New techniques for imaging photon-counting and particle detectors

Since the advent of space-based astronomy in the early 1960's, there has been a need for space-qualified detectors with sufficient sensitivity and resolution to detect and image single photons, ions or electrons. This thesis describes a research programme to develop detectors that fulfil these requirements. I begin by describing the role of detectors in space astronomy and follow with a review of detector technologies, with particular emphasis on imaging techniques. Conductive charge division image readouts offer high performance, simplicity, and flexibility and their potential is investigated in both theory and practice. I introduce the basic design concept and discuss the fundamental factors limiting performance in relation to physical design and to underlying physical processes. Readout manufacturing techniques are reviewed and a novel method presented. I describe specific space and ground-based readout applications which proved valuable in teaching lessons and raising questions. These questions initiated an experimental programme, whose goals were to understand limiting physical processes and find techniques to overcome them. Results are presented, and the innovation of the progressive geometry readout technique, which this programme also spawned, is described. Progressive geometry readout devices, such as the Vernier anode, offer dramatically improved performance and have been successfully flight-proven. I describe the development of a Vernier readout for the J-PEX sounding rocket experiment, and discuss the instrument calibration and the flight programme. First investigations into a next generation of charge division readout design are presented. These devices will use charge comparison instead of amplitude measurement to further enhance resolution and count rate capability. In conclusion, I summarize the advances made during the course of this research, and discuss ongoing technological developments and further work which will enable MCP detectors to continue to excel where characteristics such as true photon-counting ability, high spatial resolution, format flexibility, and high temporal resolution are required

Graphene as a Novel Single Photon Counting Optical and IR Photodetector

Bilayer graphene has many unique optoelectronic properties , including a tuneable band gap, that make it possible to develop new and more efficient optical and nanoelectronic devices. We have developed a Monte Carlo simulation for a single photon counting photodetector incorporating bilayer graphene. Our results show that, conceptually it would be feasible to manufacture a single photon counting photodetector (with colour sensitivity) from bilayer graphene for use across both optical and infrared wavelengths. Our concept exploits the high carrier mobility and tuneable band gap associated with a bilayer graphene approach. This allows for low noise operation over a range of cryogenic temperatures, thereby reducing the cost of cryogens with a trade off between resolution and operating temperature. The results from this theoretical study now enable us to progress onto the manufacture of prototype photon counters at optical and IR wavelengths that may have the potential to be groundbreaking in some scientific research applications.Comment: Conference Proceeding in Graphene-Based Technologies, 201

High-resolution extreme ultraviolet spectroscopy of G191-B2B: structure of the stellar photosphere and the surrounding interstellar medium

We have continued our detailed analysis of the high-resolution (R= 4000) spectroscopic observation of the DA white dwarf G191-B2B, obtained by the Joint Astrophysical Plasmadynamic Experiment (J-PEX) normal incidence sounding rocket-borne telescope, comparing the observed data with theoretical predictions for both homogeneous and stratified atmosphere structures. We find that the former models give the best agreement over the narrow waveband covered by J-PEX, in conflict with what is expected from previous studies of the lower resolution but broader wavelength coverage Extreme Ultraviolet Explorer spectra. We discuss the possible limitations of the atomic data and our understanding of the stellar atmospheres that might give rise to this inconsistency. In our earlier study, we obtained an unusually high ionization fraction for the ionized He ii present along the line of sight to the star. In the present paper, we obtain a better fit when we assume, as suggested by Space Telescope Imaging Spectrograph results, that this He ii resides in two separate components. When one of these is assigned to the local interstellar cloud, the implied He ionization fraction is consistent with measurements along other lines of sight. However, the resolving power and signal-to-noise available from the instrument configuration used in this first successful J-PEX flight are not sufficient to clearly identify and prove the existence of the two components

Characterisation of Corrosion Damage in T91/F91 steel exposed to Liquid Lead-Bismuth Eutectic

T91 samples were exposed to static liquid lead-bismuth eutectic (LBE) at 700{\deg}C for 250-500 hours in either an oxidising or reducing environment. Corrosion damage was characterised using electron microscopy techniques, which identified networks of LBE intrusion beneath LBE-wetted surfaces. Under reducing conditions these networks are uniformly distributed, while they appear patchier and deeper under oxidising conditions. The individual intrusions preferentially follow microstructural features, initially along prior-austenite grain boundaries, followed by penetration down martensite lath boundaries. Local depletion of Cr was observed within 4 {\mu}m of the intrusions and along intersecting boundaries, suggesting local Cr dissolution as the main corrosion mechanism.Comment: 14 pages, 4 figures and graphical abstract, submitted to Scripta Materiali

Einstein Probe - a small mission to monitor and explore the dynamic X-ray Universe

Einstein Probe is a small mission dedicated to time-domain high-energy astrophysics. Its primary goals are to discover high-energy transients and to monitor variable objects in the $0.5-4~$keV X-rays, at higher sensitivity by one order of magnitude than those of the ones currently in orbit. Its wide-field imaging capability, featuring a large instantaneous field-of-view ($60^\circ \times60^\circ$, $\sim1.1$sr), is achieved by using established technology of micro-pore (MPO) lobster-eye optics, thereby offering unprecedentedly high sensitivity and large Grasp. To complement this powerful monitoring ability, it also carries a narrow-field, sensitive follow-up X-ray telescope based on the same MPO technology to perform follow-up observations of newly-discovered transients. Public transient alerts will be downlinked rapidly, so as to trigger multi-wavelength follow-up observations from the world-wide community. Over three of its 97-minute orbits almost the entire night sky will be sampled, with cadences ranging from 5 to 25 times per day. The scientific objectives of the mission are: to discover otherwise quiescent black holes over all astrophysical mass scales by detecting their rare X-ray transient flares, particularly tidal disruption of stars by massive black holes at galactic centers; to detect and precisely locate the electromagnetic sources of gravitational-wave transients; to carry out systematic surveys of X-ray transients and characterize the variability of X-ray sources. Einstein Probe has been selected as a candidate mission of priority (no further selection needed) in the Space Science Programme of the Chinese Academy of Sciences, aiming for launch around 2020.Comment: accepted to publish in PoS, Proceedings of "Swift: 10 Years of Discovery" (Proceedings of Science; ed. by P. Caraveo, P. D'Avanzo, N. Gehrels and G. Tagliaferri). Minor changes in text, references update

Nano-scale corrosion mechanism of T91 steel in static lead-bismuth eutectic: a combined APT, EBSD, and STEM investigation

T91 steel is a candidate material for structural components in lead-bismuth-eutectic (LBE) cooled systems, for example fast reactors and solar power plants [1]. However, the corrosion mechanisms of T91 in LBE remain poorly understood. In this study, we have analysed the static corrosion of T91 in liquid LBE using a range of characterisation techniques at increasingly smaller scales. A unique pattern of liquid metal intrusion was observed that does not appear to correlate with the grain boundary network. Upon closer inspection, electron backscatter diffraction (EBSD) reveals a change in the morphology of grains at the LBE-exposed surface, suggesting a local phase transition. Energy dispersive X-ray (EDX) maps show that Cr is depleted in the T91 material near the LBE interface. Furthermore, we observed the dissolution of all Cr-enriched precipitates in this region. Although the corrosion is conducted in an oxygen deficient environment, both scanning transmission electron microscopy (STEM) and atom probe tomography (APT) reveal a thin surface oxide layer (presumably wüstite) at the LBE-steel interface. Using electron energy loss spectroscopy (EELS) in the STEM, as well as APT, the atomic scale elemental redistribution and 3D morphology of the corrosion interface is investigated. By combining results from these different techniques, several types of oxide phases and structures can be identified. Based on this detailed nano-scale information, we propose potential mechanisms of T91 corrosion in LBE

Study of Counting Characteristics of Porous Radiation Detectors

This paper presents the development of a new technology of registration of ionizing radiation and a new type of detectors - single-cathode multiwire porous detector with neither a gaseous nor semiconductor, but a porous dielectric substance, e.g., CsI, being used as working medium. It is shown that the performance of the multiwire porous detector is stable, ensuring highly efficient detection of both heavily ionizing particles and soft X-rays with a spatial resolution better than $\pm 60\mu m$. The continuous stable performance opens up new perspectives for using porous detectors in research as well as medicine. The obtained data are basic for the development of the theory of the phenomenon of electrons' drift and multiplication in porous dielectrics under the action of a strong external electric field.Comment: 43

The first GCT camera for the Cherenkov Telescope Array

The Gamma Cherenkov Telescope (GCT) is proposed to be part of the Small Size Telescope (SST) array of the Cherenkov Telescope Array (CTA). The GCT dual-mirror optical design allows the use of a compact camera of diameter roughly 0.4 m. The curved focal plane is equipped with 2048 pixels of ~0.2{\deg} angular size, resulting in a field of view of ~9{\deg}. The GCT camera is designed to record the flashes of Cherenkov light from electromagnetic cascades, which last only a few tens of nanoseconds. Modules based on custom ASICs provide the required fast electronics, facilitating sampling and digitisation as well as first level of triggering. The first GCT camera prototype is currently being commissioned in the UK. On-telescope tests are planned later this year. Here we give a detailed description of the camera prototype and present recent progress with testing and commissioning.Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions at arXiv:1508.0589