Showing papers by "Oswald H. W. Siegmund published in 2013"

Opaque gallium nitride photocathodes in UV imaging detectors with microchannel plates

Abstract: The optimization and performance of opaque Galium Nitride (GaN) photocathodes deposited directly on novel Microchannel Plates (MCPs) are presented in this paper. The novel borosilicate glass MCPs, which are manufactured with the help of Atomic Layer Deposition, can withstand higher temperatures enabling direct deposition of GaN films on their surfaces. The quantum efficiency of MBE-grown GaN photocathodes of various thickness and buffer layers was studied in the spectral range of ~200-400 nm for the films grown on different surface layers (such as Al2O3 or buffer AlN layer) in order to determine the optimal opaque photocathode configuration. The MCPs with the GaN photocathodes were activated with surface cesiation in order to achieve the negative Electron Affinity for the efficient photon detection. The opaque photocathodes enable substantial broadening of the spectral sensitivity range compared to the semitransparent configuration when the photocathodes are deposited on the input window. The design of currently processed sealed tube event counting detector with an opaque GaN photocathode are also described in this paper. Our experiments demonstrate that although there is still development work required the detection quantum efficiencies exceeding 20% level should be achievable in 200-400 nm range and <50% in 100-200 nm range for the event counting MCP detectors with high spatial resolution (better than 50 μm) and timing resolution of <100 ps and very low background levels of only few events cm-2 s-1.

Calibration and flight qualification of FORTIS

Abstract: The Johns Hopkins University sounding rocket group has completed the assembly and calibration of the Far-ultraviolet Off Rowland-circle Telescope for Imaging and Spectroscopy (FORTIS); a sounding rocket borne multi-object spectro-telescope designed to provide spectral coverage of up to 43 separate targets in the 900 - 1800 Angstrom bandpass over a 30′ x 30′ field-of-view. FORTIS is capable of selecting the far-UV brightest regions of the target area by utilizing an autonomous targeting system. Medium resolution (R ~ 400) spectra are recorded in redundant dual-order spectroscopic channels with ~40 cm 2 of effective area at 1216 A. The maiden launch of FORTIS occurred on May 10, 2013 out of the White Sands Missile Range, targeting the extended spiral galaxy M61 and nearby companion NGC 4301. We report on the final flight calibrations of the instrument, as well as the flight results.

Quantification of cement hydration through neutron radiography with scatter rejection

Abstract: The unique capabilities of neutrons to penetrate materials opaque to X-rays and at the same time to interact with hydrogen make neutron radiography a technique of choice for the quantification of hydrogen-containing substances. However, the interaction of neutrons with hydrogen is through scattering, not absorption and unwanted scattered neutrons can be responsible for a considerable fraction of the registered events, reducing the accuracy of hydrogen quantification, especially in imaging experiments where the sample has to be placed close to the detector to avoid image blurring due to the finite beam divergence. Various post-experiment data processing techniques were developed to correct and mitigate the effects of scattering. Another complementary 'hardware' solution described in this paper, is to simply eliminate the detrimental scattering component from the detected signal through the use of neutron collimators. Development of compact polycapillary collimators, merely a few mm thick, now enables accurate, high spatial resolution quantification of a wide range of materials which tend to strongly scatter neutrons. This paper demonstrates how it is now possible to substantially improve the accuracy of the measured water content in cement samples. A compact polycapillary neutron collimator (2 mm thick, 5 cm diameter, with ∼8 µm pores hexagonally packed on ∼10 µm centers) used for scatter rejection in our experiments. The water content within the 1×1×2 cm3 cement samples is measured by neutron transmission radiography, performed with sub-100 µm spatial resolution. The measurement of water concentration is markedly improved through the use of this collimator, proving those can be attractive for the experiments where samples must be placed in quite close proximity (e.g., only few centimeters or closer), from the neutron detector active surface.

High-Resolution Strain Mapping Through Time-of-Flight Neutron Transmission Diffraction

Abstract: The spatial resolution of time of flight neutron transmission diffraction was recently improved by the extension of photon/electron counting technology to imaging of thermal and cold neutrons. The development of novel neutron sensitive microchannel plates enables neutron counting with spatial resolution of ~55 um and time-of-flight accuracy of ~1 us, with efficiency as high as 70% for cold and ~40% for thermal neutrons. The combination of such a high resolution detector with a pulsed collimated neuron beam provides the opportunity to obtain a 2-dimensional map of neutron transmission spectra in one measurement. The results of our neuron transmission measurements demonstrate that maps of strains integrated along the beam propagation direction can be obtained with ~100 microstrain accuracy and spatial resolution of ~100 um providing there are sufficient neutron events collected. In this paper we describe the capabilities of the MCP neutron counting detectors and present the experimental results of 2-dimensional strain maps within austenitic steel compact tension (CT) crack samples measured at the ENGIN-X beamline of the ISIS pulsed neutron source.