Constant for a confidence interval of a normal variance

Let $X_1,X_2$ be a random sample from an $N(0,\theta)$ distribution, where $\theta>0$. Then the value of $k$, for which the interval $\left(0,\dfrac{X_1^2+X_2^2}k\right)$ is a $95\%$ confidence interval for $\theta$, equals