Experimental determination of photon dose from a Linear Induction Accelerator using gamma spectroscopy on photoactivated foils

The Dual-Axis Radiographic Hydrodynamic Test Facility (DARHT) at Los Alamos National Laboratory uses two Linear Induction Accelerators (LIAs) to produce an electron beam with energies from 16 - 20 MeV, currents from 1.5 - 2.0 kA, and pulse lengths from 60 - 100 ns for the analysis of dynamic systems. The beam is focused onto a heavy metal target to produce a Bremsstrahlung x-ray flash for radiography. To better interpret radiographic data, this research aims to measure the x-ray output more accurately and precisely than current diagnostics. This is accomplished by exposing thin metal foils (0.1 mm thick samples of Cu, Zn, Mo, In, and Ag) with an area of 5 cm² to photon fluxes from 10⁸ - 10¹⁰ photons/cm ² per pulse with photon energies ranging from 10 - 20 MeV. To enable simultaneous irradiation of multiple materials, we implemented a custom foil holder to ensure uniform exposure while minimizing spatial uncertainties. Gamma spectroscopy was performed on the irradiated foils using a High-Purity Germanium Detector (HPGe) with a 1.90 keV resolution at 1.33 MeV to allow for isotope identification based on characteristic emission lines. The decay curve over several half-lives was used to determine the initial activity, which in turn was used to calculate the initial photon flux and absorbed dose.