Bright Flash Neutron Radiography at the McClellan Nuclear Research Reactor

AbstractThe University of California, Davis McClellan Nuclear Research Center (MNRC) operates a 2 MW TRIGATM reactor, which is currently the highest power TRIGATM reactor in the United States. The Center was originally build by the US Air Force to detect hidden defects in aircraft structures using neutron radiography; the Center can accommodate samples as large as 10.00 m long, 3.65 m high, and weighing up to 2,270kg.The MNRC reactor can be pulsed to 350 MW for about 30ms (FWHM). The combination of a short neutron pulse with a fast microchannel plate based neutron detector enables high-resolution flash neutron radiography to complement conventional neutron radiograph

Energy-resolved neutron imaging for reconstruction of strain introduced by cold working

Energy-resolved neutron transmission imaging is used to reconstruct maps of residual strains in drilled and cold-expanded holes in 5-mm and 6.4-mm-thick aluminum plates. The possibility of measuring the positions of Bragg edges in the transmission spectrum in each 55 × 55 µm2 pixel is utilized in the reconstruction of the strain distribution within the entire imaged area of the sample, all from a single measurement. Although the reconstructed strain is averaged through the sample thickness, this technique reveals strain asymmetries within the sample and thus provides information complementary to other well-established non-destructive testing methods

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

Dynamic volume magnetic domain wall imaging in grain oriented electrical steel at power frequencies with accumulative high-frame rate neutron dark-field imaging

The mobility of magnetic domains forms the link between the basic physical properties of a magnetic material and its global characteristics such as permeability and saturation field. Most commonly, surface domain structure are studied using magneto-optical Kerr microscopy. The limited information depth of approx. 20 nanometers, however, allows only for an indirect interpretation of the internal volume domain structures. Here we show how accumulative high-frame rate dynamic neutron dark-field imaging is able for the first time to visualize the dynamic of the volume magnetic domain structures in grain oriented electrical steel laminations at power frequencies. In particular we studied the volume domain structures with a spatial resolution of ∼100 μm and successfully quantified domain sizes, wall velocities, domain annihilation and its duration and domain wall multiplication in real time recordings at power frequencies of 10, 25 and 50 Hz with ±262.5 A/m and ±525 A/m (peak to peak) applied field

Three Dimensional Polarimetric Neutron Tomography of Magnetic Fields

Through the use of Time-of-Flight Three Dimensional Polarimetric Neutron Tomography (ToF 3DPNT) we have for the first time successfully demonstrated a technique capable of measuring and reconstructing three dimensional magnetic field strengths and directions unobtrusively and non-destructively with the potential to probe the interior of bulk samples which is not amenable otherwise. Using a pioneering polarimetric set-up for ToF neutron instrumentation in combination with a newly developed tailored reconstruction algorithm, the magnetic field generated by a current carrying solenoid has been measured and reconstructed, thereby providing the proof-of-principle of a technique able to reveal hitherto unobtainable information on the magnetic fields in the bulk of materials and devices, due to a high degree of penetration into many materials, including metals, and the sensitivity of neutron polarisation to magnetic fields. The technique puts the potential of the ToF time structure of pulsed neutron sources to full use in order to optimise the recorded information quality and reduce measurement time.Comment: 12 pages, 4 figure

Noiseless, kilohertz-frame-rate, imaging detector based on micro-channel plates readout with the Medipix2 CMOS pixel chip

A new hybrid imaging detector is described that is being developed for the next generation adaptive optics (AO) wavefront sensors. The detector consists of proximity focused microchannel plates (MCPs) read out by pixelated CMOS application specific integrated circuit (ASIC) chips developed at CERN ("Medipix2"). Each Medipix2 pixel has an amplifier, lower and upper charge discriminators, and a 14-bit chounter. The 256x256 array can be read out noiselessly (photon counting) in 286 us. The Medipix2 is buttable on 3 sides to produce 512x(n*256) pixel devices. The readout can be electronically shuttered down to a terporal window of a few microseconds with an accuracy of 10 ns. Good quantum efficiencies can be achieved from the x-ray (open faced with opaque photocathodes) to the optical (sealed tube with multialkali or GaAs photocathode)

Photon counting arrays for AO wavefront sensors

Future wavefront sensors for AO on large telescopes will require a large number of pixels and must operate at high frame rates. Unfortunately for CCDs, there is a readout noise penalty for operating faster, and this noise can add up rather quickly when considering the number of pixels required for the extended shape of a sodium laser guide star observed with a large telescope. Imaging photon counting detectors have zero readout noise and many pixels, but have suffered in the past with low QE at the longer wavelengths (>500 nm). Recent developments in GaAs photocathode technology, CMOS ASIC readouts and FPGA processing electronics have resulted in noiseless WFS detector designs that are competitive with silicon array detectors, though at ~40% the QE of CCDs. We review noiseless array detectors and compare their centroiding performance with CCDs using the best available characteristics of each. We show that for sub-aperture binning of 6x6 and greater that noiseless detectors have a smaller centroid error at fluences of 60 photons or less, though the specific number is dependent on seeing conditions and the centroid algorithm used. We then present the status of a 256x256 noiseless MCP/Medipix2 hybrid detector being developed for AO