Neutrino particle is emitted in the process of ___________.

Question

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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.

Why the other options are wrong — (a) 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.  (c) 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.  (d) 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.

Concept: 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.

State PCS 2025 Q55 — Science & Technology PYQ with Answer

State PCS 2025 Q55 — Science & Technology PYQ with Answer