A reactor is provided for controlling the temperature of materials undergoing a physical or chemical process. The reactor includes a reactor having a holder adapted to hold a plurality of the materials in a plurality of 2n arrays, each array having two rows of n materials. The reactor further includes a fluid inlet adapted to receive a temperature-modifying fluid and a fluid outlet adapted to discharge the temperature-modifying fluid. The sample holder has a plurality of fluid flow channels configured such that a fluid flow channel is located adjacent to each of the two rows in each of the plurality of 2n arrays. The fluid flow channels are adapted to provide substantially equal flow rates and substantially equal fluid flow volumes as the temperature-modifying fluid travels past the arrays.