x ray image intensifier

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fromage123
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i was doing the ocr jan 2011 paper Fields, Particles and Frontiers of physics and on question 8b it asks:
'a student suggest an image intensifier uses the photoelectric effect. explain why this suggestion is incorrect.'now im sure that it does use the photoelectric effect inside the actual equipment but on the mark scheme it just says x rays produce visible light.
is this right?
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uberteknik
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(Original post by fromage123)
i was doing the ocr jan 2011 paper Fields, Particles and Frontiers of physics and on question 8b it asks:
'a student suggest an image intensifier uses the photoelectric effect. explain why this suggestion is incorrect.'now im sure that it does use the photoelectric effect inside the actual equipment but on the mark scheme it just says x rays produce visible light.
is this right?
The photoelectric effect stated in the question is not entirely incorrect, but it also is not a full explanation describing why the image is intensified.

Also realise that mark schemes are not written for the benefit of students. They are there to help the examiner and, in descriptive questions, are often nothing more than a prompt. The student is required to give a more exact answer:

X-rays are not visible spectrum em energy. The image intensifier must therefore convert the x-rays to visible spectrum photons.

The incident x-rays do indeed liberate electrons (photoelectric effect) from a target comprising a scintilator/photocathode. But electrons are not visible light either and hence the photoelectric effect is only part of the explanation.

The liberated electons are focused and accelerated by an electric field created by field coils in much the same way as an old cathode ray tv tube.

These focused photo-electrons then strike the atoms of a target phosphor plate and, in turn, excite the phosphor-atom' electrons.

Visible em-spectrum photons are finally emitted when the excited phosphor electrons drop back to their stable state.

(It is these low energy visible photons which create the final intensified image through an optic lens/CCD camera/display monitor system.)

Thus the mark schemes précis statement of 'x-rays produce visible light'.
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fromage123
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thanks for the reply, i understand how the intensifier works, but im just curious as to how the question is right. it states that the intensifier does not use the photoelectric effect event though we both agree it does- surely this question is fallacious, ambiguous at best and shouldn't be on a paper at all.
what im worried about though is that if a similar question is asked on my paper in june regarding this situation what would you recommend- ignoring the photoelectric effect in the intensifier or writing correctly that it is present?
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uberteknik
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(Original post by fromage123)
thanks for the reply, i understand how the intensifier works, but im just curious as to how the question is right. it states that the intensifier does not use the photoelectric effect event though we both agree it does- surely this question is fallacious, ambiguous at best and shouldn't be on a paper at all.
what im worried about though is that if a similar question is asked on my paper in june regarding this situation what would you recommend- ignoring the photoelectric effect in the intensifier or writing correctly that it is present?
I agree it's a poorly worded question.

The best you can do is offer the correct explanation which both acknowledges that the photoelectric effect is part of the process but not a complete answer.

The key here is to recognise the intensifier uses a multi-stage process: that high energy x-rays are converted to lower energy visible photons via the intermediary photo-electron liberation and then phosphor-luminance process.
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