Electron Beam Direction Research Articles

Crystalline effects in backscattering and Auger production

Measurements have been made of the variation of the backscattered intensity and of the Auger signal intensity as a function of the direction of the incident electron beam using a (111) Al surface and an incidence energy of 1.5 keV. The dynamical theory of electron diffraction and backscattering is able to account for the results quantitatively to within a factor of about two in the case of the quasi elastic and first plasma loss backscattered components. The variation measured for the Auger signal is significantly greater than for backscattered electrons of similar energy.

Ejected-electron spectra of potassium autoionization levels obtained by electron impact excitation at 29 eV and 500 eV incident energies

Autoionizing levels in potassium have been observed by measuring the energy spectrum of electrons at 90 degrees to the direction of an incident electron beam. Spectra were taken at 29 eV and 500 eV incident electron kinetic energies. Where possible comparisons have been made with the ultraviolet absorption spectra of Beutler and Guggenheimer and Mansfield (1975).

Ejected electron spectrum of caesium autoionizing levels obtained by electron impact excitation

The ejected electron spectrum of caesium has been observed at 90 degrees to the direction of an incident electron beam of kinetic energy 500 eV. Comparisons of the features observed have been made with the ultraviolet absorption spectra of Beutler and Guggenheimer (1934) and Connerade (see abstr. A20043 of 1970) where possible.

NEUTRON PRODUCTION WITH HIGH‐CURRENT RELATIVISTIC ELECTRON BEAMS AND DEUTERATED POLYETHYLENE TARGETS

The direction of an intense relativistic electron beam by the utilization of a Bennett cathode has made possible experiments to exploit these extremely powerful energy sources for thermonuclear fusion. Some initial experiments yielded up to 4 x 10$sup 7$ neutrons, when a small pellet of deuterated polyethylene was inserted in the beam path of an electron beam machine. Later experiments used deuterated cathode tips in such a way that collectively accelerated ions contribute to the neutron yield through a beam--target interaction. Experimental results and the theoretical analysis of that data are reported. (MOW)

Low energy electron impact excitation of autoionizing levels in cadmium vapour

The ejected electron spectrum of Cd vapour has been observed at 90 degrees to the direction of the incident electron beam. Spectra obtained at incident energies between 15 and 40 eV show multiplet structure not observed at higher energies, nor in ultraviolet absorption studies. A number of the multiplets have been assigned based on the calculation of Wilson (1968).

Energy spectra transmitted through iron slabs of bremsstrahlung produced in iron and gold targets by 0.5–1.44-MeV electrons

The bremsstrahlung spectra transmitted through iron slabs were measured as a function of incident electron energy and iron-slab thickness. In this experiment, the gold and the iron targets were bombarded by 0.5–1.44-MeV electron beams from the Van de Graaff accelerator. Bremsstrahlung produced in the target and transmitted through the iron slab, which emerged at the forward direction of the incident electron beam, was detected with a NaI(Tl) scintillator. The experimental results were in good agreement with those of the approximate numerical calculation of the forward photon spectra. From the slopes of attenuation curves of total photon number and intensity, the effective attenuation coefficients and the effective energies were obtained as a function of the incident electron energy.

Dissociative Excitation of Molecular Hydrogen by Electron Impact

Dissociative excitation of molecular hydrogen can proceed via the process ${\mathrm{H}}_{2}+e\ensuremath{\rightarrow}\mathrm{H}(2s)+\mathrm{H}+e$ and yield metastable $\mathrm{H}(2s)$ atoms that have kinetic energies near 0.3 eV ("slow") or near 4 eV ("fast"). The dissociation process has been studied using a pulsed electron gun with an energy resolution of \ifmmode\pm\else\textpm\fi{} 0.3 eV and using a metastable atom detector capable of viewing $\mathrm{H}(2s)$ atoms with an angular resolution of 1\ifmmode^\circ\else\textdegree\fi{} over a range 60\ifmmode^\circ\else\textdegree\fi{}-120\ifmmode^\circ\else\textdegree\fi{} with respect to the electronbeam direction. The measurement of the angular intensity distribution gives information about the final states that are involved in the dissociation process. (i) For slow $\mathrm{H}(2s)$ atoms, the electron energy threshold for production of the least energetic of the slow metastable atoms is 14.6 \ifmmode\pm\else\textpm\fi{}0.3 eV. The excitation function and the angular distribution of the slow $\mathrm{H}(2s)$ atoms suggest that the ${B}^{\ensuremath{'}}^{1}\ensuremath{\Sigma}_{u}^{+}$, $e^{3}\ensuremath{\Sigma}_{u}^{+}$, $D^{1}\ensuremath{\Pi}_{u}^{+}$, and $d^{3}\ensuremath{\Pi}_{u}^{+}$ excited states are involved in the formation of these metastable fragments. (ii) For fast $\mathrm{H}(2s)$ atoms, the electron energy threshold for production of the least energetic of the fast $\mathrm{H}(2s)$ atoms is near 29 eV. The angular distribution data would indicate that these atoms arise from a ${\ensuremath{\Pi}}_{u}$ state; the form of the excitation function indicates that the parent state has a multiplicity of 1. The change in energy distribution of the fast $\mathrm{H}(2s)$ atoms, measured as a function of electron-gun voltage, supports the view that the $^{1}\ensuremath{\Pi}_{u}$ state is a previously unreported doubly excited state that has an asymptotic energy of 24.9 eV.

The polarization of characteristic X radiation excited by electron impact

Using hydrogenic wavefunctions the cross sections for ionization of an atom from the magnetic substates of the L3 level are calculated in (i) the Bethe approximation, and (ii) the first Born approximation. Calculation (ii) differs from previous work of a similar nature in that it takes as axis of quantization, the direction of the incident electron beam. Hence the polarization of the resulting characteristic X radiation is estimated; it is found to be small in comparison with the degree of polarization which typically results from the excitation of optical lines, in marked disagreement with a previous calculation.

Displaced Aperture Dark Field Images by an Aberration Correction

In displaced aperture dark field microscopy, the direction of the incident electron beam with respect to the specimen is maintained unaltered between bright field and dark field images, unlike the case of usual high resolution dark field microscopy by tilted illumination. The displaced aperture technique, however, gives a strong dark field image deterioration due to field aberrations. These are prounouncedly large since the deviation of the orbits of the imaging electrons from the optical axis is very large in this case. In the present paper, the improvement of the image quality by aberration correction is discussed.1. The chromatic field aberration is composed of components mainly from the objective and intermediate lenses. It can be corrected by changing the electron orbit by means of a beam deflector, introduced in the image plane of the objective lens.

Elastic Scattering of Low-Energy Electrons at 180° in CO2

A technique is described for the study of electrons which are elastically scattered from a gas target into a small solid angle centered at 180\ifmmode^\circ\else\textdegree\fi{} with respect to the direction of the incident electron beam. The method is applied to electron scattering from C${\mathrm{O}}_{2}$. Information is obtained concerning the nature of the compound state in the 4-eV region.

Measurements of Energy and Angular Distributions of Secondary Electrons Produced in Electron-Impact Ionization of Helium

The energy and angular distributions of electrons produced in the ionization of helium by electrons with energies between 100 eV and 2 keV have been measured in a crossed-beam apparatus consisting of a fixed hemispherical energy analyzer and a rotatable electron gun. Distributions of secondary electrons (those electrons departing with energies less than one-half that of the incident primary electrons) were obtained for a wide range of energies and for angles between 30\ifmmode^\circ\else\textdegree\fi{} and 150\ifmmode^\circ\else\textdegree\fi{} with respect to the direction of the incident primary electron beam. The observed angular distributions were significantly different from the results of two early electron-impact measurements; however, they agreed to the extent expected with more recent results and with similar proton-impact data. For secondary energies above about 50 eV and for primary energies greater than 300 eV, the energy distributions (the cross sections integrated over angle) were observed to be very nearly equal to the distribution given by the Mott formula for free-electron-free-electron scattering multiplied by the number of electrons in the target.

Effect of Electric Field Direction on Pulsed Radiation-Induced Current in Crystalline Quartz

Dependence upon sample orientation of the pulsed radiation-induced current (RIC) in crystalline quartz has been studied by using 5-μsec pulses of electrons with an energy of 13 MeV. The variation with time of the RIC has been found to depend on the direction of the applied electric field with respect to the c axis of the crystal, but not appreciably on the direction of the incident electron beam. Detailed dependence of the pulsed RIC on the angle θ between the applied electric field and the c axis has been measured also for θ from 0° to 90° at intervals of 15°, and the variation with time of the RIC has been found to be large for θ<60° and very small for θ≥60°. The dependence on θ was confirmed by measuring the average RIC with a picoammeter.

Angular Distribution of Protons and Deuterons Produced by Dissociative Ionization ofH2andD2near Threshold

From observations of the kinetic-energy distribution of protons and deuterons at corresponding forward and backward angles with respect to the electron-beam direction, the forward momentum imparted to the dissociating ${\mathrm{H}}_{2}$ (${\mathrm{D}}_{2}$) molecule by the incident electron has been determined for electron energies up to 300 eV. The momentum transfer along the beam was found to remain nearly constant at all energies above threshold, although above 100 eV the values obtained for ${\mathrm{D}}_{2}$ were systematically higher than the values for ${\mathrm{H}}_{2}$. When corrected to the center-of-mass system, the angular distribution of ${\mathrm{H}}^{+}$ (${\mathrm{D}}^{+}$) near threshold was found to contain a large isotropic component and an anisotropic component which deviates, in a manner suggested by Zare, from the ${cos}^{2}\ensuremath{\Theta}$ dependence predicted from a simple dipole Born approximation.

Pulse Radiolysis Apparatus for Monitoring at 2000 Å

A pulse radiolysis apparatus with photometric monitoring has been built around an 11 MeV, 250 mA peak current, linac that delivers single 0.25 to 4 μsec pulses. The novel features of the apparatus include (1) a 450 W xenon lamp as the analyzing light source which in pulsed operation had a 25 times increased luminance; (2) a fast electronic switch that cut out the signal due to the Čerenkov radiation; (3) a secondary emission chamber that allowed the simultaneous measurement of the current and the direction of the pulsed electron beam; and (4) a system for remote controlled change of liquid samples stored in glass syringes. Reliable measurements of optical transmission could be made starting 0.2 μsec after the electron pulse at wavelengths down to 2000 Å on transient species having products of yield and absorptivity G×ε>500 mole (100 eV)−1·liter−1·cm−1.

Computed Electron Micrographs for Tilted Foils Containing Dislocations and Stacking Faults

The method of computation of electron microscope images developed by Head (1967) produces micrographs of single dislocations in about 1 min of numerical integration. In Head's original programme, the electron beam direction and the normal to the foil surface are coincident. Since this condition is not often realized in practical electron microscopy, the programme has been extended to compute the images of dislocations in tilted foils. It has been further modified to compute he images from a configuration 'containing up to two dislocations and up to three stacking faults. These programmes are thus applica.ble to a wide range of defect con� figurations in tilted thin foils. The derivation of these programmes is described and several examples of their- use in experimental situations are given. These include the "skewing" of dislocation images due to the noncoincidence of the beam direction and the foil normal, and images due to interactions between dislocations and stacking faults.

Neutron energy spectra and angular distributions from targets bombarded by 45 mev electrons

The time-of-flight method has been used to measure the energy spectra of neutrons emitted by targets of natural uranium, lead, bismuth and tungsten when bombarded by 45 MeV electrons from the Harwell linear accelerator. The spectra were measured at 90 to the electron beam and covered the energy range 40 keV-6 MeV. For targets 3 cascade shower radiation lengths in thickness, the mean energy of the neutrons was found to be 1·92 MeV for lead and 1·37 MeV for uranium and the relative yield from lead to that for uranium was 0·47 ± 0·05. For targets of lead, bismuth and tungsten the observed spectra could be represented over a large energy range by Maxwellian distributions at temperatures of 0·98, 0·82 and 0·44 MeV respectively. The angular distributions of the neutrons from lead and uranium targets were determined from foil activation measurements using the threshold reactions 31P(n, p) 31Si, 27Al(n, p) 27Mg and 27Al(n, α) 24Na. For a small uranium target forward peaking of the emitted neutrons relative to the electron beam direction was observed. The development of larger targets of uranium and lead which act as isotropic neutron sources for use in reactor physics measurements is described.

Dissociation of H2+ by Electron Impact: Calculated Angular Distribution

The angular distribution of products in the electron impact dissociation of H2+ has been calculated following closely the Born approximation treatment of E. H. Kerner [Phys. Rev. 92, 1441 (1953)]. When the translational kinetic energy of separation greatly exceeds the initial rotational energy of the molecule, as is normally the case, the fragments are ejected nearly along the molecular axis. For ``axial recoil'' the leading term in the differential cross section shows a cosine-squared anisotropy peaked about the momentum-transfer vector, independent of the initial rotational state of the molecule. This is in contrast to the findings of Kerner. The inclusion of higher-order terms in the differential cross section produces interference effects which shift the maximum of the angular distribution away from the momentum transfer vector towards larger angles. However, the contribution of the leading dipole term strongly overshadows the effects of these higher-order multipole terms except in certain cases quite close to threshold. When the angular distribution of products is measured about the electron-beam direction k0, rather than the momentum transfer vector K, the anisotropy assumes in the dipole limit a simple form proportional to IK(θ)=cos2θ′ cos2θ+12 sin2θ′ sin2θ, where θ′ is the angle included between k0 and K. Upon integrating over all possible values of K for a fixed electron-impact energy, the form of the anisotropy is found to be a maximum close to threshold, but is first degraded and then reversed in sense as the bombarding electron energy is increased. In the high-energy limit, however, the angular distribution does not approach a sine-squared anisotropy, but rather a limiting form is asymptotically approached corresponding to an average θ′ of about 68°. Comparisons are made with the related experimental studies of G. H. Dunn and L. J. Kieffer [Phys. Rev. 132, 2109 (1963)] on the dissociative ionization of H2.

Die Kantenstreifung im elektronenmikroskopischen Bild w�rfelf�rmiger MgO-Kristalle bei Durchstrahlung in Richtung der Raumdiagonale

In electron microscope images of MgO crystals orientated with their {111} lattice planes normal to the direction of the incident electron beam a sine-forme periodicity of fringe structure parallel to the crystal edges is mostly observed. This simple fringe structure cannot generally be expected since six strong diffractions (¯202), (0¯22) etc. are simultaneously excited. — In such micrographs spacing and contrast of the fringes vary strongly while from the projection images of the crystals there are no remarkable differences in crystal orientations. For this case, the region near the [111]-direction of incidence, the fringe structure was theoretically investigated by Eigenvalue method and Bethe approximation. In exact [111] direction, only two wave fields exist which produce sinusoidal fringes. Small deviations from the [111] direction keep the two wave fields prevailing, the fringe structure remaining nearly sinusoidal, but induce strong variation of spacing and contrast. The theoretical results are in qualitative agreement with the observed fringes of micrographs, but are not yet suitable for quantitative comparison because of the considerable aperture of illumination (10−3), used in the observation.

Effects of surface stress relaxation on the electron microscope images of dislocations normal to thin metal foils

Abstract The two-beam dynamical theory of Howie and Whelan is used to calculate the electron microscope image contrast from dislocations normal to a thin crystal and parallel to the direction of the incident electron beam. Bending of the lattice planes due to surface relaxation and change in lattice parameter are taken into account. Strong contrast in the form of black-white spots is predicted for dislocations with a screw component, while contrast from pure edge dislocations is expected to be very weak. Observations on platinum and copper are in good agreement with the predictions for dislocations with a screw component. It is possible from these contrast effects to determine the sense of the screw component of the dislocation.

Use of an Electron Beam for detecting Superconducting Domains of Lead in its Intermediate State

SUPERCONDUCTING domains have been detected in lead by means of an electron beam technique previously used to detect magnetic domains1. A spectroscopically pure cylinder of lead, 1.1 cm long and 0.28 cm in diameter, was mounted with its axis vertical in a modified horizontal electron diffraction camera. A 20-kV electron beam was focused about 15 cm from the cylinder and was deflected at 50 c/s in a vertical direction by means of suitable coils, so as to produce an electron shadow of the lead cylinder. Fig. 1c shows a photograph of a shadow taken with the lead in its normal state. In this case the smooth edge is merely a geometrical projection of part of the curved surface of the cylinder. The cylinder, which could be heated electrically by a non-inductively wound heater, was attached to the outside of a metal vessel containing liquid helium. A uniform magnetic field could be applied to the lead by means of a pair of small Helmholtz coils. These were mounted symmetrically on either side of the lead with their common axis horizontal and perpendicular to the mean direction of the undeflected electron beam.