Differential Pumping Research Articles

Experimental techniques for in-ring reaction experiments

As a first step of the EXL project scheduled for the New Experimental Storage Ring at FAIR a precursor experiment (E105) was performed at the ESR at GSI. For this experiment, an innovative differential pumping concept, originally proposed for the EXL recoil detector ESPA, was successfully applied. The implementation and essential features of this novel technical concept will be discussed, as well as details on the detectors and the infrastructure around the internal gas-jet target. With 56Ni(p, p)56Ni elastic scattering at 400 MeV u−1, a nuclear reaction experiment with stored radioactive beams was realized for the first time. Finally, perspectives for a next-generation EXL-type setup are briefly discussed.

Integration of RFQ beam coolers and solenoidal magnetic fields.

Electromagnetic traps are a flexible and powerful method of controlling particle beams, possibly of exotic nuclei, with cooling (of energy spread and transverse oscillations) provided by collisions with light gases as in the Radio Frequency Quadrupole Cooler (RFQC). A RFQC prototype can be placed inside the existing Eltrap solenoid, capable of providing a magnetic flux density component B(z) up to 0.2 T, where z is the solenoid axis. Confinement in the transverse plane is provided both by B(z) and the rf voltage V(rf) (up to 1 kV at few MHz). Transport is provided by a static electric field E(z) (order of 100 V/m), while gas collisions (say He at 1 Pa, to be maintained by differential pumping) provide cooling or heating depending on V(rf). The beamline design and the major parameters V(rf), B(z) (which affect the beam transmission optimization) are here reported, with a brief description of the experimental setup.

Performance test of new near-ambient-pressure XPS at Korean Basic Science Institute and its application to CO oxidation study on Pt3Ti polycrystalline surface

A performance test of near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) at Korean Basic Science Institute (KBSI) in Daejeon, Korea is carried out. The NAP-XPS at KBSI is equipped with SPECS PHOIBOS 150 analyzer and 2-dimensional-delay line detector, which provides improved detection efficiency with 1-dimensional chemical imaging mode. The in-situ gas-pressure cell is designed to operate at pressure up to 25 mbar with differential pumping scheme. A micro-focused AlKα X-ray source is utilized as photon excitation source. The NAP-XPS is composed of two separate chambers, e.g. a main preparation chamber and a measurement chamber, which hold a low energy electron diffraction, a high pressure cell, an ion sputter gun, a multi-cell E-beam evaporators, and a four-axis manipulator with heating and cooling capability. As a test run of NAP-XPS, a CO oxidation experiment is carried out on Pt3Ti polycrystalline surface. During the active phase of CO oxidation, a clear formation of Ti oxide is found on surface, revealing the intriguing role of surface metal oxide in surface chemical reaction.

Cr(Al)N/Al2O3 Superhard Coatings Prepared by Differential Pumping Cosputtering: Structure and Mechanical Properties

Various Cr(Al)N/Al2O3 coatings were fabricated in a differential pumping cosputtering system, which has two chambers for radio frequency magneton sputtering of different materials and a substrate holder rotating above the target materials. The main product of Cr(Al)N/Al2O3 layers that were sputtering deposited from CrAl and Al2O3 targets on transition buffer layers prepared on the Si substrate had a columnar structure normal to the substrate surface. The buffer layers were prepared successively in the same chamber so that their compositions were to gradually change from metal (Cr) to nitride (CrN), and appropriate as adhesion between the metal substrate and the composite nitride layer. In a Cr(Al)N/17 vol% Al2O3 coating prepared at a substrate rotational speed of ω = 12 rpm, each column comprised NaCl-type Cr(Al)N crystallites including homogeneously dispersed fine particles of amorphous (a-) Al2O3. The nanoindentation hardness (H IT) and Young’s modulus (E*) of the Cr(Al)N/Al2O3 coatings increased with increasing ω within a measured range of ω = 1–12 rpm. With increasing Al2O3 fraction, H IT and E* increased until maximum values and then decreased, showing that the hardness of the nitride coatings is improved by fabricating the nanocomposite layer. The maxima of H IT and E* were obtained at 17 vol% Al2O3 for the coatings prepared at ω = 12 rpm. The fine a-Al2O3 particles in the Cr(Al)N/17 vol% Al2O3 layer worked as obstacles against the deformation of the Cr(Al)N lattices, which yielded the superhard coating of H IT = ~43 GPa.

A study of the pressure profiles near the first pumping aperture in a high pressure photoelectron spectrometer

In a high-pressure photoelectron spectrometer, the sample is positioned close to a differential pumping aperture, behind which the pressure is several orders of magnitude lower than the pressure in the analysis chamber. To find the optimal sample position, where the path length of the photoelectrons through the high pressure region is minimized as far as possible without compromising knowledge of the actual pressure at the sample surface, an understanding of the pressure variations near the sample and the aperture is required. A computational fluid dynamics study has been carried out to examine the pressure profiles, and the results are compared against experimental spectra whose intensities are analyzed using the Beer–Lambert law. The resultant pressure profiles are broadly similar to the one previously derived from a simplistic molecular flow model, but indicate that as the pressure in the analysis chamber is raised, the region over which the pressure drop occurs becomes progressively narrower.

Latest Developments for In Situ Measurements of Gas-Solid Interactions

The power of in situ observations at elevated temperatures to elucidate the details of gas-solid interactions was realized during the very early years of the advent of transmission electron microscope [1]. Of several designs developed between 1950 and 1970, the underlying approach of using a differential pumping system, as proposed by Swann and Tighe [2], has been further developed and is now widely used in commercially-available environmental transmission or scanning transmission electron microscopes (ETEM or ESTEM). In recent years ESTEM, has been successfully employed to reveal and understand the structural and chemical changes occurring in nanoparticles under reactive environments [3,4]. However, quantitative measurements of reaction rates and chemical changes are precluded by (a) the nanoscale region used for atomic imaging, (b) uncertainties in the actual temperature of the region under observation, and (c) the large amount of data to be analyzed. Here we present various technical and analytical methods we have developed to address these issues.

A lab-based ambient pressure x-ray photoelectron spectrometer with exchangeable analysis chambers.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) is a powerful spectroscopy tool that is inherently surface sensitive, elemental, and chemical specific, with the ability to probe sample surfaces under Torr level pressures. Herein, we describe the design of a new lab-based APXPS system with the ability to swap small volume analysis chambers. Ag 3d(5/2) analyses of a silver foil were carried out at room temperature to determine the optimal sample-to-aperture distance, x-ray photoelectron spectroscopy analysis spot size, relative peak intensities, and peak full width at half maximum of three different electrostatic lens modes: acceleration, transmission, and angular. Ag 3d(5/2) peak areas, differential pumping pressures, and pump performance were assessed under varying N2(g) analysis chamber pressures up to 20 Torr. The commissioning of this instrument allows for the investigation of molecular level interfacial processes under ambient vapor conditions in energy and environmental research.

Cr(Al)N/Al2O3 nanocomposite coatings fabricated by differential pumping cosputtering

Cr(Al)N/Al2O3 nanocomposite coatings fabricated by differential pumping cosputtering

A plug-and-measure diagnostic tool for flux, resolution and beam size of a soft X-ray beamline.

A compact multi-functional diagnostic tool has been installed for commissioning beamlines at the Taiwan Light Source. For a photon beam, the instrument can measure the photon flux, energy resolution and beam size, consecutively with a photodiode or gold mesh, a windowless gas cell and a movable fluorescence screen viewport. Two gratings with ruling densities of 350 and 1000 lines mm(-1) and dispersing photons of energies from 80 to 1200 eV were calibrated with a photon flux of 10(11) photon s(-1) at slit openings of 50 µm × 50 µm; a maximum resolving power of greater than 10000 was certified with an absorption spectra of gaseous samples. Pressure differences over four orders of magnitude were achieved between the ion chamber and the flux measurement chamber with a single capillary differential pumping stage. A sequence of beam profiles was measured by moving continuously in the vicinity of the nominal focal positions. This tool is useful in commissioning or trouble-shooting at most beamlines in a synchrotron facility.

Fast and sensitive method for detecting volatile species in liquids.

This paper presents a novel apparatus for extracting volatile species from liquids using a "sniffer-chip." By ultrafast transfer of the volatile species through a perforated and hydrophobic membrane into an inert carrier gas stream, the sniffer-chip is able to transport the species directly to a mass spectrometer through a narrow capillary without the use of differential pumping. This method inherits features from differential electrochemical mass spectrometry (DEMS) and membrane inlet mass spectrometry (MIMS), but brings the best of both worlds, i.e., the fast time-response of a DEMS system and the high sensitivity of a MIMS system. In this paper, the concept of the sniffer-chip is thoroughly explained and it is shown how it can be used to quantify hydrogen and oxygen evolution on a polycrystalline platinum thin film in situ at absolute faradaic currents down to ∼30 nA. To benchmark the capabilities of this method, a CO-stripping experiment is performed on a polycrystalline platinum thin film, illustrating how the sniffer-chip system is capable of making a quantitative in situ measurement of <1% of a monolayer of surface adsorbed CO being electrochemically stripped off an electrode at a potential scan-rate of 50 mV s(-1).

Status of mass spectrometric radiocarbon detection at ETHZ

A prototype of a mass spectrometric radiocarbon detection instrument without accelerator stage was built for the first time and set into operation at ETH Zurich. The system is designed as an experimental platform to optimize performance of 14C detection at low ion energies and to study the most relevant processes that may limit system performance. The optimized stripper unit incorporates differential pumping to maintain a low gas outflow and a revised tube design to better match the phase space volume of the ion beam at low energies. The system is fully operational and has demonstrated true radiocarbon dating capabilities. The overall beam transmission through the stripper tube is about 40% for the 1+ charge state. Radiocarbon analyses with an overall precision of 0.6% were obtained on a single sample under regular measurement conditions. By analyzing multiple targets of the same sample material an uncertainty level of 0.3% has been reached. The background level corresponds to a radiocarbon age of 40,000years.

A chopper system for shortening the duration of pulsed supersonic beams seeded with NO or Br2 down to 13 μs.

A chopper wheel construct is used to shorten the duration of a molecular beam to 13 μs. Molecular beams seeded with NO or with Br2 and an initial pulse width of ≥200 μs were passed through a spinning chopper wheel, which was driven by a brushless DC in vacuo motor at a range of speeds, from 3000 rpm to 80,000 rpm. The resulting duration of the molecular-beam pulses measured at the laser detection volume ranged from 80 μs to 13 μs and was the same for both NO and Br2. The duration is consistent with a simple analytical model, and the minimum pulse width measured is limited by the spreading of the beam between the chopper and the detection point as a consequence of the longitudinal velocity distribution of the beam. The setup adopted here effectively eliminates buildup of background gas without the use of a differential pumping stage, and a clean narrow pulse is obtained with low rotational temperature.

Pressure optimization of high harmonic generation in a differentially pumped Ar or H₂ gas jet.

We experimentally studied the dependence of high harmonic generation in argon and molecular hydrogen on pressure changes in a gas jet that cause variations of the phase matching conditions and absorption. The study was performed at a peak laser intensity of ∼1.5 × 10(14) W/cm(2). To enable measurements over a wide range of pressures, we employed differential pumping with an additional cell (∼20 cm(3) volume) enclosing the gas jet. By increasing the pressure in the gas jet up to a maximum of 1.5 bars with argon or 0.5 bars with hydrogen, we observed an increase in the high harmonic (HH) yield until an optimum pressure of 0.2 bars was reached for Ar, beyond which the output began decreasing. For H2, we observed an increase of the HH output up to the maximum pressure of 0.5 bars. This pressure-dependence study allowed us to achieve a tenfold enhancement in the high harmonic yield at the optimum pressure.

Angular ion species distribution in droplet-based laser-produced plasmas

The angular distribution of the ion species generated from a laser irradiated droplet target is measured. The employed instrument was an electrostatic energy analyzer with differential pumping. Singly and doubly charged ions were detected at an argon ambient gas pressure of 2 × 10−2 mbar. The amount of Sn+ and Sn2+ and their kinetic energy is measured from 45° to 120° from the laser axis. Sn+ expands approximately isotropically, and Sn2+ expansion is peaked towards the incoming laser radiation. The singly charged ion kinetic energy is close to constant over the measurement range, while it decreases by around 30% for Sn2+. A calibrated model of the ion expansion that includes recombinations correctly predicts the mean ion charge distribution. The model is able to qualitatively estimate the influence of the laser wavelength on the mean ion charge distribution. The results show a more pronounced isotropic distribution for shorter wavelengths, and a more forward-peaked distribution for longer wavelengths. The ion charge distribution expected without the ambient gas is estimated through the measured ion kinetic energy. The presence of the ambient gas results in a decrease of the mean ion charge state and a decrease in angular anisotropy.

Development of a miniature mass spectrometer with continuous atmospheric pressure interface.

The demand for on-the-spot analysis is met by a miniature mass spectrometer which is preferred to be robust, stable, as small as possible and capable of analyzing different samples by coupling with various ionization methods. However, largely constrained by the atmospheric pressure interface (API), these aspects are difficult to be realized in one system. Herein, we describe the development of a new miniature mass spectrometer with balanced performance. The miniature mass spectrometer is small in size (30 × 30 × 18 cm) but has a continuous API, which was achieved by high-pressure ion trap operation and maximized ion transfer efficiency with the utilization of a differential pumping system. The miniature mass spectrometer was characterized and optimized in terms of stability, sensitivity, mass range, mass resolution and scan speed. Rapid analysis of mixtures was demonstrated by coupling the miniature mass spectrometer with the ambient ionization technique of paper spray. This is the smallest miniature mass spectrometer to date, which has a continuous API.

Differentially pumped quadrupole SIMS probe on FIB‐based and two‐beam microscopes

Two microscopes with focused ion beam (FIB) columns in the Department of Materials, Imperial College, have been fitted with SIMS probes (Hiden EQS 1000). Both probes have differential pumping to maintain the vacuum in the probe below 5 × 10−6 mbar irrespective of the main operating and imaging conditions of the microscopes. In both microscopes, standard operations include the injection of oxygen‐containing species close to the sputter site. This process is also a technique for SIMS analysis instrumentation to enhance positive ion yields during sputtering where an oxygen primary ion beam for dynamic SIMS analysis is not available. The aim of this paper is to describe the compatibility of the differentially pumped SIMS probe with the normal specifications of microscopes. In particular, the modern field emission gun‐SEM instrument's (Zeiss Auriga) ability to provide ultra‐high spatial resolution SIMS in combination with electron microscopy imaging, energy dispersive spectroscopy and electron backscatter diffraction. In the second instrument, an ion microscope (FEI FIB200), the installed system is additional to the existing FEI SIMS probe. Simultaneous detection of positive and negative secondary ions from the same sputtered crater is obtained on this instrument with the two SIMS probes. Copyright © 2014 John Wiley &amp; Sons, Ltd.

In situ observation on hydrogenation of Mg-Ni films using environmental transmission electron microscope with aberration correction

In situ transmission electron microscopy (TEM) was performed to observe the hydrogenation of Mg-Ni films in a hydrogen atmosphere of 80–100 Pa. An aberration-corrected environmental TEM with a differential pumping system allows us to reveal the Angstrom-scale structure of the films in the initial stage of hydrogenation: first, nucleation and growth of Mg2NiH4 crystals with a lattice spacing of 0.22 nm in an Mg-rich amorphous matrix of the film occurs within 20 s after the start of the high-resolution observation, then crystallization of MgH2 with a smaller spacing of 0.15 nm happens after approximately 1 min. Our in situ TEM method is also applicable to the analysis of other hydrogen-related materials.

Multilayered Cr(Al)N/SiOx Nanocomposite Coatings Fabricated by Differential Pumping Cosputtering

Nanocomposite films of metal nitrides such as TiN/Si3N4, TiN/BN, and CrN/AlN have attracted substantial attention as coating materials for improving hardness and saving rare metals. It is difficult to prepare composite films consisting of nitride and oxide by conventional reactive sputtering methods. Nose et al. developed a differential pumping cosputtering (DPCS) system, which can fabricate different nanocomposite films [1]. To elucidate the process and mechanism of film growth in the DPCS system, we prepared a hard coating Cr(Al)N/SiOx nanocomposite layer on the under buffer layers that were grown on a Si substrate and have been examining by means of analytical electron microscopy.

An online FT-ICR Penning-trap mass spectrometer for the DPS2-F section of the KATRIN experiment

Two Fourier-transform ion-cyclotron resonance (FT-ICR) Penning-trap mass spectrometers will be installed in the pumping section of the KArlsruhe TRItium Neutrino (KATRIN) experiment. This experiment aims at determining the electron anti-neutrino mass m(νe¯) with a sensitivity of 0.2eV (90% C.L.) by high-resolution tritium β-spectroscopy. The tritium source creates various types of ions, which have to be reduced in order to reach the required low background level. The purpose of the FT-ICR mass spectrometers is the identification of the ion flux components as well as their abundance. Furthermore, the pumping efficiency of the differential pumping section DPS2-F can be determined since these Penning traps will be installed one at the entrance and one at the exit. In this paper the operation of the FT-ICR system is described. Experimental results are presented concerning the cryogenic broad-band amplifier system for the FT-ICR detection as well as the characterisation of the mass spectrometer with respect to, e.g., noise density and detection limit.

Single-phase anatase structure and dominant metallic Ge in Ge/TiO2 multi-layer films using a differential pumping co-sputtering system

This study investigates the preparation of Ge/TiO2 multi-layer films using a differential-pumping co-sputtering system (DPCS). This system has two chambers with different atmospheres, pure Ar for the Ge target and 0.5%O2 in Ar for the TiO2 ceramic target. The optical absorption spectra of the multi-layer films obviously shift to visible and near-infrared regions with increasing Ge layer thickness, while keeping O/Ti composition ratios of 2.3±0.1 in TiO2. X-ray diffraction results indicate that the TiO2 layer forms a single-phase anatase structure in the multi-layer films. X-ray photoelectron spectroscopy also indicates that metallic Ge is dominant in the multi-layer film with negligible Ge-oxide. Therefore, DPCS provides a multi-layer film with a single-phase anatase structure in the TiO2 layer and a dominant metallic element in the Ge layer.