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2.6.1Forces, Space and Radioactivity

Modelling Radioactive Decay — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

5 min read 10 parts Lesson 33 of 37

📚 WJEC GCSE Science (Double Award) - Physics Hub / Topical Revision
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Part 1 of 8Random and spontaneous decay

1Radioactive decay
Radioactive decay the emission of radiation from an unstable nucleus
2Random process
  • Random process the nucleus that decays next, and when, cannot be known
  • Each nucleus has an equal probability of decaying.
  • Only the probability of decay in a given time can be estimated.
3Spontaneous process
  • Spontaneous process the rate of decay is unaffected by the surrounding conditions

Part 2 of 8Unstable nuclei

Cause of instability
  • Cause of instability an imbalance of the forces in the nucleus
  • The nucleus has too many protons or neutrons.
  • The nucleus is very large.
Neutron-proton balance
  • Neutron-proton balance the most stable light nuclei have about equal numbers of protons and neutrons
  • Heavier nuclei need more neutrons than protons to reduce the repulsion between protons.
  • Too many or too few neutrons makes the nucleus unstable.
Examples
  • Carbon-14 is unstable because it has two extra neutrons compared with carbon-12.
  • Hydrogen-3 (tritium) is much less stable than hydrogen-1 or hydrogen-2.
Energy
  • Energy radiation takes energy away from the nucleus, which makes the nucleus more stable

Part 3 of 8Alpha decay

Alpha decay
  • Alpha decay an alpha particle is emitted, usually from a large unstable nucleus
  • The nucleon number decreases by 4.
  • The proton number decreases by 2.
  • A new element forms.
Equation form
  • AZX → A-4Z-2Y + 42α
Equation for polonium
  • 21284Po → 20882Pb + 42α

Part 4 of 8Beta decay

Beta decay
Beta decay
  • Beta decay a neutron changes into a proton and an electron, and the electron is emitted
  • It often happens in nuclei with too many neutrons.
  • The nucleon number stays the same.
  • The proton number increases by 1.
  • A new element forms.
Beta particle numbers
  • Beta particle numbers its nucleon number is 0 and its proton number is -1
  • Total proton number is therefore conserved.
Equation form
  • AZX → AZ+1Y + 0-1β

Part 5 of 8Gamma decay

Gamma decay
  • Gamma decay a gamma ray is emitted, usually after alpha or beta decay
  • It reduces the energy of the nucleus but does not change its structure.
  • The nucleon number and proton number are unchanged.
Equation form
  • AZX → AZX + 00γ

Part 6 of 8Balancing decay equations

1
Write the parent nuclide before the arrow and the products after it.
2
Make the total nucleon number equal on both sides.
3
Make the total proton number equal on both sides.
4
Identify the daughter nucleus from its proton number.
  • Parent and daughter nuclei
    • the parent is the original nucleus and the daughter is the new element
    • The daughter has a different proton number, so it is a different element.

Part 7 of 8Decay chains

Decay series
  • Decay series a heavy nucleus such as uranium-238 may undergo several alpha and beta decays
  • The chain ends when a stable nucleus is reached.
Names of elements
  • Names of elements you do not need to know them, only to balance the numbers

Part 8 of 8More from the specification

More from the specification
Charge discrimination in transverse magnetic fields
  • Charge discrimination in transverse magnetic fields opposite curvature of alpha and beta tracks in magnetic fields provides a definitive means of identification.
Radioactive decay and half-life
  • The half-life of a radioactive isotope is the time taken for half the unstable atomic nuclei in a sample to decay,
  • or for its activity count rate to halve.
Background radiation and corrected count rate
  • Radiation from natural and artificial environmental sources constitutes background radiation,
  • which must be subtracted from total measured detector count to find source activity.
Ionising radiation and nuclear charge
  • Alpha particles (charge +2), beta particles (charge -1),
  • and gamma rays differ in ionising strength and penetrating ability depending on their atomic mass and charge.
Decay law and activity curves
  • Plotting count rate of a radioactive source against time produces an exponential decay curve characteristic of the specific isotope under investigation.
Radioactive decay and half-life
  • The half-life of a radioactive isotope is the time taken for half the unstable atomic nuclei in a sample to decay,
  • or for its activity count rate to halve.
Background radiation and corrected count rate
  • Radiation from natural and artificial environmental sources constitutes background radiation,
  • which must be subtracted from total measured detector count to find source activity.
Ionising radiation and nuclear charge
  • Alpha particles (charge +2), beta particles (charge -1),
  • and gamma rays differ in ionising strength and penetrating ability depending on their atomic mass and charge.
Decay law and activity curves
  • Plotting count rate of a radioactive source against time produces an exponential decay curve characteristic of the specific isotope under investigation.
Deflected
  • Having its direction changed.
Background radiation
  • The radiation that exists around us all the time from natural and man-made sources.
Penetrating power
  • How far radiation travels through a material before it is stopped.
Electric field
  • A region where a charge experiences a force.
Magnetic field
  • Area around a magnet where its effects can be detected.
Kinetic energy
  • Energy stored in a moving object.
Half-life
  • The time taken for half the nuclei of an isotope in a sample to decay.
Source
  • A part of the plant where substances are made or released.
Charge
  • A quantity measured in coulombs that is moved round a circuit.
Isotopes
  • Atoms of the same element with the same number of protons but different numbers of neutrons.

Key terms

TermMeaning
Radioactive decayThe spontaneous and random emission of radiation from an unstable nucleus.
Parent nucleusThe original unstable nucleus before decay.
Daughter nucleusThe new nucleus formed after decay.
Alpha decayEmission of an alpha particle, reducing the nucleon number by 4 and the proton number by 2.
Beta decayA neutron changes into a proton and an electron, raising the proton number by 1.
Gamma decayEmission of a gamma ray that lowers the energy of the nucleus but not its composition.
SpontaneousNot triggered by outside factors.

Avoid these mistakes

1
Saying beta decay changes the nucleon number — it stays the same and only the proton number rises by 1
2
Saying decay can be predicted for a single nucleus — it is random
3
Thinking temperature or pressure change the decay rate — decay is spontaneous and unaffected by conditions
4
Writing the beta particle as having a proton number of +1 — it is -1

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