Development of a ground-based facility for simulating the omnidirectional near-earth electron environment

Abstract

A method is detailed for simulating in the laboratory the omnidirectional near-earth electron environment. This technique can synthesize the directional and energy properties of an arbitrary omnidirectional electron flux by combining a sufficient number of weighted energy and direction increments. A large vacuum chamber has been built, capable of holding three-dimensional structures up to 30 cm in each dimension while maintaining pressures in the 10 −6 mm Hg range. The support structure in the chamber rotates around two axes, thereby achieving all angular orientations necessary for the omnidirectional simulation. This chamber was coupled with a beam scanning system to a 4 MeV electron Van de Graaff accelerator which was tuned to provide specified fluences of electrons at the desired energies. The system was used to irradiate a series of hollow aluminum cubes having varying wall thicknesses. Using radiochromic film dosimeters, dose profiles were mapped in the interiors for both one-wall and six-wall exposures. Results of the dose mapping are presented for several monoenergetic cases as well as for the energy spectra from a 235U fission event. The one-wall exposures show dose variations within the interior by a factor of 3.2 ± 0.6 independent of energy or wall thickness. In the six-wall exposure, distributions varied only about 15%.