Contrast and Resolution
Proton radiography uses a beam of high-energy protons from a particle accelerator to take x-ray-like images of a target—typically a piece of material, but often an entire object. Before it hits the target, the beam is pulsed, collimated, and expanded in diameter by a set of electromagnets. Thus, a pulse of protons, all moving along parallel trajectories, uniformly illuminate the target.
(Below) Penetrating the target, the speedy protons ionize atoms, so each proton continually loses a small amount of energy as it travels. With an initial energy of hundreds of million electronvolts or more, each proton will likely pass through the object with energy to spare. However, due to its interactions with the atomic nuclei, a proton will likely deviate from its initial trajectory. Collisions or near misses typically result in large directional changes, while longer-range interactions with the nuclear electric charge (Coulomb scattering) result in smaller changes. The net result is that protons passing through denser or thicker parts of the target tend to scatter more often, and so emerge from the target at larger angles from their initial trajectory.
(Below) A series of electromagnets capture, transport, and focus each exiting proton onto a detector. The figure shows the paths of protons through the so-called identity lens. Five exit points are shown, each point having five potential exit trajectories. All protons that emerge from a point at the back of the target, regardless of exit angle, get focused to the same point in the image plane. But the exit angle correlates with an areal density—the density along the path taken by the proton through the target (grams per square centimeter). If all protons were allowed to propagate to the image plane, the result would be a loss of contrast because a spot in the image plane would represent a range of areal densities. Similarly, each exit trajectory represents a proton with a different momentum, and the magnets would focus each to a different image plane. The result would be a loss of resolution (each point in the image blurs into its neighbor). But remarkably, a set of magnetic lenses can be configured so that in an intermediate plane (Fourier plane), protons with large exit angles get focused to large diameters about the beam axis, and those exiting at smaller angles get focused farther away from the beam axis. An aperture can then remove the large-angle, slowest protons. Those that continue to propagate through the lens recombine at the image plane with narrower angular and momentum distributions, improving both the contrast and resolution of the image.
(Below) This false-color pRad image is of a model airplane engine. The different intensities correspond to different areal densities. The piston and piston rod are clearly visible, as are the cooling fins. The lower image has been processed so that contrast differences now represent differences in volume density.