Statistical Inference — unbiased estimator of normal sigma

Let $x_1,x_2,…,x_n$ be a random sample from Normal distribution $N(\mu,\sigma^2)$. Assume that $\mu$ is known and $\sigma$ is unknown. Define an estimator T as $T=k\sqrt{\frac{\sum_{i=1}^n(x_i-\mu)^2}{n}}$. Then the constant k for T to be unbiased estimator of $\sigma$ is: