Different types of radiation deposit energy in biological tissues in different ways, which affects the amount of cellular damage. RBE (relative biological effectiveness) is a relative measure of the damage done by a given type of radiation per unit of energy deposited in biological tissues. Compared with higher energy photons such as cobalt-60 gamma rays, lower energy electrons and photons produce more dense clusters of ionizations, leading to more complex damage to the cell's DNA, and thus a higher RBE.

In this work, relative biological effectiveness values were derived for electrons and photons as a function of energy. There is growing evidence to support a relative biological effectiveness greater than one for low-energy electrons and photons. For example, a number of experimental studies suggest RBE values between two and three for tritium, meaning that the low-energy electrons emitted by tritium do two to three times the damage to cells per unit of energy deposited when compared to higher energy radiation. Other radiation sources with similar energy emissions may also exhibit an elevated RBE. Consideration of the potentially greater effectiveness of these radiations is important to improve the quality of risk assessments.

• Relative Biological Effectiveness of Low-Energy Electrons and Photons (PDF)(25 pp, 714 K, October 20, 2013)