\documentclass{article}
\begin{document}

\section{Radiation Damage1}
This is a test1.
\subsection{Radiation Damage2}\label{a2}
This is a test2.
\subsubsection{Radiation Damage3}
This is a test3. It is mentioned in 
section~\ref{a2} but more in appendix~\ref{bbb}.
\section{Radiation Damage1}
This is a test1.
\section{Radiation Damage1}
This is a test1.
\appendix
\section{Radiation Damage4}\label{bbb}
This is a test1.
\section{Radiation Damage5}
This is a test1.

\end{document}