Radioactivity
6 marks · NormalA radioactive source emits both alpha and gamma radiation. The source is tested in a lab using a Geiger-Muller (GM) tube, as shown below.
Describe how the GM tube is used to detect ionising radiation. Name one other way, apart from a GM tube, that ionising radiation can be detected.
A student places a thin sheet of paper between the source and the GM tube, and records the count rate. She then places a thick lead block instead and records the count rate again. Explain what you would expect to happen to the count rate in both cases, and what this shows about the properties of alpha and gamma radiation.
Alpha particles are helium nuclei. State the number of protons and neutrons in an alpha particle. How does the emission of an alpha particle affect the atomic number and mass number of a nucleus?
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(a) The GM tube detects ionising radiation by collecting the ions produced when radiation enters the tube and causes a current that is counted electronically. Another method is photographic film, which darkens when exposed to radiation.
(b) The count rate will fall to background levels with the paper, because alpha particles are stopped by paper. With the lead, the count rate will decrease but not fall to zero, because thick lead is needed to block gamma rays, while some gamma rays may still penetrate. This shows alpha is easily stopped but gamma is highly penetrating.
(c) An alpha particle has 2 protons and 2 neutrons. Emission of an alpha particle decreases the atomic number by 2 and the mass number by 4.
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Marking points
B1 (a) · 1 mark · States that a GM tube detects ionising radiation by producing a current/count when ionising particles enter the tube.
B2 (a) · 1 mark · Names photographic film or another valid detector.
B3 (b) · 1 mark · Explains that the count rate drops with paper (alpha stopped), and with lead, gamma is partly blocked.
B4 (b) · 1 mark · Links penetration properties to type: alpha stopped by paper; gamma needs lead and may partially penetrate.
B5 (c) · 1 mark · Correctly gives 2 protons and 2 neutrons in an alpha particle.
B6 (c) · 1 mark · Correctly states change to atomic and mass number from alpha emission (atomic number –2, mass number –4).
Worked solution
a) The GM tube works by detecting the ions created when radiation enters, allowing a current to be measured and counted as 'clicks' or readings. Photographic film can also be used as a detector (badges darken when exposed).
b) With thin paper, the count rate drops sharply because alpha particles cannot pass; with thick lead, the count rate decreases but does not drop to zero since gamma rays are highly penetrating and some may still pass through lead. This demonstrates that alpha has low penetration, gamma has very high penetration.
c) An alpha particle contains 2 protons and 2 neutrons. When an atom emits an alpha particle, its atomic number drops by 2, and its mass number drops by 4.