Radioactive Decays – transmutations of nuclides

Radioactivity means the emission of alpha (a) particles, beta (b) particles, or gamma photons (g) from atomic nuclei. Radioactive decay is a process by which the nuclei of a nuclide emit a, b or g rays. In the radioactive process, the nuclide undergoes a transmutation, converting to another nuclide.

The variation of radioactivity over time is called decay kinetics. The characteristics of kinetics are expressed in decay constant and half life. Variations of radioactivity in mixtures of radioactive nuclides and consecutive decays are often considered, and decay kinetics serves science and technology in many applications.

In radioactive decay processes, some of the things are conserved, meaning they do not change. The number of nucleons before and after the decay is the same (conserved). So are electric charges and energy (including mass). The relationship between nuclides is best seen in a chart based on the number of neutrons and the number of protons. Such a chart clearly shows relationships among isobars, isotopes, isotones, and isomers.

The mass number of a nuclide does not change in b and g decays, but it decreases by 4 in a decay due to the emission of a helium nucleus. Thus, there are four families of radioactive series based on mass numbers starting with 232Th, 237Np, 238U, and 235U respectively.  Masses of their family members are in the 4n, 4n +1, 4n +2, 4n +3 categories, where n is an integer. Sources of natural radioactive materials such as radium, radon, and polonium, came from the natural occurring radioactive nuclides 235U, 238U, and 232Th, whereas 237Np is a man made nuclide, because this nuclide no longer exists in the planet Earth.

Studies of radioactive decays led to theories of nuclear stability and nuclear structure. Some of these theories will be examined as we take a closer look at atomic nuclei. Concepts such as energy states of nucleons, angular momentum, parity of nuclear energy state etc. will be introduced. These concepts and theories provide the tools for the discussion of energy in radioactive decays.

We will look at radioactivity and decay kinetics, look at transmutation of nuclides in radioactive decay, the nuclide chart, which is used to discuss the four families of radioactive decay,  look at atomic nuclei closely, and look at the energy aspect in radioactive decays.

Radioactivity and Decay Kinetics

The emission of alpha (a), beta (b) or gamma (g) rays by a sample of substance is called Radioactivity. A sample may emit one or more types of radioactive ray. The number of a, b or g rays emitted per unit time is called the decay rate. The study of radioactive material requires the identification of types of rays emitted, decay rates, changes in decay rates, and the nuclides in the sample that emit the rays. The variation of decay rates over time from a fixed amount of nuclide is called decay kinetics, which is an important topic in the study of radioactivity.

Radioactivity Units, Decay Constants and Half Lives

The decay rate is measured in decays per unit time, and the SI unit for radioactivity is becquerel (Bq) which is 1 decay or disintegration per second (dps)*. The widely used unit curie (Ci) defined as the decay rate of 1.0 g of radium earlier is 3.700 x 1010 Bq.

A Summary of Decay Kinetics

No is number of nuclei at t = 0,
N is the number of nuclei at t.
A is the Radioactivity.

Integration gives

ln N = ln No – l t

  • How are decay rates related to the amounts of radioactive nuclide?
    How do decay rates vary over time?
  • Do chemical states of a radioactive nuclide (element) affect its decay rate?

Samples containing the same amount of uranium and thorium have very different decay rates. A radioactive source contains one or more radioactive nuclides. The decay rate of a sample is proportional to the amount of radioactive nuclide present. Decay rate of a nuclide is unaffected by its chemical or physical state, studies have shown.

The amount of radioactive nuclide can be expressed in unit g, mole, or number of nuclei (N). The disintegration rate of N nuclei (called activity, A, or in mathematical notation ‑ dN/dt) at any given time is proportional to N. The proportional constant is called the decay constant, l. A summary of decay kinetics is given in the text box.

* Long before SI units were established, radioactivity was compared to a quantity called curie (Ci), which was originally the radioactivity of 1.0 g of radium.  One Ci is now defined as the quantity of any radioactive material that gives 3.700 x 1010 dps or Bq.  Thus, Ci is considered a cgs (cm, gram & second) unit, and 1 Ci = 3.700 x 1010 Bq.

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