And presumably why it's pretty poor at discriminating Alpha & Beta, since they'll be quite easily stopped by the barrier as well (and probably the lens even if

And presumably why it's pretty poor at discriminating Alpha & Beta, since they'll be quite easily stopped by the barrier as well (and probably the lens even if it were exposed) IIRC actual radiation detectors use very thin mica films to protect the sensor, so they can admit the lesser penetrating particles.