Radioactivity Decay

• Background radiation is always around us — from natural sources (radon gas, cosmic rays, food) and man-made sources (medical, nuclear waste, fallout). You must subtract it to get the corrected count rate.
• Alpha (α) = 2p + 2n (helium nucleus), Beta (β⁻) = high-energy electron, Gamma (γ) = electromagnetic wave — each has different mass, charge, penetration and ionising power.
• Nuclear decay equations must balance — mass number (top) and atomic/proton number (bottom) balance on both sides, just like a chemical equation.
• Core Practical 15 investigates how lead thickness absorbs gamma radiation — count rate falls until it reaches background level.
• Radioactive decay is random (can't predict which nucleus decays when) and spontaneous (unaffected by temperature, pressure, chemical conditions).
• Activity A = ΔN/Δt = −λN — the decay constant λ is the probability per second that one nucleus decays.
• Decay follows an exponential pattern: N = N₀e^(−λt), and the number of nuclei never actually reaches zero.
• Half-life t½ is the time for half the nuclei (and half the activity) to decay: t½ = ln2 / λ.

Background radiation is low-level radiation from environmental sources that is always present around us, whether or not you're doing an experiment. It's measured in counts per second, in a unit called the Becquerel (Bq).

Think of it like background noise in a room — even in "silence," there's always a faint hum. In radioactivity, that hum comes from two broad categories of sources:

This is the trick examiners love to test: to find the true activity of a source, you measure the count rate with no source present (this is your background rate), then measure the count rate with the source present, and subtract:

How to spot it on a graph: if a count-rate-vs-time (or vs-distance) graph seems to level off at some value above zero instead of dropping to zero, that flat value is the background radiation — not evidence that the source is still emitting strongly!

unstable radioactive decay