Types of Radioactive Decay

Why This Matters

Radioactive decay isn't just about memorizing particles and numbers—it's about understanding why unstable nuclei transform and how they release energy in the process. You're being tested on the fundamental mechanisms that govern nuclear stability: the balance between protons and neutrons, the role of the strong nuclear force, and how nuclei shed excess energy or particles to reach more favorable configurations. These concepts underpin everything from medical imaging to reactor design to radiation safety protocols.

The decay types you'll encounter fall into distinct categories based on what the nucleus needs to become stable—whether that's reducing its overall size, adjusting its neutron-to-proton ratio, or simply releasing excess energy. Don't just memorize that alpha decay emits a helium nucleus; know that it's the primary mechanism for heavy nuclei to reduce their mass. Connect beta decay to neutron-proton imbalance, and recognize gamma emission as pure energy dissipation. This conceptual framework will serve you far better on exams than rote facts alone.

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Particle Emission: Reducing Nuclear Mass

When nuclei are simply too large to remain stable, they eject massive particles to shrink toward the "valley of stability." The strong nuclear force has limited range, so very heavy nuclei struggle to hold themselves together against Coulomb repulsion.

Alpha Decay

• Emits a helium-4 nucleus (2 protons + 2 neutrons)—the most tightly bound light nucleus, making it energetically favorable to eject as a single unit
• Reduces atomic number by 2 and mass number by 4—written as $^A_Z X \rightarrow ^{A-4}_{Z-2} Y + ^4_2 \alpha$
• Dominant in heavy elements ($Z > 82$) like uranium and radium; low penetration power means alpha emitters are primarily an internal radiation hazard if ingested or inhaled

Proton Emission

• Direct ejection of a single proton—decreases both atomic number and mass number by 1
• Occurs in extremely proton-rich nuclei far from stability, typically produced artificially in accelerators
• Requires overcoming the Coulomb barrier—making this decay mode relatively rare and observable mainly in nuclei at the proton drip line

Neutron Emission

• Release of one or more neutrons—decreases mass number while atomic number stays constant
• Common in fission products and nuclei beyond the neutron drip line—these nuclei have extreme neutron excess
• Neutrons are uncharged and highly penetrating—requiring hydrogen-rich materials (water, polyethylene) for effective shielding through moderation

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Beta Processes: Correcting Neutron-Proton Ratio

These decay modes don't change mass number—they convert one nucleon type to another via the weak nuclear force. The nucleus is adjusting its internal composition to reach the optimal N/Z ratio for its mass.

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

• Neutron converts to proton, emitting an electron and antineutrino—governed by $n \rightarrow p + e^- + \bar{\nu}_e$
• Atomic number increases by 1, mass number unchanged—occurs in neutron-rich isotopes like $^{14}C$ and fission products
• Beta particles have moderate penetration—stopped by a few millimeters of aluminum or plastic; continuous energy spectrum due to three-body decay

Positron Emission ($\beta^+$)

• Proton converts to neutron, emitting a positron and neutrino—requires the nucleus to supply 1.022 MeV (twice electron rest mass) for the process to occur
• Atomic number decreases by 1, mass number unchanged—occurs in proton-rich isotopes like $^{18}F$ used in PET imaging
• Positrons annihilate with electrons—producing two 511 keV gamma rays traveling in opposite directions, the basis for positron emission tomography

Electron Capture

• Inner orbital electron merges with a proton to form a neutron—competes with $\beta^+$ decay but requires no threshold energy
• Atomic number decreases by 1, mass number unchanged—same nuclear transformation as positron emission, written as $p + e^- \rightarrow n + \nu_e$
• Produces characteristic X-rays—as outer electrons fill the vacancy in the inner shell; often the only decay mode available when $\beta^+$ is energetically forbidden

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Energy Dissipation: Electromagnetic Transitions

Sometimes the nucleus has the right number of protons and neutrons but is stuck in an excited state. These modes release energy without changing nuclear composition.

Gamma Decay

• Emission of high-energy photons from excited nuclear states—no change to atomic number or mass number ($^A_Z X^* \rightarrow ^A_Z X + \gamma$)
• Typically follows other decay modes—the daughter nucleus often forms in an excited state and promptly emits gamma rays to reach ground state
• Extremely penetrating radiation—requires dense shielding (lead, concrete); gamma energies are discrete and characteristic of specific nuclear transitions

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Nuclear Fragmentation: Splitting the Nucleus

For the heaviest nuclei, incremental particle emission isn't enough—the entire nucleus can split apart. This occurs when Coulomb repulsion finally overwhelms the strong force.

Spontaneous Fission

• Heavy nucleus splits into two medium-mass fragments plus neutrons—releases approximately 200 MeV per fission event
• Occurs primarily in transuranic elements—$^{252}Cf$ is a common spontaneous fission source; competes with alpha decay in heavy actinides
• Emitted neutrons can trigger chain reactions—the foundation of both nuclear reactors and weapons; criticality depends on neutron multiplication factor

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Quick Reference Table

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Self-Check Questions

1. Which two decay modes both decrease the atomic number by 1 without changing mass number, and what determines which one occurs in a given isotope?

2. A nucleus undergoes decay and emits two 511 keV photons traveling in opposite directions. What decay process occurred, and why are the photons produced with this specific energy?

3. Compare alpha decay and spontaneous fission: both occur in heavy nuclei, so what nuclear property determines which mode dominates for a given isotope?

4. An FRQ describes a neutron-rich fission product that undergoes decay, increasing its atomic number. Identify the decay mode and explain the underlying weak interaction process.

5. Why does electron capture produce X-rays while gamma decay produces gamma rays, even though both involve photon emission? What does this distinction reveal about the origin of each radiation type?