68,70}{}_{}{}^{}\mathrm{Zn}$(${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$), $E=72\mathrm{}$ MeV; measured $\sigma (E,\theta )$; deduced binding energies of ${}_{}{}^{62}{}_{}{}^{}\mathrm{Fe}$ and ${}_{}{}^{68}{}_{}{}^{}\mathrm{Ni}$ and excited states energies. DWBA analysis, deduced spectroscopic factors.]

The energy spectra of fragments produced in the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ + ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ reaction at $E_{\mathrm{lab}}=400\mathrm{}$ MeV are found to exhibit a fully relaxed component clearly separated from the quasi-elastic component, the latter being characterized by a mean velocity near to the beam velocity. The consequences for the assignment of the deep-inelastic component in similar systems are discussed.

[NUCLEAR REACTIONS ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}({}_{}{}^{20}{}_{}{}^{}\mathrm{Ne},X)$, $X={}_{}{}^{14}{}_{}{}^{}\mathrm{N},{}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, measured $\sigma (E,\theta )$; observed fully relaxed and quasi-elastic component; natural target.]

The contribution of proton-neutron pairs in the singlet state to the cross section of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({}_{}{}^{16}{}_{}{}^{}\mathrm{O},pn){}_{}{}^{26}{}_{}{}^{}\mathrm{Al}$ at 30 MeV bombarding energy, which was measured by $\mathrm{pn}$-coincidence techniques, was found negligible.

[NUCLEAR REACTIONS ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}({}_{}{}^{16}{}_{}{}^{}\mathrm{O},pn){}_{}{}^{26}{}_{}{}^{}\mathrm{Al}$, $E=30\mathrm{}$ MeV; measured $\mathrm{pn}$ coincidences, $E_p$; deduced% of unbound deuteron emission; Ge(Li) detectors, NE 213 scintillation detectors, Si surface barrier detectors.]

Cross section and vector analyzing power data for the 2.217 MeV state in ${}_{}{}^{37}{}_{}{}^{}\mathrm{Ar}$ have been measured via the ($d,p$) reaction at an incident deuteron energy of 11 MeV. Distorted wave Born approximation analysis of the data establishes the spin parity of this relatively weak state as 7/$2^{+}$. The data are fairly well reproduced by the calculations indicating that the one-step process plays a dominant role in populating the state in the ($d,p$) reaction. The spectroscopic factor for the state is found to be 0.04±0.01. Model calculations indicate that two-step and compound nuclear processes have negligible effects on the conclusions.

[NUCLEAR REACTIONS ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ar}(\overrightarrow{d},p)$, $E=11.0\mathrm{}$ MeV; measured $\sigma \left(\theta \right)$, $i{T}_{11}\left(\theta \right)$. Deduced $J$, $\pi$, $S$. DWBA analysis.]

The 600 MeV proton-induced fission process of Ce and Yb has been investigated with the double-kinetic-energy method. Mass distributions of these and other medium-heavy and light nuclei are compared to predictions of macroscopic models. No sign of the Businaro-Gallone point, below which the saddle point becomes unstable towards mass asymmetric deformations, is observed. For Ce a stable asymmetric mass distribution is found which may be explained in terms of shell structure effects.

[NUCLEAR REACTIONS, FISSION $\mathrm{Ce}\mathrm{}(p,f)\mathrm{}$, $\mathrm{Yb}\mathrm{}(p,f)\mathrm{}$, $E=600\mathrm{}$ MeV; measured angular correlations, fragment-fragment coin.; deduced TKE, mass distribution.]

A Regge-pole analysis of the backward rise of the angular distribution observed at low incident energy in transfer reaction induced by heavy ions is proposed for a particular example.

[NUCLEAR REACTION Regge-pole analysis of $\alpha$ transfer reaction induced by heavy ions.]

When a basis space for diagonalizing a multiparticle Hamiltonian is dynamically determined from a set of variational calculations, it is then possible to treat the spurious center of mass motion problem at a level consistent with the treatment of the full solution of the problem. We propose a practical systematic method based on double diagonalization within a variationally determined subspace. The proposed method converges to an exact treatment when the variationally determined subspace spans the full model space.

[NUCLEAR STRUCTURE Spurious center of mass problem; correction to shell model bases; correction to variationally determined bases.]

It is shown that the sum of all time-ordered diagrams which contribute to the two-meson-exchange three-body potential due to the excitation of one nucleon into a ▵ isobar can be reproduced by the second order iteration of the nonrelativistic one-meson-exchange transition potential in the coupled channels approach.

[NUCLEAR STRUCTURE Two-meson-exchange three-body force, effects of $\Delta \mathrm{}\left(1236\right)\mathrm{}$, validity of coupled channels approach.]

The difference between the neutron and proton densities (${\rho }_n-{\rho }_p$) and the excess neutron density ${\rho }_{\mathrm{exc}}$ are computed for several nuclei using the Hartree-Fock approximation. Large differences are found between these two types of densities for nuclei with a small neutron excess. The above densities are related to transition densities in charge exchange reactions leading to isobaric analog states. It is argued that ${\rho }_{\mathrm{exc}}$ is the density which should be used in the computation of the cross section for such reactions in nuclei with a small number of excess neutrons.

[NUCLEAR STRUCTURE: Transition densities to isobaric analog states and charge exchange reactions.]

The width of the $6^{-}$, $T=1\mathrm{}$, $E_x=14.36\mathrm{}$ MeV state in ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ has been measured via the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}\mathrm{}(p,p)\mathrm{}{}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ reaction. The value of $\Gamma =3.7\mathrm{}\pm 0.4$ keV yields a single particle fraction $\beta _1^{}{}_{}{}^2=0.68\mathrm{}\pm 0.07$ which is shown to be consistent with the combined results for the inelastic excitation of this state by ${\pi }^{\pm }$, $p$, and $e^{-}$ and the ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}({}_{}{}^3{}_{}{}^{}\mathrm{He},d){}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ (g.s.) reaction.

[NUCLEAR REACTIONS ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}(p,p_{0,1,2}){}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$; $E_p=2.70-3.06\mathrm{}$ MeV; determined width and deduced single particle fraction for ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ state at $E_x=14.36\mathrm{}$ MeV.]

Collective rotation is examined with the gauge-invariant periodic quantization method and the assumption that the rotation takes place about a fixed axis. A rotation spectrum of the form $\frac{I^2\hslash }{2\mathcal{I}}$ where the moment of inertia $\mathcal{I}$ depends on the angular velocity of the rotation, is obtained together with a nonperturbative cranking Schrödinger equation for the intrinsic wave function.

[NUCLEAR STRUCTURE Collective rotation, moment of inertia, quantization of angular momentum, nonperturbative cranking, gauge-invariant periodic quantization method.]

Cross sections for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(p,p^{\prime })$ reaction leading to the $1^{+}$ states at 15.1 MeV ($T=1\mathrm{}$) and 12.7 MeV ($T=0\mathrm{}$) were measured at a bombarding energy of 402 MeV. These data, covering a range of momentum transfer $q$ from 0.45 to 2.65 ${\mathrm{fm}}^{-1\mathrm{}}$ ($0.6-3.7m_{\pi }$) provide a test of the effects, precursor to a pion condensate, predicted to be important for the $T=1\mathrm{}$ state near $q=2-3\mathrm{}{m}_{\pi }$. The cross sections for the two $1^{+}$ states are very similar for large $q$, perhaps indicating that the precursor effects are small.

[NUCLEAR REACTIONS ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(p,p^{\prime })$; measured $\sigma \left(\theta \right)$, $q_c\simeq \mathrm{}(2-3)\mathrm{}{m}_{\pi }$ 15.1 MeV $1{}_{}{}^{+}{}_{}{}^{}T=1\mathrm{}$ and 12.7 MeV $1{}_{}{}^{+}{}_{}{}^{}T=0\mathrm{}$ levels; $E_p=402\mathrm{}$ MeV; precursor effects discussed.]

The angle at which the analyzing power for free $\mathrm{np}$ scattering becomes zero, the zero-crossing angle ${\theta }_0$, has been measured simultaneously for the $\overrightarrow{n}p$ and $n\overrightarrow{p}$ scattering processes at 425, 565, and 665 MeV incident neutron energies. A rather strong energy dependence of the zerocrossing angle is found. Knowledge of this energy dependence is important in designing an experiment which tests for charge symmetry breaking forces by comparing high precision measurements of ${\theta }_0\left(\overrightarrow{n}p\right)$ and ${\theta }_0\left(n\overrightarrow{p}\right)$.

[NUCLEAR REACTIONS $p(n,p)n$, $E=425,565, \mathrm{and} 665$ MeV; polarized neutron beam, polarized proton target; measured analyzing powers $A(E,\theta )$; deduced zero-crossing angles; charge symmetry.]

The many-body scattering $t$ matrix is expanded into a finite and exact multiple scattering series in which each two-body scattering operator is modified by local field effects arising from all constituents of the target system not previously involved in the scattering. A natural three-body approximation is shown to follow for our first-order optical potential with local field corrections.

[NUCLEAR REACTIONS Multiple scattering, local field corrections, optical potential].

The quasi-potential procedure of Sasakawa and Austern is generalized to obtain an improved convergence of iteration series. The modified form of the quasi-potential method is compared with the earlier approaches and its effectiveness tested. Extensions to treat the general coupled-channel integro-differential equations are outlined.

[NUCLEAR REACTIONS Quasi-potential method, coupled integro-differential equations.]