At the core of every atom lies its nucleus, composed of protons and neutrons collectively referred to as nucleons. The arrangement of these nucleons is governed by the strong nuclear force, a fundamental interaction that binds them together. However, not all nuclei are stable. Certain isotopes, referred to as radionuclides, are unstable due to an imbalance between the number of protons and neutrons or an excess of energy within the nucleus. These unstable nuclei can lower their energy through nuclear transitions, which often involve the emission of radiation or particles. In this article, we will learn about the concept of nuclear transition, the mechanisms of how energy changes within a nucleus and the types of nuclear transitions that occur.

The energy levels in a nucleus are quantized, meaning that nucleons within the nucleus occupy specific energy states. When a nucleus transitions from a higher energy state to a lower energy state, it releases the excess energy in the form of electromagnetic radiation or by emitting particles. This release of energy is the hallmark of a nuclear transition, and the type of radiation or particle emitted depends on the nature of the transition.

Nuclear transitions can occur in several forms, each characterized by different processes and types of emitted particles or radiation. The most common types of nuclear transitions include:

Alpha decay occurs when a heavy nucleus emits an alpha particle, which consists of two protons and two neutrons (essentially a helium-4 nucleus). This type of nuclear transition is typically observed in very heavy elements, such as uranium, thorium, and radium. The emission of an alpha particle reduces the atomic number of the nucleus by two and its mass number by four, leading to the formation of a new element.

For example, uranium-238 undergoes alpha decay to form thorium-234, releasing an alpha particle in the process. Alpha decay is relatively slow compared to other forms of decay, but it is important in the context of radioactive dating and nuclear energy production.

Beta decay occurs when a nucleus has an imbalance between the number of protons and neutrons, leading to the conversion of a neutron into a proton or vice versa. There are two types of beta decay: beta-minus (β-) and beta-plus (β+).Beta-minus decay involves the emission of an electron (beta particle) and an antineutrino when a neutron in the nucleus transforms into a proton.

Beta-plus decay involves the emission of a positron (the electron’s antiparticle) and a neutrino when a proton in the nucleus transforms into a neutron.

An example of beta-minus decay is the transformation of carbon-14 into nitrogen-14, which is the basis for carbon dating. Beta decay plays a critical role in many natural and artificial nuclear processes, including the synthesis of elements in stars.

Gamma decay occurs when a nucleus in an excited state (high energy) transitions to a lower energy state by emitting gamma rays. Unlike alpha or beta decay, gamma decay does not involve the emission of particles that change the atomic number or mass number of the nucleus. Instead, it involves the release of high-energy photons.Gamma decay often accompanies other forms of decay.

For instance, after a nucleus undergoes alpha or beta decay, the resulting nucleus may be left in an excited state. To release the excess energy, the nucleus emits gamma radiation. Gamma rays are highly penetrating and are commonly used in medical imaging and cancer treatment.

In electron capture, an inner orbital electron is captured by the nucleus, where it combines with a proton to form a neutron and a neutrino. This process decreases the atomic number by one without changing the mass number. Electron capture is common in proton-rich nuclei and is often followed by gamma emission as the nucleus transitions to a lower energy state.

A notable example of electron capture is the decay of potassium-40 into argon-40, which is an important process used in geological dating.

Nuclear transitions are governed by quantum mechanics, which dictates that nucleons within a nucleus can only occupy specific energy levels. Transitions between these energy levels are subject to selection rules that determine whether a transition is allowed or forbidden. These rules take into account factors such as the conservation of angular momentum and parity (symmetry in space).

For example, in gamma decay, the probability of a transition occurring depends on whether the change in angular momentum between the initial and final nuclear states is compatible with the selection rules. Forbidden transitions, while not impossible, are much less likely to occur and typically result in slower decay processes.