Nuclear Decay Types

Why This Matters

Nuclear decay is the foundation of radiochemistry—it explains how unstable nuclei shed excess energy to reach more stable configurations. You're being tested on your ability to identify decay modes, predict products, write balanced nuclear equations, and understand why certain isotopes favor specific decay pathways. The key principles here are nucleon stability, energy release mechanisms, mass-energy equivalence, and penetration characteristics.

Don't just memorize which particle comes out of which decay. Know why a nucleus chooses a particular decay path (neutron-to-proton ratio, binding energy, nuclear shell effects) and how each decay type changes the nucleus. When an FRQ asks you to predict decay products or explain shielding requirements, you need to connect particle properties to real-world applications in medicine, energy, and detection.

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Particle Emission Decays

These decay modes involve the nucleus ejecting a particle to reduce instability. The type of particle emitted depends on whether the nucleus has excess mass, too many protons, or too many neutrons.

Alpha Decay

• Emits an alpha particle ($^4_2\text{He}$)—two protons and two neutrons bound together, reducing atomic number by 2 and mass number by 4
• Favored by heavy nuclei ($Z > 82$) like uranium and radium where the strong force can't overcome proton-proton repulsion across large nuclear distances
• Lowest penetration power of radioactive emissions—stopped by paper or skin, but extremely damaging if inhaled or ingested

Beta-Minus Decay ($\beta^-$)

• Converts a neutron to a proton—emitting an electron and antineutrino, increasing atomic number by 1 while mass number stays constant
• Occurs in neutron-rich nuclei where the neutron-to-proton ratio is too high for stability
• Moderate penetration power—requires plastic, glass, or aluminum for shielding; detected via Geiger counters or liquid scintillation

Beta-Plus Decay ($\beta^+$)

• Converts a proton to a neutron—emitting a positron and neutrino, decreasing atomic number by 1 with no mass number change
• Requires proton-rich nuclei with enough excess mass-energy (at least $1.022 \text{ MeV}$) to create the positron
• Critical for PET imaging—positrons annihilate with electrons, producing two 511 keV gamma rays used for medical diagnostics

Proton Emission

• Direct ejection of a proton from extremely proton-rich nuclei—decreases both atomic number and mass number by 1
• Rare decay mode occurring only when nuclei lie far beyond the proton drip line and lack sufficient energy for $\beta^+$ decay
• Competes with electron capture in proton-rich isotopes but requires nuclei with very low proton binding energy

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Energy Release Without Particle Emission

Some decay processes release energy through electromagnetic radiation or nuclear rearrangement rather than ejecting massive particles.

Gamma Decay

• Emits high-energy photons ($\gamma$ rays) as the nucleus transitions from an excited state to ground state—no change in atomic or mass number
• Follows other decay types—alpha or beta decay often leaves the daughter nucleus in an excited state that then emits gamma radiation
• Highest penetration power—requires lead, concrete, or several centimeters of dense material for effective shielding

Electron Capture

• Inner-shell electron absorbed by nucleus—combines with a proton to form a neutron, decreasing atomic number by 1 with constant mass number
• Alternative to $\beta^+$ decay for proton-rich nuclei, especially when decay energy is below the 1.022 MeV threshold needed for positron creation
• Produces characteristic X-rays as outer electrons fill the vacancy left by the captured electron—useful for detection and identification

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Nuclear Fragmentation Processes

These decay modes involve dramatic changes to nuclear structure, either splitting the nucleus or ejecting nucleons directly.

Spontaneous Fission

• Heavy nucleus splits into two lighter fragments—releases multiple neutrons plus enormous energy (approximately 200 MeV per fission event)
• Occurs without external trigger in very heavy elements like $^{252}\text{Cf}$ and $^{238}\text{U}$, though half-lives for this process are typically long
• Enables chain reactions—released neutrons can induce fission in neighboring nuclei, fundamental to reactors and weapons

Neutron Emission

• Direct release of neutrons from extremely neutron-rich or highly excited nuclei—decreases mass number without changing atomic number
• Often follows fission or nuclear reactions rather than occurring as a primary decay mode in ground-state nuclei
• Neutrons have high penetration power—uncharged particles interact only via strong force, requiring hydrogen-rich materials (water, paraffin) for moderation

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Quick Reference Table

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Self-Check Questions

1. A nucleus has a neutron-to-proton ratio significantly higher than stable isotopes of that element. Which two decay modes could restore stability, and how do their products differ?

2. Compare and contrast alpha decay and spontaneous fission: what nuclear property makes both more likely in heavy elements, and how do their energy releases compare?

3. An FRQ describes a proton-rich isotope with decay energy of 0.8 MeV. Why would this nucleus undergo electron capture rather than $\beta^+$ decay?

4. Which decay types change the atomic number without changing the mass number? For each, explain whether the atomic number increases or decreases.

5. You're designing shielding for a source that emits alpha particles, beta particles, and gamma rays. Rank these by penetration power and identify appropriate shielding materials for each.