radioactivity (urgent)

2011-05-11 6:57 am

1. Why is the answer E?

圖片參考：http://imgcld.yimg.com/8/n/HA01097219/o/701105100153313873436290.jpg

2. Some fresh foods are exposed to radiations from radioactive isotopes for a short time so that the micro-organisms in the foods can be killed. Why are the irradiated foods not harmful to people who eat them?

Ans: gamma radiations does not make the foods radioactive.
Why? Some food near the Fukushima power plant was found to have contaminated by radioactive substances. Is this caused by alpha or beta radiation or is it simply that the radio active substances go into it?

3. In order to detect cracks in an underground oil pipe, an engineer proposes adding a radioactive source to the oil. Which of the following sources is most suitable?
Ans: a gamma source with a half life of a few hours.
Why the gamma source should have a half-life of a few hours but not several years?

4. Which of the following are essential criteria in choosing radioactive sources as medical tracers in human bodies?
(1) The sources should have a short half-life.
(2) The radiation should have a weak ionizing power.
(3) The radiation emitted should not be deflected by an electric field.
Ans: (1) and (2). Why are they correct / Why is (3) incorrect?

5. There is an equation :
A = Ao e^(-kt) where Ao is the intial activity, k is the decay constant and t is the time elapsed.
How to derive this equation?

thanks

更新1:

for 2, what will happen if alpha or beta radiation is used instead of gamma radiation? Will the food become radioactivity (neglect the nutrient of the food). thanks

回答 (1)

天同

2011-05-11 6:14 pm

✔ 最佳答案

1. A head-on collision would lead to a direct bounce back of the alpha particle (as what is shown in option C). The alpha particle would not be deflected sideways. 2. You need to distinguish between “radiation” and “radioactive substances”. The term “radiation” refers to the rays or particles (e.g. alpha, beta or gamma) emitted from “radioactive substances”. In other words, “radioactive substances” emit “radiation”. The food in the question is sterilized by “radiation” (i.e. gamma rays). Since gamma ray is pure energy, once absorbed, it would not be left on the irradiated food. This is just like a patient who has received a chest X-ray would not become radioactive. In the Fukushima event, there was a release of “radioactive substances” like iodine-131 and caesium-137. When these “radioactive substances” deposited onto foodstuff, like leafy vegetables, the foodstuff was said to be “contaminated” because they were carrying “radioactive substances” that emit “radiation”. 3. A short half-life tracer enables the radioactivity to vanish after the detection is performed. Should a long half-life radioactive substance be used, the radioactivity would remain in the oil long after the detection of cracks was completed. Such situation is very undesirable in view of radiation hazard. 4. Statement (1): a short half-like tracer is preferable to be used as a tracer because the radioactivity in the patient body would vanish quickly after the medical examination is over. A long half-life tracer would give unnecessary radiation dose to the patient. [sorry...there is not enough space for me to completer the answer, as the "word limit" has exceeded. you could pm me for the other answers]

2011-05-11 10:20:42 補充：
I have emailed you the rest of the answers.

2011-05-15 22:33:01 補充：
I have e-mailed you the answer to your supplementary question

👍 0 👎 0