I wondered the same thing. I've never seen a truck bomb leave a crater like that.
air. Alpha particles will loose all their energy in just a couple inches of travel in air. Once an alpha particle is stopped, it grabs the first two free electrons it can find, and becomes a plain old atom of helium. Alpha radiation is not hazardous if the source is external to the body. Alpha particles don't penetrate deeply enough into the body to reach living tissue.