G. Amsel

Microanalysis by the direct observation of nuclear reactions using a 2 MeV Van de Graaff

Abstract The principles of nuclear microanalysis by the direct observation of the charged particles emitted from nuclear reactions induced on light nuclei are described. This method allows the determination, in short times, of very small quantities of nuclei like 16O, 18O, 14N, 19F, etc. near the surface of samples. Typical sensitivities are of the order of 1014 atom/cm2. Concentration profiles as a function of depth may be directly measured with submicron resolution. The technical details of a facility comprising a 2 MeV Van de Graaff accelerator are given. Problems related to particle detection and data recording are discussed. The various scattering chambers, which allow most of the measurements to be carried out automatically with digital control, are described. Applications to various fields in solid state physics, metallurgy and electrochemistry are presented.

The mechanism of anodic oxidation

Abstract The ion transport mechanism in the growth of oxide films during anodic oxidation has been studied. Tracer methods have been used to determine the order of the atoms in the film. The position of the labelled atoms was determined by observing the form of excitation curves near sharp resonances of the reactions O18(p, α)N15 and Al27(p, γ)Si28. Perfect conservation of the order was observed for oxygen, whereas metallic atoms show a slight mixing. A model is proposed, in which the oxygen sublattice remains stationary and the metal atoms move by vacancy diffusion and by interstitial exchange capture. A mathematical analysis of the model and its comparison to previous results are presented.

Depth profiling with narrow resonances of nuclear reactions: Theory and experimental use☆

A general analytical theory of depth profiling using narrow resonances is outlined. It is shown how some basic features of the stochastic theory of fast ion slowing down in disordered matter may be turned to advantage for a rigorous calculation of yield excitation curves corresponding to targets of various composition. Depth profiles may be extracted from the experimental data by fitting the latter with curves calculated for profiles chosen through an optimization procedure. For sufficiently broad resonances, Γ ∼ 1 keV, it may be shown that good agreement with the experimental data is obtained if the correct energy loss spectra deduced from the stochastic slowing down theory are replaced in the calculations by Gaussian spectra at all depths. This leads to very simple computer programs, fast to operate with small computers. For very narrow resonances, Γ ∼ 100 eV, the calculations require the use of the full stochastic theory. Such calculations account for the overshoot in the excitation function near the resonance energy observed for thick targets, known as the Lewis effect. Possible uses of both medium and high energy resolution resonance depth profiling in various fields of research are illustrated. The optimization of the experimental conditions (stability and energy spread of the beam, carbon deposition, etc) and the automatic recording of the data for fast operation are discussed.

Microanalysis of Flourine by nuclear reactions

The quantitative determination and depth profiling of fluorine in the surface region of various substrates using nuclear reaction microanalysis was studied. Differential cross sections and excitation functions for the reaction 19 F(p, α 0 ) 16 O and 19 F(p, α γ) 16 O were investigated in great detail, especially in the vicinity of resonances, where automatic energy scanning was used. Resonance widths and positions were measured precisely, in particular for the narrow resonances of 19 F(p, α γ) 16 O. A very narrow, Γ = 150±50 eV, resonance was found at 1088 keV. The excitation curves shown for various resonances allow one to carry out the computer calculations required for converting yield curves into depth profiles. The paper discusses how to choose among the nuclear reactions and resonances available to achieve best sensitivity, selectivity, depth resolution, ease of operation or speed of measurements, according to the problem considered, and to define the corresponding optimal experimental conditions. It appears that, in addition to the strong 872 keV, Γ = 4.2 keV resonance of 19 F(p, α γ) 16 O, the 340 keV, Γ = 2.4 keV resonance may be especially useful. The results are also of interest for hydrogen depth profiling when using the reverse reaction 1 H( 19 F, α γ) 16 O.

A new measurement of the 429 keV 15N(p,αγ)12C resonance. Applications of the very narrow width fouNd to 15N and 1H depth location: I. Resonance width measurement

High precision nuclear reaction resonance width measurement techniques were applied to the 429 keV resonance in 15N(p, αγ) 12C yielding a result which may be stated at present as: Γ=120±30) eV. Work is in progress to improve precision further. The measurements were performed using a 2 MV Van de Graaff equipped with both slit and capacitive pick-off plate feedback stabilization and fitted with an automatic energy scanning system. Energy resolutions as low as ∼40 eV fwhm were reached. Thick, nearly stoichiometric niobium nitride targets highly enriched in 15N were prepared on silicon backings by reactive sputtering in a 15N2 and argon mixture. The measurements were carried out using a ∼1μA beam, at room temperature, in 10−6 Torr liquid nitrogen trapped vacuum. The characterization of the targets as well as the fitting procedures used to extract Γ from the thick target yield curves (including Doppler effects) are presented.

Analysis and depth profiling of deuterium with the D(3He, p)4He reaction by detecting the protons at backward angles☆

Detection at backward angles of protons from D(3He, p)4He presents distinct advantages with respect to the associated α particle detection used until now for depth profiling deuterium. This geometry does not require glancing incidence for good depth resolution so that rough samples may be analyzed, while the flux and energy of the elastically scattered beam particles is reduced, their pile-up problems minimized, their filtering, when necessary, with mylar absorbers or electrostatic deflectors more favourable, etc. The various contributions to the depth resolution have been calculated taking into account detector acceptance angle, size of the probing beam, energy straggling, multiple scattering and lateral spread contributions and the way to combine them discussed in detail. The 13 MeV protons emitted at 165° for E3He=700 keV were detected using a 1500 μ deep silicon surface barrier detector with a measured overall resolution of 17 keV (fwhm). The probing beam was produced with a 350 kV HVEE accelerator using 3He2+ ions thus demonstrating the feasibility of such measurements with low energy ion implantation machines. Depth resolutions were measured and found to be in good agreement with theory. Typically, surface resolutions of 500 A were reached for incident beams normal to the target surface. This resolution could be reduced below 200 A by tilting the target with respect to the beam. The method has been applied to deuterium depth profiling in amorphous silicon hydrogenated by implantation or plasma diffusion of deuterium. A comparison with SIMS measurements is reported.

High resolution techniques for nuclear reaction narrow resonance width measurements and for shallow depth profiling

Abstract Numerous problems must be solved in order to measure and use efficiently and with high precision narrow nuclear reaction resonances with widths below some hundreds of eV for proton beams. The various techniques typically involved will be reviewed systematically, with emphasis on thier most critical stages. Their routine use will be described for high resolution depth profiling in the vicinity of the surface, i.e. in the range of some tens to hundreds of A. It will be shown that many existing facilities may be used for this purpose, specialised and highly sophisticated equipment being necessary only for advanced metrology. The discussion will cover: beam energy spread from single-ended and tandem accelerators and its optimization for proton and 15N beams; target preparation and contamination problems and in particular the contruction of standard targets for overall energy resolution measurements using well known resonances; the peculiar mathematical properties of the Breit-Wigner resonance line shape, in particular with respect to convolution with Gaussians; ion-target interactions, including Doppler effects, energy straggling and Lewis effect; data fitting using particle-solid interaction theories. Examples will be given of experiments relating to nuclei as 1H, 15N, 18O, 27Al.

The influence of the electrolyte on the composition of ‘anodic oxide films’ on tantalum

Abstract We have studied the influence of the electrolyte on the composition of dielectric films formed by anodic polarisation of tantalum. The electrolytes used were mainly aqueous solutions of salts and acids (H 2 SO 4 , H 3 PO 4 , HNO 3 etc.) at various concentrations. A new method of microanalysis of the light elements (O 16 , O 18 , N 14 and D) contained in the films has been coupled with the more classical methods of study (growth rate, capacity measurements, and the use of Redox systems in solution). The absence of large incorporation of elements other than oxygen and tantalum in films formed in aqueous salt solution and very dilute acids has been shown. In films formed in concentrated acids the incorporation of anions of the type Ax Oy n − is so important — 30 per cent of the total number of anions in the case of films formed in 95% H 2 SO 4 -that it is no longer possible to consider these films as oxides. The results obtained by deduction from the analysis of oxygen are in good agreement with those obtained from direct analysis for nitrogen. The film properties (rate of chemical attack, dielectric constant, formation field, molar volume etc.) depend on the nature of the electrolyte used for formation.

A simple, digitally controlled, automatic, hysteresis free, high precision energy scanning system for Van de Graaff type accelerators: Part I: Principle, results and applications

Abstract A digitally controlled energy scanning system is described which may be coupled in a simple and inexpensive way to any accelerator which is high voltage stabilized by a beam position sensing “slit control” system like all single ended or tandem Van de Graaff type machines. The basic idea is to “fool” the existing stabilization system by deflecting the beam at two well chosen points through small angles using electrostatic deflection plates biased to variable voltages U. It is shown that the ensuing terminal potential variation ΔV is linearly related to U, the system acting as a U → ΔV closed loop feedback amplifier with a gain G0 set only by the beam trajectory in the analyzing magnet and between the plates. G0 is hence independent of acceleration potential, beam particle mass or change state, the long term stability of G0 being set only by mechanical components. The calibration can thus be made once and for all. The system is strictly hysteresis free and provides means of varying the bombarding energy in small precisely equal steps in a fast and reproducible way. Sweeping spans up to a ΔV V = ± 8% , may be achieved with maximum differential non-linearities in the percent range. The latter may be easily corrected for. With our 2 MV Van de Graaff steps as small as 30 eV at MeV energies were routinely used with comparable mean energy stabilities and narrow, proton induced, resonances could be registered near 1 MeV with 100 eV fwhm beam energy spreads. The excitation functions may be recorded automatically with a multichannel analyzer with sweeping speeds down to 0.1 to 1 s per channel. Stability, reproducibility, residual non-linearity, transient behaviour, etc. of the device are discussed in depth. It is shown that in systems containing several magnets, beam tracking is achieved simply by fitting them with well chosen deflection plates connected to the scanning plates. Numerous applications are shown in nuclear physics, depth profiling ion beam analysis and particle solid interaction studies.

A new measurement of the 429 keV 15N(p, αλ) 12C resonance. Applications of the very narrow width found to 15N and 1H depth location: II. Applications

Abstract The unexpectedly narrow width Γp = 120 eV or ΓN15 = 1.8 keV of this resonance opens quite new possibilities for its use in 15N or 1H depth location and for hydrogen adsorption studies using 15N beams. Resonance depth profiling of 15N is possible with much better near-surface depth resolution than thought until recently. In 1H resonance depth profiling the indeep background originating from the surface contamination peak is also much smaller than previously assumed. These features are illustrated by 15N self-diffusion experiments in NbN films and by 15N adsorption experiments on copper single crystals in ultrahigh vacuum. The strong effect of Doppler broadening on the actual ultimate near-surface depth resolution of hydrogen depth profiling with 15N beams is discussed as well as the use of this resonance for the direct measurement, based on the Doppler effect, of the vibration speed distribution of hydrogen atoms adsorbed on clean surfaces.