Answer to Question #6091 Submitted to "Ask the Experts"

Q

I am a physician. In my practice in emergency medicine I grasp the "basics" that most general medical texts provide in regard to clinical radiation management. These sources are painfully lacking in detail for the questions I have, however.
That said, here we go . . . if a mass of a radioactive substance had multiple steps in decay to a stable nuclide, then I assume the substance might emit a variety of different types of radiation simultaneously. If this is true, where can one find out what types of emissions a given nuclide might simultaneously produce?

Next, do radionuclides emit alpha, beta, and/or gamma radiation with various amounts of kinetic energy? Thus, can some gamma rays carry the potential for more damage, due to higher energy, than others—and does that apply to alpha and beta radiation?

Finally, if the half-life of a radionuclide is long, doesn't it follow that the frequency of emission is slow, and thus the tissue damage would be less than for a radionuclide that emitted similar radiation (let's say primarily alpha) yet had a very short half-life?

I have many more questions, but am restraining myself.

A

I shall attempt to address your concerns/questions in the order you have presented them.

1. Regarding radionuclides that decay though multiple decay processes, each decay event is associated with one of the progeny of the original member of the decay series. As you have inferred, multiple radiations may be emitted as the various progeny decay.
   
   The major decay modes are alpha emission, beta particle emission, and electron capture (in which an inner shell electron is captured by the nucleus, transforming a proton into a neutron).
   
   Alpha decay leads to the emission of alpha particles, which consist of two neutrons plus two protons, a substantial chunk of the nucleus that carries a net positive charge of +2; the alpha particles from decay of atoms of a given radionuclide are monoenergetic.
   
   Beta particles may be either negatively charged electrons, produced from the conversion of a neutron in the nucleus into a proton plus an electron, the latter being immediately ejected, or positively charged electrons, the antiparticles of conventional negative electrons, produced by the conversion of a proton in the nucleus into a neutron plus a positive electron.

   Gamma radiation, pure electromagnetic radiation, is often emitted during the decay process; gamma-ray emission may accompany any of the major decay events. (Gamma emission is not itself a radioactive decay mode but is a nuclear deexcitation mode in which nucleons within the nucleus rearrange their positions.) The gamma rays that accompany a decay event are frequently emitted immediately after the event and appear to be virtually coincident in time with the decay event.
    
   There may also be other radiations emitted that accompany the decay process. These include conversion electrons (extranuclear electrons emitted in place of gamma rays), characteristic x rays (emitted when a vacancy is created in the electronic structure, as by electron capture, and the vacancy is filled by an electron from a higher-energy shell), and Auger electrons, sometimes emitted from the electronic structure as an alternative to characteristic x-ray emission.
   
   The individual decay events among the respective progeny in a decay chain may be separated significantly in time depending on the half-lives of the various species that comprise all the progeny in the chain and the amount of time that has passed since the parent (first) radionuclide was produced.
   
   When a long-lived radionuclide decays to a shorter-lived radionuclide, an equilibrium condition will come about such that there will be a predictable relationship among the activities of the two species. If the parent radionuclide has a much longer half-life than the daughter (the next chain member) the daughter activity will grow and will be equal to the activity of the parent within approximately seven half-lives of the daughter.
   
   For example, ⁹⁰Sr has about a 30-year half-life, and it decays to ⁹⁰Y, which has a 64-hour half-life. Within about two weeks of the preparation of pure ⁹⁰Sr, the daughter ⁹⁰Y activity will be equal to the activity of the ⁹⁰Sr.
   
   There are various compilations of data that provide specific information about the decay properties of the known radionuclides; the data include the types and energies of the various radiations emitted by each radionuclide as well as considerable other characterizing information.
   
   Published compilations include, among many others, Publication 38 of the International Commission on Radiological Protection, ICRP 38, titled Radionuclide Transformations Energy and Intensity of Emissions and Radioactive Decay Data Tables: A Handbook of Decay Data for Application to Radiation Dosimetry and Radiological Assessments, by David C. Kocher, Oak Ridge National Laboratory (1981), Technical Information Center, US Department of Energy. There are also websites where decay data are available; here is a link to one on the Health Physics Society website and another at Brookhaven National Laboratory.

2. Yes, radiations associated with the various decay processes may be born with different kinetic or radiant energies, depending on the nature of the particular radionuclide undergoing decay. Alpha particles from decay of multiple atoms of the same radionuclide along the same pathway are all born with the same kinetic energies, but alpha particles from the decay of different radionuclides expectedly exhibit different energies. For example, ²³⁸U and ²³⁹Pu are both alpha emitters but the major energies are around 4.2 MeV (million electron volts) and 5.2 MeV, respectively.
   
   Beta particles from common decay modes of the same radionuclide, however, are not born with the same kinetic energies. This is because in every beta decay event there is also produced a near massless particle called a neutrino that carries away a variable portion of the decay energy. Beta particles from decay of multiple atoms of the same radionuclide by the same pathway therefore have a range of kinetic energies, ranging from zero (when the neutrino gets virtually all the energy) up to a discrete maximum (when the neutrino gets no energy). For the ⁹⁰Sr-⁹⁰Y pair that we mentioned above, the ⁹⁰Sr may produce beta particles with energies up to a maximum of 0.546 MeV, and the ⁹⁰Y yields higher-energy beta particles up to 2.28 MeV.
   
   Gamma radiation from a specific transition within the nucleus is characterized by a discrete energy, but various transitions may be possible even for a single radionuclide, and different radionuclides typically emit gamma radiation of different energies. Gamma rays may range in energy from less than 0.1 MeV to about 10 MeV. In general, the higher the energy of the radiation, be it alpha, beta, or gamma, the more penetrating it is and often the more capable it is of delivering larger amounts of energy to selected tissues, thus enhancing the likelihood of greater tissue damage. (There are some exceptions to this when considering certain biological endpoints for which particular energies may be more damaging than other higher energies.)

3. Regarding the significance of half-life and the potential for producing damage, your inference that a long half-life is associated with a longer average time between decay events is true if we are comparing the same numbers of atoms of nuclides with different half-lives. The mean life of a radioactive atom is given by 1.44 times the half-life. Thus, a radionuclide with a one-year half-life, on average, will survive about 1.44 years before decaying while a radionuclide with a one-day half-life will survive only about 1.44 days, on average, before decaying.
   
   The amount of tissue damage associated with the presence of a radionuclide depends on the half-life as well as on the amount of radioactivity present and on the types and energies of radiations emitted. If we consider two radionuclides that emit the same radiations with the same energies and present in equal activities and in the same geometries relative to the tissue being irradiated, then the radionuclide with the longer half-life will deliver the greatest ultimate dose and produce the greatest damage. This is because we have specified equal activities for the two radionuclides, which means that the initial decay rates are the same (in order to have equal activities, however, we would require more atoms of the longer-lived nuclide compared to the shorter-lived one). Since the shorter-lived radionuclide decays away faster, it produces a lower ultimate dose than does the longer-lived nuclide.
   
   The ultimate dose is actually proportional to the product of the initial activity and the mean life of the radionuclide. (We should note that for radionuclides deposited internally within the body, biological mechanisms may also increase the removal rate from the body, and the mean effective life of an atom may be shorter than its physical half-life.)

A couple of textbooks that may be helpful are Radiation Protection: A Guide for Scientists, Regulators and Physicians by J. Shapiro and Herman Cember's Introduction to Health Physics (both are available from Amazon).

I hope this answers some of your questions.

George Chabot, PhD, CHP