Physics > Nuclear Physics > Nuclear Decay

Nuclear decay, also known as radioactive decay, is a fundamental concept in nuclear physics that describes the process through which an unstable atomic nucleus loses energy by emitting radiation. This process results in the transformation of the original atom into a different element or a different isotope of the same element. There are several types of nuclear decay, each characterized by the specific particles or radiation emitted.

1. Alpha Decay: In alpha decay, the unstable nucleus emits an alpha particle, which consists of two protons and two neutrons (essentially a helium-4 nucleus). This process decreases the atomic number by 2 and the mass number by 4. For example, the alpha decay of Uranium-238 can be expressed as:
   \[
   {}^{238}{92}\text{U} \rightarrow {}^{234}{90}\text{Th} + {}^{4}_{2}\text{He}
   \]

2. Beta Decay: Beta decay occurs in two forms—beta-minus (β^−) and beta-plus (β^+) decay:

   • In beta-minus decay, a neutron is transformed into a proton, an electron, and an antineutrino. This increases the atomic number by 1 while the mass number remains unchanged. The reaction can be represented as:
     \[
     n \rightarrow p + e^- + \bar{\nu}_e
     \]
     For example, Carbon-14 undergoing beta-minus decay can be written as:
     \[
     {}^{14}{6}\text{C} \rightarrow {}^{14}{7}\text{N} + e^- + \bar{\nu}_e
     \]

   • In beta-plus decay, or positron emission, a proton is converted into a neutron, a positron, and a neutrino. This decreases the atomic number by 1 while the mass number remains unchanged:
     \[
     p \rightarrow n + e^+ + \nu_e
     \]
     An example is the decay of Magnesium-23:
     \[
     {}^{23}{12}\text{Mg} \rightarrow {}^{23}{11}\text{Na} + e^+ + \nu_e
     \]

3. Gamma Decay: Gamma decay involves the emission of gamma rays (high-energy photons) from an excited nucleus as it transitions to a lower energy state. This type of decay occurs without a change in the number of protons or neutrons, thus the atomic number and mass number remain unchanged. For instance, when Cobalt-60 decays, it initially undergoes beta decay to Nickel-60, which is often left in an excited state. The subsequent transition to a lower energy state involves gamma emission:
   \[
   {}^{{60}{27}\text{Co} \rightarrow {}^{60}{28}\text{Ni}}* + e^- + \bar{\nu}_e
   \]
   \[
   {}^{60}{28}\text{Ni}^* \rightarrow {}^{60}{28}\text{Ni} + \gamma
   \]

4. Other rare forms of decay: There are other less common decay modes, such as proton decay, neutron decay, and double beta decay, which are subjects of ongoing research and can provide insights into the fundamental symmetries of physics.

The study of nuclear decay is crucial in various fields such as radiometric dating in geology and archaeology, medical diagnostics and treatments, nuclear energy production, and understanding the processes occurring in stellar environments. The decay rates are typically characterized by the half-life of the isotope, which is the time required for half the atoms in a sample to undergo decay, represented mathematically as:
\[
N(t) = N_0 e^{-\lambda t}
\]
where \( N(t) \) is the quantity of substance remaining, \( N_0 \) is the initial quantity, \( \lambda \) is the decay constant, and \( t \) is time.

Understanding nuclear decay not only provides insights into the stability of elements but also impacts practical applications in technology, medicine, and environmental science.