Soft radiation is a term used to describe high-energy electromagnetic radiation, typically high energy X-rays or gamma rays. The term refers to the ability of the rays to penetrate a given thickness of material, typically a lead shield.
Soft radiation is able to cross 5 gcm−2 of brass but not 167 gcm−2 of lead (composed by p with energy greater than 0.4 MeV, μ with E>80 MeV and e and γ with E>10 MeV)[1]