High Speed Gas Density Measurements Using Proton Induced X-Rays

The need for high speed gas density measurements in interior ballistic environments has led to the development of a technique which uses the attenuation that a gas produces in transiting characteristic x-ray fluxes to determine its density. To assess the technique an apparatus has been constructed that generates KΑ and Kß x-rays by proton impact on metallic targets and subsequently directs them through beryllium windows across the bore of a high pressure gas chamber (0.0 - 0.1 gm/cm
³
), and on to a detector. A comparison between the loss of x-ray counts due to the presence of the gas and predictions based upon tabulated mass-absorption coefficients permits a simple evaluation of the gases density. This evaluation is sensitive only to the gross atomic composition and not its detailed chemistry. Calibration of the apparatus was performed with 26 < Z < 40 characteristic x-rays in static pure O
₂
and N
₂
. Following that work Cu x-rays produced by a 1-2 μamp/cm
²
beam of 1.5 MeV protons were used to study density changes in N
₂
with < l × 10
^-3
second resolution. These results indicate that with a small commercial proton accelerator one can create x-ray fluxes intense enough for a density measurement with an error of a few percent at a time resolution of 10
^-5
seconds. A new apparatus and accelerator are now being installed to evaluate the x-ray technique in the hostile environments of high temperature and pressure that are found in gun tubes.