Other way around, alpha(a helium nucleus) will not penetrate the plastic case, beta(electron moving near the speed of light) probably not much useful penetration, gamma(photon above x-ray in energy) zings right through is ionizing radiation and will trigger a CCD or CMOS sensor.

BTW internal alpha emitting radionuclide ingestion is highly ionizing and dangerous although nearly harmless if radiated against the dead outer layer of skin.

A radiation detector app, obviously uncalibrated, could be a great tech demonstrator to keep Maemo/Meego in peoples heads even if Nokia is desperate to ditch the project.