Depending upon the cell type to be studied following an exposure to gamma-radiations, inclusion of more than one housekeeping gene in gene expression analyses is likely to increase confidence in experiments. For example, certain studies have used both beta-actin (ACTB) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) as reference genes. 

What is your target tissue. Plant material, animal tissue or cell line. In plants, Ubi/actin/elongation factor and in animals GAPDH should work well.

Initially we will use human cell lines and ultimately patients samples pre-, post radiation.

Thanks for the suggestions so far.