Ch 3 · 5.2.1 Detection of Radioactivity · O Level Physics 5054
Topic 3 of 8 in Nuclear Physics — O Level Physics 5054 — 59 lessons.
Introduction
Nuclear radiation is invisible, silent and has no smell. To study it, or to check whether a place is safe, we need instruments that turn each particle into a click, a spark or a visible track. (Syllabus section 5.2.1.)
Key ideas — Detecting alpha particles
🧑🏫 Teacher's explanation
Imagine you are walking through a dense, foggy forest. If you carry a high-powered torch, the beam becomes visible as it hits the water droplets in the fog, creating a distinct line of light. Alpha particles act like these torches. Because they are relatively large and carry a +2e charge, they are extremely 'clumsy' and collide with many air molecules, stripping away electrons (ionisation). This creates a trail of charged ions that act as seeds for condensation.
📖 Definitions
🔍 Worked example — Easy
A cloud chamber is used to observe alpha particles. Describe the visual appearance of an alpha particle track in a cloud chamber.
🔍 Worked example — Standard
A Geiger-Müller tube records a total count rate of 55 counts/min when a sample is present. The background radiation is measured to be 5 counts/min. Calculate the corrected count rate of the sample.
🔍 Worked example — Exam-style
A student measures the background radiation over 10 minutes and finds 120 counts. They then measure a sample for 2 minutes and find 450 counts. Calculate the corrected count rate of the sample in counts/s.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Detecting beta particles and gamma radiation
A Geiger–Müller (GM) tube connected to a counter (or ratemeter):
🧑🏫 Teacher's explanation
Imagine a dark room filled with invisible, tiny ping-pong balls flying around randomly. If you stand in the middle with a bucket, you might catch a few balls even if no one is throwing them; those are 'background' balls. If someone starts throwing balls at you much faster, you can figure out their speed by subtracting the 'background' rate from your total count.
📖 Definitions
🔍 Worked example — Easy
A student uses a Geiger-Müller tube to measure a radioactive source. The total count rate is 45 counts/min. The background radiation is measured to be 5 counts/min. Calculate the corrected count rate of the source.
🔍 Worked example — Standard
A GM tube detects 120 counts in 60 seconds. The background radiation is 0.5 counts/s. Calculate the corrected count rate in counts/s.
🔍 Worked example — Exam-style
A researcher measures a sample for 5 minutes and gets 1500 counts. The background radiation is 10 counts/min. Calculate the corrected count rate in counts/min.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) The student measures the background radiation and finds it to be 20 counts/min. (b) The student then measures the total count rate from the source and finds it to be 150 counts/min. Calculate the corrected count rate of the source in counts/min. [2] [2 marks]
Key ideas — Count rate
The count rate is the number of counts recorded per unit time, in counts/s or counts/minute.
🧑🏫 Teacher's explanation
Imagine you are standing by a busy road and you want to know how many cars pass by every minute. You don't just count the cars that pass while you are looking; you also have to account for the cars that were already driving through the area before you started your stopwatch. In nuclear physics, the 'cars' are radioactive particles, and the 'background traffic' is the natural radiation that is always present around us.
📖 Definitions
🔍 Worked example — Easy
A Geiger-Müller tube records a total count rate of 45 counts/min when a radioactive source is present. The background radiation is measured to be 5 counts/min. Calculate the corrected count rate of the source.
🔍 Worked example — Standard
A student measures 120 counts in 30 seconds from a sample. The background radiation is 2 counts/s. Calculate the corrected count rate in counts/s.
🔍 Worked example — Exam-style
A sample is placed near a detector. Over 5 minutes, the detector records 650 counts. The background radiation is measured as 15 counts/min. Calculate the corrected count rate in counts/min.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) The student records a total of 400 counts over 2 minutes. (b) The background radiation is measured to be 5 counts/min. Calculate the corrected count rate of the sample in counts/min. [3] [3 marks]
Key ideas — Background radiation
Background radiation is the low level of ionising radiation that is around us all the time, from natural and artificial sources, even with no radioactive source nearby.
🧑🏫 Teacher's explanation
Imagine you are trying to listen to a friend whispering in a crowded, noisy cafeteria. Even if your friend stops talking, you still hear a constant low hum of chatter from other tables. In physics, that 'hum' is background radiation. It is the constant, unavoidable presence of ionizing radiation that exists everywhere on Earth, even when no radioactive source is present in the room.
📖 Definitions
🔍 Worked example — Easy
A student measures the background radiation using a Geiger-Müller tube and finds a count rate of 15 counts/min. What is the background radiation level in counts/s?
🔍 Worked example — Standard
A Geiger-Müller tube records a total count rate of 120 counts/min when a radioactive source is present. The background radiation was measured to be 20 counts/min. Calculate the corrected count rate of the source in counts/min.
🔍 Worked example — Exam-style
A scientist measures a total count rate of 45 counts/s when a sample is present. The background radiation is 0.5 counts/s. Calculate the corrected count rate of the sample in counts/s, giving your answer to 2 significant figures.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) The student measures the background radiation and finds it to be 0.20 counts/s. (b) When the source is placed near the tube, the count rate increases to 1.5 counts/s. Calculate the corrected count rate of the source in counts/s. [2] [2 marks]
Key ideas — Corrected count rate
A GM tube always picks up background radiation as well as radiation from a source. To find the count rate due to the source alone:
🧑🏫 Teacher's explanation
Imagine you are trying to listen to a friend whispering in a crowded, noisy cafe. The 'whisper' is the signal you actually want to hear, but the 'clatter of plates and background chatter' is the noise that is always there. If you want to know exactly how loud your friend is being, you can't just measure the total noise; you have to subtract the noise level of the cafe first.
📖 Definitions
🔍 Worked example — Easy
A student uses a Geiger-Müller tube to measure a radioactive source. The background count rate is 0.02 counts/s. The total count rate measured with the source present is 0.55 counts/s. Calculate the corrected count rate.
🔍 Worked example — Standard
A technician measures a background radiation level of 15 counts/min. When a sample is placed near the detector, the reading is 145 counts/min. Calculate the corrected count rate in counts/min.
🔍 Worked example — Exam-style
A sample's total count rate is 2.4× 10^-2 counts/s. The background radiation is measured as 4.0× 10^-3 counts/s. Calculate the corrected count rate in counts/s, giving your answer to 2 significant figures.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) The student measures the background radiation and finds it to be 0.05 counts/s. (b) The student then measures the total count rate with the source present and finds it to be 1.25 counts/s. Calculate the corrected count rate of the source. [2] [2 marks]
Worked example
A GM tube records 360 counts in 2.0 minutes with no source present. With a source, it records 1800 counts in 1.0 minute. Find the corrected count rate from the source.
🔍 Worked example — Easy
A GM tube measures 40 counts in 20 seconds when no source is present. Calculate the background count rate in counts/s.
🔍 Worked example — Standard
A source gives a total count rate of 500 counts/min. The background radiation is measured at 50 counts/min. Calculate the corrected count rate in counts/min.
🔍 Worked example — Exam-style
A GM tube records 360 counts in 2.0 minutes with no source present. With a source, it records 1800 counts in 1.0 minute. Find the corrected count rate from the source in counts/s.
⚠️ Common errors
Common mistakes
Practice
Answers: (1) A Geiger–Müller tube and counter. (2) Ionising radiation present all around us all the time; any three: radon gas, rocks and buildings, food and drink, cosmic rays. (3) 95 - 0.50 = 94.5 counts/s. (4) The alpha particle ionises the air; alcohol vapour condenses on the ions, leaving a visible track.