How are activity and atom count linked?

Activity is the decay constant times the number of atoms, and the constant comes from the half-life.

Does a gamma emission change the element?

No, a gamma ray only removes energy, so both mass number and atomic number stay the same.

S5 Physics Radioactivity, Paper 4: beta decay, activity and safety

2026-11-02

Covers half-life both ways, beta minus and gamma changes, activity from atom count, comparing two sources and a dose time limit in ten graded items.

Focus　Count half-lives first, use A equals lambda N for activity, and dose equals rate times time for safety questions.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Radioactivity and Nuclear Decay
Level	Four basic, three advanced, two challenge and one error-repair item

02　Questions (10 items)

FoundationA nuclide has a half-life of 12 years. How much of 48 g is left after 36 years?
FoundationA source has an activity of 800 Bq. Find its activity after four half-lives.
FoundationIodine 131, mass number 131 and atomic number 53, emits a beta minus particle. Give the mass number and atomic number of the new nucleus.
FoundationWhat happens to mass number and atomic number when a nucleus emits a gamma ray?
IntermediateCarbon fourteen has a half-life of 5730 years. How old is a sample with 12.5 per cent of its original activity?
IntermediateA nuclide with a half-life of 8 days falls from 64 g to 4 g. How long does that take?
IntermediateA sample holds 6.0 times ten to the twenty radioactive atoms with a half-life of 5 days. Estimate its activity in becquerels.
ChallengeSources A and B start with the same activity, with half-lives of 20 minutes and 90 minutes. After 180 minutes how many times larger is the activity of B?
ChallengeA dose rate of 8 mSv per hour is measured 0.5 m from a source. How many minutes may a worker stay to keep below 1 mSv?
Error redoA student says 24 g with a 6 hour half-life “disappears completely after 18 hours”. What actually remains?

03　Answers and one-line notes

#	Answer	One-line note
1	6 g	Thirty six years is three half-lives, so 48 g halves three times to 6 g.
2	50 Bq	Activity halves each half-life, so 800 divided by sixteen is 50 Bq.
3	Mass number 131 and atomic number 54	In beta minus decay a neutron turns into a proton, so the mass number stays and the atomic number rises by one.
4	Neither changes, only the energy falls	Gamma rays carry energy but no mass number or charge, so neither number changes.
5	17190 years	Twelve and a half per cent is one eighth, three half-lives, so 5730 times 3 gives 17190 years.
6	32 days	Going from 64 g to 4 g takes four halvings, so 8 times 4 gives 32 days.
7	About 9.6 times ten to the fourteen becquerels	The decay constant is about 1.6 times ten to the minus six per second, so the activity is about 9.6 times ten to the fourteen becquerels.
8	128 times	A has gone through nine half-lives and B two, so the ratio of activities is 512 over 4, that is 128.
9	7.5 minutes	Dose is rate times time, so one millisievert allows one eighth of an hour, that is 7.5 minutes.
10	3 g	Each half-life only halves the amount, so three of them leave 3 g, never exactly zero.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

Book an S5 Physics tutor Tutor Registration →

Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.