The isobar-doorway model is formulated for $\overline{K}$-nucleus scattering. A $\overline{K}$-nucleus optical potential is derived. Using the closure approximation, binding energies and widths of the $\Lambda \mathrm{}\left(1405\right)\mathrm{}$ and $\Lambda \mathrm{}\left(1520\right)\mathrm{}$ in nuclear matter are obtained from $\overline{K}$-atomic data and elastic scattering.

NUCLEAR REACTIONS Meson induced reactions. Properties of hyperon resonances.

Phenomenological fits to low-energy $\pi$-nucleus scattering have uniformly required a greatly enhanced $s$-wave repulsion in the optical potential, for which no satisfactory theoretical explanation has been given. Evaluation of nonlinear terms in field-theoretic Lagrangians with broken chiral symmetry and inclusion of necessary binding effects leads to a substantial, repulsive ${\rho }^2\mathrm{}\left(r\right)\mathrm{}$ potential, even on an isospin zero target. This potential may account for the previously unexplained effect.

NUCLEAR REACTIONS Pion-nucleus elastic scattering; field theoretic optical potential. Binding corrections. ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ targets.

We analyze, using basic geometrical arguments, the $A$ dependence of the total pion absorption cross section, the effective number of nucleons sharing the pion momentum and energy, and the proton yields from ${\pi }^{+}$ and ${\pi }^{-}$ induced reactions. The results are consistent with the pion penetrating some distance through the nuclear volume and annihilating on a pair of nucleons.

NUCLEAR REACTIONS transport theory applied to true pion absorption in nuclei.

We show that virtual pion rescattering in a nuclear medium gives rise to an effective $S$-wave $\pi \mathrm{NN}$ interaction Hamiltonian. The first few terms in the density expansion of the strength parameter are estimated and shown to be appreciable in nuclear matter.

NUCLEAR REACTIONS Pion-nucleus interactions: $S$-wave interaction.

Low-spin states of ${}_{}{}^{250}{}_{}{}^{}\mathrm{Cf}$ have been investigated by measuring $\gamma$ rays and conversion electrons associated with the electron capture decay of 2.22-h ${}_{}{}^{250}{}_{}{}^{}\mathrm{Es}$. Mass-separated ${}_{}{}^{250}{}_{}{}^{}\mathrm{Es}$ samples produced by the ${}_{}{}^{249}{}_{}{}^{}\mathrm{Cf}(d, n)$ reaction were used for these measurements. The $\gamma$-ray spectra were measured with a 25-${\mathrm{cm}}^3$ coaxial Ge(Li) spectrometer and the electron spectra were measured with a cooled Si(Li) detector. Multipolarities of intense transitions in ${}_{}{}^{250}{}_{}{}^{}\mathrm{Cf}$ were deduced and $log\mathrm{ft}$ values of electron capture transitions were derived from measured electron capture intensities. On the basis of the results of the present investigation the following bandheads were identified in ${}_{}{}^{250}{}_{}{}^{}\mathrm{Cf}$: $E$ (keV),$K$, $I\pi =871.6, 2, 2-; 1031.9, 2, 2+; 1154.2, 0, 0+; 1175.5, 1, 1-; 1210.0, 2, 2-; 1244.4, 2, 2+; 1266.5, 0, 0+; \mathrm{and} 1658.1, 2, 2+$. The 2.22-h state in ${}_{}{}^{250}{}_{}{}^{}\mathrm{Es}$ has been given a spin-parity assignment of 1- with configuration ${\mathrm{}\left\{n\right[734\mathrm{}]\mathrm{}\frac{9}{2-}; p[633\left]\mathrm{}\frac{7}{2+}\right\}}_{1-}$.

RADIOACTIVITY ${}_{}{}^{250}{}_{}{}^{}\mathrm{Es}$ [from ${}_{}{}^{249}{}_{}{}^{}\mathrm{Cf}(d, n)$]; measured $T_{\frac{1}{2}}$, $E_{\gamma }$, $I_{\gamma }$, $E_{\mathrm{ce}}$, $I_{\mathrm{ce}}$. ${}_{}{}^{250}{}_{}{}^{}\mathrm{Cf}$ deduced levels, $log\mathrm{ft}$ (EC), $\gamma$ multipolarity, $I$, $\pi$. Mass-separated ${}_{}{}^{250}{}_{}{}^{}\mathrm{Es}$. Cooled Si(Li) electron spectrometer.

In the analysis of time-reversal and Mössbauer absorption experiments, it is important to consider atomic processes which interfere with the direct nuclear transition. Interaction of the photon with the atomic electrons causes the radiation to acquire a phase shift, specified by the interference parameter $\xi \left(L\pi \right)$. We present theoretical expressions for $\xi$ and compare our calculated values with experiment. Satisfactory agreement is obtained. In particular, an apparent violation of time-reversal invariance in the 129-keV transition of ${}_{}{}^{191}{}_{}{}^{}\mathrm{Ir}$ is fully explained by these effects.

RADIOACTIVITY Atomic effects on nuclear electromagnetic transitions; time-reversal violation; dispersion of Mössbauer lines.

The nuclide ${}_{}{}^{45}{}_{}{}^{}\mathrm{K}$ has been produced by fragmentation of a uranium carbide target with 600 MeV protons and also, as a daughter product of the disintegration of ${}_{}{}^{45}{}_{}{}^{}\mathrm{Ar}$, by the $\mathrm{V}(p, 6pxn)\mathrm{Ar}$ spallation reaction on a vanadium carbide target. Its $\beta$ decay to ${}_{}{}^{45}{}_{}{}^{}\mathrm{Ca}$ has been studied via delayed $\gamma$-ray singles and coincidence techniques. Evidence is found for approximately 10 new $\beta$ branches. Excitation energies in ${}_{}{}^{45}{}_{}{}^{}\mathrm{Ca}$ and $\gamma$-branching fractions have been determined for 16 levels. Spectroscopic data deduced from the $\beta$ decay selection rules are compared to theoretical predictions.

RADIOACTIVITY ${}_{}{}^{45}{}_{}{}^{}\mathrm{K}$ from $p+U$ at 600 MeV and from $\mathrm{V}(p, 6pxn){}_{}{}^{45}{}_{}{}^{}\mathrm{Ar}\left({\beta }^{-}\right)$, natural targets, mass separated, measured delayed $E_{\gamma }$, $I_{\gamma }$, $\gamma -\gamma$ coincidences, $T_{\frac{1}{2}}$, deduced decay scheme, $log\mathrm{ft}$, $J$, $\pi$.

Gamma-ray angular correlations have been measured for several cascades populated in the decays of ${}_{}{}^{147,149}{}_{}{}^{}\mathrm{Eu}$ to ${}_{}{}^{147,149}{}_{}{}^{}\mathrm{Sm}$. The general features of the decay schemes have been confirmed, and the spin of the 1054-keV level in ${}_{}{}^{147}{}_{}{}^{}\mathrm{Sm}$ was determined to be $\frac{3}{2}$. Multipole mixing ratios for several gamma transitions have been deduced.

RADIOACTIVITY ${}_{}{}^{147}{}_{}{}^{}\mathrm{Eu}$ [from ${}_{}{}^{147}{}_{}{}^{}\mathrm{Sm}(d, 2n)$], ${}_{}{}^{149}{}_{}{}^{}\mathrm{Eu}$ [from ${}_{}{}^{149}{}_{}{}^{}\mathrm{Sm}(p, n)$]; measured $\gamma \gamma \left(\theta \right)$. ${}_{}{}^{147,149}{}_{}{}^{}\mathrm{Sm}$ deduced $J$, $\pi$, $\delta \left(\frac{E2\mathrm{}}{M1\mathrm{}}\right)$.

The possibility of abnormally occupying plane-wave Hartree-Fock determinants is studied analytically for several many-fermion systems interacting with short- and long-ranged pair potentials in three dimensions. Lower energy states are found in some cases, but they always depend sensitively on the shape of the potential as well as on the amount of space exchange present in the interaction.

NUCLEAR STRUCTURE Plane-wave Hartree-Fock, nuclear matter.

We describe the odd-mass indium nuclei $A=119, 121$ within the framework of the unified-model taking into account the coupling of single-hole and one-particle-two-hole proton configurations with quadrupole and octupole vibrations of the underlying core. Besides the energy spectra, spectroscopic factors for pickup and electromagnetic transition properties [branching ratios, $T_{\frac{1}{2}}\left(\frac{1}{2_1^{+}}\right)$; $\delta (\frac{3}{2_1^{-}}\to \frac{1}{2_1^{-}})$] are calculated and compared with experimental data, mainly for ${}_{}{}^{119}{}_{}{}^{}\mathrm{In}$: The yrast structure of the vibrational multiplet states has also been studied in lowest order perturbation theory and in the unified-model description. These calculations are also approached from a deformed zero-order description giving the total potential energy surfaces of all odd-mass In nuclei. Extensive band-mixing calculations produce energy spectra for the $\frac{1}{2^{+}}$ rotational-like band and are used to calculate static moments and transition rates. The equivalence with the former, spherical description is pointed out.

NUCLEAR STRUCTURE Unified-model calculations, ${}_{}{}^{119}{}_{}{}^{}\mathrm{In}$ and ${}_{}{}^{121}{}_{}{}^{}\mathrm{In}$, level schemes, branching ratios $T_{\frac{1}{2}}\left({\frac{1}{2}}_1^{+}\right)$, $\delta ({\frac{3}{2}}_1^{-}\to {\frac{1}{2}}_1^{-})$; yrast structure of vibrational multiplet, deformed description; total potential energy surfaces; Coriolis bandmixing.

The unusually large $E2\mathrm{}$ strength associated in the literature with the 1459→ 110 keV, $\frac{3}{2^{-}}\to \frac{1}{2^{-}}$ transition in ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ is found to be due to unreliable measurements of the $\frac{E2\mathrm{}}{M1\mathrm{}}$ mixing ratio. The published values of the mixing ratio have been reexamined. New measurements of the angular distributions and linear polarizations of the 1459→ 110 keV and 1459 keV→g.s. $\gamma$ rays have been carried out, using the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}(p, p^{\prime }\gamma ){}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ reaction to populate the 1459 keV level. The best value of the mixing ratio from the former and present measurements is $\delta =0.248\mathrm{}\pm 0.020$, corresponding to an $E2\mathrm{}$ strength for the 1459→ 110 keV transition of 18.7±1.9 Weisskopf units.

NUCLEAR STRUCTURE ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$; measured $\gamma \left(\theta \right)$, $P\left(90\mathrm{}°\right)$; deduced $\delta \left(\frac{E2\mathrm{}}{M1\mathrm{}}\right)$ for 1459 → 110.

Electron-deuteron scattering observables, including the tensor polarizations $T_{20}$, $T_{2\pm 1}$, $T_{2\pm 2}$, are calculated for a variety of $N-N$ potential models. The main goal is to determine how the tensor polarization experiments can help distinguish competing potential models with respect to off-shell behavior and tensor force strength. The pair meson-exchange-current correction of Gari and Hyuga is included in the analysis as well as a correction suggested by Friar for energy-dependent potentials. It is found that whereas electron-deuteron elastic scattering cross sections do not readily distinguish different potentials, the tensor polarizations do. The tensor polarization $T_{20}$ mainly distinguishes potentials with different deuteron $s$-wave momentum distributions, but does not distinguish potentials with different $d$-state probabilities ($P_D$). The tensor polarization $T_{2\pm 1}$, since it factors essentially into a product of the magnetic and quadrupole form factors, allows for the extraction of the quadrupole form factor which is closely related to the tensor force strength and $P_D$. The tensor polarization $T_{2\pm 2}$ is completely determined by the elastic scattering cross sections and magnetic form factor, and yields no additional information on the $N-N$ force unobtainable from elastic scattering measurements. The tensor polarizations $T_{20}$ and $T_{21}$ have maximum values of order unity in the region $q=2\mathrm{} \mathrm{to} 5$ ${\mathrm{fm}}^{-1\mathrm{}}$. The calculations indicate that measurements of $T_{20}$ for $2\lesssim q\lesssim 5$ ${\mathrm{fm}}^{-1\mathrm{}}$, and $T_{2\pm 1}$ for $3\lesssim q\lesssim 5$ ${\mathrm{fm}}^{-1\mathrm{}}$ could yield important information on the off-shell behavior and tensor force strength. The meson-exchange-current and energy-dependence corrections are important and must be taken into account to extract potential properties from such experiments.

NUCLEAR STRUCTURE ${}_{}{}^2{}_{}{}^{}\mathrm{H}$: form factors, tensor polarization calculated: dependence on off-shell behavior and $P_D$; seven potential models; pair currents included.

A Galilean-noninvariant correction to ordinary one-boson-exchange potentials is derived. It gives rise to a three-body force in the three-nucleon system. Its importance for the properties of the three-nucleon bound state is tested and found to be small. This result justifies the common practice which neglects the dependence of the one-boson-exchange potentials on the total momentum of the interacting pair in many-nucleon systems.

NUCLEAR STRUCTURE Three-body gound state dependence of nucleon-nucleon potential on total pair momentum.

Collective and single-particle contributions to transverse form factors of rotational nuclei have been computed in the projected Hartree-Fock approach using the density matrix expansion effective Hamiltonian. Results on several odd-$A$ rare earth nuclei (${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$, ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$, ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tb}$) are presented and compared to the available experimental data. Theoretical predictions for transverse electric form factors of ${}_{}{}^{166}{}_{}{}^{}\mathrm{Er}$ are also presented. A comparison is made of the resulting form factors as $q\to 0$ with those extracted from measured transition probabilities $B\left(E\right(M)\mathrm{}\lambda ; I_i\to I_f)$.

NUCLEAR STRUCTURE ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tb}$, ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$, ${}_{}{}^{166}{}_{}{}^{}\mathrm{Er}$, ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$; calculated elastic and inelastic transverse form factors. Angular momentum projected Hartree-Fock approximation.

The boson expansion theory, based on which successful explanations were made earlier of collective properties of Ru, Pd, Sm, and other isotopes, is applied here to the Os and Pt isotopes. The nuclei are characterized by the fact that their energy spectra and the $E2\mathrm{}$ transitions embody in themselves a very strong $\gamma$-unstable nature, nevertheless having quadrupole moments $Q\left(2_1^{+}\right)$ which are large, being negative in Os and positive in the Pt isotopes. It is shown that all these properties can be predicted using the present approach.

NUCLEAR STRUCTURE ${}_{}{}^{186-194}{}_{}{}^{}\mathrm{Os}$, ${}_{}{}^{188-198}{}_{}{}^{}\mathrm{Pt}$, energy levels, $B\left(E\right)\text{'}\mathrm{s}$, branching ratios, magnetic moments, static quadrupole moments, boson expansion theory.

Particle identity in quantum mechanics makes it impossible to test the Pauli exclusion principle by looking for "forbidden" x rays or $\gamma$ rays. Such experiments do, however, test particle stability. Evidence for the indistinguishability of electrons and nucleons is discussed.

NUCLEAR STRUCTURE Particle identity and the Pauli exclusion principle, particle stability.

A recent in-beam study indicates that the first-excited state in the $N=82\mathrm{}$ nucleus, ${}_{}{}^{146}{}_{}{}^{}\mathrm{Gd}$, is $3^{-}$, rather than $2^{+}$ as systematics had indicated. A subsequent investigation, which utilized ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}(\alpha , 2n\gamma )$ excitation function measurements, has provided evidence that the lowest $2^{+}$ state in ${}_{}{}^{146}{}_{}{}^{}\mathrm{Gd}$ lies at 1971 keV, about 392 keV above the $3^{-}$ level. It seems therefore that, as in the case of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, and ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ where the first-excited states are not $2^{+}$, ${}_{}{}^{146}{}_{}{}^{}\mathrm{Gd}$ may have a doubly closed-shell structure. Previously, we investigated levels in ${}_{}{}^{146}{}_{}{}^{}\mathrm{Gd}$ populated in the decay of 23-s ${}_{}{}^{146}{}_{}{}^{}\mathrm{Tb}$. Data from these decay studies were reexamined, and a weak 1971.8-keV $\gamma$ ray was observed to follow the decay of ${}_{}{}^{146}{}_{}{}^{}\mathrm{Tb}$. This transition most probably deexcites the $2^{+}$ ${}_{}{}^{146}{}_{}{}^{}\mathrm{Gd}$ level seen in the ${}_{}{}^{144}{}_{}{}^{}\mathrm{Sm}(\alpha , 2n\gamma )$ reaction. In the light of the newly available information, a modified ${}_{}{}^{146}{}_{}{}^{}\mathrm{Tb}$ decay scheme is presented and discussed.

RADIOACTIVITY ${}_{}{}^{146}{}_{}{}^{}\mathrm{Tb}$, measured $T_{\frac{1}{2}}$, $E_{\gamma }$, $I_{\gamma }$; ${}_{}{}^{146}{}_{}{}^{}\mathrm{Gd}$, deduced levels, $J^{\pi }$.

Relative intensities for $K$ x rays and gamma rays emanating from ${}_{}{}^{245}{}_{}{}^{}\mathrm{Cm}$ have been measured using several Ge photon detectors. The absolute intensity for the dominant 175-keV gamma ray in ${}_{}{}^{241}{}_{}{}^{}\mathrm{Pu}$ was determined to be 9.5 photons per 100 ${}_{}{}^{245}{}_{}{}^{}\mathrm{Cm}$ alpha decays with an uncertainty of ± 7%. Eleven gamma rays have been placed as transitions among levels in ${}_{}{}^{241}{}_{}{}^{}\mathrm{Pu}$.

RADIOACTIVITY ${}_{}{}^{245}{}_{}{}^{}\mathrm{Cm}$; measured $E_{\gamma }$, $I_{\gamma }$, $I_{K-x}$. Deduced level energies in ${}_{}{}^{241}{}_{}{}^{}\mathrm{Pu}$. Chemically separated isotopically pure source.

Energy spectra of pions produced by the ($p, {\pi }^{+}$) reaction on ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, and ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}$ leading to discrete final states have been measured with 170-210 keV resolution. Except for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, final states that can be reached by the simple transfer of a neutron tend to be favored compared to final states of more complicated structure, which is suggestive of a one-step reaction mechanism. These ($p, {\pi }^{+}$) spectra are surprisingly similar to ($d, p$) spectra at much lower momentum transfer. For ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, single-particle and 2p-1h final states are populated about equally and there is little similarity between the ($p, {\pi }^{+}$) and ($d, p$) spectra.

NUCLEAR REACTIONS ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}(p, {\pi }^{+}){}_{}{}^{11}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}(p, {\pi }^{+}){}_{}{}^{12}{}_{}{}^{}\mathrm{B}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}(p, {\pi }^{+}){}_{}{}^{13}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}(p, {\pi }^{+}){}_{}{}^{14}{}_{}{}^{}\mathrm{C}$, $E_p=200\mathrm{}$ MeV; measured $\sigma \left(E_{\pi }\right)$ at ${\theta }_{\mathrm{lab}}=25\mathrm{}°$.

The contributions of the $N^{*}\mathrm{}\left(1688\right)\mathrm{}$ resonance to the two-nucleon scattering phase parameters are compared with the effects of the $\Delta \mathrm{}\left(1236\right)\mathrm{}$ resonance, and are found to be small.

NUCLEAR REACTIONS Nucleon-nucleon interaction. $N^{*}\mathrm{}\left(1688\right)\mathrm{}$ effects.