iGCSE Physics (0625) 5.2.1 Detection of radioactivity -Exam Style Questions Paper 2 - New Syllabus
▶️ Answer/Explanation
Detailed solution:
First, calculate the average background count over 10 minutes: $\frac{170 + 164 + 185}{3} = 173$ counts.
Convert this to a background count rate per minute: $\frac{173}{10} = 17.3$ counts per minute.
The measured total count rate near the source is $949$ counts per minute.
The corrected count rate is found by subtracting the background rate from the total rate: $949 – 17.3 = 931.7$ counts per minute.
Rounding to the nearest whole number gives $932$ counts per minute.
This matches Option A, which represents the true activity of the source alone.
▶️ Answer/Explanation
Detailed solution:
Background radiation refers to the low-level ionizing radiation constantly present in the environment from natural and artificial sources. Natural contributors include cosmic rays (high-energy electromagnetic rays and particles), food and drink containing radioactive isotopes like $K-40$, and rocks containing uranium or thorium. Radon gas, mentioned in the stem, is also a primary natural source. Conversely, seismic waves are mechanical waves that travel through the Earth’s layers due to geological activity; they do not involve nuclear decay or ionizing radiation. Therefore, seismic waves do not contribute to the detected count rate of background radiation.
▶️ Answer/Explanation
Detailed solution:
The total reading shown by a detector includes both the radiation from the source and the ever-present background radiation.
Background radiation comes from natural sources such as cosmic rays and radioactive rocks.
To find the radiation specifically emitted by the source, we must subtract the background count from the total measured count.
The mathematical relationship is: $\text{corrected count rate} = \text{measured count rate} – \text{background count rate}$.
Therefore, taking away the background radiation count rate provides the accurate value for the source alone.
This process ensures that the data reflects only the activity of the radioactive sample being studied.