HKDSE Physics: Radioactivity and Nuclear Decay Lesson-Prep Guide (for tutors)

2026-11-01

Tutor lesson-prep guides · Secondary physics

Students can divide by two once, then divide by three when the question says three half-lives. This lesson drills one habit: every half-life multiplies the activity by one half, never divides by n.

Objectives — Students can (1) state the composition, charge and penetrating power of alpha, beta and gamma and say which plate each deflects towards; (2) compute activity after n half-lives with powers of two; (3) complete alpha and beta decay equations conserving nucleon number and proton number.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Powers of two	What is 2 cubed?	8
Atomic structure	Nucleon number minus proton number	neutron number
Charge	Charge on an alpha particle	+2 (helium nucleus)

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three radiations	composition and charge	students build the table
8-20 min	Penetration and deflection	paper, aluminium, lead; electric field	draw three paths
20-34 min	Half-life	multiply by one half each time	get 1000 Bq
34-46 min	Decay equations	conserve both numbers	write two equations
46-56 min	Attenuation and safety	half-value layer, shielding, dose	get 25%
56-60 min	Exit check	three questions, marked at the door	independent

03 Teaching script (read it out)

Scene 1: Three radiations, one table

Alpha is a helium nucleus (two protons, two neutrons, charge +2), beta is a fast electron (charge -1), gamma is an electromagnetic wave with no mass and no charge.
Penetration: alpha stops at paper or a few centimetres of air; beta needs a few millimetres of aluminium; gamma needs centimetres of lead or thick concrete.
In an electric field alpha bends towards the negative plate, beta towards the positive plate, gamma goes straight. Ionising power runs the other way: alpha strongest, gamma weakest.

Ask the student: why is alpha the most dangerous inside the body even though it penetrates least? (All its energy is dumped in a very short track.)

Scene 2: Half-life: multiply by one half, never divide by n

A source of 8000 Bq with a half-life of 8 days: after 24 days that is 3 half-lives, so activity = 8000 / 2^3 = 1000 Bq. Eight days later (day 32) it is 500 Bq.
Reverse direction: a count rate falls from 6400 to 800 in 30 minutes. 6400 / 800 = 8 = 2^3, so three half-lives, half-life = 30 / 3 = 10 minutes.
Carbon-14 has a half-life of 5730 years, so after 11460 years 25% remains.

Ask the student: what fraction is left after four half-lives? (1/16, about 6.25%.)

Scene 3: Decay equations and attenuation

Alpha decay: mass number down 4, proton number down 2, for example U-238 to Th-234 plus He-4. Beta decay: mass number unchanged, proton number up 1 because a neutron turns into a proton, for example C-14 to N-14 plus an electron.
Gamma attenuates by half-value layers: two layers (2.4 cm of lead) leave 25%, three leave 12.5%. Protection is time, distance and shielding, in that order of cheapness.

Warn: the beta particle comes from the nucleus, not from the electron shells.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	activity after n half-lives	A = A0 / 2^n with n = t / T-half	1000 Bq
Reverse	half-life from two count rates	check the ratio is a power of two	10 min
Equation	complete an alpha or beta decay	conserve mass and proton number	Th-234 + He-4
Comparison	properties of the three radiations	charge, penetration, ionising power	alpha to negative plate

Sanity check (teach this): Activity can only fall with time; an answer larger than the starting value means the student multiplied instead of dividing. Half-life is constant and does not change with temperature, pressure or chemical state, so any answer that depends on temperature is wrong.

05 Independent practice (3 questions)

Question	Answer	What to watch
activity of a 8000 Bq source after 32 days (half-life 8 days)	500 Bq	32 / 8 = 4 half-lives
count rate 6400 falling to 800 in 30 minutes, find the half-life	10 minutes	6400 / 800 = 8
change in proton number after beta decay	plus 1	a neutron becomes a proton

06 Common mistakes and what to say

Mistake	Student writes	Say this
tripling instead of halving repeatedly	answers 8000 / 3	Every half-life halves it; three half-lives mean divide by eight.
alpha treated as an electron	alpha charge -1	Alpha is a helium nucleus, two protons, so +2.
proton number left unchanged in alpha decay	U-238 to Th-238	Both mass number and proton number must balance.
heating speeds up decay	higher temperature, faster decay	Half-life is nuclear, not chemical; temperature does nothing.

07 Homework (paper, section and question numbers)

DSE physics practice paper (atoms and nuclear energy) — Section B questions 6 to 8 (half-life calculations)
Secondary physics chapter index — all chapters

08 Exit check (last 5 minutes)

Question	Answer	Decision
activity after three half-lives from 8000 Bq	1000 Bq	all three right, move to nuclear energy
plate an alpha particle bends towards	the negative plate	one wrong, re-teach charge next lesson
proton number change in beta decay	plus 1	two wrong, restart act one

09 Tutor FAQ

Do students need to memorise nuclides?

No. The exam tests conservation and the standard examples; other data is given.

How is gamma written in an equation?

Gamma changes neither number, so it is written as a photon emitted by an excited nucleus.

Are these numbers from a real source?

They are teaching numbers: 8000 Bq and a 8-day half-life, chosen so the division is clean.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.