ENERGY LOSS AND ENERGY STRAGGLING OF LIGHT IONS IN FULLERITE

To determinate the stopping cross section in fullerite a feasible approach, taking into account the high radiation sensitivity and mechanical fragility of fullerite films, should be employed. In this work, the stopping cross sections of ¹H, ³H, ⁴He and ⁷Li ions for several selected energies were measured by Rutherford backscattering, neutron depth profiling and alpha spectroscopy using sandwich structures of samples composed of fullerite deposited on a firm substrate (Si, steel) with an intermediate marker (Au, N, Li, B, Pu). In addition, ion transmission through a thin C film supporting a fullerite layer was also utilized. The measured stopping cross sections follow the theoretical predictions calculated for carbon, but are systematically (10–35%) higher than the theoretical ones (with the exception of 5 — 5.5 MeV ⁴He). The observed deviation of the experimental data can partly be explained by the chemical state effects in fullerite, which accounts for about 20–50% of the difference. The measured energy straggling exceeds Bohr's value by a factor of about 2 for alpha spectroscopy and ion transmission, and 2.5 or 9.5 for Rutherford backscattering and neutron depth profiling, respectively. The discrepancy can be explained by a thickness variation, such as surface roughness of the fullerite films.     

Accurate determination of quantity of material in thin films by Rutherford backscattering spectrometry

Ion beam analysis (IBA) is a cluster of techniques including Rutherford and non-Rutherford backscattering spectrometry and particle-induced X-ray emission (PIXE). Recently, the ability to treat multiple IBA techniques (including PIXE) self-consistently has been demonstrated. The utility of IBA for accurately depth profiling thin films is critically reviewed. As an important example of IBA, three laboratories have independently measured a silicon sample implanted with a fluence of nominally 5 × 10(15) As/cm(2) at an unprecedented absolute accuracy. Using 1.5 MeV (4)He(+) Rutherford backscattering spectrometry (RBS), each lab has demonstrated a combined standard uncertainty around 1% (coverage factor k = 1) traceable to an Sb-implanted certified reference material through the silicon electronic stopping power. The uncertainty budget shows that this accuracy is dominated by the knowledge of the electronic stopping, but that special care must also be taken to accurately determine the electronic gain of the detection system and other parameters. This RBS method is quite general and can be used routinely to accurately validate ion implanter charge collection systems, to certify SIMS standards, and for other applications. The generality of application of such methods in IBA is emphasized: if RBS and PIXE data are analysed self-consistently then the resulting depth profile inherits the accuracy and depth resolution of RBS and the sensitivity and elemental discrimination of PIXE.     

Thin film analysis via accelerator-based nuclear methods

Methods for depth profiling layered structures in the 10–100 nm range via ion beam analysis, such as Rutherford backscattering spectrometry, channeling, nuclear reaction analysis and elastic recoil detection analysis, are described in connection with ion-beam synthesized silicides (β-FeSi_2, TaSi₂) and epitaxial regrowth of amorphous SiO₂. Methods using implanted radioactive marker isotopes in nanometer thin films, such as perturbed angular correlation spectroscopy, will be sketched.     

Design considerations for thin film coated semiconductor thermal neutron detectors—I: basics regarding alpha particle emitting neutron reactive films

Semiconductor-based thermal neutron detectors provide a compact technology for neutron detection and imaging. Such devices can be produced by externally coating semiconductor-charged-particle detectors with neutron reactive films that convert free neutrons into charged-particle reaction products. Commonly used films for such devices utilize the ¹⁰B(n,)⁷Li reaction or the ⁶Li(n,)³H reaction, which are attractive due to the relatively high energies imparted to the reaction products. Unfortunately, thin film or “foil” type thermal neutron detectors suffer from self-absorption effects that ultimately limit neutron detection efficiency. Design considerations that maximize the efficiency and performance of such devices are discussed. Theoretical and experimental results from front coated, back coated, and “sandwich” designs are presented.     

Test of He-based neutron polarizers at NIST

Neutron spin filters based on polarized ³He are useful over a wide neutron energy range and have a large angular acceptance among other advantages. Two optical pumping methods, spin-exchange and metastability-exchange, can produce the volume of highly polarized ³He gas required for such neutron spin filters. We report a test of polarizers based on each of these two methods on a new cold, monochromatic neutron beam line at the NIST Center for Neutron Research.     

A continuous electrical resistivity measurement in thin films

The continuous electrical resistivity measurement, while an interesting parameter is being changed, can be a useful tool for in-situ thin film analysis as most changes in films are accompanied by changes in the electrical resistivity. In-situ measurements in a tube furnace at atmospheric pressure during different heat treatments are presented for oxidation tracing in Cr films and the TiN/CrN multilayer and for detection of interactions in Ni/Si}, Ti/Si} and Ni/Al multilayers. Results of electrical resistivity measurements c correlated well with weight gain measurements, X-ray diffraction, Rutherford backscattering spectroscopy and Auger electron spectroscopy depth profiling. It is therefore shown that the measurement of the resistivity with its time and temperature derivatives can represent a useful basis for the application of other analytical methods.     

Neutron energy spectrum measurements of neutron sources with an NE-213 spectrometer

The neutron energy spectra of the following sources were measured using a fast-neutron spectrometer with NE-213 liquid scintillator: ²⁵²Cf, Am-Be and T(d, n)⁴He from a Van de Graaff accelerator (400 keV). The measured proton recoil pulse-height data were unfolded using the FANTI code to obtain the neutron energy spectrum. The spectrometer gives neutron spectra in the range of 2–16 MeV, with 6% intrinsic efficiency and a resolution between 4% and 11%. The ²⁵²Cf neutron energy spectrum was measured and the results obtained showed good agreement with the spectrum usually published in the literature, which can be fitted by the expression N(E) =√E exp(− E/T) with the constant T = 1.42 MeV.     

Compaction of Fullerite Films after High Fluence Ion and Electron Bombardment

Thin fullerite layers are irradiated with 0.1 to 16 MeV electrons and 250 to 500 keV light ions at high fluence, and GeV heavy ions at low fluence. The subsequent analysis by Rutherford backscattering spectrometry revealed that in all cases the energy loss of the analyzing particles in the irradiated samples is enhanced, in comparison to the corresponding pristine samples. This is interpreted as resulting from a compactation of the fullerite films, and indicates an irradiation-induced phase transition towards amorphous, or even diamond-like carbon.     

Ion-activated interface adsorption of oxygen during silver deposition on silicon

The process of ion-activated oxygen adsorption on silicon has been investigated using an experimental concept with simultaneous deposition of silver films. Auger electron spectroscopy in combination with sputter depth profiling is subsequently performed to determine the amount of oxygen adsorbed at the Ag---Si interface. Noble gas ions (⁴He⁺, ²⁰Ne⁺ and ⁴⁰Ar⁺)with energies between 50 and 175 keV were used, and it was found that for substrate temperatures of 300–700 K the oxygen adsorption depends strongly on ion mass, ion energy and ion flux density. For flux densities of 5 × 10¹¹ cm⁻² s⁻¹ or less, adsorption dominates and, in particular, for light-ion bombardment the majority of adsorbed oxygen atoms form chemical bonds with the silicon surface atoms (Si---O). However, for heavy ions, physical sputtering starts to compete and limits the effective rate of adsorption. At sufficiently high ion fluxes the adsorption starts to decrease, and for all ions and energies used in this work it is found that, if the electronic energy deposition density exceeds a critical value of about 1.2 × 10²¹ eV cm⁻² s⁻¹, dissociation of the Si---O bonds prevails with a corresponding loss in the adsorbed oxygen quantity.     

Nuclear fusion between heavy ions by the gamma-ray spectroscopy method

The in-beam and off-beam gamma-ray spectroscopy method, as a tool for the study of nuclear fusion between heavy ions, is described. These methods allow the study of the production and decay of the compound nucleus, inelastic scattering and transfer reactions. In this work we report the determination of fusion cross sections at energies near and below the Coulomb barrier for a series of systems: targets of ^46,50Ti, ⁵⁹Co and ^64,66Zn bombarded by ¹⁴N and ¹⁶O projectiles. Cross sections from 0.4 to 1000 mb were measured.     

SOURCES: a code for calculating (alpha,n), spontaneous fission, and delayed neutron sources and spectra

SOURCES is a computer code that determines neutron production rates and spectra from (alpha,n) reactions, spontaneous fission and delayed neutron emission owing to the decay of radionuclides in homogeneous media, interface problems and three-region interface problems. The code is also capable of calculating the neutron production rates due to (alpha,n) reactions induced by a monoenergetic beam of alpha particles incident on a slab of target material. The (alpha,n) spectra are calculated using an assumed isotropic angular distribution in the centre-of-mass system with a library of 107 nuclide decay alpha-particle spectra, 24 sets of measured and/or evaluated (alpha,n) cross sections and product nuclide level branching fractions, and functional alpha particle stopping cross sections for Z < 106. Spontaneous fission sources and spectra are calculated with evaluated half-life, spontaneous fission branching and Watt spectrum parameters for 44 actinides. The delayed neutron spectra are taken from an evaluated library of 105 precursors. The code outputs the magnitude and spectra of the resultant neutron sources. It also provides an analysis of the contributions to that source by each nuclide in the problem.     

Stoichiometry determination of VOx thin films by O-RBS spectrometry

This paper reports on the stoichiometry determination of epitaxially grown vanadium monoxide (VO_x) thin films on MgO(100) substrates. The epitaxial growth was confirmed by RHEED, LEED and XRD techniques. The oxygen content of VO_x thin films, as a function of oxygen flux, was determined using Rutherford backscattering spectrometry. The ¹⁸O isotope was used for film growth, in order to distinguish between the oxygen of film and substrate. The upper and lower stoichiometry limit found are consistent with the ones known for bulk material.     

Feasibility study of a He-magnetometer for neutron electric dipole moment experiments

We report on a ³He-magnetometer capable of detecting tiny magnetic field fluctuations of less than 10⁻¹⁴ T in experiments for measuring the electric dipole moment (EDM) of the neutron. It is based on the Ramsey technique of separated oscillating fields and uses nuclear spin-polarized ³He gas which is stored in two vessels of V10 l in a sandwich-type arrangement around the storage bottle for ultra-cold neutrons (UCN). The gas is polarized by means of optical pumping in a separate, small discharge cell at pressures around 0.5 mbar and is then expanded into the actual magnetometer volume. To detect the polarization of ³He gas at the end of the storage cycle the gas is pumped out by means of an oil-diffusion pump and compressed again into the discharge cell where optical detection of nuclear polarization is used.     

Measurement of the Am–Li radionuclide neutron source spectrum

The neutron energy spectrum of an ²⁴¹Am–Li radionuclide source has been measured in a low scatter room at the National Physical Laboratory, using a variety of spectrometers namely: a Bonner sphere set, a ³He ionisation chamber, and a set of proton recoil proportional counters. Calculations with the Monte Carlo multi-particle transport code MCNP have been used to explain some of the features seen in the spectrum. The data have been analysed using different unfolding techniques, and a final spectrum has been derived based on all the available measurements.     

A new low-field determination of the proton gyromagnetic ratio inwater

The proton gyromagnetic ratio in water has been measured by the low field method using atomic magnetic resonance in ⁴He, a multicurrent solenoid and an induction method for the dimensional measurement of the solenoid. The result for air and vacuum environment is γ'_p (low, air)=2.67515451×10⁸ s^-1 T^-1 (0.18×10^-6), γ'_p (low, vacuum)=2.67515418×10⁸ s^-1 T^-1 (0.18×10^-6), based on the adopted values K_J-90=483597.9 GHz/V, R_K-90=25812.807 Ω, and measured values γ(⁴ He)/γ'_p=658.200556, γ(⁴He) (air)=1760.78819×10⁸ s^-1 T^-1 (0.18×10^-6), K_J-90, R_K-90     

Uniaxial and cubic anisotropy constants in ion-implanted Sm,Tm,Ga:YIG bubble thin films

The effects of ion implantation into (SmTmY)3(GaFe)5O12garnet thin films have been studied by ferromagnetic resonance. He+ ions were used for implantation with doses ranging from 3 to 4 × 10¹⁵He+/ cm²and implantation energy ranging from 150 to 175 Kev. The uniaxial and cubic anisotropy constants have been studied as a function of temperature from 0°C to 100°C for both as-grown and ion-implanted films. The implantation has effectively changed an easy-axis anisotropy to that of an easy plane in the implanted layers of the films. A cubic anisotropy constant K1with values varying from 4 × 10³to 6 × 10³erg/cm³at room temperature has been observed in these films. The results also indicated that to the first order, the implanted region in the film was essentially magnetically uncoupled from the bulk of the film.     

The possibility of tailoring the ne vs cLi relationship in lithium niobate optical waveguides

We present results of our study of concentration profiles of lithium (c_Li) in annealed proton exchanged (APE) waveguiding layers as measured by the neutron depth profiling (NDP) method. This non-destructive method, based on the ⁶Li(n,)³H reaction induced by thermal neutrons, allowed easy monitoring of c_Li profiles in a large number of samples fabricated under various fabrication conditions. Our systematic study revealed that, though every particular waveguide could be characterised by very similar mirror-shaped extraordinary refractive index (n_e) as well as c_Li depth profiles, in contrast with up to now generally accepted opinion, there was no linear relationship which unambiguously attributed Δn_e to Δc_Li. The most important fabrication step appeared to be the post-exchange annealing, during which the lithium atoms were transported towards the sample surfaces. The annealing regime pre-destined not only the depth distribution of the lithium atoms but, as a consequence of it, also other properties of the waveguiding region. That knowledge allows us to fabricate the APE waveguides with a priori given properties for a wide range of special applications. We have also formulated the n_e vs c_Li semi-empirical relationship, which was proved to fit all our fabricated APE waveguides.     

Characterization of hydrogenated amorphous carbon films by nuclear magnetic resonance techniques

Different nuclear magnetic resonance (NMR) methods have been used to investigate the structure of hydrogenated amorphous carbon films. Besides measuring the sp²-to-sp³ ratio by means of ¹³C cross-polarization magic-angle spinning total suppression of spinning sidebands NMR we studied the distribution of covalently bound hydrogen over sp²- and sp³-type carbon atoms using dipolar dephasing techniques. The reliability of the dipolar dephasing measurements is discussed and the results are compared with the data derived from a simple statistical model. Moreover they are compared with previous IR spectroscopy measurements. ¹H combined rotation-and-multipulse NMR spectra are presented which do not show resolved proton resonances in the case of as-deposited films but correspond in their line shape to the proton distribution measured with dipolar dephasing. Finally a few considerations concerning a refined structural model are presented.     

A novel technique for fabrication of metallic structures on polyimide by selective electroless copper plating using ion implantation

The successful use of palladium ion implantation into polyimide to seed an electroless plated film of copper on the polyimide surface is reported. Polyimide (Hitachi PIX 3400) was implanted with palladium ions to doses of 1.5 × 10¹⁵ − 1.2 × 10¹⁷ ions cm⁻² using a MEVVA ion implanter. The implanted ions acted as sites for nucleation of copper film. A copper film was then deposited on implanted polyimide using a commercial electroless plating solution. The ion energy was kept low enough to facilitate a low critical ‘seed’ threshold dose that was measured to be 3.6× 10¹⁶ Pd ions cm⁻². Test patterns were made using polyimide to study the adaptability of this technique to form thick structures. Plated films were studied with optical microscopy, Rutherford Backscattering Spectrometry (RBS) and Profilometry. The adhesion of films was qualitatively assessed by a ‘scotch tape test’. The film growth (thickness) was observed to be linear with plating time. A higher implantation dose led to greater plating rates. The adhesion was found to improve with increasing dose.     

卢瑟福背散射分析

对卢瑟福背散射分析的基本原理作了概要的介绍，论述了背散射分析的最佳实验条件，质量分辨率和分析灵敏度，列举了卢瑟福背散射分析在材料表面层和薄膜研究中的应用例子。