Photo pair productions of electrons, muons, and heavy leptons and bremsstrahlung of electrons and muons are reviewed. Atomic and nuclear form factors necessary for these calculations are discussed. Straggling of electrons in matter and other effects due to finite target thickness are considered. Tables of radiation lengths of all materials and the energy dependence of photon absorption coefficients of many materials are presented. Problems associated with production of particles by photon and electron beams are also discussed.

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Pair Production and Bremsstrahlung of Charged Leptons

Positive muons implanted into diamond and zincblende-structured semiconductors often form hydrogen-like paramagnetic muonium (${\ensuremath{\mu}}^{+}\ensuremath{-}{e}^{\ensuremath{-}}$) states whose characteristics can be investigated with the "muon spin rotation" ($\ensuremath{\mu}\mathrm{SR}$) technique. In contrast to the case of hydrogen, which is not known to form a paramagnetic state in semiconductors, two coexisting types of muonium states are seen. "Mu" with a large isotropic hyperfine interaction, and "${\mathrm{Mu}}^{*}$" with a small [111]-axially symmetric hyperfine interaction. Both "spectroscopic" properties of these states, such as the electronic $g$ factors and the nuclear hyperfine interactions, and "dynamic" properties, such as their diffusion rates and their rates of interconversion, are accessible with $\ensuremath{\mu}\mathrm{SR}$. Direct information about the site of the muonium states is available using the channeling effect of the positron from muon decay in a crystalline host. The techniques for probing semiconductors with positive muons are described in this review, and the results they have provided to date are critically discussed. The considerable amount of theoretical work that has been invested in microscopic models of Mu and ${\mathrm{Mu}}^{*}$ is also summarized.

Muonium states in semiconductors

We show that the nonrelativistic quark model leads to an exceptionally simple sum rule involving a form factor determined by high-energy inelastic electron-proton scattering. The simplicity of this result appears to be a unique consequence of the peculiar quark charges.

Sum rule for high-energy electron - proton scattering

Quarks and Leptons Beyond the Third Generation

Proton-nuclei cross sections at 20 GeV

Prompt publication of brief reports of important discoveries in physics may be secured by addressing them to this department. Closing dates for this department are, for the first issue of the month, tke eighteenth of the preceding month, for the second issue, the tkird of the month. Because of the late closing dates for the section no proof can be shown to authors. The Board of Editors does not hold itself responsible for the opinions expressed by the correspondents.

New Evidence for the Existence of a Particle of Mass Intermediate Between the Proton and Electron

Evidence for Spin Zero of the η From the Two Gamma-Ray Decay Mode

Data are presented on the details of the interaction $\ensuremath{\gamma}+p\ensuremath{\rightarrow}p+{\ensuremath{\rho}}^{0}$ observed in a 12-in. hydrogen bubble chamber exposed to a bremsstrahlung photon beam of 6-BeV maximum energy at the Cambridge Electron Accelerator. The energy dependence of the cross sections, the production angular distributions, and the decay angular distributions of the ${\ensuremath{\rho}}^{0}'\mathrm{s}$ are compared with the predictions of the one-pion-exchange (OPE) mechanism and with a diffraction or multiperipheral model proposed by Berman and Drell. These data, as well as a comparison with ${\ensuremath{\omega}}^{0}$ and with ${\ensuremath{\rho}}^{0}+{N}^{*}(1238)$ production, reject the OPE model and favor a diffraction mechanism.

J. C. Street

Papers

New Evidence for the Existence of a Particle of Mass Intermediate Between the Proton and Electron

Evidence for Spin Zero of the η From the Two Gamma-Ray Decay Mode

PHOTOPRODUCTION OF HYDROGEN OF /rho/$sup 0$ MESONS BETWEEN THRESHOLD AND 6 Bev

Small-Angle Proton Scattering at 3 Bev

Gamma-Ray Proton Interactions between 0.5 and 4.8 BeV