MPSC 2025 Prelims Paper-I · Question 55 of 100

Correct — option (2), beta-decay. In beta-minus decay, a neutron inside an unstable nucleus transforms into a proton while emitting an electron (the beta particle) and an antineutrino; in beta-plus decay, a proton transforms into a neutron while emitting a positron and a neutrino. Wolfgang Pauli first proposed the neutrino's existence in 1930 specifically to resolve a puzzle in beta decay: the emitted electrons were observed to carry a continuous range of energies rather than a single fixed value, which appeared to violate conservation of energy and momentum unless a third, then-undetected particle also carried away some of the energy and momentum in every decay event. Enrico Fermi built this into his 1934 theory of beta decay and named the particle the neutrino ('little neutral one'); it was experimentally confirmed by Clyde Cowan and Frederick Reines in 1956. Because the neutrino (or antineutrino) is intrinsic to how beta decay balances energy, momentum and lepton number, its emission is a defining feature of beta decay specifically, not of the other decay or radiation processes listed.

• (1)α-decay — Alpha decay is the emission of an alpha particle — a helium-4 nucleus, made of two protons and two neutrons bound together — from a heavy unstable nucleus, reducing its mass number by 4 and its atomic number by 2. It is a two-body process (parent nucleus splitting into a daughter nucleus and the alpha particle) that conserves energy and momentum without needing any additional emitted particle, so no neutrino is involved.
• (3)γ-decay — Gamma decay is the emission of a high-energy photon (gamma ray) as a nucleus drops from an excited energy state to a lower one, typically right after an alpha or beta decay has already changed the nucleus's proton or neutron count. It changes neither the mass number nor the atomic number of the nucleus and involves only the emission of electromagnetic radiation, not any neutrino.
• (4)X-ray — X-rays are electromagnetic radiation, typically produced outside the nucleus by decelerating electrons (bremsstrahlung) or by electrons dropping between atomic energy levels to fill an inner-shell vacancy (characteristic X-rays) — an atomic-electron process, not a nuclear-decay process at all, and it does not involve neutrino emission.

Beta decay is one of the three classical types of radioactivity (alongside alpha and gamma decay) and comes in two forms: beta-minus decay, in which a neutron converts to a proton, an electron, and an antineutrino (n to p + e- + antineutrino), and beta-plus decay, in which a proton converts to a neutron, a positron, and a neutrino (p to n + e+ + neutrino). The neutrino (or antineutrino) is essential to the process because it is what allows beta decay to conserve energy, linear momentum and lepton number simultaneously — without it, the continuous energy spectrum observed for the emitted electrons would be unexplainable within known conservation laws, which is exactly the puzzle that led Pauli to propose the particle's existence in the first place.

MPSC's physics section frequently tests the specific distinguishing feature of each type of radioactive decay — what is emitted, and what changes (or does not change) in the nucleus's mass number and atomic number — since this is the standard way nuclear-physics recall questions are framed. Knowing that the neutrino is specifically a beta-decay signature, arising from the historical puzzle of continuous beta-electron energy spectra, is the precise fact this question rewards.

• Beta-minus decay: a neutron converts to a proton, emitting an electron and an antineutrino; beta-plus decay: a proton converts to a neutron, emitting a positron and a neutrino.
• Wolfgang Pauli proposed the neutrino's existence in 1930 to explain the continuous energy spectrum of beta-decay electrons without violating conservation of energy and momentum.
• Enrico Fermi formalised the theory of beta decay in 1934 and named the particle the neutrino; it was experimentally detected by Cowan and Reines in 1956.
• Alpha decay emits a helium-4 nucleus, gamma decay emits a photon, and neither involves neutrino emission; only beta decay does.

• Assuming neutrino emission accompanies all radioactive decay, rather than being specific to beta decay
• Confusing gamma decay (a photon-emission process that changes neither mass number nor atomic number) with beta decay (a particle-transformation process that changes atomic number)
• Treating X-rays as a nuclear radioactivity phenomenon rather than an atomic-electron phenomenon

MPSC's science section frequently asks which particle or radiation is characteristic of a specific type of radioactive decay, testing precise recall of what changes in the nucleus and what is emitted for alpha, beta and gamma decay individually.

No directly related past PYQ was found.

• practice — not a real PYQ

  Who first proposed the existence of the neutrino, to explain the continuous energy spectrum observed in beta decay ?

  • (a)Enrico Fermi
  • (b)Wolfgang Pauli
  • (c)Ernest Rutherford
  • (d)James Chadwick

  Answer(b) Wolfgang Pauli — he proposed the neutrino in 1930 to preserve conservation of energy and momentum in beta decay; Fermi later formalised the theory and named the particle in 1934.

• practice — not a real PYQ

  Which type of radioactive decay changes a nucleus's excited energy state without changing either its mass number or its atomic number ?

  • (a)Alpha decay
  • (b)Beta-minus decay
  • (c)Beta-plus decay
  • (d)Gamma decay

  Answer(d) Gamma decay — it is the emission of a photon as an already-formed nucleus drops from an excited state to a lower energy state, leaving its proton and neutron count unchanged.