Richard C. Schirato

Scattering muon radiography and its application to the detection of high-Z materials

We have proposed a new method for nuclear material contraband detection based on cosmic ray muon radiography. The method is safe, because it does not include the generation of any artificial radiation, but uses naturally produced high-energy muons. Results obtained with our prototype experiment, and from simulations demonstrate the feasibility of the method for the detection of high-Z materials hidden inside different types of ordinary cargo. Detector technology for muon detection is mature and enables cost-effective design for a muon radiography apparatus. Image reconstruction is not trivial and may be done in various ways. We developed the PoCA algorithm for image reconstruction and the MC (muon crossing) algorithm for the detection of high-Z material. Our simulations of a full-scale muon radiography system suggest high efficiency of detection in 1 minute of exposure with a low number of false positives for a 20 kg of uranium hidden inside 11 tons of uniformly distributed iron.

Evaluation of COTS silicon carbide photodiodes for a radiation-hard, low-energy x-ray spectrometer

Wide band-gap semiconducting materials such as silicon carbide (SiC) are attractive alternatives for radiation detection in more rugged environments due to low leakage currents, superior radiation hardness, and temperature insensitivity. However, the development of these technologies has been hindered by serious materials problems that restrict the quality and size of detector grade crystals. Recent developments in epitaxial growth of thin crystals on a substrate are yielding detector-grade material of reasonable dimension. While applications for ionizing radiation detectors are still limited primarily to small academic and R&D ventures, an active market exists for very thin epitaxial SiC photodiodes for the purpose of detecting UV light in high-temperature environments. The purpose of this project is to evaluate the utility of these commercially available SiC UV photodiodes for the purpose of detection and measurement of low-energy x-rays. We present results from bench-top electronic characterization, radioactive source measurements, x-ray source measurements at Los Alamos National Lab, and responsivity measurements performed at the National Synchrotron Light Source at Brookhaven National Lab.

Time gating for energy selection and scatter rejection: High-energy pulsed neutron imaging at LANSCE

The Los Alamos Neutron Science Center (LANSCE) is a linear accelerator in Los Alamos, New Mexico that accelerates a proton beam to 800 MeV, which then produces spallation neutron beams. Flight path FP15R uses a tungsten target to generate neutrons of energy ranging from several hundred keV to ~600 MeV. The beam structure has micropulses of sub-ns width and period of 1.784 ns, and macropulses of 625 μs width and frequency of either 50 Hz or 100 Hz. This corresponds to 347 micropulses per macropulse, or 1.74 x 104 micropulses per second when operating at 50 Hz. Using a very fast, cooled ICCD camera (Princeton Instruments PI-Max 4), gated images of various objects were obtained on FP15R in January 2015. Objects imaged included blocks of lead and borated polyethylene; a tungsten sphere; and a tungsten, polyethylene, and steel cylinder. Images were obtained in 36 min or less, with some in as little as 6 min. This is novel because the gate widths (some as narrow as 10 ns) were selected to reject scatter and other signal not of interest (e.g. the gamma flash that precedes the neutron pulse), which has not been demonstrated at energies above 14 MeV. This proof-of-principle experiment shows that time gating is possible above 14MeV and is useful for selecting neutron energy and reducing scatter, thus forming clearer images. Future work (simulation and experimental) is being undertaken to improve camera shielding and system design and to precisely determine optical properties of the imaging system.

Neutron Imaging at LANSCE—From Cold to Ultrafast

In recent years, neutron radiography and tomography have been applied at different beam lines at Los Alamos Neutron Science Center (LANSCE), covering a very wide neutron energy range. The field of energy-resolved neutron imaging with epi-thermal neutrons, utilizing neutron absorption resonances for contrast as well as quantitative density measurements, was pioneered at the Target 1 (Lujan center), Flight Path 5 beam line and continues to be refined. Applications include: imaging of metallic and ceramic nuclear fuels, fission gas measurements, tomography of fossils and studies of dopants in scintillators. The technique provides the ability to characterize materials opaque to thermal neutrons and to utilize neutron resonance analysis codes to quantify isotopes to within 0.1 atom %. The latter also allows measuring fuel enrichment levels or the pressure of fission gas remotely. More recently, the cold neutron spectrum at the ASTERIX beam line, also located at Target 1, was used to demonstrate phase contrast imaging with pulsed neutrons. This extends the capabilities for imaging of thin and transparent materials at LANSCE. In contrast, high-energy neutron imaging at LANSCE, using unmoderated fast spallation neutrons from Target 4 [Weapons Neutron Research (WNR) facility] has been developed for applications in imaging of dense, thick objects. Using fast (ns), time-of-flight imaging, enables testing and developing imaging at specific, selected MeV neutron energies. The 4FP-60R beam line has been reconfigured with increased shielding and new, larger collimation dedicated to fast neutron imaging. The exploration of ways in which pulsed neutron beams and the time-of-flight method can provide additional benefits is continuing. We will describe the facilities and instruments, present application examples and recent results of all these efforts at LANSCE.

Development and Characterization of a High-Energy Neutron Time-of-Flight Imaging System

Los Alamos National Laboratory has developed a prototype of a high-energy neutron time-of-flight imaging system for the nondestructive evaluation of dense, massive, and/or high atomic number objects. High-energy neutrons provide the penetrating power, and thus the high dynamic range necessary to image internal features and defects of such objects. The addition of the time gating capability allows for scatter rejection when paired with a pulsed monoenergetic beam, or neutron energy selection when paired with a pulsed broad-spectrum neutron source. The Time Gating to Reject Scatter and Select Energy system was tested at the Los Alamos Neutron Science Center’s weapons nuclear research facility, a spallation neutron source, to provide proof of concept measurements and to characterize the instrument response. This paper will show results of several objects imaged during this run cycle. In addition, results from system performance metrics, such as the modulation transfer function and the detective quantum efficiency measured as a function of neutron energy, characterize the current system performance and inform the next generation of neutron imaging instrument.

Dislocations in LaBr 3 crystals

Using a many-body embedded ion method potential for La-Br system, molecular dynamics simulations have been performed to study dislocations in the UCl3 type of LaBr3 crystal including identification of dislocation line energy, core structure, migration mechanism, and mobility. We found that dislocations with the < 0001 > Burgers vector can move under shear stresses, but they retain perfect dislocations during the motion rather than dissociated partials as commonly seen in metal systems. Unlike the < 0001 > edge dislocations whose mobility increases with temperature, the < 0001 > screw dislocations may become sessile at high temperatures due to thermally activated dissociation of the core. Dislocations with the <1120 > Burgers vector were found to be sessile due to non-planar dissociation at the core. Because the < 0001> dislocations can only slip on the {1 1 00 } prism plane and often only the edge dislocations are operative, the stresses created during any thermal mechanical processes cannot be effectively relieved by the plastic deformation mechanism. Considering that LaBr3 tend to cleave along the {1 1 00 } prism plane, the simulations shed some lights on why this material is so brittle and how large LaBr3 crystals tend to fracture during growth.

Characterization of vapor-deposited Lu 2 O 3 :Eu 3+ scintillator for x-ray imaging applications

The europium-doped lutetium oxide (Lu2O3:Eu) transparent optical ceramic has excellent scintillation properties, namely very high density (9.5 g/cm3), high effective atomic number (67.3), light output comparable to thallium-doped cesium iodide (CsI:Tl), and emission wavelength (610 nm) for which silicon-based detectors have a very high quantum efficiency. If microcolumnar films of this material could be fabricated, it would find widespread use in a multitude of highspeed imaging applications. However, the high melting point of over 2400°C makes it extremely challenging to make microcolumnar films of this material. We have recently fabricated and characterized microcolumnar films of Lu2O3:Eu. These results are presented in this paper.

Front Matter: Volume 7080

This PDF file contains the front matter associated with SPIE Proceedings Volume 7080, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.