Section 18A. An employee in a jail or house of correction of a county who, while in the performance of duty, receives bodily injuries resulting from acts of violence of patients or prisoners in his custody, and who as result of such injury is entitled to benefits under chapter one hundred and fifty-two, shall be paid, in addition to the benefits of said chapter one hundred and fifty-two, the difference between the weekly cash benefits to which he is entitled under said chapter one hundred and fifty-two and his regular salary, without such absence being charged against available sick leave credits, even if such absence may be for less than eight calendar days duration.