If I try to horizontally align two nodes by giving the same y-coordinate, the text $D_{\textrm{X-ray}}$ and $D_{\textrm{C-ion}}$ is still not aligned properly, which has apparently to do with the letter height of the subscripts. Removing [above] tag does not change that behaviour. How do I align both nodes to the D? Correctly align vertical text on a baseline in pgfplots deals with a similar issue, but I don't know how to apply the solution to my example.
\begin{figure}[ht]
\centering
\begin{tikzpicture}
\begin{semilogyaxis}[samples=200,ymin=0.001,ymax=1,xmin=0,xmax=5,unbounded coords=discard,xlabel=Dose (Gy),
ylabel=Survival,
legend entries={X-rays, Carbon ions},
legend style={draw=none, nodes=right},
legend pos= north east]
\addplot [thick, dashed] plot (\x, {1*(exp((-0.8*\x)-(0.12*\x*\x)))});
\addplot [thick] plot (\x, {1*(exp((-2*\x)))});
\draw[semithick] (axis cs:0,0.01) -> (axis cs:2.302,0.01);
\draw (axis cs:0,0.0095) -- (axis cs:3.701,0.0095);
\node[above] at (axis cs:0.5,0.01) {\small $D_{\textrm{C-ion}}$};
\node[above] at (axis cs:2.8,0.01) {\small $D_{\textrm{X-ray}}$};
\draw[semithick] (axis cs:2.302,0.01) -- (axis cs:2.302,0.001);
\draw (axis cs:3.701,0.0095) -- (axis cs:3.701,0.001);
\node at (axis cs:1.15,0.003) {\small $\textrm{RBE} = \frac{3.7\textrm{ Gy}}{2.3\textrm{ Gy}} \approx 1.6$};
\end{semilogyaxis}
\end{tikzpicture}
\end{figure}
