Proton Induced Characteristic X-Ray Radiography

A new accelerator-based technique for measuring the densities of rapidly changing multiphase media, characteristic x-ray radiography (CXR), has been developed. It has been successfully tested in a particularly violent regime, the interior of a discharging firearm, where it has produced the first quantitative measurements of the propellant´s density. The method is based upon a timewise continuous determination of the attenuation that an energetically well-defined flux of x-rays experiences as it traverses the media. The ARRADCOM-BRL apparatus obtains the required x-rays by impacting a beam of 2.35 MeV protons upon a metal foil; a process that copiously produces only brems-strahlung-free x-ray line spectra that are characteristic of the metal. The x-rays enter and exit the firearm via beryllium windows mounted diametrically across the weapon´s bore. When the weapon discharges the propellant intercepts the transiting x-ray flux causing correspondingly attenuated output currents in a scintillator/photomultiplier type detector. The present method has several important advantages. The well-defined line spectrum of x-rays simplifies the relation between transmission and density to an easily calculated exponential function of the propellant´s atomic composition, the weapon´s geometry, and the x-ray energy. The results are insensitive to the detailed chemistry and phase of the propellant. Furthermore, by merely changing the proton´s target material, one may select the x-ray energy that will optimize the sensitivity and minimize errors for the specific conditions under study. Density measurements now underway are producing results with time resolutions of a few tens of microseconds.