In the present Rapid Communication, we report the discovery of a rotational band in ${}_{}{}^{105}{}_{}{}^{}\mathrm{Pd}$ and possibly one in ${}_{}{}^{104}{}_{}{}^{}\mathrm{Pd}$ that can be interpreted as arising from superdeformed shapes. The moments of inertia, $J^{\mathrm{}\left(1\right)\mathrm{}}$ and $J^{\mathrm{}\left(2\right)\mathrm{}}$, of these bands are similar to those measured in the Ce-Nd region, once the $A^{\frac{5}{3}}$ mass dependence is removed. This implies a deformation $\epsilon \sim 0.35-0.4$. If so, this is the third mass region where superdeformed bands have been found at high spins.

The finite-temperature Hartree-Fock-Bogoliubov cranking equation predicts the most probable shape for ${}_{}{}^{158}{}_{}{}^{}\mathrm{Yb}$ nuclei. At a spin-dependent critical temperature, there is a transition from prolate collective rotation to oblate noncollective rotation. For spins above $39\hslash$, there is oblate noncollective rotation at all temperatures. However, inclusion of thermal shape fluctuations produces an average shape which is prolate collective at spins 40 and $50\hslash$ and thermal excitation energy $E^{*}\approx 10$ MeV.

Electron scattering form factors for stretched transitions are computed using radial wave functions from realistic nuclear potentials, including the unbound nature of final states above particle decay thresholds. The calculated form factors are compared to data for $4^{\mathrm{-}}$ states in ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, $6^{\mathrm{-}}$ states in ${}_{}{}^{24}{}_{}{}^{}\mathrm{Mg}$, ${}_{}{}^{26}{}_{}{}^{}\mathrm{Mg}$, and ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, $8^{\mathrm{-}}$ states in ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$, ${}_{}{}^{54}{}_{}{}^{}\mathrm{Fe}$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, and ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$, the ${10}^{\mathrm{-}}$ state in ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, and the ${14}^{\mathrm{-}}$ state in ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. We assess the fraction of the single-particle sum rule strengths using these realistic nuclear potentials in place of the standard results using harmonic oscillator wave functions. Appreciably greater fractions are obtained for low mass nuclei in the present work, totalling 105% of the sum strength for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ and 81% for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$. Much less damping of the magnetic strength is thus experimentally observed than is the case when oscillator wave functions are used for comparison. The results of including meson exchange currents in the analysis are also discussed.

Using a modified Lee model for the nucleon-nucleon interaction we show the equivalence of this model to an interaction based on the quark compound bag model recently proposed by Bhasin and Gupta. It is found that their choice of the sign of the parameter coupling the hadronic to quark degrees of freedom leads to unphysical results. By investigating the model with the correct sign and more general form factors, we observe that elementary degrees of freedom in terms of the elementary particle (e.g., six-quark or dibaryon) eigenstate components of the two-nucleon system, will not be resolved by studying the on-shell behavior of the T matrix, but may be seen by studying effects that depend on the detailed structure of the two-nucleon wave function.

Using the two-body K matrix extended to the whole complex-momentum plane, we construct the S matrix as a function of the strength λ of attractive Yukawa potentials. By varying λ we study pole trajectories for the s, p, and d waves.

Compton scattering off the deuteron is studied for photon energies up to about 100 MeV. This energy limit reflects the fact that only intermediate nucleon-nucleon (NN) states are considered. The NN propagator is constructed in a separable potential model, the parameters of which are fitted to describe the experimental NN scattering phase shifts. The problem of gauge invariance of the Compton amplitude is analyzed and the role of nonlocal currents is discussed. The low-energy theorem is satisfied. Our approach enables a direct calculation of both the real and imaginary part of the Compton scattering amplitude. It turns out that the strongest multipoles are dominated by the Born terms. Numerical results are compared to a previous dispersion theoretical calculation, and we find a strong disagreement between both calculations. We are led to conclude that certain assumptions made in the dispersion theoretical calculation are not justified.

The N=81 odd-mass ${}_{}{}^{139}{}_{}{}^{}\mathrm{Ce}$ nucleus has been studied from the (p,nγ) reaction on lanthanum to look for the single neutron-hole states and also the states arising from hole-core coupling. Ten excited states of ${}_{}{}^{139}{}_{}{}^{}\mathrm{Ce}$ have been studied by gamma-ray singles measurements at different angles with Ge(Li) detectors following the bombardment of a lanthanum target with protons of energies 4.00 to 6.50 MeV. A level scheme for ${}_{}{}^{139}{}_{}{}^{}\mathrm{Ce}$ has been proposed from the in-beam gamma-ray spectroscopy. The $A_2$ and $A_4$ parameters of the transitions have been measured from the angular distributions of the gamma rays. Relative excitation functions of the states with respect to the 1347.4 keV ${(7/2\mathrm{}}^{+}$ state have been measured. The experimentally measured excitation cross sections of the individual states have been compared with the values obtained from theoretical calculation on the basis of the compound nuclear statistical theory of Hauser and Feshbach.

Cross sections were measured for the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$(n,α${)}^{90}$${\mathrm{Y}}^{\mathrm{m}}$, ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$(n,${\alpha }^{90}$${\mathrm{Y}}^{\mathrm{m}+\mathrm{g}}$, and ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$(n,2n${)}^{92}$${\mathrm{Nb}}^{\mathrm{m}}$ reactions from threshold up to 10.6 MeV. Use was made of the activation technique in combination with high-resolution γ-ray spectroscopy, except for the product ${}_{}{}^{90}{}_{}{}^{}\mathrm{Y}_{}^{\mathrm{g}}{}_{}{}^{}$ where radiochemical separation and ${\beta }^{\mathrm{-}}$ ray counting were applied. The quasi-monoenergetic neutrons were produced via the ${}_{}{}^2{}_{}{}^{}\mathrm{H}$(d,n${)}^3$He reaction using a deuterium gas target at a compact cyclotron. Statistical model calculations taking into account precompound effects (reported in the literature) agree reasonably well with the experimental excitation functions for the formation of both ${}_{}{}^{90}{}_{}{}^{}\mathrm{Y}_{}^{\mathrm{m}}{}_{}{}^{}$ and ${}_{}{}^{90}{}_{}{}^{}\mathrm{Y}_{}^{\mathrm{m}+\mathrm{g}}{}_{}{}^{}$. The isomeric cross-section ratio for the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$(n,α${)}^{90}$${\mathrm{Y}}^{\mathrm{m}}$,g reactions increases with energy. The model calculation gives somewhat lower values for the ratio.

The reactions ${}_{}{}^{50}{}_{}{}^{}\mathrm{Cr}$(p,p’γ${)}^{50}$Cr and ${}_{}{}^{50}{}_{}{}^{}\mathrm{Cr}$(p,γ${)}^{51}$Mn have been used to investigate resonances in ${}_{}{}^{51}{}_{}{}^{}\mathrm{Mn}$ in the range 2.5<$E_{\mathrm{p}<3.1\mathrm{}}$ MeV (7.7<$E_x$<8.3 MeV). Eighty resonances, some of which are unresolved doublets, were found. Gamma-ray spectra were measured for thirty resonances which populated bound levels in ${}_{}{}^{51}{}_{}{}^{}\mathrm{Mn}$ up to an excitation energy of 5.174 MeV. Angular distributions were measured in the (p,p’γ) and (p,γ) channels for forty-eight resonances. These measurements and capture spectra allow spin assignments to be made for about sixty resonances. A ${(9/2\mathrm{}}^{+}$ core excited state has been found at $E_{\mathrm{p}=2.798\mathrm{}}$ (8.012 MeV) and its decay measured to high spin bound levels including two previously unreported levels at 4.463 and 4.741 MeV.

The isomeric cross-section ratio ${\sigma }_m$/(${\sigma }_m$+${\sigma }_g$) for the formation of ${}_{}{}^{73\mathrm{m}}{}_{}{}^{}$,gSe was determined in the ${}_{}{}^{70}{}_{}{}^{}\mathrm{Ge}$(α,n) reaction over the energy range of 13–27 MeV, in the ${}_{}{}^{\mathrm{nat}}{}_{}{}^{}\mathrm{Ge}$${(}^3$He,xn) process over 13–24 MeV, in the ${}_{}{}^{75}{}_{}{}^{}\mathrm{As}$(p,3n) reaction over 25–45 MeV, and in the ${}_{}{}^{75}{}_{}{}^{}\mathrm{As}$(d,4n) reaction over 28–56 MeV. Measurements were done radiochemically using the ‘‘stacked-foil’’ technique. In (α,n) and ${(}^3$He,xn) processes the ratio is relatively high at low incident particle energies but decreases with increasing energy. In (p,3n) and (d,4n) reactions, on the other hand, it is practically constant. Statistical model calculations taking into account precompound emission were carried out on the four processes investigated in this work as well as on the ${}_{}{}^{74}{}_{}{}^{}\mathrm{Se}$(n,2n) reaction reported in the literature. The total cross section (${\sigma }_m$+${\sigma }_g$) is described well by the calculation. The calculated isomeric cross-section ratio depends strongly on the input level scheme of the product nucleus. In general, however, the statistical model, under a suitable set of global assumptions, can reproduce the isomeric cross-section ratio in all the five nuclear processes.

Improved corrections for inscattering and photon regeneration were made to observed total absorption cross sections of uranium reported earlier. Electron-pair creation cross sections ${\sigma }_K$ for 10 and 20 MeV photons on ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ were obtained from the adjusted total cross sections. Recently published theoretical values of ${\sigma }_K$ at these energies agree within about 1% with the revised experimental values. The discrepancy at 10 MeV was previously thought to be about 4%. The remaining discrepancy may be due to neglect of the finite size of the nucleus in the calculations, or to slightly inaccurate screening or radiative corrections.

Inelastic electron scattering at incident energies between 170 and 260 MeV was used to identify and study M8 transitions in ${}_{}{}^{52}{}_{}{}^{}\mathrm{Cr}$. A strong transition to an $8^{\mathrm{-}}$ state at $E_x$=15.47 MeV was observed, as well as a number of weaker transitions. The results are compared with a single particle-hole shell model calculation that uses a model space of the form [(${\mathit{f}}_{7/2}^{11}$×${\mathit{g}}_{9/2}$${]}_{8\mathrm{-}}$. The shell model calculation and systematics in neighboring nuclei were used to determine the isospin of the observed states. The experimentally determined strengths exhaust 60.8% and 34.5% of the T=3 and T=2 sum rules, respectively.

Energy levels of ${}_{}{}^{101}{}_{}{}^{}\mathrm{Ru}$ have been studied by the ${}_{}{}^{100}{}_{}{}^{}\mathrm{Ru}$(d,p${)}^{101}$Ru reaction at an incident deuteron energy of 12 MeV. Outgoing particles were momentum analyzed by a magnetic spectrograph and detected in nuclear emulsion plates, with an energy resolution of 7.5 keV. A total of 68 levels up to 3.2 MeV excitation energy was identified, about two-thirds of them reported for the first time. Experimental angular distributions were compared to distorted-wave Born approximation predictions and reduced spectroscopic factors obtained. The total l=2 and 75% of l=0, 4, and 5 spectroscopic strengths were located. Attention is drawn to transitions to low-lying states in ${}_{}{}^{101}{}_{}{}^{}\mathrm{Ru}$ (below $E_{\mathrm{exc}=0.75\mathrm{}}$ MeV) with l=3 and l=1 character.

Elastic angular distributions for ${}_{}{}^{12}{}_{}{}^{}$C${+}^{12}$C and ${}_{}{}^{16}{}_{}{}^{}$O${+}^{12}$C have recently been measured in the c.m. energy range between 70 and 700 MeV. The wide angular range covered by the measurements exhibits, in every case, a distinct Fraunhofer crossover followed at larger angles by a farside-dominated ‘‘smooth falloff,’’ whose appearance is a clear sign that these systems (unlike heavier ones) are surface transparent. A recent optical-model analysis of these data has produced a family of refractive ($V_0$/$W_0$≳6) potentials common to all the angular distributions, whose analysis is the subject of this and the following paper. The present paper identifies key interference dips in the farside cross sections which signify the occurrence of deeply penetrating trajectories. These dips are responsible for reducing or even eliminating potential ambiguities, and the following paper exhibits and identifies the interfering farside trajectories via a semiclassical analysis of the optical potential amplitudes. The net result is an unusually clear and detailed physical understanding of highly complex angular distributions.

The optical potential fits summarized in the preceding paper are subjected to a semiclassical analysis of the Ford-Wheeler–Knoll-Schaeffer type. The important broad dips in their farside cross sections, which are essential in greatly reducing potential ambiguities, are found (in partial agreement with a suggestion of Goldberg’s) to be mainly weak ‘‘Airy’’ or rainbow minima, that serve to identify deeply penetrating trajectories. The semiclassical analysis also permits the identification and understanding of a new category of discrete and continuous potential ambiguities, and suggests the manner in which specific features of the angular distributions (such as spacings and depths of various angular minima) determine the Woods-Saxon parameters found by a chi-squared search.

High spin states in ${}_{}{}^{81}{}_{}{}^{}\mathrm{Sr}$ were investigated in the reactions ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$${(}^{28}$Si,2pn) at 103.6 MeV and ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$${(}^{29}$Si,p2n) at 95.2 MeV using the techniques of in-beam γ-ray spectroscopy. Measurements were made of γ-γ coincidences and γ-ray directional correlations from oriented nuclei. The lifetimes of 12 states and lifetime limits of 10 others were determined with the Doppler shift attenuation method. The data have been analyzed in terms of the Woods-Saxon cranking model. The first crossing observed in the yrast band is attributed to the alignment of a pair of $g_{9/2}$ protons. The irregularities observed in the $J^{\mathrm{}\left(2\right)\mathrm{}}$ moment of inertia in the $K^{\pi }$${=(5/2\mathrm{}}^{\mathrm{-}}$ and ${1/2}^{\mathrm{}(+)\mathrm{}}$ bands are interpreted as evidence for the alignment of a $g_{9/2}$ proton pair as well. The coexistence of prolate and oblate configurations at low spin is discussed in terms of the polarization of the soft even-even core by the odd neutron.

Breakup processes in 220- and 341-MeV ${}_{}{}^{20}{}_{}{}^{}$Ne${+}^{197}$Au reactions were studied by performing coincidence measurements of the secondary fragments. Projectile-like fragments and light charged particles corresponding to primary stripping, pickup, and inelastic channels were examined. The projectile-like fragments were detected near the classical grazing angle. Kinematic reconstructions of the three-body final states were used to deduce the excitations in the primary projectile-like and target-like fragments. At both bombarding energies, the stripping channels produced relatively cold ejectiles, with the excitation residing mostly in the target recoil. However, the pickup channels tended to leave the target cold, while populating higher excitations in the ejectile. The data are consistent with nucleon transfer as the dominant mode for generating excitation, with the partition of excitation energy between the fragments governed by the direction of the transfer. An extended version of the optimum-Q-value model of Siemens et al. predicts target-like excitations in good agreement with data at 11 MeV/nucleon, but with deviations occurring at 17 MeV/nucleon.

A statistical model analysis of heavy-ion, xnγ data puts constraints on the electromagnetic dipole strength functions at low energies, $E_{\gamma }$∼0.5–4 MeV. The M1 strength is inferred to be up to an order of magnitude smaller than at the higher energies of neutron capture gamma rays. The analysis supports the Kadmensky-Markushev-Furman shape of the E1 strength function, with a low-energy tail of the giant resonance that is suppressed relative to the shape based on a Lorentzian. The data considered include the yrast feeding pattern in the N=92 isotones ${}_{}{}^{160}{}_{}{}^{}\mathrm{Er}$, ${}_{}{}^{162}{}_{}{}^{}\mathrm{Yb}$, and ${}_{}{}^{164}{}_{}{}^{}\mathrm{Hf}$, and in ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$ up to spin 60ħ.

The Q value of the ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$${(}^7$Li, ${}_{}{}^8{}_{}{}^{}\mathrm{He}$) ${}_{}{}^{39}{}_{}{}^{}\mathrm{Sc}$ reaction has been measured, and a value of -37.40(4) MeV was obtained. The corresponding mass excess of ${}_{}{}^{39}{}_{}{}^{}\mathrm{Sc}$ is -14.14(4) MeV, which makes ${}_{}{}^{39}{}_{}{}^{}\mathrm{Sc}$ proton unbound by 630(40) keV. Under the assumption of a quadratic form, the b and c coefficients of the isobaric multiplet mass equation for 39 (T=(3/2) were obtained. They agree well with systematics and the predictions of the isospin-nonconserving Hamiltonian model of Ormand and Brown.