Sec 4 Physics Radioactivity notes

Radiation is invisible, but it betrays itself in three classic ways. A photographic film badge worn by a worker darkens in proportion to the dose received — the film is developed to read the exposure afterwards. A cloud chamber shows the radiation’s path directly: ionising particles seed visible vapour trails (thick, short tracks for alpha; thin, wispy ones for beta). A Geiger-Müller tube turns each burst of ionisation inside the tube into a click and a count on a ratemeter — the counting workhorse.

Even with no source on the bench, a detector keeps ticking: the background radiation — cosmic rays from space, radon gas from the ground, traces of uranium in rocks and building materials, tiny amounts of carbon-14 in all living things, and a slice from medical and industrial use. Before any experiment, measure the background rate and subtract it: a reading of 1200 counts per minute with a source, and 200 without, means the source itself delivers 1000 counts per minute. Skipping this step is the classic radioactivity-calculation error.

An alpha particle is a helium nucleus: 2 protons + 2 neutrons, written ⁴₂He or α. It is heavy and doubly charged, so it rips through matter ionising fiercely — and is stopped within a few centimetres of air or by a sheet of paper. A beta particle is a fast electron (⁰₋₁β) thrown out when a neutron turns into a proton; it is light and fast, less ionising, and travels a metre or so in air, stopped by a few millimetres of aluminium. Gamma rays are high-energy electromagnetic waves with no mass or charge: weakly ionising but hugely penetrating, needing several centimetres of lead (or concrete) to cut the intensity down.

In electric and magnetic fields the charges betray themselves: alpha (positive) and beta (negative) deflect in opposite directions, alpha far less because it is vastly heavier; gamma passes straight through undeflected. Memory line for penetration: paper < aluminium < lead stops alpha < beta < gamma; for ionising power the ranking reverses. Half the exam questions on this topic are one of those two rankings in disguise.

Nuclide notation: A/Z X, with A the nucleon number (protons + neutrons) and Z the proton number. Isotopes share Z but differ in A — same element, different mass (carbon-12 and carbon-14 are both carbon because both have Z = 6). A nuclear equation is checked like any other: total A balances and total Z balances across the arrow.

The two decays you must write from memory. Alpha decay: the nucleus spits out ⁴₂He, so A falls by 4 and Z falls by 2 — uranium-238 → thorium-234 + alpha. Beta decay: a neutron becomes a proton, an electron is ejected, so A is unchanged and Z rises by 1 — carbon-14 → nitrogen-14 + β⁻. Gamma emission changes neither A nor Z: the nucleus just sheds excess energy. Write the daughter, write the particle, check both totals — balancing marks are the cheapest marks in Paper 2.

You cannot say which nucleus decays next — that is the randomness. But the average is reliable: the half-life is the time for half the undecayed nuclei in a sample to decay, equivalently the time for the (background-corrected) activity to fall to half. After n half-lives, remaining fraction = 1/2ⁿ. A decay graph falls steeply and flattens towards zero, halving every half-life — read off the time for the rate to halve and you have measured T½.

Method for every calculation: (1) count the half-lives: n = time elapsed ÷ half-life; (2) divide by 2, n times; (3) if a count rate is given, subtract background first and add it back if the question asks for the meter reading. The value never reaches exactly zero — each halving leaves half of something — and cooling, heating or chemistry changes nothing about the rate.

The isotope is chosen to fit the job. Medical tracers (technetium-99m): gamma escapes the body to a detector and the short half-life clears the dose quickly. Sterilising syringes and food: gamma penetrates the sealed packaging and kills bacteria. Thickness gauges for metal foil: beta is partly absorbed by the foil, so the count falling on the detector tracks the thickness. Carbon dating: the known half-life of carbon-14 (about 5700 years) compares living and dead material. Alpha is the worst penetrator, so it only works inside the body (targeted therapy) or close by — as in the smoke alarm’s ionisation chamber.

All ionising radiation can kill or damage living cells — radiation sickness at high dose, cancer risk from damaged DNA at lower doses. Safety follows from dose = intensity × time: handle with tongs (distance), work behind lead screens, wear film badges to log exposure, keep exposure time short, and store sources in lead-lined boxes. Strong sources are never handled bare, and alpha sources are harmless outside the body but dangerous inhaled or ingested — a fact examiners love.