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The reaction {sup 58}Ni({pi}{sup +},{ital pp}) was studied with good energy resolution for an incident pion energy of 160 MeV. The angular correlation of the outgoing protons was measured for {theta}{sub lab}=30{degree}, 75{degree}, and 130{degree}. From these angular correlations the contribution from direct two-nucleon absorption was extracted. A Monte Carlo calculation modeling the absorption process in terms of initial-state scattering of the pion before absorption, two-nucleon absorption, and final-state scattering of the outgoing nucleons is compared to the data. We conclude that within this framework the two-nucleon mechanism can account for less than half of the total absorption cross section in {sup 58}Ni. We report results for the low-energy coincidence proton spectra (down to 5 MeV) which suggest that most of these protons come from evaporation. Angular correlation data for the reaction {sup 58}Ni({pi}{sup {minus}},{ital pp}) are also presented.

The energy spectra of neutrons between 2 and 28 MeV emitted following radiative pion capture in carbon, oxygen, and calcium fall off exponentially. An enhancement, however, is observed for neutron energies below 5 MeV in carbon and oxygen. Neutron yield peaks at $n\ensuremath{-}\ensuremath{\gamma}$ angles of 180\ifmmode^\circ\else\textdegree\fi{}, indicating a quasifree absorption mechanism in this process.

Angular distributions have been measured for ..pi../sup +- / elastic scattering at 50 MeV from /sup 58/Ni, /sup 60/Ni, and /sup 64/Ni. A target isospin dependence is observed in the ..pi../sup +/ angular distributions in the region of the diffraction minimum, while the effect is less pronounced for ..pi../sup -/. The ..pi../sup -/ angular distributions for these isotopes have a very deep minimum compared with those for ..pi../sup +/. The angular distributions are compared with predictions of the second-order Michigan State University potential. The potential describes the ..pi../sup +/ data much better than the ..pi../sup -/ data, though qualitative agreement was obtained for both. The discrepancy between the experimental and calculated cross sections is discussed. The Coulomb distortion of the nuclear amplitude is shown to be important in the region of the diffraction minimum. The ..pi../sup +//..pi../sup -/ difference introduced by this distortion is responsible for the striking energy dependence of the cross sections near the diffraction minimum between 50 and 80 MeV and can account for the poorer agreement for ..pi../sup -/.

Measurements of positive-pion elastic scattering cross sections at 20 MeV have been made in the angular range 30\ifmmode^\circ\else\textdegree\fi{}-150\ifmmode^\circ\else\textdegree\fi{}. Angular distributions were obtained for targets of $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{40}\mathrm{Ca}$, $^{90}\mathrm{Zr}$, $^{120}\mathrm{Sn}$, and $^{208}\mathrm{Pb}$. The data were fit with a phenomenological optical model potential calculation to obtain the real and imaginary $s$- and $p$-wave strength parameters. These parameters are compared to those obtained from data at 30, 40, and 50 MeV. Cross sections calculated from an optical model potential based on pionic atom data are in good agreement with our data at 20 MeV.NUCLEAR REACTIONS Elastic scattering of 20-MeV ${\ensuremath{\pi}}^{+}$ from $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{40}\mathrm{Ca}$, $^{90}\mathrm{Zr}$, $^{120}\mathrm{Sn}$, and $^{208}\mathrm{Pb}$. Angular distributions $30\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}{\ensuremath{\theta}}_{L}\ensuremath{\le}160\ifmmode^\circ\else\textdegree\fi{}$. Optical model analysis of differential cross sections.

80 MeV positive and negative pions inelastically scattered from targets of /sup 12/C and /sup 40/Ca have been studied at laboratory angles between 50/sup 0/ and 120/sup 0/. Angular distributions are presented for scattering from the 4.44 MeV (2/sup +/) state in /sup 12/C, the 3.7 MeV (3/sup -/), 13.4 MeV, and 17.5 MeV states in /sup 40/Ca. The cross sections are compared with distorted wave calculations using potentials obtained from elastic scattering.

Differential cross sections for the reaction ..pi../sup +/+d..-->..p+p were measured for pion laboratory energies of 65, 72.5, 80, 95, 110, 125, and 140 MeV. The integrated cross sections for these energies were found to be 7.51 +- 0.19, 7.91 +- 0.16, 8.64 +- 0.15, 9.61 +- 0.15, 11.3 +- 0.2, 11.7 +- 0.3, and 11.3 +- 0.3 mb, respectively. These values indicate that the centroid and width of the energy dependence for the resonance in this reaction is lower and greater, respectively, than predicted by most theoretical models of the process. Comparisons made with predictions by Blankleider and Afnan show a qualitative difference between experimental measurements and calculations of the P/sub 4/(costheta) term in the polynomial expansion of the differential cross section. This may indicate a more important role for rho-like exchange in the reaction mechanism than that included by the calculations.

Differential cross sections for the elastic scattering of positive pions on $^{12}\mathrm{C}$ at 49.9 MeV were measured. The results differ from those deteermined by other investigators. The data are analyzed in terms of ${\ensuremath{\pi}}^{+}$ nucleus partial waves and are compared to various model calculations.NUCLEAR REACTIONS $^{12}\mathrm{C}({\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{+})^{12}\mathrm{C}$. $E=49.9$ MeV, $\ensuremath{\theta}=25\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}160\ifmmode^\circ\else\textdegree\fi{}$ measured $\frac{d\ensuremath{\sigma}}{d\ensuremath{\Omega}}(\ensuremath{\theta})$. Partial wave and optical model analyses.

Angular distributions of ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ elastic scattering on $^{40}\mathrm{Ca}$ at 64.8 MeV are presented. The data are analyzed using a first-order, nonlocal optical model based on a Kisslinger $t$ matrix with an off-shell form factor. This model is unable to fit the data for any value of the off-shell parameter $\ensuremath{\alpha}$ unless the nuclear matter radius is allowed to vary from that derived from electron scattering. The resulting best-fit radii are as much as 0.40 fm larger than the electron scattering value for ${\ensuremath{\pi}}^{+}$ and 0.55 fm larger for ${\ensuremath{\pi}}^{\ensuremath{-}}$. For a given $\ensuremath{\alpha}$ the ${\ensuremath{\pi}}^{\ensuremath{-}}$ best fit radius is also consistently larger by about 0.2 fm than that for ${\ensuremath{\pi}}^{+}$. In addition, the fits show no significant preference for any value of $\ensuremath{\alpha}$ between 400 and 900 MeV/c. Inclusion of the normally omitted second-order potential terms in the optical model does not significantly alter the results.NUCLEAR REACTIONS ${\ensuremath{\pi}}^{+}$ and ${\ensuremath{\pi}}^{\ensuremath{-}}$ elastic scattering from $^{40}\mathrm{Ca}$ at 64.8 MeV; angular distributions, $15\ifmmode^\circ\else\textdegree\fi{}l{\ensuremath{\theta}}_{\mathrm{lab}}l127\ifmmode^\circ\else\textdegree\fi{}$; optical model analysis.

Differential cross sections for elastic scattering of 40 MeV positive pions from /sup 12/C, /sup 16/O, /sup 40/Ca, /sup 90/Zr, and /sup 208/Pb were measured at 18 angles in the angular range 25/sup 0/--160/sup 0/. Relative uncertainties of 2.5% to 7% and overall normalization uncertainties of 4% to 7% were obtained. An optical potential fit of the data indicates that while the magnitudes of the strength parameters, except for Re b/sub 1/, are different from values obtained from free ..pi..N scattering amplitudes, the variation of these magnitudes with target mass is roughly as predicted by the impulse approximation. Coulomb effects, especially in the cases of /sup 90/Zr and /sup 208/Pb, are extremely important, but impossible to unambiguously separate from the strong interaction in the optical model.

Absolute differential cross sections have been measured for the ..pi../sup +/ + d ..-->.. p + p reaction at pion energies of 40, 50, and 60 MeV. The angular distributions are fitted with the functional form C/sub 0/ + C/sub 2/cos/sup 2/theta + C/sub 4/cos/sup 4/theta.

Differential cross sections for elastic scattering of positive pions on $^{16}\mathrm{O}$ at 40.0 and 49.7 MeV were measured at about 10\ifmmode^\circ\else\textdegree\fi{} intervals for laboratory angles from 25\ifmmode^\circ\else\textdegree\fi{} to 160\ifmmode^\circ\else\textdegree\fi{}. Relative uncertainties between angles are 3%, with the overall normalization known to 7% for each angular distribution. The angular distributions are similar in shape, being dominated by a minimum at about 60\ifmmode^\circ\else\textdegree\fi{}. The differential cross sections are presented along with the results of a partial wave analysis which gives good fits at both 40.0 and 49.7 MeV. The data are also fitted by a simple phenomenological first-order Kisslinger nonlocal optical model calculation.NUCLEAR REACTIONS $^{16}\mathrm{O}({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{+})^{16}\mathrm{O}$. $E=40.0, 49.7$ MeV, $\ensuremath{\theta}=25\ensuremath{-}160\ifmmode^\circ\else\textdegree\fi{}$, measured $\ensuremath{\sigma}(\ensuremath{\theta},E)$. Partial-wave and optical model analyses of $\ensuremath{\sigma}(\ensuremath{\theta},E)$.

Elastic scattering cross sections for ..pi../sup + -/ scattering from /sup 12/C, /sup 13/C, and /sup 14/C are presented for pion energies of 65 and 80 MeV and for scattering angles from 20/sup 0/ to 120/sup 0/. Energy dependent isospin effects are observed. The cross sections were fit with the Kisslinger potential, and the potential strength parameters show an (N-Z)/A dependence similar to that predicted by the impulse approximation at 65 MeV, but not at 80 MeV. Calculations using the Siciliano potential, which contains the explicit isospin dependence of the Lorentz-Lorenz-Ericson-Ericson effect plus isoscalar, isovector, and isotensor terms, indicate the importance and energy dependence of absorption effects. Finally a ''model independent'' parametrization of the neutron density suggests a neutron excess at the surface of /sup 13/C but not /sup 14/C.

Study of the elastic scattering of 600-MeV protons from light nuclei. Differential cross sections have been obtained for the scattering of protons from hydrogen, deuterium, helium-3, and helium-4. Polarization was measured for deuterium and He-4 nuclei. The p-p cross-section data are in excellent agreement with the predictions from the Livermore phase shifts. Small-angle p-D, p-He-3 elastic scattering data are compared with calculations based on the multiple-scattering theories of Watson and Glauber.