I haven't had to typeset full decay chains either. I used this in my dissertation (xelatex+withesis.cls):
\newcommand{\leftexp}[2]{{\vphantom{#2}}^{#1}{#2}}
\newcommand{\isotope}[2]{\leftexp{\text{#2}}{\text{#1}}}
and used it in this context:
\begin{equation}
\begin{split}
\isotope{H}{2} + \isotope{H}{2} & \xrightarrow{50\%} \isotope{H}{3} (1.01~\textrm{MeV}) + \isotope{H}{1} (3.02~\textrm{MeV}) \\
& \xrightarrow{50\%} \isotope{He}{3} (0.82~\textrm{MeV}) + \textrm{n} (2.45~\textrm{MeV})
\end{split}
\end{equation}
This could be modified to include Z and N numbers. Anything fancier than this would probably require TikZ or inkscape.