Why is alpha radiation considered more biologically effective than gamma radiation at the same absorbed dose?

Alpha radiation is considered more biologically effective than gamma radiation at the same absorbed dose because of differences in how they deposit energy within living tissue.

The biological impact of radiation depends on the concentration of molecular damage produced at critical sites, such as DNA. Alpha particles are heavy, charged particles that produce ionizations that are characteristically clustered closely together along their path. This high density of energy deposition, known as high linear energy transfer (LET), makes the resulting damage more likely to be irreparable.12

In contrast, gamma rays are sparsely ionizing electromagnetic radiation. They interact with matter less strongly, resulting in ionizations that are more widely spaced. Because of this, gamma radiation typically requires multiple tracks to cause significant damage, such as two-break chromosome aberrations, whereas a single alpha particle track can often produce such damage directly. Consequently, charged particles like alpha particles have a higher relative biologic effectiveness (RBE) than gamma rays.1324