Answer to Question #9646 Submitted to "Ask the Experts"

Category: Radiation Basics

The following question was answered by an expert in the appropriate field:

Radionuclide decay tables often provide energy released in MeV. Alpha particles, with 4 amu (atomic mass units) and relatively low speed (0.1 c) normally have the highest energy released, even though beta particles and gamma radiation have a much higher potential for tissue damage. This seems counterintuitive. For particulate radiation, it would make sense that tissue damage might be a function of particle momentum, rather than energy released, but how do we explain the danger of gamma radiation, which has no mass? I cannot find any explanation regarding the biological significance of energy released (MeV). How does energy released relate to the potential for tissue damage?

The amount of energy released in a nuclear decay event is an important indicator of potential tissue dose and ultimately possible tissue damage, but additional critical factors affecting the extent of energy deposition and damage are (1) the fraction of the energy of the radiation of concern that is released that is delivered to tissue and (2) the amount of energy deposited per unit path length traversed by the particulate radiation itself or by the charged particles set free by the radiation interactions; this quantity is referred to as the linear energy transfer (LET).

Your statement that beta particles and gamma radiation have a much higher potential for tissue damage than do alpha particles is often true if the radiation source is external to the body, but it is not true if the radioactive material is taken into the body. In the latter case vulnerable tissue may be in intimate contact with the radioactive material and this provides for the possible total deposition of all of the kinetic energy of charged particles released in affected tissues. Thus, alpha particles, with a dual charge and high mass deposit all of their energy within a very short distance from their emission point, often within tens of micrometers. The LET of alpha radiation is very high. Beta particles will traverse greater distances, but usually only several millimeters in soft tissue. The beta particles exhibit LETs that are much lower than the LETs for alpha particles. Gamma radiation may deposit all, some, or none of its radiation in the body before exiting. The gamma radiation itself is much more penetrating than alpha or beta radiation; when it interacts it does so by setting free electrons in the material with which it interacts. These electrons behave similarly to beta radiation and have comparable LETs. The differences in LET result in differences in biological effectiveness in inducing some types of tissue damage.

When sources are external to the body, the situation is somewhat different. Most alpha particles are stopped in the dead layer of the skin, thus presenting no hazard to live tissue. Beta radiation often has sufficient energy to penetrate several mm and thus produce some dose to superficial tissues such as the live skin cells and the lens of the eye. Gamma radiation, which interacts less strongly, may penetrate to various depths in the body with the possibility of distributing energy throughout body tissues. Thus, for the external radiation case, 5 MeV alpha particles present no dose or tissue damage threat; 1 MeV beta particles represent a potential dose and tissue damage impact, especially to skin and to the lens of the eye, and 1 MeV gamma rays may deliver energy throughout the depth of the body and have dose consequences for the whole body.

In every case in which radiation interacts in the body, the delivered dose to a tissue mass of interest is proportional to the energy deposited in that mass; the average absorbed dose to the tissue is the total energy deposited in the tissue divided by the tissue mass. In the cases of alpha radiation and beta radiation incident on tissue, it is common for all of the charged particle energies to be deposited in the localized tissue. In the case of gamma rays or x rays, it is unlikely that all of the gamma ray energy will deposit locally, but the energy deposited per unit mass, on average, used for typical dose calculations, is proportional to the incident photon energy.

Depending on what biological effect endpoints are of concern, the delivered absorbed dose may be multiplied by a modifying factor (referred to as a relative biological effectiveness (RBE) factor by radiation biologists and others) that accounts for the differing biological impact of different LET radiations. The RBEs for beta radiation and gamma radiation are pretty much the same, generally taken as 1.0, while the RBE for alpha radiation is typically set at about 20 for radiation protection purposes. Thus, for a given absorbed dose to the same tissue from each of the three radiation types noted, the biological dose impact would expectedly be about twenty times greater for the alpha radiation than for the beta or gamma radiation.

Because alpha and beta radiations have relatively short ranges in tissue, such particles incident on tissue will deposit all or most of their energy in that tissue. Gamma radiation similarly incident on tissue may travel a significant distance in the body before interacting (e.g., a 1 MeV gamma ray travels, on average, about 14 cm in soft tissue before interacting). Such gamma rays may have an overall greater dose impact to the body than do the alpha or beta particles, whose interactions are confined to smaller volumes of tissue. The alpha or beta radiation may produce higher doses to localized tissues than do gamma rays, but the biological significance of the dose from the gamma radiation may be greater – i.e., the dose implications of a modest whole body dose may be greater than the dose implications of greater localized doses.

Regarding the presentation of decay data, the total decay energy is often presented. For the case of pure alpha decay most of the decay energy is associated with the alpha particle, typically about 98 percent, and a small amount, typically about 2 percent, represents the kinetic energy of the recoil ion produced in the alpha decay process. For beta decay the decay energy is split among the beta particle, the antineutrino (or neutrino in the case of beta + decay), and the recoil atom, the latter being very small, usually just a few electron volts. When the neutrino receives no energy, the beta particle receives practically all of the decay energy, and this quantity defines the maximum energy of the beta particle. Thus, the decay energy is an important quantity useful in assessing the potential kinetic energies of particles of concern. More detailed decay schemes show the specific energies and yields associated with gamma rays, x-rays and other radiations.

Because radiation energies are critical to predicting and evaluating the outcomes of radiation interactions, including the dose impacts of exposure to particular radiation fields, it is desirable to have the energy information available. Particle momentum is certainly a useful and necessary quantity for certain analyses, such as determining the magnitude of the kinetic energy of a recoiling ion produced in alpha decay (the alpha particle and recoil ion momenta are set equal to each other to obtain the relative velocities of the two particles that is use to obtain the relative kinetic energy split between the two).

In summary, the decay energies and the energies of the radiations produced are very useful and necessary to evaluate the dose impacts of these radiations as well as other effects of interest, such as expected energy deposition in a radiation detector. Other quantities, such as particle momenta, may be useful and descriptive in some situations but are not generally as helpful as the radiation energies.

I’ve gone on a bit long here, but I thought some elaboration was necessary to address your questions. I hope it helps.

George Chabot, PhD, CHP

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