The photoproduction of wide-angle electron pairs from hydrogen and carbon has been measured as a test of quantum electrodynamics (QED), using the 5-GeV electron synchrotron NINA. The results are consistent with the predictions of QED up to an invariant mass of the electron-positron pair of 500 MeV/${\mathit{c}}^2$.

Incoherent photoproduction of ${\rho }^0$ mesons on the nucleus is calculated assuming vector dominance. The results are sensitive to the $\rho$-nucleon total cross section which is found to decrease with energy. One finds a relatively weak dependence on energy of the incoherent effective nucleon number.

The decay rate of the 8.88-MeV state of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ is estimated with a parity-nonconserving force involving $\rho$ exchange. It is pointed out that this decay can be of crucial importance for testing part of the nonleptonic weak interaction, because isospin conservation in the $\alpha$ decay limits the interaction responsible for the parity mixing in the initial state to the $\Delta I=0\mathrm{}$ part arising from the strangeness-nonchanging nonleptonic weak currents.

A partial-wave analysis is made of the $\pi \rho$ mass enhancement in the $A_1$ region for the Reggeized Deck amplitude of Berger. It is found that the state is predominantly $1^{+}S$ wave in agreement with the semilocal-duality interpretation of Chew and Pignotti, but the amplitude does not provide a local description of $A_1$ resonance production.

A method of polarizing channeled deuterons by their passage through a monocrystalline nickel foil magnetized to saturation and subsequently through a region of weak magnetic field is described. A beam with tensor polarization $P_{\mathrm{ZZ}}=-0.32\mathrm{}\pm 0.010$ is obtained.

The suppression of drift instabilities in a magnetoplasma having high electron temperature is obtained by avoiding transverse temperature gradients.

A finite segment of an infinite, harmonically bound, weakly coupled oscillator chain evolves to a final state of equilibrium as a consequence of the statistical influence of the heat bath—the rest of the chain—on the system, via the equations of motion. Information theory eliminates the need for coarse graining or ergodicity. The entropy, defined in terms of the reduced Liouville function of the system variables, is shown to evolve to the equilibrium value, and equipartition follows directly.

A recent generalization by Sherman of Griffiths' second inequality on correlations in Ising ferromagnets is further generalized. Straightforward proofs are provided both for the original inequality and its generalization.

The liquid structure factor of helium-4 has been determined in the momentum-transfer range 0.2 to 0.8 ${\mathrm{\AA }}^{-1\mathrm{}}$. At 0.32 K the structure factor exhibits a gentle but decided change of slope near 0.4 ${\mathrm{\AA }}^{-1\mathrm{}}$ in qualitative agreement with certain theoretical predictions. Below 0.4 ${\mathrm{\AA }}^{-1\mathrm{}}$ the liquid structure factor agrees with that deduced from the Bijl-Feynman dispersion relation. At 4.99 K and 680 mm pressure we have shown that helium gas is distinctly nonideal.

The nuclear magnetic susceptibility of solid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ was measured between temperatures of 5.3 and 800 mK for several molar volumes. In each case, the susceptibility followed a Curie-Weiss law with a negative Weiss constant, hence a negative exchange interaction $J$. Magnitudes of $J$ as a function of molar volume agree with previous measurements.

Using the cooling power of the adiabatic solidification of ${\mathrm{He}}^3$, we have reached temperatures near 2 mK and have measured the temperature dependence of the susceptibility of solid ${\mathrm{He}}^3$ down to 6 mK. The magnetic ordering is antiferromagnetic, with a Weiss $\theta =3.0\mathrm{}\pm 0.3$ mK corresponding to a Néel temperature of 2.1 mK for bcc ${\mathrm{He}}^3$ at 34 atm.

A simple model for a new instability resulting from particles trapped in a large-amplitude electrostatic wave is invoked to explain the generation of satellite frequencies in a recently reported experiment by Wharton, Malmberg, and O'Neil. The model predicts satellites on the large-amplitude wave at a frequency separation proportional to $\sqrt{E}$, where $E$ is the amplitude of the large wave. The predicted growth rates reasonably account for the observed growth of these satellites.

The absolute reflectance of liquid helium was measured as a function of wavelength between 1000 and 518 Å. The main feature of the data is a large peak in reflectance located at 574 Å and having a width (at half-maximum) of 15.5 Å. The absolute reflectance at the maximum is 0.12±0.01. The peak appears to correspond to the $1{}_{}{}^1{}_{}{}^{}S_0^{}{}_{}{}^{}\to 2{}_{}{}^1{}_{}{}^{}P_1^{}{}_{}{}^{}$ atomic transition. Very small structure near 597 Å may correspond to a forbidden atomic transition.

High-resolution optical studies of alkali halides at liquid-helium temperature have revealed complex structures associated with the exciton peaks. A sideband picture which involves the emission either of localized vibrations or of crystal phonons, depending upon the exciton radius, is proposed.

We have measured the time dependence of the hot-electron current following a step variation in the applied electric field in $n$-InSb at 4.2°K. We obtain the average electron energy as a function of electric field, as well as the energy dependence of both the mobility and the energy relaxation rate.

An absorption line of oxygen in silicon has been observed at 29.3 ${\mathrm{cm}}^{-1\mathrm{}}$ at 1.8°K. Assignment to oxygen has been confirmed by observation of the oxygen isotope effect. Effects of uniaxial stress on the line have been measured. Additional lines observed at 37.7, 43.5, and 48.9 ${\mathrm{cm}}^{-1\mathrm{}}$ when the crystals are heated to 35°K are assigned to a bending vibrational mode of ${\mathrm{Si}}_2$O.

We consider frequency conversion of x rays via the nonlinear interaction of short-wavelength radiation with crystalline solids. Phase-matched parametric down-conversion of Mo $K\alpha$ in diamond is computed to be observable with presently accessible sources.

The optical absorption coefficient for ferromagnetic semiconductors has been calculated for an indirect transition in which spin-fluctuation scattering takes the place of the usual phonon scattering. The resulting absorption coefficient is found to be proportional to $〈{\left|S_{{\overrightarrow{\mathrm{Q}}}_Z}\right|}^2〉$, where $\overrightarrow{\mathrm{Q}}$ is the wave-vector difference between the valence-band maximum and the conduction-band minimum. The fact that this decreases with decreasing temperature below the Curie point can lead to an apparent blue shift of the absorption edge similar to what is observed in Cd${\mathrm{Cr}}_2$${\mathrm{S}}_4$.