Relative biological effectiveness

Photons and beta particles have a low linear energy transfer (LET) coefficient, meaning that they ionize atoms in the tissue that are spaced by several hundred nanometers (several tenths of a micrometer) apart, along their path.

Radiation weighting factors have not been developed for internal sources of heavy ions, such as a recoil nucleus.

The ratio of these doses is the RBE of R. Instead of death, the endpoint might be the fraction of cells that become unable to undergo mitotic division (or, for bacteria, binary fission), thus being effectively sterilized — even if they can still carry out other cellular functions.

Early on it was found that X-rays, gamma rays, and beta radiation were essentially equivalent for all cell types.

Therefore, the standard radiation type X is generally an X-ray beam with 250 keV photons or cobalt-60 gamma rays.

However, since alpha particles cannot traverse the outermost dead layer of human skin, they can do significant damage only if they come from the decay of atoms inside the body.

[4] Measurements of RBE with external sources also neglect the ionization caused by the recoil of the parent-nucleus due to the alpha decay.

The parent nucleus is required to recoil, upon emission of an alpha particle, with a discrete kinetic energy due to conservation of momentum.

Thus, all of the ionization energy from the recoil-nucleus is deposited in an extremely small volume near its original location, typically in the cell nucleus on the chromosomes, which have an affinity for heavy metals.

It had been noticed that those effects depended both on the type and energy spectrum of the radiation, and on the kind of living tissue.