Small focal spot in X-ray imaging

[BobP]

Unfortunatley I cannot post this question in the Homework section (not sure why) so I hope its OK if I post here and show work as appropriate

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown >[/color]

QUESTRION State two design features in an X-ray tube that help maintain a high intensity output while keeping a small focal spot size

MY ANSWER
(1) The nickel block supporting the cathode helps focus the x-ray beam
(2) the rotating anode means heat dissapation is possible and a small focal spot can be used.

Are these both correct? Are there others? thanks

 

[sophiecentaur]

The nickel block supporting the cathode helps focus the x-ray beam

How? I think it would focus the electron beam, perhaps.

 

[BobP]

How? I think it would focus the electron beam, perhaps.

It does. but doesn't that reduce focus spot size?

 

[sophiecentaur]

It does. but doesn't that reduce focus spot size?

I was thinking in terms of focussing the Xray beam. Does a small Xray source help with making the X-rays more 'intense'? (Answer is presumably Yes because of the 'optics' of a simple collimator).
It is possible to focus X-rays in an Xray telescope so it may be possible to use the same technique to produce an intense Xray beam.

 

[sophiecentaur]

I think I see the problem now. You have used this sentence.:

The nickel block supporting the cathode helps focus the x-ray beam

when you probably meant 'focus the electron beam'
The bit about rotating the cathode sounds like good sense if you want to use a heavy current.
There is no diagram or reference, so the nickel block bit could mean anything.

 

[BobP]

I think I see the problem now. You have used this sentence.:

when you probably meant 'focus the electron beam'
The bit about rotating the cathode sounds like good sense if you want to use a heavy current.
There is no diagram or reference, so the nickel block bit could mean anything.

Yes. That is what I meant. Sorry for the confusion

I've been reading that x-ray filament size also has an affect but not sure how. do you know anything about this?

 

[sophiecentaur]

do you know anything about this?

Only the basics of the way electrons behave and the formation of X-rays by their interaction with metal targets. You are asking a more specific question and that's why I asked for details or a reference. The question in the OP will have had some context, surely?

 

[BobP]

Only the basics of the way electrons behave and the formation of X-rays but their interaction with metal targets. You are asking a more specific question and that's why I asked for details or a reference. The question in the OP will have had some context, surely?

The question was in a past exam paper and the first part asked me to draw the design of an x-ray system. no other context unfortunatley

 

[sophiecentaur]

The question was in a past exam paper and the first part asked me to draw the design of an x-ray system. no other context unfortunatley

The question would, presumably expect you to have some past knowledge of these things? Do you have a diagram in your notes somewhere? I can't imagine you would have needed to invent a system during an exam.

 

[BobP]

The question would, presumably expect you to have some past knowledge of these things? Do you have a diagram in your notes somewhere? I can't imagine you would have needed to invent a system during an exam.

I''ve attached the only diagram from our notes.
This question was not covered by my course at all which is why I am confused about its presence in a past exam

 

[sophiecentaur]

its presence in a past exam

If the question was in a past exam paper then probably the course content has changed. If you were given a copy by your tutor then he / she probably didn't scan the paper in too much detail.
The electron optics are very simple in that diagram (unlike what you get with an 'electron gun' and focussing electrodes in a CRT, for instance, which produce a very low beam divergence ). Electrons will spray off that cathode in the vague direction of the anode and not be focussed particularly. The mitred edge of the rotating anode just maximises the access of the electrons to the target at the same time as maximising the area of the radiated (omnidirectional) X-rays. (hence the 45 degrees) The design looks pretty simple but I am sure that a practical source would have some subtle features to get a more intense beam of electrons on the anode. The requirement for cooling seems to be paramount, here and that's a clever system. I have heard the whirr as an Xray machine prepares for an exposure and I guess I was hearing the anode building up its rotation.
The Xray optics are very basic in the diagram. They show a simple 'cone' of rays from the anode but the 'spot' wouldn't be very small (in that basic diagram) and the resulting Xray shadow from the test piece would be blurred unless the beam were further collimated.
You'll have to read around an let us know what you find. I have seen information about the linac sources for the X-rays used in Radiotherapy and there are some pretty clever design features.

 

[BobP]

You'll have to read around an let us know what you find. I have seen information about the linac sources for the X-rays used in Radiotherapy and there are some pretty clever design features.

Thanks :) I will do