Microsphere Plate Research Articles

Building a Casimir metrology platform with a commercial MEMS sensor

The Casimir Effect is a physical manifestation of quantum fluctuations of the electromagnetic vacuum. When two metal plates are placed close together, typically much less than a micron, the long wavelength modes between them are frozen out, giving rise to a net attractive force between the plates, scaling as d−4 (or d−3 for a spherical-planar geometry) even when they are not electrically charged. In this paper, we observe the Casimir Effect in ambient conditions using a modified capacitive micro-electromechanical system (MEMS) sensor. Using a feedback-assisted pick-and-place assembly process, we are able to attach various microstructures onto the post-release MEMS, converting it from an inertial force sensor to a direct force measurement platform with pN (piconewton) resolution. With this system we are able to directly measure the Casimir force between a silver-coated microsphere and gold-coated silicon plate. This device is a step towards leveraging the Casimir Effect for cheap, sensitive, room temperature quantum metrology.

Cosima – High Resolution Time-of-Flight Secondary Ion Mass Spectrometer for the Analysis of Cometary Dust Particles onboard Rosetta

The ESA mission Rosetta, launched on March 2nd, 2004, carries an instrument suite to the comet 67P/Churyumov-Gerasimenko. The COmetary Secondary Ion Mass Anaylzer – COSIMA – is one of three cometary dust analyzing instruments onboard Rosetta. COSIMA is based on the analytic measurement method of secondary ion mass spectrometry (SIMS). The experiment’s goal is in-situ analysis of the elemental composition (and isotopic composition of key elements) of cometary grains. The chemical characterization will include the main organic components, present homologous and functional groups, as well as the mineralogical and petrographical classification of the inorganic phases. All this analysis is closely related to the chemistry and history of the early solar system. COSIMA covers a mass range from 1 to 3500 amu with a mass resolution m/Δm @ 50% of 2000 at mass 100 amu. Cometary dust is collected on special, metal covered, targets, which are handled by a target manipulation unit. Once exposed to the cometary dust environment, the collected dust grains are located on the target by a microscopic camera. A pulsed primary indium ion beam (among other entities) releases secondary ions from the dust grains. These ions, either positive or negative, are selected and accelerated by electrical fields and travel a well-defined distance through a drift tube and an ion reflector. A microsphere plate with dedicated amplifier is used to detect the ions. The arrival times of the ions are digitized, and the mass spectra of the secondary ions are calculated from these time-of-flight spectra. Through the instrument commissioning, COSIMA took the very first SIMS spectra of the targets in space. COSIMA will be the first instrument applying the SIMS technique in-situ to cometary grain analysis as Rosetta approaches the comet 67P/Churyumov-Gerasimenko, after a long journey of 10 years, in 2014.

X-ray tests of a microchannel plate detector and amorphous silicon pixel array readout for neutron radiography

High-performance large area imaging detectors for fast neutrons in the 5–14MeV energy range do not exist at present. The aim of this project is to combine microchannel plates or MCPs (or similar electron multiplication structures) traditionally used in image intensifiers and X-ray detectors with amorphous silicon (a-Si) pixel arrays to produce a composite converter and intensifier position sensitive imaging system. This detector will provide an order of magnitude improvement in image resolution when compared with current millimetre resolution limits obtained using phosphor or scintillator-based hydrogen rich converters. In this study we present the results of the initial experimental evaluation of the prototype system. This study was carried out using a medical X-ray source for the proof of concept tests, the next phase will involve neutron imaging tests. The hybrid detector described in this study is a unique development and paves the way for large area position sensitive detectors consisting of MCP or microsphere plate detectors and a-Si or polysilicon pixel arrays. Applications include neutron and X-ray imaging for terrestrial applications. The technology could be extended to space instrumentation for X-ray astronomy.

New system for secondary electron detection in variable‐pressure scanning electron microscopy

A novel secondary electron detection system combining a two-stage detector head and a differential pumping system is presented. The detector head consisted of a scintillation Everhart-Thornley detector and a microsphere plate, separating it from the lower vacuum in the intermediate chamber (below 0.1 mbar). The system was arranged asymmetrically, which should contribute to a lower gas leakage through the plate and a longer life span of the plate. The system offered all the advantages of the scintillator detector in a wide range of gas pressures, from high vacuum to those of the order of 10 mbar, typical of high-pressure scanning electron microscopy.

A novel trochoidal analyzer for the Belfast EBIT

Presently, Queens University Belfast is constructing a unique permanent magnet EBIT which grants unparalleled access to the photons from the trap region. However measurements of ions would still require their transfer through the collector to other measurement areas. To allow more representative sampling, a new in situ analyzer is proposed to be situated immediately at the collector end of the trap region. This analyzer may detect ions by expanding the radius of their cyclotron motion using electric fields until the ions strike Microsphere Plates (MSPs). It may also be possible by using trochoidal motion to detect product electrons. This is important as these have never been detected before in an EBIT. The preliminary design is described in this paper.

Development of a time-of-flight telescope for ERDA at the Jožef Stefan Institute

A new type of time detector in annular geometry for time-of-flight (TOF) telescope was designed and constructed. The start signal is triggered by the secondary electrons emitted from a thin carbon foil placed perpendicularly to the path of the fast ions. The secondary electrons are accelerated through the central hole of the microsphere plate (MSP) by the electric field between the carbon foil and the electrode at the rearside of the MSP. They are reflected to the active surface of the MSP by the mirror electrode. The stop signal is generated by a similar time detector with microchannel plates where the thin gold entrance window of the silicon detector serves as the secondary electron source. The performance of both time detectors was studied. The TOF detector performance was demonstrated in TOF-elastic recoil detection analysis (ERDA) measurements with a 9600 keV Cl 5+ ion beam.

Micro-sphere plate as an electron detector at low vacuum

The application of a micro-sphere plate (MSP) to secondary electron detection in environmental scanning electron microscopy (ESEM) and possible ways of using the MSP is discussed and experimental measurements made of significant MSP parameters. In the preferred solution the MSP plays the role of a semi-permeable wall separating a standard scintillator detector from the low vacuum chamber. The gas conductance measured for an MSP of dimensions 1.4 mm thick and 12 mm diameter was 0.03 dm 3/s, which is sufficiently low and suitable for this application. A survey of the MSP multiplication coefficient for various gases and vapours revealed that the coefficient increases at increasing gas pressures and has a value of the order of 10 2–10 3 in the pressure range 10 −2–10 −1 mbar. The results suggest that the MSP may be suitable as a secondary electron detector for ESEM.

Evaluation of a microsphere plate detector for an orthogonal acceleration matrix-assisted laser desorption/ionisation time-of-flight mass spectrometer

A single 70 mm diameter microsphere plate (MSP) detector was evaluated in an orthogonal acceleration time-of-flight mass spectrometer fitted with a matrix-assisted laser desorption/ionisation source. The ion energy of the spectrometer was approximately 20 kV. The results indicated that the detector performance was excellent, for a single plate device, in terms of background count-rate, temporal characteristics of the pulses and the gains for single ions. A background count rate was observed to be less than 1 count s −1 cm −2 and the single-ion pulse width was about 800 ps at half height. The plate displayed a gain in the range 1×10 6 to 3×10 6 for all masses tested. The single ion gain measurements showed no significant decrease with mass while the summed signal intensity (total observed counts) decreased strongly at high mass. The gain distribution data were interpreted in conjunction with this observation to indicate that the detector efficiency was poor for high mass ions.

A 20 kV orthogonal acceleration time-of-flight mass spectrometer for matrix-assisted laser desorption/ionization

A compact time-of-flight mass spectrometer has been constructed with the reflecting analyzer region orientated to be orthogonal to the desorption axis of a matrix-assisted laser desorption/ionization source. The instrument is designed with a detector that is large enough to accommodate a correspondingly large desorption velocity spread without the need for collisional cooling. Laser desorbed positive ions from a positively biased probe enter a field free fill region of an orthogonal accelerator, initially at ground potential. The ions are then orthogonally accelerated to reach a final potential of −20 kV and a kinetic energy of ∼21 keV. The space focus of the reflectron geometry is adjusted to lie in the ion mirror to allow easy conversion to the linear mode. The detector of the current configuration is a 70 mm diameter microsphere plate. The spectrometer has a total length of less than 1 m and, in good agreement with simulations, it provides a typical resolution of 8000 (full-width at half-maximum). Detection limits determined with ∼10–50 laser shots were in the low femtomole range up to m/z ∼ 10000. Beyond this limit, sensitivity appears to be lowered by decreasing detector efficiency and the increasing velocity spread of the desorbed ions. Mass calibration of the instrument is very simple and reproducible. Measured mass accuracy with external calibration is better than 100 ppm over several days. Limitations in mass accuracy are attributed to the drift of the power supplies and timing jitter. A focusing method for improving high mass sensitivity by a factor of typically 20 is briefly described and demonstrated with the detection of molecular ions of myoglobin (m/z ∼ 17000).

Microsphere plate detectors used with a compact Mott polarimeter for time-of-flight studies

A compact retarding-potential Mott polarimeter combined with microsphere plates (MSP) as electron detectors was built to perform spin-resolved time-of-flight electron spectroscopy. The comparison of the performance of MSP and channeltron detectors shows that the MSP detector has a better time resolution but a lower efficiency. The overall time resolution of the system was determined to be 350 ps using synchrotron radiation pulses.

Performance of a microsphere plate electron multiplier in APFIM applications

The microsphere plate (MSP) is a recently introduced electron multiplier, that is comparable in principle of operation to the well-known microchannel plate. The performance of a MSP detector has been evaluated for field ion imaging and single atom detection in time-of-flight atom probe mass spectrometry. The image performance of a MSP is not sufficient for obtaining high-quality field ion images, due to the lack of spatial resolution as well as the granularity observed in the images. Gain, pulse height resolution, and temporal resolution obtained from one double thickness MSP are sufficient for use in a time-of-flight single atom sensitivity detector. The detection efficiency was estimated to be ∼42% for ions in the energy range between 10 and 40 keV. The operation of the MSP detector was successfully demonstrated in a typical atom probe application.

Application of the post-source pulse-focusing technique in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: optimization of the experimental parameters and their influence on the capability of the method.

This paper describes experiments demonstrating the performance of a linear time-of-flight mass spectrometer equipped with the post-source pulse-focusing (PSPF) technique. The influence of various experimental parameters like the shape of the PSPF pulse on the performance of the instrument is discussed. The application of an ion lens in the acceleration region allows the focusing of high mass ions on the detector with a high yield. The observed mass resolution for matrix-assisted laser desorption/ionization (MALDI) generated ions is about 7000 (FWHM) and the mass accuracy using external standards for calibration is about 0.05% or better, in this work demonstrated for molecules with a mass up to 5700 Da. The percentage of the complete mass spectrum which can be detected with increased resolution and accuracy depends on the geometry of a particular setup. In order to cover a broad mass range several parameter sets have to be applied one after the other. Instead of the commonly employed microchannel plates, microsphere plates were used for detection of the ions in these experiments in order to test their applicability in combination with MALDI.

Microsphere electron multiplier applicability to picosecond time correlated single photon counting

Electron transit-time spreads (TTS) measurements in the new microsphere plate electron multiplier is reported on. TTS’s of 120 and 150 ps were measured for a 0.7- and 1.4-mm-thick plates, respectively. The results are compared and discussed with regard to the widely used microchannel-plate multipliers.

Further microsphere plate studies

The microsphere plate is a new type of electron multiplier similar to the microchannel plate in dimensions and mode of operation, but manufactured from microscopic glass beads sintered to form a wafer of user definable thickness. We present further measurements taken using a detector utilizing one or more microsphere plates with a charge division anode for imaging purposes. Charge division anode systems are well-suited to the microsphere plate as they benefit from the relatively large charge footprint which occurs during multiplication unlike other read-out systems, such as phosphors, whose performance is degraded by a large charge footprint. A comparison between the imaging performance of single thickness and double thickness MSPs is given and we investigate the mechanism causing variability of quantum efficiency with position using a bare microsphere plate illuminated with ultraviolet light. The potential advantages in the imaging performance of a hybrid detector using microchannel and microsphere plates in tandem are discussed and imaging performance measurements with such a combination are presented.

Spatial uniformity and UV sensitivity of microsphere plates (MSPs)

We have recently reported on a detailed study of basic microsphere plate (MSP) operation [A.S. Tremsin et al., Nucl. Instr. Meth. A 368 (1996) 719]. In this paper, we describe, with specific regard to imaging applications, the spatial uniformity of MSP quantum efficiency and gain. Measurements were made using 0.7 and 1.4 mm thick MSPs, both “bare” (nichrome-electroded) and coated with an Al layer for enhanced UV detection efficiency. As might be expected from the granular structure of the MSP, the relative variation in pulse gain dG G was found to decrease with scale size, falling from ∼12% when averaged over 0.18 mm areas to ∼7% for 1 mm square cells. The efficiency of the Al-coated MSP to 2540 Å radiation was one order of magnitude higher than that of the bare plate. The details of image transfer from the input to the output face of the MSP were measured by scanning a fine slit across the working diameter of the detector.

An electron multiplier capable of working at low vacuum: The microsphere plate

A new type of electron multiplier that has recently been developed, the microsphere plate (MSP), does not have the problem of ion feedback, which limits the operation of the known microchannel plates. Due to its structure, the MSP can amplify charge current even at pressures as high as 0.1 mbar.

The Microsphere Plate: a new type of electron multiplier

Microsphere Plates (MSPs), a new type of electron multiplier, consisting of sintered disks of glass beads, have recently become available from El-Mul Technologies Ltd. The principles of MSP operation are similar to those of microchannel plates (MCPs). We present a survey of the gain, resistance, dark noise, count rate capability, charge abstraction lifetime and image characteristics of a number of standard microsphere plates (of thickness 0.7 and 1.4 mm), operated both singly and as two-stage multipliers. Modal gains lay in the range 2.4–43 pC with pulse height relative FWHM values as low as 52%. MSP dark noise is relatively low (⋍0.5 counts cm 2s −1) for well-chosen lower level discriminator settings. The image response is globally uniform, having local variations in image intensity on sub-mm scale, corresponding to the structure of MSP. Spatial resolution of an MSP detector may be better than 250 μm FWHM. No dependence of the gain on the angle of radiation incidence was observed for an angular range of 90°±25° (relative to the MSP surface), while the 2540 Å UV relative detection efficiency varied by 10–15% over the same range. Lifetest measurements revealed that after removal of 8 × 10 −4C cm −2, the detector modal gain fell from 8 pC to half of its initial value. This fall in gain was compensated by raising the bias voltage.