Radioactive Decay
Section: Nuclear Physics | Syllabus: Cambridge AS Level Physics 9702
What is Radioactive Decay? Radioactive Decay the spontaneous process by which unstable atomic nuclei lose energy and matter by emitting radiation, such as alpha (α) particles, beta (β) particles, or gamma (γ) rays.
Key Characteristics of Radioactive Decay Spontaneous: Happens by itself without any external trigger or influence Random: We cannot predict exactly when a particular nucleus will decay Exam Tip Always describe radioactive decay as spontaneous and random .
These are key syllabus terms. Change of Element During α-decay or β-decay, the nucleus changes to that of a different element (because the number of protons changes). Why Nuclei Are Radioactive Isotopes of an element may be radioactive due to: 1.
Excess of Neutrons in the Nucleus Too many neutrons compared to protons makes the nucleus unstable Example: Carbon-14 has 6 protons and 8 neutrons (excess neutrons make it radioactive) 2. The Nucleus Being Too Heavy Very large nuclei (high nucleon number) are generally unstable Example: Uranium-238 and all other uranium isotopes are radioactive Example: Comparing Carbon Isotopes 12 6 C: 6 protons, 6 neutrons → Stable (balanced ratio) 14 6 C: 6 protons, 8 neutrons → Radioactive (excess neutrons) Carbon-14 decays to become more stable by converting a neutron to a proton (beta decay).
FIG 5.18: Belt of nuclear stability Graph with x-axis: proton number, y-axis: neutron number. Stable nuclei form a band. Nuclei above the band (excess neutrons) undergo beta decay. Heavy nuclei beyond Z=82 undergo alpha decay.
Alpha Decay In alpha decay, an unstable nucleus emits an alpha particle, transforming into a different element. What Happens During Alpha Decay Nucleus emits an alpha particle ( 4 2 He) Nucleon number (A) decreases by 4 (loses 2 protons + 2 neutrons) Proton number (Z) decreases by 2 (loses 2 protons) The element changes to one that is 2 places lower in the periodic table Nucleus becomes more stable Number of excess neutrons reduces General Alpha Decay Equation Example: General Alpha Decay Equation A Z X → A-4 Z-2 Y + 4 2 He Where X is the parent nucleus and Y is the daughter nucleus Example: Alpha Decay of Radium-226 Example 1: Alpha decay of Radium-226 226 88 Ra → 222 86 Rn + 4 2 He Check the equation: Mass numbers: 226 = 222 + 4 ✓ Atomic numbers: 88 = 86 + 2 ✓ Radium (Z=88) changes to Radon (Z=86) Example: Alpha Decay of Polonium-210 Example 2: Alpha decay of Polonium-210 210 84 Po → 206 82 Pb + 4 2 He Analysis: Polonium-210 (84 protons) → Lead-206 (82 protons) Nucleon number: 210 → 206 (decrease of 4) Proton number: 84 → 82 (decrease of 2) FIG 5.19: Alpha decay visualization Before and after: Left shows large nucleus labeled with A, Z.
Arrow showing alpha particle (2p+2n) leaving. Right shows smaller nucleus labeled A-4, Z-2. Conservation of nucleons shown visually. Remember In alpha decay: A decreases by 4, Z decreases by 2. The element changes to a new element.
Beta Decay In beta decay, a neutron in the nucleus transforms into a proton, emitting a beta particle (electron). What Happens During Beta Decay A neutron converts into a proton inside the nucleus An electron (beta particle) is emitted from the nucleus Nucleon number (A) stays the same Proton number (Z) increases by 1 The element changes to a different element Nucleus becomes more stable Number of excess neutrons reduces The Change in the Nucleus During β-Emission Beta Decay Process neutron → proton + electron Exam Tip The syllabus requires you to know this exact change: neutron → proton + electron .
This explains why Z increases by 1 while A stays the same. FIG 5.20: Beta decay process Neutron decay inside nucleus: neutron (grey) transforming, proton (red) remains in nucleus, electron (beta particle) shoots out.
Each particle labeled with mass and charge. General Beta Decay Equation Example: General Beta Decay Equation A Z X → A Z+1 Y + 0 -1 e Where X is the parent nucleus and Y is the daughter nucleus Example: Beta Decay of Carbon-14 Example 3: Beta decay of Carbon-14 14 6 C → 14 7 N + 0 -1 e Check the equation: Mass numbers: 14 = 14 + 0 ✓ Atomic numbers: 6 = 7 + (-1) ✓ Carbon (Z=6) changes to Nitrogen (Z=7) One neutron converted to one proton Example: Beta Decay of Strontium-90 Example 4: Beta decay of Strontium-90 90 38 Sr → 90 39 Y + 0 -1 e Analysis: Strontium-90 (38 protons) → Yttrium-90 (39 protons) Nucleon number: 90 → 90 (stays the same) Proton number: 38 → 39 (increase of 1) Number of neutrons: 52 → 51 (decrease of 1) Remember In beta decay: A stays the same, Z increases by 1.
The element changes to the next element in the periodic table. Gamma Emission Gamma radiation is often emitted after alpha or beta decay when the nucleus has excess energy. What Happens During Gamma Emission Nucleus releases excess energy as a gamma ray photon No change in nucleon number (A) No change in proton number (Z) The element stays the same Nucleus becomes more stable (lower energy state) Often follows alpha or beta emission Effects of Decay on Sta…
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