Petkau effect
The Petkau effect is an early counterexample to linear-effect assumptions usually made about radiation exposure. It was found by Dr. Abram Petkau at the Atomic Energy of Canada Whiteshell Nuclear Research Establishment, Manitoba and published in Health Physics March 1972. The Petkau effect was coined by Swiss nuclear hazards commentator Ralph Graeub in 1985 in this book Der Petkau-Effekt und unsere strahlende Zukunft.
Petkau had been measuring, in the usual way, the radiation dose that would rupture a simulated artificial cell membrane. He found that 3500 rads delivered in hours would do it. Then, almost by chance, Petkau repeated the experiment with much weaker radiation and found that 0.7 rad delivered in hours also ruptured the membrane. This was counter to the prevailing assumption of a linear relationship between total dose or dose rate and the consequences.
The radiation was of ionizing nature, and produced negative oxygen ions. Those ions were more damaging to the simulated membrane in lower concentrations than higher because in the latter, they more readily recombine with each other instead of interfering with the membrane. The ion concentration directly correlated with the radiation dose rate and the composition had non-monotonic consequences.