. Isotopes have the same proton number but different neutron numbers.
An unstable nucleus can emit ionising radiation in a random process. Random means the exact decay time of one nucleus cannot be predicted, though a large sample follows a predictable statistical pattern.
Alpha, beta and gamma
An alpha particle is a helium nucleus with two protons and two neutrons. It is strongly ionising but has low penetration, stopped by paper or a short distance in air.
A beta-minus particle is a fast electron emitted from a nucleus. It has moderate ionising and penetrating ability and is stopped by a few millimetres of aluminium.
Gamma is high-frequency electromagnetic radiation. It is weakly ionising compared with alpha but highly penetrating, reduced by thick lead or concrete.
No radiation is made completely harmless by saying a material “stops” it. Shielding reduces exposure, and suitable thickness depends on type and intensity.
A penetration investigation records count rate with no absorber and with paper, aluminium and lead of controlled geometry. Background count should be measured and considered. A simulation can test the same comparison without handling a physical source.
Background, half-life and uses
Background ionising radiation comes from rocks and soil, radon gas, cosmic rays from space, living material, medical exposure and human-made residues. A detector reading therefore includes source plus background.
Half-life is the time for the number of undecayed nuclei, or the activity or count rate due to the source, to fall to half its value. Different isotopes have different half-lives. After n half-lives, the remaining fraction is (1/2)n. Random fluctuations mean individual count readings may not lie exactly on a smooth curve.
Medical uses include tracers, imaging and treatment. Industrial uses include thickness monitoring and tracers for leaks. Choice depends on penetration, ionising ability and half-life. A tracer should last long enough for measurement but not remain active unnecessarily; a treatment source must deliver a controlled dose to the target.
Irradiation, contamination and risk
Irradiation means exposure to radiation from an external source. Removing or switching off the source ends exposure, and the object does not automatically become radioactive. Contamination means unwanted radioactive material is on or inside an object, so it continues to irradiate until removed or decayed.
Ionising radiation can damage cells and tissue and cause mutations, increasing cancer risk. Reduce dose by shortening exposure time, increasing distance, using shielding, handling sources remotely and preventing contamination.
Radioactive waste can remain hazardous for long periods. Risk reduction includes secure containment, shielding, monitoring, appropriate transport and storage chosen for the isotope's activity and half-life.
Fission and fusion
In fission, a uranium-235 nucleus absorbs a neutron and splits. It produces two radioactive daughter nuclei, a small number of neutrons and energy as kinetic energy of the products. Released neutrons can cause further fissions, though detailed chain-reaction control is outside this Single Award boundary.
Reactor shielding absorbs or reduces radiation escaping the reactor and protects workers and surroundings. Shielding is distinct from the other reactor components omitted here.
Fusion joins light nuclei and is the energy source for stars. Fission, fusion and radioactive decay can all release nuclear energy, but their nuclear changes are different.
Radiation choice map
Worked application
A detector records 920 counts per minute near a source and background is 40 counts per minute. The corrected source count is 880 counts per minute. After 12 hours the detector records 260, giving corrected count 220, exactly one quarter of 880. Two half-lives have passed, so the half-life is 12/2=6 hours. Background must be subtracted before taking the fraction. A later single reading may fluctuate around the predicted value because decay is random, so repeated counts over equal intervals improve the estimate. The reported half-life should reflect the time resolution and scatter in the measurements.
Common misconceptions
“Mass number is the number of neutrons.” It is protons plus neutrons.
“Random decay means half-life cannot be predicted.” Individual decays are random, but large samples follow statistical decay.
“Gamma is a fast charged particle.” It is electromagnetic radiation with no charge.
“The most penetrating radiation is always most ionising.” Alpha and gamma show the opposite pattern.
“Background can be ignored because it is small.” It can distort half-life and penetration comparisons.
“After one half-life every other nucleus decays.” Half-life describes the sample statistically.
“Irradiated objects always become radioactive.” External exposure does not automatically contaminate them.
“Fission and fusion are the same split process.” Fission splits heavy nuclei; fusion joins light nuclei.
Assessment guidance
Read nuclide notation by locating proton and nucleon numbers before calculating neutrons. Radiation comparisons should include nature, ionisation, penetration and suitable shielding. In count data, subtract background before ratios or half-life estimates. Use repeated halving rather than assuming linear decline. For uses, link the radiation property and half-life to the task and risk control. Distinguish contamination from irradiation by asking whether radioactive material remains. Fission answers need neutron absorption, splitting, daughter nuclei, extra neutrons and kinetic energy. Do not import detailed reactor-control components or decay equations excluded from this Single Award section.
Retrieval practice
Decode five nuclide symbols and identify isotope pairs.
Compare alpha, beta-minus and gamma by nature, ionisation, penetration and shielding.
Plan a penetration comparison that accounts for background and geometry.
Find half-life from corrected count-rate data.
Choose and justify radiation for one medical and one industrial use.
Distinguish contamination and irradiation in three scenarios.
Explain uranium-235 fission, reactor shielding and stellar fusion.
