IP Physics Notes (Upper Secondary, Year 3-4): 17) Radioactivity

Study guide30 Sep 2025, 00:00 ZUpdated 30 Nov 2025

Trace nuclear structure, alpha/beta/gamma emissions, half-life maths, and safety protocols for IP nuclear physics.

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Q: What does IP Physics Notes (Upper Secondary, Year 3-4): 17) Radioactivity cover?
A: Trace nuclear structure, alpha/beta/gamma emissions, half-life maths, and safety protocols for IP nuclear physics.

Quick recap -- Unstable nuclei shed energy by emitting alpha, beta, or gamma radiation. Identify the emission, update the nuclide notation, and use half-life reasoning to track activity changes.

The core idea is simple: Radioactivity is unstable nuclei changing by emitting radiation.

Use it as a working check: Alpha changes mass and proton number, beta changes proton number, gamma changes energy only, and half-life halves activity after each equal time interval.

Then go one layer deeper: Use the decay equations, half-life example, and safety notes to practise balancing nuclear notation, reading activity changes, and choosing shielding or handling methods.

Keep your practice loop tight via our IP Physics tuition hub-it links each topic here to quizzes, diagnostics, and WA-style problem sets.

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These notes align with SEAB GCE O-Level Physics (6091) content used in IP programmes (exams from 2026).

Status: SEAB O-Level Physics 6091 syllabus (exams from 2026) checked 2025-11-30 - scope unchanged; remains the reference for these notes.

Rutherford Scattering Recap

Most alpha particles passed straight through gold foil -> atoms mostly empty space.
Some deflected at large angles -> positive charge concentrated in tiny nucleus.
A few rebounded -> nucleus is dense and carries most mass.
Led to nuclear model: central nucleus (protons + neutrons) with orbiting electrons.

Nuclear Notation & Isotopes

Nuclide represented as $^A_Z \text{X}$

Reviewed by

Chee Wei Jie·Academic Advisor (Physics)

Radiation	Nature	Charge	Mass (amu)	Ionising power	Penetration
Alpha ( $\alpha$ )	Helium nucleus ( $^4_2 \text{He}$ )	+2	4	Very strong	Stopped by paper / few cm air
Beta ( $\beta^-$ )	High-speed electron ( $e^-$ )	-1	0	Moderate	Stopped by few mm aluminium
Gamma ( $\gamma$ )	EM wave	0	0	Weak	Requires thick lead/concrete

Emission	What leaves the nucleus	Daughter nucleon number	Daughter proton number	Common trap
Alpha	$^4_2\text{He}$	$A - 4$	$Z - 2$	Subtracting 4 from both $A$ and $Z$ .
Beta-minus	$^0_{-1}\beta$	$A$ stays the same	$Z + 1$	Changing mass number even though a neutron becomes a proton.
Gamma	$\gamma$ energy	$A$ stays the same	$Z$ stays the same	Treating gamma as a new particle with mass or charge.

Data given	First reading to choose	Second reading to choose	Common trap
Activity-time graph	A point on the smooth curve, such as $\pu{800 Bq}$ .	The later point where activity is $\pu{400 Bq}$ .	Reading the half-life from the time-axis intercept.
Count-rate table with background already corrected	Any reliable count rate.	The row where corrected count rate has halved.	Using raw counts from unequal time intervals.
Graph that does not start at $t = 0$	A convenient point inside the plotted range.	The point half as high on the same curve.	Assuming the half-life must be measured from zero time.
Several noisy readings	Use the trend or best-fit curve.	Read the halved value from the trend, not one stray point.	Letting random count variation decide the half-life.

Question clue	First calculation	Why it matters
Raw counts over a time interval	Divide raw counts by time to get count rate	Longer counting times naturally give more counts, so compare rates first.
Background count rate is given	Subtract background rate from measured rate	The source is not responsible for all detected pulses.
Count rate after several equal time intervals	Correct for background first, then count halvings	Half-life applies to the source activity, not to background radiation.
Two source readings are compared	Use the same time unit and subtract background for both	A higher raw count can simply come from a longer measurement time.

Application clue	Source property to choose	Why it fits	Common trap
Medical tracer inside the body	Short half-life and penetrating enough to be detected outside the body	Activity falls quickly after diagnosis, while the detector can still pick up the signal	Choosing a long half-life because it is easier to detect for longer
Smoke detector	Alpha source with short range in air	Smoke particles reduce ionisation between the plates and change the current	Choosing gamma because it is most penetrating
Paper or aluminium thickness control	Beta source	Too much material absorbs more beta radiation, so the detector count changes with thickness	Choosing alpha, which is stopped too easily, or gamma, which passes through too easily
External radiotherapy	Strongly penetrating radiation directed at the tumour	Radiation can reach tissue below the skin when aimed carefully	Forgetting shielding and minimised exposure for healthy tissue

IP Physics Notes (Upper Secondary, Year 3-4): 17) Radioactivity

Rutherford Scattering Recap

Nuclear Notation & Isotopes

Types of Radioactive Emission

Nuclear Equations

Decay equation balancing checkpoint

Half-Life & Activity

Worked Example: Half-Life Count

Half-life graph checkpoint

Detecting Radiation

Background Count Checkpoint

Applications & Risks

Source choice checkpoint

Practice Quiz

Key Takeaways

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