Why is energy released in decay so important?

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