J.S. Forster

ERDA with very heavy ion beams

Abstract The use of very heavy ion beams such as 127I or 197Au with 1–2 MeV/u makes ERDA (elastic recoil detection analysis) a quite universal technique for thin film analysis capable of analyzing simultaneously light and heavy elements including H with almost constant sensitivity. Due to the strong increase of the recoil cross section with the projectile atomic number typically less than 1012 ions are required to get sensitivities in the 100 ppm range. Detector systems with particle identification are advantageous for ERDA, which can be improved additionally regarding depth resolution and detection efficiency, if the kinematic energy spread is corrected. The characteristic features of heavy ion ERDA will be briefly outlined and illustrated by several examples using a position sensitive detector system. Some general problems and limitations, which are caused by heavy ion beams, will also be discussed.

FORWARD ELASTIC RECOIL MEASUREMENTS USING HEAVY IONS

The application of the elastic recoil detection technique utilizing heavy ions for the analysis of semiconductor samples is demonstrated. With this technique the depth profiles of the primary constituents as well as profiles of all impurities can be measured in one spectrum. Depending on the target material, a depth resolution down to 20 nm can be achieved. All elements except hydrogen can be detected with almost the same sensitivity, namely ∼1×1015 at/cm2 with 136 MeV I in a 30° recoil geometry. For hydrogen, the sensitivity is about four times better.

Resonant coherent excitation of N6+ and Mg11+ in planar channeling: anisotropies in ionization probabilities and X-ray emission

The thrust of the conference in which this paper is presented is toward inelastic interactions of ions with surfaces. The work described in this paper deals with inelastic interactions of ions in planar channeling inside the crystal. The authors are, however, not too far off the mark because the planar channeling potential is made up from the sum of two (sheet) surface potentials. For the nitrogen experiment, a beam of N{sup 6+} ions at an energy of 3.25 MeV/amu was supplied by the ORNL EN Tandem accelerator. After suitable collimation, it was passed through an Au crystal (1,800 {angstrom} thick). The emergent charge-state distribution was analyzed by electrostatic deflection followed by a solid-state position-sensitive detector. Charge states 5, 6 and 7 were simultaneously detected. In the case of Mg{sup 11+} the beam was supplied at an energy of 25 MeV/amu by the CSCC Tandem Cyclotron Facility at Chalk River, Canada, and passed through an Ni crystal 4,000 {angstrom} thick which was epitaxially grown with the surface parallel to a (100) plane. The beam was charge state analyzed by a Q3D magnetic analyzer in which charges 11+ and 12+ were registered. At the energy in question, almost no 10+ fraction ismorexa0» visible. Two Si(Li) X-ray detectors aimed at 90{degree} and 45{degree} recorded the emission of Mg{sup 11+} Ly{alpha} (n = 2 {yields} 1) X-rays. In conclusion, there is strong evidence that once a given ionic state is excited, a large fraction of the ions remain in that state and are either ionized or radiate from that state. The result is counter to the expectation that electron-ion collisions in the channel would have a large enough cross section to destroy the alignment before either of these two events could take place. At this point, the reasons for the observed retention of anisotropy are open to conjecture.«xa0less

Elastic recoil detection (ERD) with extremely heavy ions

Abstract Extremely heavy-ion beams such as 209Bi in elastic recoil detection (ERD) make ERD a uniquely valuable technique for thin-film analysis of elements with mass ≤ 100. We report ERD measurements of compositional analysis of dinosaur eggshells and bones. We also show the capability of the ERD technique on studies of thin-film, high-temperature superconductors.

Quantitative calibration of intense (α, α) elastic scattering resonances for 12C at 5.50–5.80 MeV and for 16O at 7.30–7.65 MeV

Abstract Extremely strong (α, α) elastic scattering resonances — more than 100 times the Rutherford value — occur in 16O at 7.35–7.65 keV and in 12C at 5.50–5.80 MeV, thus allowing the normal RBS technique to be extended to low-Z components, such as oxygen and carbon. Both resonances have been calibrated over the appropriate 300 keV energy regime with an overall accuracy of ±4%. Some recent examples of their use in profiling oxygen and carbon in various substrates are given.

APPLICATION OF ION-BEAM-ANALYSIS TECHNIQUES TO THE STUDY OF IRRADIATION DAMAGE IN ZIRCONIUM ALLOYS

Abstract Ion-beam-analysis techniques are being used to provide an understanding of the nature of collision cascades, irradiation-induced phase changes, lattice location of solute atoms and defect-solute atom interactions in various zirconium alloys. In zirconium intermetallic compounds, such as Zr3Fe, Zr2Fe, ZrFe2, and Zr3(Fex,Ni1 − x), electron and ion irradiations have been used to obtain detailed information on the crystalline-to-amorphous transformation occurring during the irradiation. Transmission-electron-microscopy (TEM) observations have provided information on the nature of the damage produced in individual cascades, the critical dose required for amorphization, and the critical temperature for amorphization. In a study on the electron-energy dependence of amorphization in Zr3Fe, Zr2Fe and ZrCr2 in situ high-voltage-electron-microscope investigations were combined with high-energy forward-elastic-recoil measurements to yield information on the threshold displacement energies for Zr and Fe or Cr in these lattices, as well as the role of secondary displacements of lattice atoms by recoil impurities (C,O) at low electron energies. In Zr implanted with 56Fe ions and subsequently bombarded with 40Ar ions at 723 K, subsequent secondary-ion-mass-spectrometry (SIMS) analyses were used to monitor the effect of irradiation on the migration of Fe in the Zr lattice. In addition, ion-channeling investigations have been used to determine the lattice sites of solute atoms in Zr as well as the details of the interaction between the solute atoms and the irradiation-produced defects.

DEFLECTION OF HIGH ENERGY CHANNELED CHARGED PARTICLES BY ELASTICALLY BENT SILICON SINGLE CRYSTALS

An experiment has been carried out to observe the deflection of charged particles by planar channeling in bent single crystals of silicon for protons with energy up to 180 GeV. Anomolous loss of particles from the center point of a three point bending apparatus was observed at high incident particle energy. This effect has been exploited to fashion a “dechanneling spectrometer” to study dechanneling effects due to centripital displacement of channeled particle trajectories in a bent crystal. The bending losses generally conform to the predictions of calculations based on a classical model.

Heavy ion channeling

Abstract We discuss recent experiments on heavy ion channeling at high energies. Resonant-capture and loss of electrons has been studied by measurement of the charge state of ions transmitted through a thin Si crystal. The energy loss has been used to define the transverse energy of axially channeled ions and to measure the strength of the resonant processes as a function of transverse energy. We therefore first discuss briefly the average energy loss and the straggling in energy loss for channeled and random penetration. Dielectronic KLL capture (inverse KLL Auger emission) has been measured in the energy range 12–18 A MeV for Br 33+ (He like) ions and the observed dependence of the resonance strength on transverse energy is consistent with the calculated variation of the density of valence electrons seen by the ions. Coherent excitation leading to electron loss has been measured for Si 13+ (H like) ions channeled along a 〈111〉 axis at about 21.7 A MeV, where the energy in the ion rest frame of virtual photons in the 7th harmonic of the axial potential matches the energy of the transition of the bound electron from the ground state to the first excited state. A detailed theoretical discussion of this process is given. The dependence on transverse energy is very strong because the field decreases rapidly with distance from an atomic string. There is good agreement between calculations and experiment, both concerning the dependence of the strength of the resonance on transverse energy and concerning the Stark splitting, which is caused mainly by the zeroth harmonic of the transverse force from 〈111〉 atomic strings.

GeV CHANNELING IN BENT CRYSTALS WITH SLOWLY VARYING CURVATURE

In a recent Fermilab experiment, the deflection of charged particles by planar channeling in bent single crystals in the 100 GeV/c regime was observed. The experiment shows the necessity of taking into account the variation of curvature along the bend. Here a theory is developed to treat the case of slowly varying curvature and it is shown that the predictions are in qualitative agreement with the experimental data.

Effects on channeling of radiation damage due to 28-GeV protons

For an irradiation of 2.9 × 1019 protons at a beam energy of 28 GeV, the channeling minimum yield in a silicon single crystal increased from 2.3% to 4.1%. The radiation damage occurred with a proton fluence of (4.1 ± 1.4) × 1020cm2. The degradation was measured with MeV-range He ions using Rutherford backscattering. The relevance to bent crystal extraction of TeV beams is discussed.