Let $A B C$ be the triangle with $A B = 1 , A C = 3$ and $\angle B A C = \frac { \pi } { 2 }$. If a circle of radius $r > 0$ touches the sides $A B , A C$ and also touches internally the circumcircle of the triangle $A B C$, then the value of $r$ is $\_\_\_\_$.