Quite a few basic errors in the physics here. "High energy electrons" are beta decay, and are stopped by a single sheet of paper. Your "boron-doped concrete" is for absorbing neutrons or protons, while the lead absorbs gamma (and x-rays) -- but Nickel-63's decay mode doesn't emit these. Beam therapy for cancer treatment can be photons (x-rays), electrons, or protons ... that's why composite shielding is used.
For some people, this is what they think when they read "High Energy Electrons"