J. C. Allison

A dark-field imaging condition established through pre- and post-object beam collimation has been tested for a variety of different settings at the proton radiography facility (pRad) at Los Alamos National Laboratory. This technique facilitates imaging of thin samples and materials with small areal density changes with 800 MeV protons that would otherwise appear nearly transparent for high energy protons. In the configuration that was examined, the pre-object (upstream) collimator is utilized to remove protons with large scattering angles prior to the target object. The post-object collimation entails a combination of a conventional collimator, which eliminates protons with large scattering angles after target interaction, and an inverse collimator. The inverse collimator removes unscattered protons traveling on the beam axis and those with smaller scattering angles up to a desired angle. This establishes a dark-field condition by permitting solely protons that exhibit scattering angles from target interaction within a designated range to contribute to the image. This study explores the potential of combining different sized collimators for pre- and post-object collimation. The objective is to optimize contrast for a range of samples with minimal areal density variations. A series of dark-field conditions are explored employing a remote-controlled collimator wheel and the contrast-to-noise ratio, spatial resolution, and proton transmission are compared for those combinations. It is found that the contrast-to-noise ratio can be substantially enhanced for thin samples and low areal density objects, thereby facilitating capturing high-contrast images of objects that were previously invisible to high energy protons.

A new high energy proton radiography facility PRIOR-II (Proton Microscope for FAIR) has been designed, constructed, and successfully commissioned at the GSI Helmholtzzentrum für Schwerionenforschung (Darmstadt, Germany) pushing the technical boundaries of charged particle radiography with normal conducting magnets to the limits. The setup is foreseen to become a new and powerful user facility for carrying out fundamental science experiments in the fields of plasma and shock wave physics, material science, and medical physics. It will help address several unsolved scientific challenges, which require high-speed and precise non-invasive diagnostic methods capable of probing matter with up to 100 g/cm2 areal density. PRIOR-II is specifically designed to utilize the full timing capabilities of the SIS-18 synchrotron at GSI for ultra-fast dynamic experiments with up to 4 GeV protons and will also be fielded at the future FAIR facility, where higher proton energies and beam intensities will be available. This will enable experiment geometries with even higher areal densities, more flexible experiment timing, and further enhanced spatial resolution.

Protons from the Los Alamos Neutron Science Center have been used for pulsed radiography in dynamic experiments for the past 25 years. Pulses of protons are imaged on a scintillator, and the light from these images is captured by fast gated cameras. The need for fast, bright scintillators has led to some compromises in image quality due to tiling the scintillators and backgrounds with totally internally reflected light. We show how large-grain scintillator screens, made using a thin plastic binder, solve these problems.

Neutron radiography is a technique uniquely suited to applications in nuclear diagnostics, non-destructive testing, and subcritical experiments. The spatial resolution of neutron radiographs is degraded by optical blur in the imaging system and the neutron source size, where the ideal source is point-like to optimize the point-spread function. A potential neutron source for radiography is the dense plasma focus (DPF), a coaxial Z-pinch that produces thermonuclear and beam-target neutrons. To assess if the source size is suitable for radiography, a neutron imaging system was used to measure the source size of the 4 MA Sodium DPF at the Nevada National Security Site operating with deuterium–tritium gas-fill. The source size was measured using the edge-spread function of tungsten objects, each having a rolled (convex) edge. The spot size was found to be 7–12 mm full-width at half-max (FWHM) assuming a Gaussian source, though comparison is presented for Lorentzian and Bennett distributions. The average FWHM was found to be 8.6 ± 1.2 mm vertically and 10.8 ± 1.2 mm horizontally with respect to the image plane, averaging over varied edges and alignments. The results were sensitive to source alignment and edge metrology, which introduced notable uncertainties. These results are consistent with separate experimental measurements as well as magnetohydrodynamics simulations of this DPF, which suggest that neutron production can originate from pinches ∼5–7 mm off-axis. These results suggest that the DPF should be used for radiography at low magnification (M < 1) where spot size does not dominate spatial blur.