Multi-MeV X-ray Source Characterization with Finite Aperture Collection

The Dual-Axis Radiograph Hydrodynamic Test (DARHT) facility conducts flash radiography experiments on dense, high-Z objects. The accelerators produce intense relativistic electron beams of 16-20MeV end-point energies and 0.5-4kA currents which are focused into sub-millimeter FWHM spots on tantalum x-ray converter targets. MCNP simulations indicate the spectrum of x-rays is dominated by bremsstrahlung x-rays up to the beam end-point energy in the forward direction but shifts to a larger fraction of fluorescence x-rays in the reverse direction. The high-energy x-rays require shielding from about 6cm to 3cm in the forward and reverse directions, respectively, to reach less than 1% of the initial signal. Penumbral imaging can be used to characterize the source geometry but is not affected by the thick collimator. We have developed a deconvolution and inversion method that accounts for finite aperture effects including off-axis collection. The method is demonstrated with the forward and reverse spectra and indicates a source that can be decomposed into two Gaussians. The inner Gaussian dominates the signal amplitude and has a 1-sigma width of approximately 450um, similar to forward-directed pinhole imaging measurements.