E. Lægsgaard

Axial channeling radiation from MeV electrons

Abstract Axial channeling radiation from MeV electrons in Si resulting from spontaneous transitions between transverse energy states is studied theoretically and experimentally. Transitions between tightly bound states are well described within the single-string approximation with a thermally averaged Hartree-Fock continuum potential. This applies both for line energies and for absolute intensities as functions of incidence direction for the electron beam. For weakly bound states the extension of the calculations to describe itinerant states in the tight-binding approximation reproduces the band widths obtained from more complete many-beam calculations. For a thick crystal the discrete lines are Lorentzians with widths determined by incoherent scattering, whereas the finite crystal thickness modifies the line shape and the line width in thinner specimens. For tightly bound states the incoherent scattering is due to thermal displacements of atoms, and the formula presented for the scattering probabilities gives satisfactory agreement with observed line widths when correlations of displacements are included.

Position-sensitve semiconductor detectors

Abstract Solid-state radiation detectors for simultaneous determination of charged particle position and energy are reviewed. Particular emphasis is devoted to the operational conditions, which are of importance for the performance of such devices, and examples of distortion and resolution reflecting inherent limitations are given for one- as well as for two-dimensional detectors. Several applications of such detectors are considered, and illustrations of the present status are given with relevant results obtained with various detectors.

Z1 scaling for impact-parameter dependence of inner-shell ionization by heavy ions

Abstract The Z 1 dependence of inner-shell ionization has been investigated by measuring the impact-parameter dependence of Cu-K-shell ionization by H, Be, C, and O, at energies corresponding to 2 MeV/amu. Large deviations from a scaling with Z 1 2 are observed. By scaling results for ionization of Cu by protons, it is attempted to test quantitatively an explanation based upon the modification of the K-electron binding and wavefunction, due to the proximity of the projectile to the Cu nucleus. The scaling is successful for close collisions, but significant discrepancies are found at larger impact parameters. From measurements with oxygen at different energies, the deviations are found to depend strongly on projectile velocity, almost disappearing at the lowest bombarding energy. Possible explanations are considered in connection with a discussion of the binding correction, based on adiabatic perturbation theory. Also the origin of the charge-state effect observed for oxygen (≈20% difference between O 8+ and O 6+ ) is discussed.

Impact parameter dependence of K-shell ionization.

Abstract The impact-parameter-dependent probability of K-shell ionization by protons has been measured in the energy range 0.5–2.5 MeV for copper, silver, and gold targets. The data are used to test the scaling relations predicted from semiclassical calculations in first Born approximation, with unperturbed projectile trajectory and nonrelativistic hydrogenic electron wave functions. A systematic procedure is described for correcting such calculations for the inadequacy of these approximations, and the correction scheme is tested by comparison partly with experiments, partly with more complete theoretical treatments. The relation between ionization by scattered particles and by particles emitted in nuclear decays or absorbed in compound-nucleus reactions is discussed, and quantitative estimates are derived for the monopole term, which dominates for projectile velocities much lower than the K-shell electron velocity.

Incoherent Scattering of Electrons and Linewidth of Planar-Channeling Radiation

A theoretical description of planar-channeling radiation is given, with special emphasis on the scattering processes contributing to the radiation linewidth. Owing to coherence between scattering in the initial and final states for the radiation process, the contribution to the linewidth from processes with small momentum transfer (e.g., plasmon excitation) is strongly suppressed, and the linewidth is dominated by thermal interband scattering. Also contributions from nonsystematic reflections and from finite crystal thickness are evaluated. Experimentally, planarchanneling radiation has been studied for 4 MeV electrons in nickel, at different target temperatures, and the radiation energies and linewidths confirm the predictions. The influence of correlation of vibrations is revealed by a variation of the linewidth with the angle to a major axis in the plane. The radiation intensity as a function of incidence angle to the plane is strongly modified by multiple interband scattering, and a comparison between measured and calculated intensities serves as a further check on the theoretical treatment.

Accurate determination of cross sections for K-shell ionization by proton impact

Abstract Precise (3–10%) measurements of K-shell-ionization cross sections for protons have been performed for Ti, Fe, Cu, Se, Zr, and Ag. Experimental conditions are emphasized and possible errors are discussed. Comparison with theoretical values corrected for binding, Coulomb, and relativistic effects show good agreement.

POSITION SENSITIVE DETECTORS MADE BY ION IMPLANTATION IN SILICON.

Abstract Implanted dopant ions are used to form the resistive layer in position sensitive p-n junction particle detectors. Position resolution of 0.2 mm fwhm and linearity of ±0.3 mm are obtained.

Temperature dependence of axial-channeling radiation

Abstract The 2p-1s and 3p-1s lines of channeling radiation have been measured at three different temperatures, 110 K, 300 K, and 500 K, for 3.5 MeV electrons channeled along a 〈111〉 direction in silicon. A variation of about 20% of the photon energy is observed, in good agreement with calculations. The variation reflects a temperature dependence of the 1s level due to the thermal smearing of the continuum string potential. The measured linewidths also agree with theoretical estimates. They are dominated by thermal scattering, with an estimated contribution of ~ 10% from electronic scattering.

Position-Sensitive Semiconductor Particle Detectors Fabricated by Ion Implantation

The advantages and problems associated with the use of ion implantation for the fabrication of position-sensitive particle detectors are discussed. Analysis of the noise and pulse risetime properties of such devices shows the desirability of using a high sheet resistance in the distributing resistive layer for optimum position resolution while restricting the RC of the detector to values approximately equal to the clipping times used in the measuring circuits in order to obtain good position linearity. From both detector noise and pulse rise-time considerations it is found desirable to use equal pulse shaping in the energy and the position signal measuring systems. Detectors were made by implantation of 60 keV boron and phosphorus ions into 4000 ohm-cm n-type silicon. Representative results are shown for a 4×32 mm2 one dimensional detector which shows integral and differential position nonlinearity of less than one percent and position resolution of less than 0.2 mm. Extension to one dimensional detectors up to 50 mm and to 10×10mm two dimensional detectors is discussed.

Equilibrium charge distributions of light ions in helium, measured with a position-sensitive open electron multiplier

Abstract Equilibrium charge distributions have been measured for 2≦Z≦18 ions with energies from 100 to 400 keV. Equilibrium was established by passing accelerated ions through a helium gas. The different charge states of the beam were separated in an electric field and detected by an open position-sensitive electron multiplier, constructed for this experiment. The results show that the equilibrium charge distribution depends strongly on the atomic number of the projectile and that the mean charge ι is an oscillating function of Z at fixed velocity. For ions with Z ι is larger in helium than after passage through a carbon foil.