4PH1

Properties of Radiation

Radioactivity & Particles · 2 question types

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 8% of your exam marks.

stable

Low

Stable8%

Properties of alpha, beta and gamma radiation and nuclear equations tested consistently.

Use a Geiger–Müller (GM) tube to investigate the penetrating power of alpha, beta and gamma radiation by placing different absorbers (paper, aluminium, lead) between a sealed source and the detector

Independent variable: the absorbing material (and its thickness for aluminium)
Dependent variable: the count rate registered by the GM tube, measured in counts per minute (cpm) or counts per second (cps)
Control variables: the same source, fixed distance between source and tube, same location (so background level is the same), same measurement time per reading

Equipment	Purpose
Sealed radioactive sources (one α, one β, one γ) in a school-approved holder	The sources to be tested
Long-handled tongs	Allow safe handling of the sources without bringing them close to the body
GM tube and digital counter	Detects ionising radiation and counts the pulses
Selection of absorbers: sheet of paper, several aluminium sheets at 0.5 mm thickness intervals, several lead sheets	Placed between source and detector to test penetration
Lead-lined storage box for the sources	Reduces stray exposure when sources are not in use
Ruler and clamp stand	Fixes the source-to-tube distance
Stopwatch	Times each measurement

With no source present, take three one-minute readings of the background count and find the mean. Subtract this from every later reading so the measured value is the count due to the source alone
Set up the GM tube and source 3 cm apart on the bench, with the tube pointing at the source
Without any absorber in place, record the count for one minute. This is the unblocked count for that source
Place a single sheet of paper between the source and tube and record the count for one minute
Replace the paper with 0.5 mm of aluminium foil, then 1.0 mm, then 1.5 mm, recording the count at each thickness
Replace the aluminium with several thicknesses of lead in turn and record the count for each
Repeat the whole sequence for the other two sources

The unblocked count tells you the total emission from the source. Compare each absorber's count to it:
- If paper drops the count near to the background level, the source is emitting alpha
- If paper has no effect but a few mm of aluminium drops the count to near background, the source is emitting beta
- If even thick lead only partially reduces the count, the source is emitting gamma
Many sources emit more than one type of radiation. A source that drops with paper and again with aluminium is emitting both alpha and beta, for example

Never directly handle radioactive sources; use tongs at arm's length
Keep the unboxed source pointing away from yourself and others
Spend as little time as possible with the source out of its lead box
Return the source to its lead-lined container immediately after each measurement