Photographing Radioactivity with a Webcam
randombio.com
randombio.com
First, "alphas" are not necessarily "low-energy particles". Energy and particle type are orthogonal. You can have a 1 MeV alpha, beta, or gamma, or a 1 keV alpha, beta, or gamma.
Second, energy certainly contributes to how far particles can penetrate, but alphas have a massive charge (2+) which allows them to easily interact with materials they pass through. This means they are easily stopped by the glass protecting the sensor, as the electromagnetic forces occur at longer ranges than nuclear forces.
This is pretty cool though and reminds me of that video of the museum (in Germany, I think?) where you can see trails beta (or alphas?) physically zipping inside an "aquarium".