Showing papers by "Anton S. Tremsin published in 2010"

Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

Abstract: Results of neutron transmission Bragg edge spectroscopic experiments performed at the SNAP beamline of the Spallation Neutron Source are presented. A high resolution neutron counting detector with a neutron sensitive microchannel plate and Timepix ASIC readout is capable of energy resolved two dimensional mapping of neutron transmission with spatial accuracy of ~55 μm, limited by the readout pixel size, and energy resolution limited by the duration of the initial neutron pulse. A two dimensional map of the Fe 110 Bragg edge position was obtained for a bent steel screw sample. Although the neutron pulse duration corresponded to ~30 mA energy resolution for 15.3 m flight path, the accuracy of the Bragg edge position in our measurements was improved by analytical fitting to a few mA level. A two dimensional strain map was calculated from measured Bragg edge values with an accuracy of ~few hundreds μistrain for 300s of data acquisition time.

Large-format high-spatial resolution cross-strip readout MCP detectors for UV astronomy

Abstract: We have implemented cross strip readout microchannel plate detectors in a 40mm diameter active area format, open face (UV/particle) configuration. These have been tested with a field programmable gate array based parallel channel electronics for event encoding which can process high input event rates (> 5 MHz) with high spatial resolution. Using small pore MCPs (6μm) operated in a pair, we achieved spatial resolution of 100 mm) with high event rate throughput (~ MHz) while retaining high spatial resolution. We will discuss our plans to scale our 47 mm square anodes to 100 mm and our ideas for the next front end ASIC that combines a state-of-the-art, fast charge sensitive amplifier with fast sampling analog storage and built in ADCs.

Gallium nitride photocathodes for imaging photon counters

Abstract: Gallium nitride opaque and semitransparent photocathodes provide high ultraviolet quantum efficiencies from 100 nm to a long wavelength cutoff at ~380 nm. P (Mg) doped GaN photocathode layers ~100 nm thick with a barrier layer of AlN (22 nm) on sapphire substrates also have low out of band response, and are highly robust. Opaque GaN photocathodes are relatively easy to optimize, and consistently provide high quantum efficiency (70% at 120 nm) provided the surface cleaning and activation (Cs) processes are well established. We have used two dimensional photon counting imaging microchannel plate detectors, with an active area of 25 mm diameter, to investigate the imaging characteristics of semitransparent GaN photocathodes. These can be produced with high (20%) efficiency, but the thickness and conductivity of the GaN must be carefully optimized. High spatial resolution of ~50 μm with low intrinsic background (~7 events sec-1 cm-2) and good image uniformity have been achieved. Selectively patterned deposited GaN photocathodes have also been used to allow quick diagnostics of optimization parameters. GaN photocathodes of both types show great promise for future detector applications in ultraviolet Astrophysical instruments.

High Resolution Stroboscopic Neutron Radiography at the FRM-II ANTARES Facility

Abstract: The rapidly developing field of high resolution neutron radiography primarily concentrates on non-destructive studies of stationary objects with relatively long exposure times required to achieve adequate neutron statistics. The combination of a high intensity neutron beam with a high temporal and spatial resolution detector, enables the investigation of dynamic processes in a stroboscopic mode, where image frames are synchronized with the sample or acquired continuously at high acquisition frame rates. Although neutron statistics in the acquisition frames as short as <; 10 μs is considered quite low (typically <; 1000 n/cm2 at the sample), repetitive processes can still be studied with high resolution by integrating a large number of frames synchronized to the process. In this paper we demonstrate the stroboscopic imaging capabilities of the highly collimated thermal neutron beamline ANTARES together with a high resolution detector with neutron-sensitive microchannel plates and the Medipix2 readout. The dynamics of water uptake due to capillary forces as well as the two-phase flow of an air-water mixture is investigated, and stroboscopic imaging of an operating beam chopper and a spinning fan is performed, with sub-100-μm spatial resolution and with acquisition frames varying between 10 μs and 200 ms. The results of these experiments demonstrate the future potential for performing high resolution neutron radiography of fast and/or repetitive processes, such as water flow and uptake, operation of fuel injection nozzles, as well as many others.

Cross strip microchannel plate imaging photon counters with high time resolution

Abstract: We have implemented cross strip readout microchannel plate detectors in 18 mm active area format including open face (UV/particle) and sealed tube (optical) configurations. These have been tested with a field programmable gate array based parallel channel electronics for event encoding which can process high input event rates (> 5 MHz) with high spatial resolution. Using small pore MCPs (6 μm) operated in a pair, we achieve gains of >5 x 10 5 which is sufficient to provide spatial resolution of <35 μm FHWM, with self triggered event timing accuracy of ~2 ns for sealed tube optical sensors. A peak quantum efficiency of ~19% at 500 nm has been achieved with SuperGenII photocathodes that have response over the 400 nm to 900 nm range. Local area counting rates of up to >200 events/mcp pore sec -1 have been attained, along with image linearity and stability to better than 50 μm.

A high resolution neutron counting sensors in strain mapping through a transmission bragg edge diffraction

Abstract: High resolution neutron counting sensors with microchannel plates coupled to a Timepix readout enable high spatial (∼55 µm) and temporal (∼1 µs) accuracy for each detected thermal and cold neutron. One of the attractive applications for those sensors is the high resolution strain mapping in engineering samples through transmission Bragg edge diffraction. The unique combination of high detection efficiency (up to 70%), high spatial and temporal resolution of MCP detectors enable ∼100 µm strain mapping with ∼100 μstrain accuracy. We present the results of proof of principle measurements performed at ROTAX beamline at ISIS spallation neutron source. Strain map of a bent steel sample is measured with very high spatial resolution. The same sensors enable high resolution non-destructive studies in such diverse areas as neutron microtomography, dynamics of fuel injection, material composition, archaeology, water propagation and many others.