The spin correlation parameter $A_{\mathrm{nn}}\left(\theta \right)$ for $\mathrm{pp}$ elastic scattering has been measured at 796 MeV from 30° to 90° c.m. The typical relative uncertainties of ±0.01 are almost an order of magnitude better than those of previous data near this energy. The absolute normalization uncertainty is ±2%. Data are consistent with previous measurements near this energy.

NUCLEAR REACTIONS ${}_{}{}^1{}_{}{}^{}\mathrm{H}\mathrm{}(p,p)\mathrm{}{}_{}{}^1{}_{}{}^{}\mathrm{H}$, $E=79.6\mathrm{}$ MeV; measured $A_{\mathrm{nn}}\left(\theta \right)$, $A\left(\theta \right)$; $\theta =30\mathrm{} \mathrm{to} 90°$ c.m.

The three-body approach to the single-scattering optical potential proposed by Tandy, Redish, and Bollé is tested in the context of $\pi -d$ scattering where the various approximations are compared with the exact solution of the Faddeev equations. For $\pi -d$ scattering we find that the integral equations derived for the Kerman-McManus-Thaler form of the single-scattering optical potential are markedly superior to those of the Watson form. Our analysis includes a study of the convergence properties of the various multiple-scattering series encountered.

NUCLEAR REACTIONS $\pi -d$ optical potential; TRB, KMT, and Watson multiple-scattering series; Faddeev solution comparison.

We present a critical comparison for two types of three-dimensional covariant equations for the three-particle system obtained by the Blankenbecler-Sugar reduction technique with the Wightman-Gårding momenta and the usual Jacobi variables. We also discuss the relations between the relativistic and nonrelativistic equations in the low-energy limit.

NUCLEAR REACTIONS Relativistic Faddeev equations, Blankenbecler-Sugar reduction technique, nonrelativistic limit.

We report here the results of the numerical calculations of the gamma-neutrino angular correlation coefficients in the capture of polarized muons by the ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ nucleus. Closed expressions for these coefficients are derived taking into account the relativistic terms in the muon capture Hamiltonian, in the impulse approximation. Relations among these coefficients, the average and the longitudinal polarization of the recoil nucleus in muon capture, are derived and are shown to be independent of nuclear models and muon-capture coupling constants. By comparing with experimental data, we obtain a value for the induced pseudoscalar coupling constant $g_P$, as $\mathrm{}(13.5+3.5,-5.5)\mathrm{}{g}_A$, indicating a possible absence of quenching of $g_P$ in the ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$ nucleus. The effect of meson exchange currents on these correlation coefficients are discussed.

NUCLEAR REACTIONS ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}({\mu }^{-},\gamma {\nu }_{\mu }{}_{}{}^{28}{}_{}{}^{}\mathrm{Al})$, $\gamma$-neutrino angular correlation, polarized muon capture, particle-hole model, induced pseudoscalar, second class current, induced tensor coupling constants, meson exchange corrections.

On symmetry grounds and by a systematic analysis of one-meson exchange diagrams, we obtain the two-body axial meson-exchange current operator which is needed for computing meson-exchange corrections in nuclear systems. Special attention is given to the time-component of this operator and the experimental evidence of its importance.

RADIOACTIVITY Structure of axial meson-exchange currents; meson-exchange corrections to $0^{+}$ - $0^{-}$ transitions in $\beta$ decay and $\mu$ capture of $A=12\mathrm{} ({}_{}{}^{12}{}_{}{}^{}\mathrm{B},{}_{}{}^{12}{}_{}{}^{}\mathrm{C},{}_{}{}^{12}{}_{}{}^{}\mathrm{N})$ and $A=16\mathrm{} ({}_{}{}^{16}{}_{}{}^{}\mathrm{O},{}_{}{}^{16}{}_{}{}^{}\mathrm{N})$ systems.

The ${9/2}^{+}$ ground state of ${}_{}{}^{89}{}_{}{}^{}\mathrm{Mo}$ has been found to decay with a half-life of 2.15±0.20 min to the ground state and levels at 658.6, 1155, and 1272 keV in ${}_{}{}^{89}{}_{}{}^{}\mathrm{Nb}$. Spin and parity assignments are discussed as are the levels of the $N=48\mathrm{}$ and $N=52\mathrm{}$ odd $Z$ isotones.

RADIOACTIVITY ${}_{}{}^{89}{}_{}{}^{}\mathrm{Mo}_{}^g{}_{}{}^{}$ from ${}_{}{}^{92}{}_{}{}^{}\mathrm{Mo}\mathrm{}(p,p3n)\mathrm{}$, measured $t_{\frac{1}{2}}$, $E_{\gamma }$, $I_{\gamma }$. ${}_{}{}^{89}{}_{}{}^{}\mathrm{Nb}$ deduced levels. Ge detector. Enriched target.

Beta-delayed protons have been observed from the decays of the mass separated $T_z=-2\mathrm{}$ nuclides ${}_{}{}^{20}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Si}$, and ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ca}$. From these proton spectra the mass excesses of the lowest $T=2\mathrm{}$ states in the $T_z=-1\mathrm{}$ nuclei ${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Al}$, and ${}_{}{}^{36}{}_{}{}^{}\mathrm{K}$ are determined to be 13420±50 keV (${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$), 5903±9 keV (${}_{}{}^{24}{}_{}{}^{}\mathrm{Al}$), and - 13168±22 keV (${}_{}{}^{36}{}_{}{}^{}\mathrm{K}$). The complete $A=20,24, \mathrm{and} 36$ isospin quintets have all their members bound against isospin allowed particle-decay modes, providing a stringent test of the isobaric multiplet mass equation. Good agreement is observed for all these quintets using only the quadratic form of this equation.

RADIOACTIVITY ${}_{}{}^{20}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{36}{}_{}{}^{}\mathrm{Ca}$ (mass separated); measured $\beta$-delayed protons; deduced $T_{\frac{1}{2}}$ and proton branching; derived mass excesses of the lowest $0^{+}$, $T=2\mathrm{}$ states in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Na}$, ${}_{}{}^{24}{}_{}{}^{}\mathrm{Al}$, and ${}_{}{}^{36}{}_{}{}^{}\mathrm{K}$; deduced coefficients of the isobaric multiplet mass equation.

A semirealistic parametrization in momentum space is used for the matrix elements of the effective nucleon-nucleon interaction. In this model, the nucleons only interact in a relative $s$-wave state and essentially via an exponential potential. In the Hartree-Fock approximation, algebraic expressions are derived for the self-energy and the effective mass. The strength and the range of the nucleon-nucleon interaction are chosen in such a way that these quantities are in fair agreement with the empirical values. We calculate analytically the frequency dependence of the imaginary part of the polarization and correlation contributions to the self-energy. The real part of these contributions can then be computed very accurately with the help of dispersion relations. Their frequency dependence can most conveniently be described in terms of a frequency mass $\overline{m}$. It is found that $\overline{m}$ has an enhancement peak, typically 30 MeV wide, centered on the Fermi energy. The contributions of the correlation and of the polarization graphs to this local enhancement are disentangled. The contribution to $\overline{m}$ of the polarization graph peaks somewhat above the Fermi energy, and that of the correlation graph somewhat below the Fermiy energy. The total peak is approximately symmetric about the Fermi energy. We identify the range of excitation energy of those core excited states which mainly contribute to the enhancement. The enhancement peak is more pronounced at low density and for long-range interactions. The usual effective mass also displays an enhancement at the Fermi surface. The momentum distribution in the correlated ground state is calculated. The effect of introducing a hard core in the nucleon-nucleon interaction is studied.

NUCLEAR STRUCTURE Second order approximation to self-energy of nucleons in nuclear matter.

The perturbational approach that has been used for the calculation of the triton bound state is applied to ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. The Coulomb potential is fully taken into account within the limitation of the Reid soft-core-3 potential. We obtain 5.775 MeV as the binding energy of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. The Coulomb energy difference with ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ is 0.625 MeV. The Coulomb effects to the charge form factor and the two-body correlation function are calculated. Qualitative discussions are given for the complex behavior of the Faddeev components expressed in terms of the spectator momentum and the relative distance of the interacting pair.

NUCLEAR STRUCTURE ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{H}$ bound state. Exact solution of the Faddeev equation by a perturbative approach. Binding energy of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$: 5.775 MeV. Coulomb energy difference with triton: 0.625 MeV. Coulomb effects to charge form factor and correlation function.

The parametrization of the spin-orbit part of the Woods-Saxon potential was optimized taking into account the single-particle energies corrected for nucleon-nucleus interaction and the high-spin spectra of ${}_{}{}^{204}{}_{}{}^{}\mathrm{Pb}$ and ${}_{}{}^{212}{}_{}{}^{}\mathrm{Rn}$. With the spin-orbit potential strength parameters $\lambda =36.0\mathrm{}$ and the radius parameter ${\left(r_0\right)}_{\mathrm{so}}=1.3\mathrm{}$ fm for both protons and neutrons, both types of empirical data are reproduced well. The configurations of the calculated high-spin states are identical with the shell-model results.

NUCLEAR STRUCTURE Woods-Saxon single-particle levels, high-spin excitations, lead region nuclei.

Three different methods for deriving heavy ion polarization potentials are critically compared. It is argued that their possible applications depend strongly on whether they reproduce the elastic wave function or just the elastic phase shifts.

NUCLEAR REACTIONS Heavy ion polarization potentials.

The decay rate of an exciton is calculated using the imaginary part of both the phenomenological optical potential of Becchetti and Greenless and the microscopic potential of Jeukenne et al., The decay rate thus calculated is found to be about the same for proton- and neutron-induced reactions and weakly sensitive to mass number. Kalbach's two body matrix element, obtained from the analysis of precompound reactions using the Williams exciton model, also leads to a decay rate in agreement with the phenomenological potential estimate. However, the exciton model of Gadioli et al. is inconsistent with optical model estimates. Further, remarks are made on the concept of a mean free path in finite nuclear matter.

NUCLEAR REACTIONS Exciton decay rate from phenomenological and microscopic optical models and exciton model for the precompound reactions.

The mass dependence of excitation energies of the $2_1^{+}$, $2_2^{+}$, and $4_1^{+}$ states of the even-mass Hg isotopes, and of some related $B\left(E2\mathrm{}\right)\mathrm{}$ values, shows a marked discontinuity at ${}_{}{}^{200}{}_{}{}^{}\mathrm{Hg}$. Analysis of $B(E2;0_1^{+}\to 2_1^{+})$ values in terms of an extended interacting boson approximation model suggests that this discontinuity is due to a change in the proton and neutron distributions at ${}_{}{}^{200}{}_{}{}^{}\mathrm{Hg}$. Apart from ${}_{}{}^{200}{}_{}{}^{}\mathrm{Hg}$, the data favor $\gamma$-soft models rather than the rigid triaxialrotor model.

NUCLEAR STRUCTURE Analysis of levels in ${}_{}{}^{196-204}{}_{}{}^{}\mathrm{Hg}$ using extended IBA model.

The kinematical two-body fragmentation model is generalized to multibody fragmentation. Comparison with statistical model is given and possible experiments to distinguish the two models are suggested.

NUCLEAR REACTIONS ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ beam fragmentation, $p=1,2\mathrm{}$ GeV/nucleon, target independent momentum distribution, fragment mass dependence, extension of pseudo-two-body.

Inelastic scattering of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ particles at 110-140 MeV from ${}_{}{}^{90,92}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{116,118,120}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{114}{}_{}{}^{}\mathrm{Sm}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ nuclei exhibits the presence of a new giant resonance at $E_x\sim 118A^{-\frac{1}{3}}$ MeV with a width of about 7 MeV. The angular distributions are consistent with those for $L=3\mathrm{}$ transfer and octupole energy-weighted sum-rule fractions of 47-95% are exhausted for the resonance.

NUCLEAR REACTIONS ${}_{}{}^{90,92}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{116,118,120}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}$, ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, ${}_{}{}^3{}_{}{}^{}\mathrm{He}\prime$), $E=110\mathrm{}-140$ MeV. Measured $\sigma (\theta ,E)$. Deduced giant octupole resonance, $E_x$, ${\beta }_3$, EWSR. Enriched target.

The transition amplitude for ($p,2p$) reactions initiated by protons with energies in excess of 100 MeV is cast into a form which facilitates identification of components associated with the standard (direct) knockout mechanism, and with semidirect processes in which excited states of the target, notably the giant resonances, mediate the transitions. Calculations of both direct and semidirect amplitudes are presented, for the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}\mathrm{}(p,2p)\mathrm{}$ reaction, in an asymmetric energy-sharing geometry for which a half distorted-wave approximation is appropriate. Estimates of the semidirect effects, using coupling strengths deemed appropriate from analyses of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(p,p^{\prime })$ data, indicate that under favorable kinematic conditions such effects should be discernable in correlations.

NUCLEAR REACTIONS Scattering theory including resonance effects, applied to ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}\mathrm{}(p,2p)\mathrm{}$ asymmetric energy-sharing geometry, $E=98.7\mathrm{}$ MeV; calculated $\sigma \left(\theta \right)$; optimal resonance effects estimated.

The $\alpha$-cluster pickup reaction ($d,{}_{}{}^6{}_{}{}^{}\mathrm{Li}$) has been studied at $E_d=55\mathrm{}$ MeV on targets of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$. Members of the ground-state rotational bands in ${}_{}{}^{228}{}_{}{}^{}\mathrm{Ra}$ and ${}_{}{}^{234}{}_{}{}^{}\mathrm{Th}$ are excited and absolute reduced $\alpha$ widths obtained from finite-range distorted-wave analysis are in good agreement with values deduced from $\alpha$ decay. In addition three excited groups of states are very strongly populated in both nuclei with spectroscopic strength per group comparable with those of the respective ground state bands. These groups are apparently excited rotational bands with band heads at $E_x=700\mathrm{}\pm 40, 1070\pm 60, \mathrm{and} 1390\pm 60$ keV in ${}_{}{}^{228}{}_{}{}^{}\mathrm{Ra}$ and $E_x=810\mathrm{}\pm 30, 1150\pm 40, \mathrm{and} 1470\pm 40$ keV in ${}_{}{}^{234}{}_{}{}^{}\mathrm{Th}$. The selective and strong excitation in this particular multi-nucleon transfer reaction of several excited bands is not predicted by existing theoretical models. An attempt has been made to describe the systematics of excited $0^{+}$ states in the actinide region with the interacting boson model. Excitation energies are reasonably well described but intruder states are present and transfer strengths are not reproduced properly. The observation of strong $\alpha$-cluster pickup to excited rotational bands suggests coherent contributions from both neutron and proton pair excitations which can lead to strong four-body correlations and/or to new types of collective excitations which favor quartet structure. It is found that about 25% of the nuclear charge (matter) at $r\simeq 10.6$ fm must be associated with $\alpha$ particles. This high $\alpha$-clustering probability indicates $\alpha$-particle condensation in low-density nuclear matter.

NUCLEAR REACTIONS ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}(d,{}_{}{}^6{}_{}{}^{}\mathrm{Li})$, $E=54.8\mathrm{}$ MeV; measured $\sigma \left(\theta \right)$; DWBA analysis; ${}_{}{}^{228}{}_{}{}^{}\mathrm{Ra}$, ${}_{}{}^{234}{}_{}{}^{}\mathrm{Th}$ deduced levels, $S_{\alpha }$, $\gamma _{\alpha }^{}{}_{}{}^2$ (10.5 fm), $\alpha$-clustering probabilities.

A large number of experimental angular distributions have been studied for particles emitted into the continuum in preequilibrium nuclear reactions in order to study their systematics. For pure multistep direct reactions it has been found that to first order the shapes of these angular distributions are determined by the energy of the outgoing particle. In addition, there is evidence that the division of the cross section into multistep direct and multistep compound components with the latter having a symmetric angular distribution is a meaningful one. The shapes of the angular distributions can be described in terms of Legendre polynomials. The polynomial coefficients are given by simple phenomenological relations involving the energy of the outgoing particle. The same reduced coefficients are used for multistep direct and multistep compound processes except that only even order polynomials are considered in the latter case. The present formulation has been shown to have significant predictive ability for light ion reactions.

NUCLEAR REACTIONS $A(a,b)\mathrm{}$, $A={}_{}{}^{12}{}_{}{}^{}\mathrm{C} \mathrm{to} {}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$, $a=p,d,{}_{}{}^3{}_{}{}^{}\mathrm{He},{}_{}{}^4{}_{}{}^{}\mathrm{He}$, $b=n,p,d,t,{}_{}{}^3{}_{}{}^{}\mathrm{He},{}_{}{}^4{}_{}{}^{}\mathrm{He}$, $E=18\mathrm{}-80$ MeV, $E_b=4\mathrm{}-60$ MeV. Deduced systematics of $\frac{\sigma (E_b,\theta )}{\sigma \left(E_b\right)}$ in continuum. Parametrized $\frac{\sigma (E_b,\theta )}{\sigma \left(E_b\right)}$ in continuum with energy dependent Legendre polynomial coefficients.

The Griffin or exciton model for preequilibrium nuclear reactions has been extended so that particle unbound states in the equilibrating system are distinguished from bound states. This permits the preequilibrium cross section to be divided into components due to multistep direct and multistep compound processes for calculation of emitted particle angular distributions. Preequilibrium models traditionally assume that all states of a given particle-hole number are equally likely whereas present work indicates that unbound states are preferentially populated. Nevertheless, the emission energy spectra calculated are not significantly changed from earlier versions of the model.

NUCLEAR REACTIONS Extended Griffin preequilibrium statistical model to calculate multistep direct and multistep compound cross sections separately. Tested equal a priori probabilities assumption.