Xeplict said:
What is the reason alpha rays can't travel long distances?
Alpha Radiation
Contrary to popular belief Alpha Radiation is actually quite strong. The reason it does not travel far is because it is stopped by a few inches of air or even a thin sheet of paper. Of the 3, Alpha, Beta, and Gamma Radiation it is the strongest but is easiest to stop.
Beta is weaker than Alpha but is harder to stop, it can pass through several meters of air. While Gamma is the hardest to stop and can pass through a thin sheet of lead easily. You would need 10 centimeters thick block of lead to stop. Luckily Gamma is not strong and is actually the weakest of the three.
So to answer your question:
Alpha Rays do not travel long distances because they are so easy to stop that air itself stops it before it can get to far.