Radioactivity

What is Radioactivity?

Radioactivity, also known as radioactive decay or nuclear disintegration, is a spontaneous process in which an unstable parent nucleus emits a particle or electromagnetic radiation (such as gamma rays) and transforms into a more stable daughter nucleus. This process continues until a stable nuclear configuration is achieved.

Radioactive decay is usually accompanied by the emission of energetic particles or gamma-ray photons. These decays follow a general formalism based on the activity A(t) of the radioactive material and the decay constant λ.

Decay Constant and Activity

The decay constant, denoted as λ, is a characteristic parameter for each radioactive decay process, with dimensions of reciprocal time (s^–1). It represents the probability that any individual atom in a large collection of identical radioactive atoms will decay in a given time interval.

The activity of a radioactive substance, A(t), is defined as the total number of decays (disintegrations) per unit time. The activity is given by the product of the number of radioactive atoms N(t) and the decay constant λ, as:

A(t) = λN(t)

The SI unit of activity is the becquerel (Bq), where 1 Bq = 1 s^–1. This is the unit used to describe the rate of decay. Another older unit is the curie (Ci), which was defined as the activity of 1 gram of ²²⁶Ra, approximately 3.7 × 10¹⁰ disintegrations per second. The relationship between becquerels and curies is:

1 Ci = 3.7 × 10¹⁰ Bq

Note: The curie (Ci) is an older unit, and the becquerel (Bq) is the preferred SI unit for radioactivity.

Specific Activity

The specific activity a is defined as the activity A per unit mass m of a substance:

a = A/m = λN/m

The units of specific activity are Bq/kg (SI unit) or Ci/g (old unit). The specific activity depends on the decay constant λ and the atomic mass number A of the radioactive atom.

Types of Radioactive Decay

There are several types of radioactive decay processes, including:

• Alpha decay (α): The emission of an alpha particle (helium nucleus, ⁴He). This typically occurs in heavy elements such as uranium and radium.
• Beta decay (β): The emission of a beta particle (electron or positron), where a neutron decays into a proton or vice versa.
• Gamma decay (γ): The emission of a gamma ray photon, which usually follows other types of decay as a way to release excess energy from the nucleus.

Each type of decay has a characteristic half-life and decay constant, which are essential for understanding the rate of decay and the behavior of radioactive materials over time.

Radioactive Decay Series

In some cases, a daughter nucleus from a radioactive decay may still be unstable and undergo further decay. This process continues in a sequence, known as a decay series, until a stable isotope is formed. A well-known example is the decay of uranium-238 (²³⁸U), which eventually transforms into lead-206 (²⁰⁶Pb).