It refers to the alpha carbon which is the first
aliphatic carbon in the trypt
amine molecule looking from the
amino group. The next carbon is beta carbon, and if there was a third aliphatic carbon it would be the gamma carbon, but as the third carbon is a part of the
aromatic indole nucleus it isn't. The carbons labeled with greek letters are always aliphatic and the ordering is usually relative to the functional group that forms the suffix of the name, hence looking from the
amino group. Compare the structures of tryptamine, AMT and AET. The first has a hydrogen in the alpha position, the second a methyl group and the third an ethyl group.
These matters are also discussed in
http://www.erowid.org/library/books_online/pihkal/pihkal142.shtml
in relation to phenethylamines. As you can see it has nothing to do with radioactivity.