Pioneering medical work done in the late 1940s and early 1950s established cobalt-60 as a useful isotope, as the relatively high-energy gamma rays produced when it undergoes beta decay are able to penetrate the skin of the patient, and deliver a greater portion of the dose directly to the tumor. The high neutron efficiency of the NRX's heavy water-moderated design, coupled with the high neutron flux of the reactor, made it relatively inexpensive for AECL to produce medical-grade cobalt-60. For example, the cost of the entire unit used to perform the first cobalt-60 treatment was about $50,000. By way of contrast, it would cost $50,000,000 just to produce enough radium (which had been previously used as a therapy source) to perform the same procedure.