The position vectors of vertices \(A\), \(B\) and \(C\) of triangle \(ABC\) are \(\hat{i}-\hat{j}-3\hat{k}\), \(2\hat{i}+\hat{j}-2\hat{k}\), and \(-5\hat{i}+2\hat{j}-6\hat{k}\) respectively. The length of the bisector \(AD\) of angle \(BAC\), where \(D\) is on the line segment \(BC\), is