Sterilizable separator tubes and methods of utilization of same are presented. Methods include providing a separator tube containing a separator substance that can be polymerized to a desired hardness following sterilization using radical generating radiation. The separator substance is formulated to have a density between that of the average densities of separable fractions derived from a sample fluid such as blood, and to be flowable. Upon centrifugation of a separator tube containing a fluid sample, the separator substance forms a barrier between the fractions. The barrier subsequently hardens to form a solid barrier when triggered by a suitable energy source.