Geant4 example rdecay02 radioactive particles do not travel far

Hi I am running geant4 examples in radioactivity. I am running rdecay02 example. I am studying how far radioactive particle travels if I use u238c.mac file.

for that I have added code which checks x,y,z position at each step along with A and Z. I kept the medium (Target and Detector) as Air. So that radioactive particles can travel as far as possible.

But I found out that although detector length is 20cm. all the radioactive particles travel only till around 20 micrometers (um).

Why is that?