[单选题]
A student uses the arrangement shown below to
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The SI unit of radioactive decay constant is $s^{−1}$, which is the same as that of frequency.When the sheet of paper is in the position shown, a count rate of 720 Bq is recorded. When she lowers the thick sheet of paper so as to obstruct the direct path between the source and the detector, the count rate drops to 18 Bq. Which of the following is the most likely explanation for the data collected?
A. Only alpha particles are emitted. The remaining activity is from background radiation.
B. An equal number of alpha and beta particles are emitted.
C. Only beta particles are emitted.
D. There is not enough data to draw a reasonable conclusion.
参考答案:
A
本题详细解析:
Explanation:
A. The drop in the count rate is considerable but does not completely drop off to zero. The remaining count can easily be explained by the presence of background radiation. This indicates that the emission from the source is almost completely blocked by the sheet of paper. Alpha is the only type of radiation that can be nearly completely stopped by a sheet of paper.
B. If both alpha and beta were being emitted in equal numbers, the sheet of paper would block the alpha but an appreciable amount would penetrate through the paper as beta radiation. So the count rate after the paper is inserted should be larger than the recorded value.
C. If only beta radiation was emitted, the count rate would not have significantly diminished after the paper was inserted because beta radiation is able to penetrate through paper and if that is the only type of radiation, there is no reason for the count to fall that far.
D. The information is sufficient to draw a reasonable conclusion. That conclusion is expressed in answer A.
