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Coincidence measurements of electrodisintegration products have been used to simultaneously study the $^{12}\mathrm{C}$+$^{12}\mathrm{C}$, $^{16}\mathrm{O}$+$^{8}\mathrm{Be}$, and $^{20}\mathrm{Ne}$+$\ensuremath{\alpha}$ decay channels of $^{24}\mathrm{Mg}$. Asymmetric fission into $^{16}\mathrm{O}$+$^{8}\mathrm{Be}$, is concentrated between 18 and 28 MeV in $^{24}\mathrm{Mg}$ and exhibits resonances with cross sections ten times those of symmetric fission. There is little correlation among resonances in the three decay channels. The fission yields are not consistent with statistical decays from giant resonances, and suggest highly clustered states in $^{24}\mathrm{Mg}$.

The location of the giant monopole resonance and, therefore, the compressibility is unknown in light nuclei. The forward-backward (e, e′p) angular correlation asymmetry from 12 C has been measured and found to be very sensitive to monopole strength in the giant dipole resonance region. Similar reactions will, thus, provide a sensitive tool in the search for monopole strength in light nuclei.

A precision luminosity monitor is described which has been used successfully in an experimental test of quantum electrodynamics at the 2.6 GeV electron-positron storage ring (SPEAR-I) at the Stanford Linear Accelerator Center. Design considerations and construction details are given, together with a discussion of radiative corrections and an evaluation of the response of the monitor.

We report measurements of e+e−→e+e−, e+e−→γγ, and e+e−→μ+μ− at angles close to 90°, relative to Bhabha scattering at 3.7°, at a center-of-mass energy equal to 5.2 GeV. The results are found to be consistent with the predictions of quantum electro-dynamics.

A multiwire proportional chamber system is described which has been used successfully in an experiment at the 2.6 GeV electron-positron storage ring (SPEAR) at the Stanford Linear Accelerator Center. Detailed descriptions of the chamber construction and test procedures are given, together with a description of the electronic read-out circuitry. The performance of the system during the experiment is reported and on-line displays of typical events are shown.

We report measurements of the reactions e/sup +/e/sup -/ ..-->.. e/sup +/e/sup -/, e/sup +/e/sup -/ ..-->.. ..gamma gamma.., and e/sup +/e/sup -/ ..-->.. ..mu../sup +/..mu../sup -/ at angles close to 90/sup 0/, relative to Bhabha scattering, at a center-of-mass energy of 5.2 GeV. The results are found to be in agreement with the predictions of quantum electrodynamics.

As a test of the validity of quantum electrodynamics (QED) for heavy virtual leptons, cross sections for wide-angle electron-pair photoproduction have been measured using hydrogen and carbon targets. Electron pairs were detected in two independent mirror-image spectrometers which identified production angles and momenta. A symmetrical arrangement was used in which interference between Bethe-Heitler and virtual Compton graphs is identically zero. Data were obtained at electron production angles from 5\ifmmode^\circ\else\textdegree\fi{} to 7\ifmmode^\circ\else\textdegree\fi{} and momenta from 1 to 2 GeV/c, and were compared with theory by means of a Monte Carlo calculation. Up to the maximum invariant mass of the electron pair 489 MeV/${\mathit{c}}^{2}$, the data are consistent with the predictions of QED for both targets, confirming the results of other recent wide-angle pair-production and bremsstrahlung experiments in this energy range. Expressing the results in terms of a cutoff parameter $\ensuremath{\Lambda}$, where any positive deviation from QED is proportional to ${\ensuremath{\Lambda}}^{\ensuremath{-}4}{{Q}_{l}}^{4}$ (where ${Q}_{l}$ is the mass of the virtual lepton), one obtains $\ensuremath{\Lambda}g0.76$ GeV/${\mathit{c}}^{2}$ with 95% confidence. For a similar negative deviation, $\ensuremath{\Lambda}g0.55$ GeV/${\mathit{c}}^{2}$.

The study of elastic electron scattering from nuclei, at angles near 180°, permits an examination of nuclear magnetic multipole distributions as well as the testing of certain aspects of quantum electrodynamics. Apparatus has therefore been designed to enable such experiments to be performed at the Stanford Mark III linear accelerator. The special problems connected with 180° elastic scattering are discussed. In general, it is necessary to subtract from the data, contributions from ordinary charge (Coulomb) scattering. This background is mainly due to electrons which scatter at angles other than 180° and yet enter the detector because of multiple scattering in the target, finite beam spread and finite solid angle. 180° scattering is achieved by passing the incident beam through a magnetic field before it strikes the target. Background scattered electrons then pass through the field a second time, bending away from the incident beam, and enter a spectrometer. The momentum range restrictions of the magnet optics and the optimization of the solid angle are discussed. A novel method is presented of varying the scattering angle around 180° without causing significant phase space restrictions. Practical details of the apparatus are discussed and an outline of the data reduction methods used in such experiments is presented.

We have measured the cross section for elastic electron scattering from ${\mathrm{He}}^{4}$ at values of ${q}^{2}$, the four-momentum transfer squared, ranging from 0.5 to 20.0 ${\mathrm{F}}^{\ensuremath{-}2}$. Previous Stanford measurements extended up to ${q}^{2}=6.2$ ${\mathrm{F}}^{\ensuremath{-}2}$ and were consistent with a Gaussian shape for the charge distribution of the ${\mathrm{He}}^{4}$ nucleus. Our results confirm the earlier data but show a deviation from the Gaussian model for ${q}^{2}g6$ ${\mathrm{F}}^{\ensuremath{-}2}$. At ${q}^{2}=10$ we observe a diffraction minimum of the scattering cross section. Among the models which have been found to fit the new data is the Fermi three-parameter charge distribution. The rms radius ${〈{r}^{2}〉}^{\frac{1}{2}}$, the half-density radius ${r}_{\frac{1}{2}}$, and the 90% to 10% skin thickness $t$ of the charge distribution have been calculated for the models which adequately fit the data. The results indicate that the surface of the ${\mathrm{He}}^{4}$ charge distribution is less diffuse than implied by the nuclear harmonic-oscillator model.

A design for a secondary electron beam suitable for electron scattering experiments at the National Accelerator Laboratory is presented. Using standard NAL beam transport magnets it will operate at up to 300 GeV/c with the following properties: acceptance 9.5μsr%Δp/p(fw), momentum bite ±2%, momentum resolution of momentum hodoscope Δp/p=±0.30% (hw at base), final spot size ≤±3.0mm×±5.5mm(hw at base). Hodoscopes are also included for measuring the angles at which electrons pass through the experimental target with resolutions in both planes of better than ±0.1 mr. These properties are achieved by a periodic beam structure which includes correction of 2nd order aberrations by means of sextupoles. The yield of electrons is estimated to be ∽108/1013 incident protons when the energy of the electrons is about one half the energy of the protons. It is emphasized that synchrotron radiation by electrons in the beam transport magnets is very significant in reducing pion contamination of the final beam spot. It is shown that superconducting magnets to enhance this effect are superfluos. Pion impurity is ≤0.01% at every energy and may be made essentially zero at energies greater than 160 GeV. The beam is designed with four focussing stages, the first two of which provide a beam spot of sufficient quality for tagged photon experiments.

The spectra of electrons scattered inelastically from 4 He have been measured at incident energies from 150 MeV to 400 MeV for scattering angles from 38° to 90°. Through the use of a liquid 4 He target, a high momentum resolution (≈0.25%) was obtained, and the 20 MeV 0 + state of the α-particle was observed for the first time in electron scattering. The excitation energy and the total width of this state were determined and are in good agreement with the results from other experimental methods. It was found that the total disintegration cross section appears be smaller than 2(σ p +σ n ) by more than an order of magnitude at the lowest q 2 (≈ 0.33 fm −2 ). In earlier work the assumption of a total cross section of 2(σ p +σ n ) was found to be quite accurate for higher q 2 . As a new result, the partial radiative width for the 0 + state is determined to be Γ rad = 1.1±0.3 meV.

The absolute differential cross section for the production of bremsstrahlung has been measured for photons emitted at 93° by 27.6 MeV electrons scattered by 27Al. The shape of the energy spectrum is in good agreement with theory but the ratio of the experimental to theoretical cross section predicted by the formula of Biel and Burhop is 1.23 ± 0 07. Full details of the experiment and analysis are given.

The matrix method for analysing a pulse height distribution, obtained with a spectrometer of finite resolution, is reviewed. It is shown how the statistical errors in the energy spectrum depend on those in the experimental pulse height distribution and the response matrix of the spectrometer. Accuracy of calibration may limit the number of channels which may be used in a given analysis. A simple first order correction is developed for the quadrature error involved in the matrix method and the consequent improvement in results demonstrated. An expression for the upper limit to the number of channels which may be usefully employed in a given analysis, is suggested. The design of experiments, in which continuous spectra are measured, and the choice of suitable spectrometers, are facilitated by these results.

The long-standing discrepancies between theory and experiment in the magnetic elastic and inelastic scattering of electrons from the deuteron at high momentum transfer are reexamined. It is concluded that the predictions using most deuteron models agree with the elastic scattering data if one employs the presently accepted dipole fit (with scaling) for the proton and neutron form factors. A second conclusion is that the inelastic scattering to the $^{1}S_{0}$ state is not affected by small changes in the nucleon form factors, but is completely dominated by dynamic effects such as those due to meson-exchange diagrams.

The efficiency of a Faraday cup for 29 MeV electrons has been measured to within 0.2% by a null method using a toroidal coil transformer. Similar accuracy can readily be obtained with all beams having pulse currents ⩾ 50 μ A. The method enables integrated beam currents to be determined absolutely without using a total-absorption cup.

The use of a photon monochromator for calibrating a total absorption NaI(Tl) spectrometer is described. Response functions have been obtained for photon energies up to 28 MeV and used to construct a response matrix. A particular advantage of this calibration method is that the usual integration of response functions over the energy channels is eliminated. The measured matrix, after suitable smoothing and grouping, was inverted and used in the measurement of a 28 MeV bremsstrahlung spectrum. Agreement of this measurement with theory provides confidence in the calibration. Formulae are given for the errors, due to counting statistics, in the experimentally determined spectrum, special attention being given to the effect of smoothing the matrix and pulse height spectrum and of grouping the channels in the bremsstrahlung spectrum.

We have studied elastic electron scattering from 4He at values of q2, the momentum transfers squared. ranging from 0.5 fm−2 to 10.0 fm−2. Among the models which have been found to fit the new data is modified Gaussian distribution with a half density radius of 1.19 fm and a skin thickness of 1.61 fm.

We have searched for virtual excited states of 3 He by inelastic electron scattering at E = 200 MeV and θ = 60°. No evidence for narrow states excited with cross section greater than 3 × 10 −33 cm 2 /sr has been found for excitation energies up to 17 MeV.

The purpose of the paper is to present a summary of recent developments in high energy electron physics at Stanford. The material is divided into the following topics: nucleon form factors; deuteron form factors; (coincidence studies of elastic scattering, inelastic continuum studies); nuclear physics; (Ca40, Ca44 Investigations, study of the inelastic continuum in He3, magnetic octupole moments in light nuclei); shower studies at 900 MeV.

The dead space between channels in a HutchinsonScarrott pulse-height analyzer is eliminated by connecting a small capacitor (10 to l5 pF) between the channel oscillator and the linear sweep so that the sweep has superimposed positive pips. This method prevents a coincidence occurring whsn a channel pip is due, thus the input pulse is held and recorded in the following channel. This sorting action is normal for each new channel. However, a pulse may not be counted in the next channel until the sweep again passes this level so that a time interval is created during which comparison is prohibited. This interval covers the dead time completely, due to the inherent delay of the circuits, and counts are no longer lost. The small displacement of the channel boundaries can be corrected. Results when measuring a uniform distribution of pulses are reduced from 3% to less than 10-4%. (N.W.R.)

The magnetic form factors of the $1p$-shell nuclei ${\mathrm{Li}}^{6}$, ${\mathrm{Li}}^{7}$, ${\mathrm{Be}}^{9}$, ${\mathrm{B}}^{10}$, ${\mathrm{B}}^{11}$, ${\mathrm{N}}^{14}$, have been measured by high-energy electron scattering at an angle of 180\ifmmode^\circ\else\textdegree\fi{}. Incident electron energies up to 230 MeV (${q}^{2}\ensuremath{\simeq}5.2$ ${\mathrm{F}}^{\ensuremath{-}2}$) were used. The incident- and scattered-electron paths were separated by use of a uniform magnetic field. Back-ground due to charge scattering was subtracted by varying the scattering angle around 180\ifmmode^\circ\else\textdegree\fi{}. At the higher momentum transfers the scattering from those nuclei having spin $\ensuremath{\ge}\frac{3}{2}$, was dominated by the octupole moment distributions. Results are interpreted in terms of harmonic-oscillator and finite-square-well shell models. Parameters dependent only on the coupling of the angular momenta in the $1p$ shell may be obtained from the data. They show a general trend from $L\ensuremath{-}S$ coupling to $j\ensuremath{-}j$ coupling as the mass number increases, in agreement with calculations of magnetic dipole moments and energy-level schemes. The outstanding exception is ${\mathrm{Li}}^{7}$, whose form factor is accurately described by the extreme independent-particle model. ($L\ensuremath{-}S$ coupling is quite incompatible with the data.) The ${\mathrm{Li}}^{6}$ results may also be interpreted in terms of an "alpha plus deuteron" model. Numerical results for the harmonic-well strengths are obtained and compare favorably with electron-charge-scattering values. It is also possible to estimate magnetic octupole moments, where applicable, with fair accuracy, although these estimations are to a certain extent model-dependent.