Please comment if you know where to find an online version of the captioned Health Physics paper by Dr. Geoffrey Smith et al. The NIH PubMed entry doesn’t even have an abstract. Dr. Smith describes the research area as:
Low-level Radiation Effects in the Waste Isolation Pilot Plant Permian-age Salado Formation. We are studying the effects of shielding cells from normal levels of radiation by growing them 650 meters underground at the WIPP site in a pre-World War II 6-in thick steel chamber. The effort is to test the Linear No-threshold Theory from the “other side of background”, in a radiation-shielded environment that is well below natural levels of radiation (Smith et al. 2010). Additionally, the Permian-age halite is being examined for biochemical evidence of ancient life.
This study could make an important contribution to verification or refutation of the Linear No-threshold (LNT) hypothesis.
SeekerBlog 
I’m not sure if my earlier comment made it to you so here goes again. Sorry if redundant.
I was not able to find the full report but there is a very brief summary on an Indian newspaper.
http://www.thehindu.com/sci-tech/science/is-radiation-a-must-for-cells-normal-growth/article2201199.ece
Thanks heaps Hiroshi! Your comment was buried under spams.
This work looks promising. I will try to contact the lead author – perhaps they will allow access to undated version.