Answer to Question #217045 in Atomic and Nuclear Physics for Maddy

Question

Briefly explain why each of the types of radioactive emissions has a different potential for causing detrimental effects as it passes through biological tissues. 

Answer 1

There are two types of ionizing radiation: nonparticulate (gamma and X rays) and particulate (alpha and beta particles, neutrons and protons). Both forms can transfer energy into a substance. If the energy is high enough, the incoming radiation may eject electrons from atoms along its path through the material. This process is called ionization. Not all types of radiation are equally penetrating, and the depth that a particular radiation penetrates into material depends on the energy and the type of radiation. Almost all types of ionizing radiation are much more easily stopped by dense material (such as lead) than by water or tissue in the human body. In general, X rays and gamma rays are more penetrating than the particulate types of radiation such as beta and alpha particles.

Beta particles are electrons and typically penetrate into the tissue only a centimeter or so. Their limited range means that they can damage internal organs only when ingested or inhaled, but they can be an external hazard to exposed skin if they are present in sufficient concentrations. Alpha particles are much larger and heavier than beta particles and have a greater electrical charge. This makes it even more difficult for them to penetrate tissue. A typical alpha particle from radioactive materials, such as plutonium, will not even penetrate the external dead layer of skin tissue. Radioactive materials that emit alpha particles are a hazard only if they are inhaled or ingested and get into the cells of the body in sufficiently large concentrations. Because X rays and gamma rays travel as very-high-energy electromagnetic waves, they can penetrate the human body quite easily. Either external or internal sources of gamma radiation can be hazardous to the whole body because of the extraordinary penetrating ability of the radiation that they emit.

This fact is important because the spatial distributions of ionization in material for gamma rays, beta particles, and alpha particles are different. Alpha particles interact very readily with the matter that they penetrate. They are called high-linear-energy-transfer (high-LET) radiation because they dissipate their energy rapidly, producing very short, dense tracks of ionization. Because of their high-LET characteristics alpha particles can be much more damaging, for a given absorbed dose, than low-LET radiations such as beta particles and gamma rays. Low-LET radiations ionize the atoms in their paths much less frequently and produce tracks that are much less densely ionized.

It is possible to compare the biologic effects from different types of radiation by using radiation weighting factors. The factor for alpha particles is about 20 and that for gamma and beta radiation is approximately 1, indicating that it takes about 20 times more gamma or beta radiation than alpha radiation to cause a given effect. The dosimetry measurement that allows the differences in biologic effectiveness of various types of radiation to be combined is called the equivalent dose. It is calculated by multiplying the absorbed dose by the radiation weighting factors. The unit of equivalent dose is the sievert or the rem. One sievert (Sv) equals 100 rem.