Analysis of polystyrene and carbon foam morphologies using proton beams

The energy distribution of a proton beam transmitted through low-density polystyrene and carbon foams is determined by the target morphology. The sensitivity in analyzing materials using scanning transmission ion microscopy and ion beam tomography is affected by the shape of the transmitted spectrum. For the measurements considered, a sufficiently high incident proton energy is used so that the principal energy loss mechanism is by electronic collisions. The passage of protons through such materials is simulated by a Monte Carlo code. The numerical calculation requires no adjustable parameters, but relies upon geometric models which characterize different foam cell structures. The effect of the morphology in a particular foam on the experimental proton transmission energy spectra can be derived using the code. We have also derived analytic expressions for the average material thickness and standard deviation corresponding to the Monte Carlo cell models. These expressions give insight into the functional dependence of the measured transmission spectrum in terms of the fundamental properties of the materials. (orig.)