What determines whether a nucleus undergoes alpha, beta, or gamma decay?

The type of decay depends on the nucleus's instability and its position relative to the 'band of stability' on the N-Z plot. Nuclei that are too heavy (Z > 83) or have too many protons relative to neutrons tend to undergo alpha decay to reduce their mass and proton count. Nuclei with an excess of neutrons (high N/Z ratio) undergo beta-minus decay, converting a neutron to a proton. Nuclei with an excess of protons (low N/Z ratio) undergo beta-plus decay or electron capture, converting a proton to a neutron. Gamma decay typically occurs when a nucleus is in an excited state after another decay, releasing excess energy without changing its composition.

Why are neutrinos and antineutrinos emitted during beta decay?

Neutrinos and antineutrinos are emitted during beta decay to conserve fundamental physical quantities: energy, linear momentum, and angular momentum (spin). Early experiments showed that beta particles were emitted with a continuous spectrum of kinetic energies, which seemed to violate energy conservation. Wolfgang Pauli hypothesized the existence of a neutral, nearly massless particle (the neutrino/antineutrino) that carries away the 'missing' energy and momentum, ensuring that the total energy and momentum of the system remain conserved. This particle was later experimentally confirmed.

How do alpha, beta, and gamma radiations interact with electric and magnetic fields?

Alpha particles, being positively charged (+2e), are deflected by both electric and magnetic fields. The direction of deflection follows the right-hand rule for positive charges. Beta particles (electrons or positrons) are also charged (-e or +e, respectively) and are deflected, but in the opposite direction to alpha particles (for electrons) or in the same direction but with a larger curvature due to their much smaller mass and higher velocity. Gamma rays, being uncharged photons, are not affected by electric or magnetic fields and pass through them undeflected.

Can a nucleus undergo multiple types of decay simultaneously?

A single nucleus typically undergoes one specific type of decay at a time. However, a radioactive isotope can have multiple decay modes, meaning different nuclei of the same isotope might decay via different pathways (e.g., some undergoing beta-minus, others electron capture). Also, a decay chain often involves a sequence of different decay types. For instance, a heavy nucleus might first undergo alpha decay, and the resulting daughter nucleus might then undergo beta decay, followed by gamma decay to reach a stable state. So, while a single event is one type, a series of events can involve multiple types.

What is the difference between an electron emitted in beta decay and an electron from an atomic shell?

The key difference lies in their origin and energy spectrum. An electron emitted in beta decay (a beta particle) is created *within* the nucleus when a neutron transforms into a proton. It is a product of the weak nuclear force. These beta particles are emitted with a continuous spectrum of kinetic energies. An electron from an atomic shell, on the other hand, is one of the electrons orbiting the nucleus. If it's ejected, it's typically due to processes like ionization, photoelectric effect, or internal conversion. In internal conversion, the ejected electron has a discrete, specific kinetic energy, unlike the continuous spectrum of beta decay electrons.

Why is gamma decay often associated with alpha or beta decay?

Gamma decay is frequently associated with alpha or beta decay because these particle emissions often leave the daughter nucleus in an excited, unstable energy state. Imagine a nucleus as having discrete energy levels, similar to electron shells in an atom. When an alpha or beta particle is emitted, the nucleus might not settle directly into its lowest possible energy state (ground state). Instead, it might be left in a higher-energy, 'excited' state. To transition from this excited state to the more stable ground state, the nucleus releases the excess energy by emitting a gamma-ray photon. Thus, gamma decay acts as a 'de-excitation' process following a primary particle decay.

Alpha, Beta, Gamma Decay

Physics

NEET UG

Version 1Updated 23 Mar 2026

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Radioactive decay is a spontaneous process by which an unstable atomic nucleus loses energy by emitting radiation, transforming into a different nucleus or a lower energy state of the same nucleus. This process is governed by the fundamental forces of nature and occurs without external influence. The three primary modes of radioactive decay are alpha ( $\alpha$ ) decay, beta ( $\beta$ ) decay, and gam…

Quick Summary

Radioactive decay is the spontaneous process by which unstable atomic nuclei transform into more stable forms by emitting radiation. The three primary types are alpha, beta, and gamma decay, each with distinct characteristics and effects on the nucleus.

Alpha decay involves the emission of a helium nucleus ( $^4_2\text{He}$ ), reducing the atomic number (Z) by 2 and mass number (A) by 4. Beta decay involves the transformation of a nucleon: beta-minus ( $\beta^-$ ) decay sees a neutron convert to a proton, emitting an electron ( $e^-$ ) and an antineutrino, increasing Z by 1 while A remains constant.

Beta-plus ( $\beta^+$ ) decay involves a proton converting to a neutron, emitting a positron ( $e^+$ ) and a neutrino, decreasing Z by 1 while A remains constant. Electron capture is an alternative to beta-plus, where an inner electron is captured, converting a proton to a neutron, also decreasing Z by 1.

Gamma decay involves the emission of high-energy photons (gamma rays) from an excited nucleus, without changing A or Z, simply lowering the nucleus's energy state. These decays adhere to conservation laws of mass number, atomic number (charge), energy, and momentum.

The emitted radiations have varying ionizing and penetrating powers, crucial for their applications and hazards.

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Key Concepts

Alpha Decay Equation and Changes

Alpha decay involves the emission of an alpha particle ( $^4_2\text{He}$ ). This means the parent nucleus loses…

Beta-minus Decay Equation and Changes

Beta-minus decay occurs when a neutron in the nucleus transforms into a proton, emitting an electron ( $e^-$ …

Beta-plus Decay Equation and Changes

Beta-plus decay involves a proton in the nucleus transforming into a neutron, emitting a positron ( $e^+$ or…

Gamma Decay and Nuclear De-excitation

Gamma decay is a process where an excited nucleus releases its excess energy in the form of a high-energy…

Alpha Decay —

^A_Z\text{X} \rightarrow ^{A-4}_{Z-2}\text{Y} + ^4_2\text{He}

. A decreases by 4, Z decreases by 2. High ionizing, low penetrating. Positively charged.

Beta-minus Decay —

^A_Z\text{X} \rightarrow ^A_{Z+1}\text{Y} + e^- + \bar{\nu}_e

. A unchanged, Z increases by 1. Moderate ionizing, moderate penetrating. Negatively charged.

Beta-plus Decay —

^A_Z\text{X} \rightarrow ^A_{Z-1}\text{Y} + e^+ + \nu_e

. A unchanged, Z decreases by 1. Moderate ionizing, moderate penetrating. Positively charged.

Electron Capture —

^A_Z\text{X} + e^- \rightarrow ^A_{Z-1}\text{Y} + \nu_e

. A unchanged, Z decreases by 1. No particle emission, only X-rays/Auger electrons.

Gamma Decay —

^A_Z\text{X}^* \rightarrow ^A_Z\text{X} + \gamma

. A unchanged, Z unchanged. Low ionizing, high penetrating. No charge, no mass (photon). De-excitation process.

Conservation Laws — A, Z (charge), energy, momentum are conserved in all decays.

Alpha: A-4, Z-2. (Heavy, Helium nucleus) Beta-minus: A-same, Z+1. (Neutron to Proton, Electron) Beta-plus: A-same, Z-1. (Proton to Neutron, Positron) Gamma: A-same, Z-same. (Energy release, Photon)

Penetrating Power Order: Alpha < Beta < Gamma (APBG) Ionizing Power Order: Alpha > Beta > Gamma (AIBG)