mc about radioactivity

tommyli006

2010-08-20 8:45 am

請看下圖
http://i820.photobucket.com/albums/zz127/forupload_2010/-1.jpg

Q8 answer is B
Q10 answer is B
Q11 answer is C

Although I know the answers of them, I have a few questions about them.
For Q8. I dont understand the statement(3). Why count rate of T1 decreases? And actually i dont quite understand what the set up is doing. Please explain!
For Q10. Why statement(2) is incorrect? I want to know what the track consist of?
For Q11. Why statment(1) is incorrect? I know gamma ray wont deflect under a magnetic field but beta particles will have a large deflection. Thus, we can distinguish them, am i right?

更新1:

Sorry, I have put the wrong answer for Q11, the answer actually is option B If the answer given is B,from what you say, for Q11 the statments(1) and (3) are both correct. Right?

回答 (1)

天同

2010-08-20 5:57 pm

✔ 最佳答案

Q8: The diagram shown is incomplete. The switch S mentioned in the question is not shown on the diagram, hence the polarities of P1 and P2 are unknown. Therefore, it would not be able to determine if statements (1) or (2) are correct.

Since GM tube T1 is facing directly on the radioactive source, when switch S is open, all radiation that come from the source, irrespective it is alpha, beta or gamma, will reach T1. But when an electric field is produced between the two plates, radiation of charged particle nature (i.e. alpha or beta) will be deflected by the field towards T2, hence the reading on T2 increases. Now, there will not be any alpha or beta reaching T1, because all of them have been deflected by the electric field towards T2. Clearly, the reading on T1 decreases.

Q10. The supersaturated vapour in a diffusion cloud chamber is not necessarily water vapour. Some diffusion cloud chamber uses alcohol and hence the vapour is that from alcohol. It follows that the droplets left by the passage of charged particles will be alcohol droplets, not water droplets. Q11. Statement (1) is right. Your given answer is option C, which states that statements (1) and (2) are correct.

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