Recoil Spectrometry Research Articles

Digital pulse analysis for fast neutron recoil spectroscopy with a 4He scintillation detector

Helium-4-based fast neutron scintillation detectors are an attractive alternative to pulse-shape discrimination-capable organic scintillators for fast neutron detection and spectroscopy, as the response of the detectors to gamma rays is intrinsically limited to low energy deposition. Consequently, the neutron recoil distribution can be measured with these detectors without the need for pulse shape analysis. In this work, the response of an Arktis S670 4He scintillation detector to D-D, D-T, and 252Cf neutrons was measured. The detector has a unique construction and readout mechanism, with multiple output channels observing the same scintillation event, and an analysis method was developed to aggregate the outputs from all channels into a single list. The D-D and D-T neutron responses were used to perform a two-point energy calibration, which yielded a near-zero intercept, suggesting that the 4He scintillation medium behaves linearly to a higher energy than previously reported, and that a two-point calibration is sufficient for nuclear recoil energies below 9 MeV. Furthermore, the detector was measured to have 16.7-ns FWHM time resolution when using the developed custom analysis, a reduction of 4.9 ns when compared to the conventional pulse analysis.

Hydroxylation of the Zn terminated ZnO(0 0 0 1) surface under vacuum conditions

Under vacuum conditions, the polar surface ZnO(0 0 0 1) evolves in time forming a layer with H and additional O which changes the top layer stoichiometry and other properties such as its work function. In this work we present a study of the evolution of the ZnO(0 0 0 1) surface in ultra-high vacuum performed with time of flight direct recoil spectroscopy to detect the adsorbed H and O, plus other standard techniques including photoelectron spectroscopy and low energy electron diffraction. The experimental results are complemented with density functional theory (DFT) calculations for surfaces perfectly terminated and with triangular defects plus O adatoms. We show that the ZnO surface reaches a stable condition at an OH coverage of around 0.25 ML. We also studied the effect of increasing the partial pressure of H2 and of H2O on the layer growth. We observe that during the formation of the OH layer the work function decreases. Analysis of this variation of the work function with DFT reveals the role of the O adatoms in the adsorption process. Finally we show that the OH coverage decreases continuously with the increase of the sample temperature, without showing a well-defined desorption peak, which is in agreement with earlier predictions of the thermodynamics evolution. We believe that the present results contribute to clarify the behavior of the ZnO(0 0 0 1) surface under vacuum and will be useful in future works, particularly where H cannot be detected by standard techniques.

How oxygen passivates polycrystalline nickel surfaces.

The passivation of polycrystalline nickel surfaces against hydrogen uptake by oxygen is investigated experimentally with low energy ion scattering (LEIS), direct recoil spectroscopy (DRS), and thermal desorption spectroscopy (TDS). These techniques are highly sensitive to surface hydrogen, allowing the change in hydrogen adsorption in response to varying amounts of oxygen exposure to be measured. The chemical composition of a nickel surface during a mixed oxygen and hydrogen exposure was characterized with LEIS and DRS, while the uptake and activation energies of hydrogen on a nickel surface with preadsorbed oxygen were quantified with TDS. By and large, these measurements of how the oxygen and hydrogen surface coverage varied in response to oxygen exposure were found to be consistent with predictions of a simple site-blocking model. This finding suggests that, despite the complexities that arise due to polycrystallinity, the oxygen-induced passivation of a polycrystalline nickel surface against hydrogen uptake can be approximated by a simple site-blocking model.

Predictive Atomistic Model for Hydrogen Adsorption on Metal Surfaces: Comparison with Low-Energy Ion Beam Analysis on Tungsten

We present an analytical thermodynamic model of a surface in contact with a gas phase, which enables us to determine the surface coverage of the adsorbate depending on the temperature, pressure, and chemical potential of the gas. This model is applied to both the W(110) and W(100) surfaces of tungsten in contact with hydrogen. The thermodynamic model is built upon data computed by density functional theory that provide the complete electronic and vibrational energetics of both surfaces. It is further compared to experimental measurements of hydrogen coverage during isobar exposure at various temperatures acquired via low-energy ion scattering and direct recoil spectroscopy techniques. On W(110), the agreement is quantitative provided that an additional degree of freedom is added to the model. This degree of freedom accounts for the translational motion of the adsorbate along the 1D channel on the surface. On W(100), surface reconstruction makes the energetics of the system more complex; the full details of the experimental isobar are not well reproduced, although the overall consistency is obtained. We end up with a thermodynamic model based on DFT data having accurate predictive capabilities to determine the hydrogen coverage of tungsten at any p and T.

Development of the materials analysis and particle probe for Proto-MPEX.

The Prototype Material Plasma Exposure eXperiment (Proto-MPEX) is a linear plasma device being used in plasma source research and development (R&D) for the proposed MPEX. Once the R&D is completed, this device can also be used to perform plasma-material interaction studies. To perform these studies, a new materials analysis and particle probe (MAPP) has been constructed. The MAPP's components are a sample holder and manipulator and a custom vacuum chamber with ports to facilitate surface chemistry diagnostics. The MAPP's overall design enables rapid sample turnaround and in vacuo surface characterization. The surface analysis vacuum chamber has ports for x-ray photoelectron spectroscopy, thermal desorption spectroscopy, back-scatter ion scattering spectroscopy, forward-scatter ion scattering spectroscopy, and direct recoil spectroscopy. The sample manipulator and holder is a Lesker/UHV Multi-Centre Analytical Stage, which is used to place the samples in the exposure region of the Proto-MPEX or the analysis position in the MAPP vacuum chamber. The sample holder has a heating capability of up to 1200 °C for heated exposure and for desorption studies. In this work, we present the MAPP's design and the first tungsten sample exposure with ex situ analysis that shows a surface deposition layer on the exposed target, highlighting the need for additional in situ measurements on the Proto-MPEX.

Oxidation and passivation of U(AlxSi1-x)3 alloy at elevated temperatures

The surface composition of U(AlxSi1-x)3 alloy (x = 0.57) and its interactions with oxygen, at elevated temperatures, were studied, utilizing Auger electron spectroscopy, X-Ray photoelectron spectroscopy and direct recoil spectrometry. Heating the alloy in ultra-high vacuum, results in aluminum (and some silicon) segregation to the surface, forming, above 700 K, a ∼0.6 nm self-assembly capping layer. Exposing the surface alloy to oxygen, at temperatures up to 500 K, causes oxidation of the uranium and the aluminum components, while silicon is only slightly oxidized. Above 600 K, only the aluminum segregated overlayer is oxidized, forming a passivation layer that inhibits further oxidation of the alloy.

Experimental characterization of hydrogen adsorption sites for H/W(111) using low energy ion scattering

Low energy ion scattering (LEIS) and direct recoil spectroscopy (DRS) are among the few experimental techniques that allow for the direct detection of hydrogen on a surface. The interpretation of LEIS and DRS measurements, however, is often made difficult by complexities that can arise from complicated scattering processes. Previously, these complexities were successfully navigated to identify the exact binding configurations of hydrogen on a few surfaces using a simple channeling model for the projectile ion along the surface. For the W(111) surface structure, this simple channeling model breaks down due to the large lateral atomic spacing on the surface and small interlayer spacing. Instead, our observed hydrogen recoil signal can only be explained by considering not just channeling along the surface but also scattering from subsurface atoms. Using this more complete model, together with molecular dynamics (MD) simulations, we determine that hydrogen adsorbs to the bond-centered site for the W(111)$+\text{H}$(ads) system. Additional MD simulations were performed to further constrain the adsorption site to a height $h=1.0\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{3.33333pt}{0ex}}\AA{}$ and a position ${d}_{\mathrm{BC}}=1.6\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{3.33333pt}{0ex}}\AA{}$ along the bond between neighbors in first and second layers. Our determination of the hydrogen adsorption site is consistent with density functional theory simulation results in the literature.

Photon-recoil spectroscopy: Systematic shifts and nonclassical enhancements

In photon recoil spectroscopy, signals are extracted from recoils imparted by the spectroscopy light on the motion of trapped ions as demonstrated by C. Hempel et al., Nature Photonics 7, 630 (2013) and Y. Wan et al., Nature Communications 5, 3096 (2014). The method exploits the exquisite efficiency in the detection of phonons achievable in ion crystals, and is thus particularly suitable for species with broad non-cycling transitions where detection of fluorescence photons is impractical. Here, we develop a theoretical model for the description of photon recoil spectroscopy based on a Fokker-Planck equation for the Wigner function of the phonon mode. Our model correctly explains systematic shifts due to Doppler heating and cooling as observed in the experiment. Furthermore, we investigate quantum metrological schemes for enhancing the spectroscopic sensitivity based on the preparation and detection of nonclassical states of the phonon mode.

Nuclear recoil spectroscopy of levitated particles

We propose a new method for the detection and characterization of nuclear decay processes. Specifically, we describe how nuclear decay recoil can be observed within small particles levitated in an optical trap with high positional resolution. Precise measurements of the magnitude of each recoil as well as their rate of occurrence can provide accurate information about the isotopic composition of a radioactive sample. We expect that this new technique for nuclear material characterization will be especially useful in the area of nuclear forensic analysis.

Unveiling the origin of photoluminescence in nanoporous anodic alumina (NAA) obtained by constant current regime

Photoluminescence of Nanoporous Anodic Alumina (NAA) films is characterized by an intense and broad band in the blue region when excited in UV, and its origin and mechanisms are still debatable. In this work, the NAA films were prepared by galvanostatic anodization of electropolished Al foils (99.99%) by applying 5 mA/cm2 for 90 min at 20 °C, in three different electrolytes: oxalic acid, phosphoric acid, and an equimolar mixture of oxalic and phosphoric acids. After anodization, the Al substrate was removed from NAA porous layer in CuCl2 + HCl solution. The NAA membranes were characterized by Scanning Electron Microscopy (SEM) and Rutherford Backscattering Spectroscopy (RBS). The fluorescence measurements were performed in a spectrofluorimeter at λ = 320 nm as excited wavelength. SEM images showed the changes in morphology as being a function of electrolyte composition. The NAA membrane prepared in oxalic acid presented the smallest pore diameter and those prepared in phosphoric acid presented more defects and branched pores. The presence of impurities inside the porous layer structure was investigated by Rutherford Backscattering Analysis using different conditions (default RBS, C resonance using 1.7 MeV protons and H by Forward Recoil Spectrometry) to evaluate their concentration and elemental depth profile. The relative quantities of Al, O, P and C were evaluated. A model of an inner and an outer layer of NAA oxide with different Al/O proportions is proposed based on the results. Incorporation of oxalate ions was not observed in the oxide film galvanostatically prepared in oxalic acid. This is usually attributed to be the origin of the PL emission in NAA films produced in oxalate media. The results showed that the photoluminescence (PL) emission of NAA might be associated with the presence of recombination centers from oxygen defects inside the oxide structure.

Neutron Diagnostics for Tokamak Plasma: From a Plasma Diagnostician Perspective

Neutron measurement is considered to be one of the major diagnostic methods in tokamak plasma. The neutron diagnostics provide key information on plasma physics, machine protection and control issues. In this article, the commonly used techniques of neutron emission diagnostics for tokamak plasma are presented. The requirements and limitations imposed on the diagnostic systems are discussed. Several basic techniques of total neutron yield and neutron spectrum measurements such as neutron counters, neutron activation technique, time-of-flight spectroscopy or magnetic proton recoil spectroscopy are described in a practical way, from a plasma diagnostician point of view. Primary physical quantities that can be derived from those measurements are pointed out and discussed.

Status of Ion Beam Modification and Analysis of Materials at STU MTF

Abstract The new Ion Beam Centre (IBC) equipped with 6 MV tandem ion accelerator and 500 kV ion implanter systems was built at the Slovak University of Technology, Faculty of Materials Science and Technology (STU MTF). The facility provides Ion Beam Modification of Materials (IBMM) and Ion Beam Analysis (IBA), which includes Rutherford Backscattering Spectrometry (RBS), Particle Induced X-ray Analysis (PIXE), Elastic Recoil Spectrometry (ERDA) and Nuclear Reaction Analysis (NRA). Presented are selected experimental procedures carried out in the IBC during the first year of operation. They present examples of a typical IBA performed, such as thin film characterisation in nm to tens of µm range, elemental depth profiles and sensitivity to the light elements enhancement by non-Rutherford cross-section regime application along with the crystalline sample channelling spectra and boron content measurement.

Saturation of tungsten surfaces with hydrogen: A density functional theory study complemented by low energy ion scattering and direct recoil spectroscopy data

Herein, we investigate the saturation limits of hydrogen on the (110) and (100) surfaces of tungsten via Density Functional Theory (DFT) and complement our findings with experimental measurements. We present a detailed study of the various stable configurations that hydrogen can adopt upon the surfaces at coverage ratios starting below 1.0, up to the point of their experimental coverage ratios, and beyond. We provide the many low-energy configurations that exist at all coverages along with the energy landscape they form. Our findings allow us to estimate that the saturation limit on each surface exists with one monolayer of hydrogen atoms adsorbed. In the case of (110) this corresponds to a coverage ratio of one hydrogen atom per tungsten atom, while in the case of (100) a full monolayer is present at a coverage ratio of 2.0 hydrogen atoms per tungsten atoms. Preliminary Low Energy Ion Scattering (LEIS) and Direct Recoil Spectroscopy (DRS) measurements complement this work on the W(110) surface. These results and some previously published measurements obtained on the W(100) surface confirm the findings obtained by DFT. In particular, the saturation limits on each surface, the preferred adsorption sites on both surfaces up to saturation, and the reconstruction of the bare and unsaturated (100) surface.

Surface characterization of U(AlxSi1-x)3 alloy and its interaction with O2 and H2O, at room temperature

Surface characterization and the interactions of U(AlxSi1-x)3 alloy (x = 0.57) with oxygen and water vapor were studied, utilizing X-Ray Photoelectron Spectroscopy and Direct Recoil Spectrometry, at room temperature. The U 4f spectrum of U(AlxSi1-x)3 alloy exhibits weak correlation satellites, suggesting an itinerant description of the U 5f states for this compound. The Al and Si 2p lines are chemically shifted to lower binding energies. Exposing the alloy to oxygen and water vapor results in oxidation of mainly the uranium and aluminum components, while silicon is only slightly oxidized. Oxygen was found to be a stronger oxidizer than water vapor and the trend is consistent with the more negative enthalpies of formation of metal oxides produced by the O2 reaction, as compared to H2O. During oxygen exposure, fast oxidation occurs by oxide islands nucleation and lateral growth, followed by oxidation of the sub-surface, up to ∼4 nm, at 1000 L exposure. Water initially reacts with the surface by full dissociation and oxide islands formation, which is then covered by hydroxides. Only a minor increase in the oxide thickness of up to ∼2.5 nm, was observed after coalescence.

Effects of ion irradiation damage on the initial interactions of oxygen with polycrystalline gadolinium

The effects of surface and near-surface defects, induced by Ar+ ion irradiation, on the interactions of oxygen with polycrystalline gadolinium surfaces were investigated over the temperature range 140–300K, utilizing Auger electron spectroscopy, direct recoil spectrometry, work function change measurements and density functional theory calculations. This enabled the distinction between the topmost surface and subsurface processes involved in this reaction. It turns out that the formation of bulk gadolinium oxide (at the near-surface region) involves in general three sequential steps: (i) surface random chemisorption, (ii) clustering of the chemisorbed atoms and the formation of a type of “surface oxide”, (iii) dissolution of surface oxygen atoms into the sub-surface and nucleation of the “bulk” oxide. However, the time duration of these steps becomes shorter (i.e. faster kinetics) as the corresponding rate controlling parameters (i.e. surface reactivity and temperature) become more favorable. Consequently, all the above steps are discernable only for the less favorable kinetic parameters (annealed surfaces at low temperatures). For the more active surfaces (sputtered or annealed at higher temperatures) the very initial random surface chemisorption step is practically absent and a two-step process occurs, with an initial stage associated with direct clustering and formation of the “surface oxide”, followed by the subsurface penetration and bulk oxide formation. It was also found that the irradiation induced damage does not affect significantly the topmost surface accumulation of oxygen. However, it accelerates appreciably the penetration of oxygen into the near-surface region and the development of the bulk oxide.

ERDA at the 9 MV Tandem and at the 3 MV Tandetron of IFIN-HH

Recoil spectrometry using heavy ions proposed in 1976 by L’Ecuyer has evolved into a universal IBA technique. Few years later an experimental setup for simultaneous light and medium heavy element detection including a compact ΔE(gas)–Er(solid) telescope, was developed at the Tandem accelerator of IFIN-HH. To increase the resolution, an integrated preamplifier was mounted close to the ionization chamber. The calibration procedure for the telescope and the software for the quantitative evaluation of the data are briefly presented. Recently, a 3MV Tandetron accelerator has been installed and commissioned at the IFIN-HH. Among several ion-beam techniques for detection and depth profiling of hydrogen isotopes, Elastic Recoil Detection Analysis (ERDA) technique using a low energy 4He beam, proposed by Doyle and Peercy, is particularly advantageous. By measuring simultaneously both the H or D recoiling at a forward angle and backscattered 4He ions, a rather complete characterization of the sample can be achieved. Selected results from our investigations, obtained using these facilities, are presented.

Unexpectedly large difference of the electron density at the nucleus in the $$ 4p\, ^2{\mathrm {P}}_{{1}/{2},{3}/{2}}$$ 4 p 2 P 1 / 2 , 3 / 2 fine-structure doublet of Ca $$^+$$ +

We measured the isotope shift in the $^2$S$_{1/2}$-$^2$P$_{3/2}$ (D2) transition in singly-ionized calcium ions using photon recoil spectroscopy. The high accuracy of the technique enables us to resolve the difference between the isotope shifts of this transition to the previously measured isotopic shifts of the $^2$S$_{1/2}$-$^2$P$_{1/2}$ (D1) line. This so-called splitting isotope shift is extracted and exhibits a clear signature of field shift contributions. From the data we were able to extract the small difference of the field shift coefficient and mass shifts between the two transitions with high accuracy. This J-dependence is of relativistic origin and can be used to benchmark atomic structure calculations. As a first step, we use several ab initio atomic structure calculation methods to provide more accurate values for the field shift constants and their ratio. Remarkably, the high-accuracy value for the ratio of the field shift constants extracted from the experimental data is larger than all available theoretical predictions.

The influence of ion irradiation on hydrogen chemisorption and diffusion on gadolinium surfaces

The role of irradiation induced surface defects on the chemisorption process of hydrogen and inward penetration of chemisorbed H atoms was studied on sputtered, with 4.5 keV Ar + ions and on annealed gadolinium surfaces. A combination of Direct Recoil Spectrometry (DRS) and Contact Potential Difference (CPD) enabled to distinct between topmost surface and subsurface processes. Simulations of irradiation-induced defects, which assume two types of defects-vacancies due to sputtering and Frenkel pairs, taking account of the annealing processes occurring at different temperatures, have reasonably reproduced the experimental Ar scattering peaks in the DR spectra. It has been concluded that the presence of surface defects has a significant effect on the binding energies of the chemisorbed hydrogen, resulting in hydrogen trapping on the surface and affects the surface to subsurface inward penetration process.

Detection of motional ground state population of a trapped ion using delayed pulses

Efficient preparation and detection of the motional state of trapped ions is important in many experiments ranging from quantum computation to precision spectroscopy. We investigate the stimulated Raman adiabatic passage (STIRAP) technique for the manipulation of motional states in a trapped ion system. The presented technique uses a Raman coupling between two hyperfine ground states in 25Mg+, implemented with delayed pulses, which removes a single phonon independent of the initial motional state. We show that for a thermal probability distribution of motional states the STIRAP population transfer is more efficient than a stimulated Raman Rabi pulse on a motional sideband. In contrast to previous implementations, a large detuning of more than 200 times the natural linewidth of the transition is used. This approach renders STIRAP suitable for atoms in which resonant laser fields would populate nearby fluorescing excited states and thus impede the STIRAP process. We use the technique to measure the wavefunction overlap of excited motional states with the motional ground state. This is an important application for force sensing applications using trapped ions, such as photon recoil spectroscopy, in which the signal is proportional to the depletion of motional ground state population. Furthermore, a determination of the ground state population enables a simple measurement of the ion's temperature.

Dissociative adsorption of molecular deuterium and thermal stability onto hydrogenated, bare and ion beam damaged poly- and single crystalline diamond surfaces

In this work we report on dissociative adsorption of deuterium (D2) on bare, hydrogenated and ion beam bombarded polycrystalline and single crystalline diamond surfaces. Polycrystalline diamond films with an average grain size of ~300nm were deposited on silicon substrates by hot filament chemical vapor deposition technique from methane/hydrogen gas mixture. Deposited films were characterized using Raman spectroscopy, atomic force microscopy and scanning electron microscopy to estimate the phase composition and microstructure. High resolution electron energy loss spectroscopy and direct recoil spectrometry were used to study hydrogen (deuterium) bonding configuration of the upper surface region. Near surface amorphization was achieved by 1keV Ar+ implantation at ~1×1015ions/cm2 at room temperature (RT). As deposited and Ar+ bombarded films are annealed to 500–1000°C in ultra-high vacuum conditions and also under D2 partial pressure of 5×10−6Torr. For comparison, key experiments were repeated on the single crystal (100) diamond. Our results clearly show the preferential dissociative adsorption of D2 on low hybridized carbon (sp/sp2) states with activation temperature as low as RT, but with a lower thermal stability compared to pure diamond CD bonds.