Exam-style question
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In an absorption experiment for alpha particles, a thin sheet of paper is placed between the source and the detector. Which of the following best describes the effect of increasing the paper thickness on the detected alpha count rate?.
- A.The count rate increases because more alpha particles are reflected back to the detector.
- B.The count rate remains unchanged because alpha particles are not affected by paper.
- C.The count rate decreases because alpha particles lose energy and are stopped by the paper.
- D.The count rate fluctuates randomly due to the stochastic nature of radioactive decay.
Model answer
What a good answer should say
- The count rate decreases because alpha particles lose energy and are stopped by the paper.
Explanation
Why this works
Alpha particles are massive, doubly charged ions that interact strongly with matter. In an absorption experiment, increasing the thickness of a material such as paper increases the probability that alpha particles will collide with atoms and lose kinetic energy, eventually being stopped before reaching the detector.
This leads to a systematic decrease in the detected count rate, which is the key observation used to identify alpha radiation.
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