  In this paper we study the singular limit for critical points of boundary
reactions \begin{equation*}
  (-\Delta)^{\frac{1}{2}}u = \frac{1}{\varepsilon}(u-u^3) \quad \text{in } U
\subset \textbf{R}^n . \end{equation*}
  We show the existence of a $(n-1)$-rectifiable energy concentration set.
Furthermore, we show that the limit of the energy measures can be associated to
a stationary, $(n-1)$-rectifiable varifold supported in the concentration set.
This is analogous to a result of Hutchinson and Tonegawa for phase transitions.
