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Theme E — Nuclear and Quantum Physics

E.3.13 Evidence for the Neutrino — IB Physics HL Revision Notes & Topical Questions

5 min read 9 parts Lesson 30 of 45 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

E.3.13 Evidence for the Neutrino — IB PHYSICS HL notes: Explain how the continuous beta energy spectrum and conservation laws provided evidence for the neutrino.Neutrino — a subatomic particle with no charge and negligible mass emitted from the nucleusAntineutrino — the antiparti…

  • Production
  • Interaction
  • Alpha and gamma
  • Conclusion
  • Missing energy
  • Hypothesis

Part 1 of 7Neutrinos and antineutrinos

Neutrino
  • Neutrino a subatomic particle with no charge and negligible mass emitted from the nucleus
Antineutrino
  • Antineutrino the antiparticle of the neutrino
Production
  • Production electron antineutrinos accompany β− decay and electron neutrinos accompany β+ decay
Interaction
  • Interaction it interacts only through the weak force, so it passes through matter almost without interacting

Part 2 of 7The puzzle of beta decay

Alpha and gamma
  • Alpha and gamma particles come out with sharp, well-defined energies, giving spikes on a number–energy graph
Beta
  • Beta the number of particles against kinetic energy gives a continuous curve from almost zero up to a maximum
Conclusion
  • Conclusion beta particles have a continuous range of energies
    • (electrons or positrons)
Missing energy
  • Missing energy with only two bodies the electron would have a single energy

Part 3 of 7Pauli's proposal

1930
  • Pauli suggested a third, very light, uncharged particle is emitted with the electron and shares the decay energy
Fermi
  • Fermi later named it the neutrino and built a theory of beta decay
Hypothesis
  • Hypothesis it accounts for the conservation of energy in beta decay
Sharing
  • Sharing the energy is shared between the beta particle and the neutrino or antineutrino

Part 4 of 7Conservation laws that need it

Energy
  • Energy the energy not given to the electron is carried by the neutrino, explaining the continuous spectrum
Momentum
  • Momentum in cloud-chamber photographs the electron and recoiling nucleus do not move in exactly opposite directions
  • Momentum is conserved only if a third particle carries some away
Spin
  • Spin a neutron, proton and electron all have spin ½
  • n → p + e− alone cannot conserve spin, and an antineutrino with spin ½ restores it
Lepton number
  • Lepton number the electron (+1) is balanced by an antineutrino (−1) in β− decay

Part 5 of 7Detection

1First detected
  • 1956 by Cowan and Reines near a nuclear reactor
2Why hard
  • Why hard it has no charge, almost no mass and interacts only weakly
3Prediction confirmed
  • Prediction confirmed the hypothesis made a testable prediction and its discovery confirmed it

Part 6 of 7Why it is good evidence

One idea
  • One idea the neutrino explains the energy spectrum, the momentum balance and the spin
Testable
  • Testable it predicted a particle with certain properties that could be found
Q value
  • Q value the decay energy equals the mass difference times c2, shared randomly between electron, antineutrino and the recoiling nucleus

Part 7 of 7More from the specification

More from the specification
Neutrino hypothesis verification
  • Neutrino hypothesis verification identifying continuous beta emission spectra provided a compelling theoretical means to deduce undetected neutral neutrinos.
Continuous Beta Spectrum and Prediction of the Neutrino
  • In beta decay, emitted beta particles exhibit a continuous energy spectrum up to a maximum endpoint energy;
    • to satisfy conservation of energy and linear momentum, Wolfgang Pauli postulated the emission of a neutral, virtually massless particle called the antineutrino (ν̄_e), represented in the nuclear decay equation: n → p + e⁻ + ν̄_e.
Radioactive Activity and Exponential Decay Constant
  • The activity A of a radioactive sample is the rate of decay of unstable parent nuclei,
    • (A = -dN/dt = λ·N)
  • where λ is the decay constant (s⁻¹); sample activity and undecayed nuclei decrease exponentially over time according to N = N₀·e^(-λ·t).
Half-Life and Nuclear Binding Energy
  • The half-life t₁/₂ is related to the decay constant by t₁/₂ = ln 2 / λ; for long-lived radioisotopes, radioactive decay transforms nuclei toward states of higher nuclear binding energy per nucleon.
    • (such as uranium-238 with a half-life of 4.5 billion years)
Binding energy
  • The minimum energy required to separate the nucleons of a nucleus to infinity.
Background radiation
  • Radiation that is always present in the environment.
Decay equation
  • An equation in nuclide notation with balanced nucleon and proton numbers.
Ionising radiation
  • Radiation with enough energy to remove electrons from atoms
Penetrating power
  • How far radiation travels through a material.
Radiation
  • Transfer by electromagnetic waves, requiring no medium
Radioactive
  • Describes a nucleus that decays spontaneously and randomly.
Activity
  • Number of decays per second of a sample.
Half-life
  • Time for half the nuclei of an isotope to decay.
Constant
  • Not changing.
Isotope
  • Atom of the same element with a different number of neutrons.
Isotopes
  • Nuclei with the same proton number but different neutron numbers.
Source
  • Reservoir at TH providing heat QH

Key terms

TermMeaning
NeutrinoA neutral, almost massless particle emitted in β+ decay.
AntineutrinoThe antiparticle of the neutrino, emitted in β− decay.
Continuous beta spectrumThe range of beta kinetic energies from zero to a maximum.
Weak interactionThe force through which neutrinos interact with matter.
Lepton numberA conserved quantity that is +1 for electrons and neutrinos and −1 for their antiparticles.

Avoid these mistakes

1
Saying the neutrino was found because beta energies were discrete — they are continuous.
2
Mixing up neutrino with antineutrino — β− decay emits an antineutrino.
3
Thinking the neutrino carries electric charge — it is neutral.
4
Believing neutrinos are easy to detect — they interact only weakly.
5
Forgetting the neutrino also conserves momentum and spin, not only energy.

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