Characterizing the X-ray spectra from scraping relativistic electrons in linear accelerators *

Finite rise and fall times in linear induction accelerators produce off energy electrons, which are 10% or more of the electron pulse. These lower energy, relativistic electrons are over focused by transport magnets and strike the stainless steel beam pipe, producing Bremsstrahlung X-rays. Some of the low energy electrons may exit the beam pipe into the accelerator hall. On the DARHT Axis-I accelerator we are performing a series of experimental measurements supplemented calculations to characterize the X-ray spectra from the lower energy electrons incident on the beam pipe after the acceleration cells. The nominal energy of the 1.7 kA beam is 19.8 MeV at this point ~30 m downstream of the injector. Greater than 10⁷ photons are produced by both the head and tail of the beam, a dose equivalent of ~10 mrad at a distance of 1 m. The X-rays may be dominated by Fe K-edge emission on top of a Bremsstrahlung continuum. We will distinguish the contribution of the stray electrons to the scattered signal with magnetic dipoles and characterize the X-ray spectra using filtered and diffractive X-ray optic techniques.