Chi squared two df is exponential

Let $X_1,X_2$ be a random sample from $N(0,{\sigma }^2)$ distribution, where $\sigma >0$ and $N(\mu ,{\sigma }^2)$ denotes a normal distribution with mean $\mu$ and variance ${\sigma }^2$. Suppose, for some constant $c,(c(X_1^2+X_2^2),\infty )$ is a confidence interval for variance ${\sigma }^2$ with confidence coefficient 0.95. Then the value of c is equal to