Enhanced radiation shielding by space-charge fields in insulating slabs

Abstract

Space charge and the resulting electric fields accumulate in good insulators during irradiation. In high quality insulators the resulting large electric field will alter the trajectories of high energy electrons. The altered trajectories result in decreased flux of high energy electrons in some regions of material, thus providing enhanced shielding effectiveness. We experimentally and theoretically investigate the effect with high energy electron irradiation (0.7–2 MeV) of planar polymethylmethacrylate shields. For the slab shield cases which we investigated, the shielding effectiveness is enhanced by as much as a factor of 2. By allowing the electric field to extend into vacuum adjacent to the insulator, one obtains improved shielding effectiveness without adding mass.

Multimode charge transport in electron implanted PMMA

“…The conduction decay time constant at first becomes longer and then is reduced at higher implantations, perhaps caused by greater amounts of non-linear radiation induced conductivity. [6]. Note also for all three implantation levels the initial internal planar charge amount, Q o , corresponds to 90% of the total incident applied planar charge as determined by the Faraday cup tests.…”

“…This increased transport appears to result from at least two factors; (1) radiation induced conductivity that is often modeled as non-linear, and (2) an increase in space-charge induced electric fields at greater amounts of implanted charge, which may also modify trajectories and deposition during implantation. [6] However, at low implantation doses of 200 Gy or less the implanted planar charge is less than 0.1 µcoulombs/cm 2 and resultant internal voltages are small compared to beam energy. In such situations implantation trajectories will not be notably altered from those for charge-free conditions.…”

“…In such situations implantation trajectories will not be notably altered from those for charge-free conditions. [6] It is known that electrical transport in PMMA may result from carrier drift in the ambient field, a so called mobility model. Transport may also result from sufficient carrier density to support ohmic-like conduction.…”

“…Transport may also result from sufficient carrier density to support ohmic-like conduction. [6] While various transport models exist, such as Poole-Frenkel for electron hopping, and ion hopping, and space-charge-limited theories, in general it has been found that conduction in polymers is thermally activated according to the Arrhenius law [7][8][9].…”

“…The thin electrodes did not influence the implantation process [1] and were always short-circuited to eliminate surface charges from air ions. [6] A single sample was used to avoid material uncertainties. …”