Study of ion feedback in multi-GEM structures

We study the feedback of positive ions in triple and quadruple Gas Electron Multiplier (GEM) detectors. The effects of GEM hole diameter, detector gain, applied voltages, number of GEMs and other parameters on ion feedback are investigated in detail. In particular, it was found that the ion feedback is independent of the gas mixture and the pressure. In the optimized multi-GEM structure, the ion feedback current can be suppressed down to 0.5% of the anode current, at a drift field of 0.1 kV/cm and gain of 10⁴. A simple model of ion feedback in multi-GEM structures is suggested. The results obtained are relevant to the performance of time projection chambers and gas photomultipliers.     

Bipolar ion detector based on sequential conversion reactions

This work demonstrates the feasibility of a novel scintillation detector with greater detection efficiency than that of chevron-type microchannel plate (MCP) detectors. The detection mechanism involves sequential conversion reactions induced by ion-surface impacts. Identical detection conditions can be utilized to monitor both positive and negative ions in mass spectrometers. The proposed detector comprises an ion beam guiding device, a negatively biased washer-shaped conversion dynode, and an aluminum-coated scintillation detector. The beam guide changes the electric field around the washer-shaped conversion dynode, and it allows the primary and secondary ions to propagate toward the scintillation phosphor and the conversion dynode, respectively. The detection is achieved by the detection of electron-induced luminescence on a phosphor. The amplification efficiency of this bipolar ion detector increases as the conversion dynode voltage increases. For ions with a mass-to-charge ratio of up to 90 000, the sensitivity of the BID is 1.4-14.4 times that of the MCP. Further improvement of the sensitivity can be achieved by increasing the conversion dynode voltage or the ion acceleration voltage. Results of this study demonstrate that this detector is a promising alternative for efficient ion detection.     

Charge transfer and charge broadening of GEM structures in high magnetic fields

We report on the measurements of charge transfer in Gas Electron Multipliers (GEM) structures in high magnetic fields. These were performed in the framework of the R&D work for a Time Projection Chamber at a future Linear Collider. A small test chamber has been installed in the aperture of a superconducting magnet with the GEM structures mounted perpendicular to the B-field direction. The charge transfer is derived from the electrical currents monitored during irradiation with an ⁵⁵Fe source. No significant loss of primary ionisation charge is observed, and an improved ion feedback suppression is achieved for high magnetic fields. Additionally, the width of the charge cloud released by individual ⁵⁵Fe photons is measured using a finely segmented strip readout after the triple GEM structure. Charge widths between 0.3 and 0.5 mm RMS are observed, which originate from the charge broadening inside the GEM amplification. This charge broadening is only partly suppressed at high magnetic fields.     

An analytic solution for energy loss and time-of-flight calculations for intermediate-energy light ions

Particle identification in intermediate heavy-ion collisions, using a modern 4π detector which contains several active layers, relies on a parametrisation or numerical integration of the energy loss in thick layers of detector material for different ions. Here an analytical solution applicable over an energy range of a few MeV up to a 100A MeV and for ions up to at least Z=8 is presented. Also, the consequences for time-of-flight measurements (TOF) in detectors behind several thick layers of detector material are discussed. The solution is applied to the data of the Huygens detector, which uses a TPC (dE/dx) and plastic scintillators for particle identification (E and TOF or dE/dx and TOF).     

A time-of-flight mass reflectron with controlled ion-energy spread in the packet generated by an ion source

A new principle is proposed for eliminating the energy spread of ions in a packet generated by the ion source of a time-of-flight (TOF) mass reflectron. The ion source is treated as an element of the optic tract, in which the ion trajectories are conjugated with those in the electrostatic mirror, while the primary temporal focus near the source is absent. By properly selecting parameters of the emission-reflection system comprising a combination of the ion source and electrostatic mirror, the total time of ion flight from the source to detector can be controlled so as to be independent of the ion energy. Numerical simulations have been used to find the optimum parameters of a three-electrode electrostatic mirror possessing rotational symmetry, which simultaneously ensure the TOF control (to within third-order terms) and the spatial focusing of ions in the detector plane.     

Transport of Caesium ions through thin poly-p-xylylene films

We describe an experimental setup for investigating the transport of alkali ions through poly(p-xylylene) membranes. It consists of an ion source where a continuous beam of alkali ions is generated from a surface emitter. Via ion-optics the ions are guided to the main chamber where the interaction with the free-standing membranes of variable thickness is investigated as a function of the impact energy. For a cesium ion beam the transmission of ions exhibits a maximum at an impact energy of several hundred volts, depending on the membrane thickness. The transport of the ions most likely proceeds through pores or porosities in the membrane. At the highest impact energies employed (around 2000 eV) the transmission of electrons is observed, most likely due to collision processes causing kinetic electron emission.     

Comparative studies on the CO2- signal in tooth enamel and carbonates

When a swift ion is slowed down through a plastic detector it creates a latent track. In nuclear track detectors, this latent track can be specifically etched by an appropriate chemical solution. This enlargement process is due to a higher etch velocity (VT) along the ion's path than in the non-damaged part of the detector. The etched track velocity is definitely linked to the damage created by the incoming ion in the detector material. A relationship between the physical parameters of the energy deposition and the variation in this etched track velocity with the ion energy cannot easily be explained. We present here our study on the chemical damage created by several ions in a cellulose nitrate type detector and our first attempt to simulate them by the use of the hit theory.     

Evaluation and reduction of ion back-flow in multi-GEM detectors

In gaseous photomultipliers, avalanche generated ions back-flowing to the photocathode can drastically limit the detector operation and lifetime. This is especially the case for photocathodes with low electron emission threshold, where impinging ions induce ion feedback effects by secondary electron emission.We present ways of reducing ion-backflow to the photocathode, and thus suppress ion-feedback effects in multi-stage Gas Electron Multiplier detectors. We studied the effect of the various electric fields on the ion transport in the detector and present our results on active ion gating with a dedicated gating electrode.     

Time and energy resolved ion mass spectroscopy studies of the ion flux during high power pulsed magnetron sputtering of Cr in Ar and Ar/N2 atmospheres

Mass spectroscopy was used to analyze the energy and composition of the ion flux during high power pulsed magnetron sputtering (HIPIMS/HPPMS) of a Cr target in an industrial deposition system. The ion energy distribution functions were recorded in the time-averaged and time-resolved mode for Ar⁺, Ar²⁺, Cr⁺, Cr²⁺, N₂⁺ and N⁺ ions. In the metallic mode the dependence on pulse energy (equivalent of peak target current) was studied. In the case of reactive sputtering in an Ar/N₂ atmosphere, variations in ion flux composition were investigated for varying N₂-to-Ar flow ratio at constant pressure and HIPIMS power settings. The number of doubly charged Cr ions is found to increase linearly with increasing pulse energy. An intense flux of energetic N⁺ ions was observed during deposition in the reactive mode. The time evolution of ion flux composition is analyzed in detail and related to the film growth process. The ionization of working gas mixture is hampered during the most energetic phase of discharge by a high flux of sputter-ejected species entering the plasma, causing both gas rarefaction and quenching of the electron energy distribution function. It is suggested that the properties (composition and energy) of the ion flux incident on the substrate can be intentionally adjusted not only by varying the pulse energy (discharge peak current), but also by taking advantage of the observed time variations in the composition of ion flux.     

Macromolecular ion accelerator

Presented herein are the development of macromolecular ion accelerator (MIA) and the results obtained by MIA. This new instrument utilizes a consecutive series of planar electrodes for the purpose of facilitating stepwise acceleration. Matrix-assisted laser desorption/ionization (MALDI) is employed to generate singly charged macromolecular ions. A regular Z-gap microchannel plate (MCP) detector is mounted at the end of the accelerator to record the ion signals. In this work, we demonstrated the detection of ions with the mass-to-charge (m/z) ratio reaching 30,000,000. Moreover, we showed that singly charged biomolecular ions can be accelerated with the voltage approaching 1 MV, offering the evidence that macromolecular ions can possess much higher kinetic energy than ever before.     

Analysis of megadalton ions using cryodetection MALDI time-of-flight mass spectrometry

Presented are initial results from the first commercially available matrix-assisted laser desorption/ionization time-of-flight mass spectrometer specifically designed for the sensitive detection of very high mass ions (macromizer, Comet AG). This new instrument utilizes a 16-element superconducting tunnel junction detector coupled with a fully adjustable gimbal-mounted ion source/focusing region that allows unparalleled sensitivity for detection of singly charged high molecular weight ions. Using this new technology, the singly charged ions in the megadalton region are detected from immunoglobulin M and von Willebrand factor proteins. This detector technology also measures the kinetic energy of the particles impacting the detector, which can be correlated to the charge of the particles. Immunoglobulin G and streptavidin were used to demonstrate the ability of the macromizer instrument to detect high-mass ions and to discern the charge state of the ions.     

Improvement of the wire gain near the frame of a time projection chamber

We have quantitatively studied the behaviour of a time projection chamber (TPC for the DELPHI detector) at small distances from the frame (less than 1 cm). The severe drop in gain near the frame can be overcome by shaping the electric field with guard rings.     

Low energy alphas in the drift detector

The Directional Recoil Identification From Tracks project is a US–UK endeavor to build and operate a low pressure negative ion TPC (NITPC) to search for weakly interacting massive particles (WIMPs) thought to make up the dark matter in our Galaxy. Low energy (∼10 keV) alpha events from U and Th decays within the walls and wires of the detector can enter the active volume of the detector and be confused for WIMP interactions. This paper presents data on and a model of low energy alphas in a NITPC operated at 40 Torr CS₂ with the aim of understanding and removing this potentially serious background. A comparison of the data to this model reveals good agreement with range predictions of SRIM2000 and allows us to calculate the energy dissipation per ion pair, W=19.0±0.5 eV for low energy alphas in CS₂.     

Progress in GEM simulation

The charge induction mechanism and signal formation on the electrodes of the Gas Electron Multiplier (GEM) detector are discussed in detail. The method of simulating the effective gas gain, transparency and charge losses in GEM structures is developed. Results on simulation of various GEM geometries are consistent with published experimental data.     

离子在矩形波四极场中的运动与四极场离子阱质谱的数字化操作

矩形波驱动的四极场同样可用于对离子进行质量分析和存储。本文推导了离子在矩形波四极场中的运动规律,并以常用于正弦波四极场的马绍方程参量a,q来表出离子的稳定性。运用赝势阱概念探讨了离子在这种四极场中的本征振动,推导了本征振动频率的近似公式。在理论研究的基础上,提出了数字化离子阱质谱的概念,并运用离子光学模拟,研究了实际离子阱几何结构下的共振出射以及数字化质量扫描的可行性。     

离子能流密度对类金刚石薄膜成膜的影响

着重讨论了离子轰击能量影响类金刚石薄膜成膜过程的机理 ,特别引入了离子能流密度即在单位时间内成膜表面的单位面积上接收到的离子轰击能量的概念 ,这就能更加正确地解释离子轰击在成膜过程中所起的作用。采用了具有特色的等离子体分解碳氢化合物结合高能离子轰击基片的方法制备类金刚石薄膜。这种兼有PVD和CVD各自优点的PCVD方法 ,能独立地分别调节轰击基片的离子能量和离子流密度。实验结果表明 ,存在一个离子能流密度的阈值。超过该阈值 ,才能得到结构致密、均匀 ,与基片结合强度好 ,硬度很高的类金刚石薄膜 ,扫描电镜对膜层形貌的显微成像和维氏硬度测试展示了离子能流密度对成膜的影响。作为佐证 ,还对薄膜进行了喇曼光谱测试和分析。     

Surface morphology analysis of natural single crystal diamond chips bombarded with Ar ion beam

We have studied the surface morphology of natural single crystal diamond chips machined by 0.5-3.0 keV Ar⁺ ion beam irradiation at ion incidence angles of 0°, 30°, 45°, 60°, and 80° with ion doses from 3.4 × 10¹⁸ ions/cm² to 6.8 × 10¹⁸ ions/cm². The surface of diamond chips machined with 0.5 and 1.0 keV Ar⁺ ion beam, at angles of ion incidence from 0° to 45° can be made smooth. Results show that the machined surface at ion dose of 6.8 × 10¹⁸ ions/cm² and beam energy of 0.5 and 1.0 keV become ultra-smooth (surface roughness SR = 0.1 nm rms) compared with unprocessed surface (SR = 0.15-2.1 nm rms). Results also confirm the ripple formation on diamond surface at ion incidence angles of 60°-80° by 0.5-3.0 keV Ar⁺ ion beam. Therefore, the technique of smoothing by choosing ion beam irradiation parameter can be applicable to nano-finishing of diamond tools without ripple formation. This technique can also be applicable in mass production if the diamond surface is mechanically pre-finished.     

A comparative study of ion exchange kinetics in zinc/lead-modified zeolite-clinoptilolite systems

The kinetics of zinc and lead ions removal by modified zeolite-clinoptilolite has been investigated. The rate of the ion exchange process for lead ions is faster than for zinc ions, as well as the time needed to reach the equilibrium. The ion exchange capacity of zeolite of lead ions is doubly higher than that of zinc ions. Diffusion models according to the Vermeulen's approximation, the parabolic diffusion model and the homogeneous diffusion model have been tested with the experimental data of ion exchange for zinc and lead. For both systems examined, the best fit of the models proposed with the experimental data was shown by the Vermeulen's approximation and the homogeneous diffusion model with t-->t(infinity). The diffusion coefficients are calculated from kinetic models of lead ions they are of the order of 10(-6)cm(2)/min, constant for all examined initial concentrations and not dependent on time. The diffusion coefficients in the system of zinc ions is of the order of 10(-8)cm(2)/min, also independent of initial concentrations, but decreasing with time from the beginning of ion exchange to the equilibrium.