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Re: [ccp4bb] Conservation of momentum - was "Is anomalous signal a different wavelength?"
 

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CCP4bb <-- 2007 <-- June 2007 <-- 04 June 2007
Previous message:
Subject: ERROR: can't read "array(ef2ccp4)": no such element in array
From: Tiancen Hu tony_htc {- at -} 126 {- dot -} COM
Date: 2007-06-04 		Next message:
Subject: B-factor & Space gr questions!
From: "Ibrahim M {- dot -} Moustafa" I {- dot -} moustafa {- at -} PSU {- dot -} EDU
Date: 2007-06-04


Subject: Re: Conservation of momentum - was "Is anomalous signal a different wavelength?"
From: "Nave, C (Colin)" c {- dot -} nave {- at -} DL {- dot -} AC {- dot -} UK
Date: 2007-06-04
Apologies for the late posting (been away). Interesting question from James and an interesting answer from Ian!

There should be a radiation pressure effect here resulting in a transfer of energy and momentum to the sample. The effect is very small (the term includes flux density divided by speed of light). Although this will not make your crystal fly away, the effect is proposed for space travel (using large "solar sails"). A very small change in energy of the photon results. This can be explained by quantum or more classical theory.

The question referred to elastically scattered photons. In fact the Compton effect could also be considered an elastic scattering phenomena as the total energy and momentum of the particles (photon plus electron) is conserved. There is confusion about the terminology here between different disciplines. The difference is that a significant amount of the photons energy is transferred to the electron.

Colin

-----Original Message-----
From: CCP4 bulletin board [mailto:CCP4BB@JISCMAIL.AC.UK]On Behalf Of Ian
Tickle
Sent: 31 May 2007 10:53
To: CCP4BB@JISCMAIL.AC.UK
Subject: Re: [ccp4bb] Is anomalous signal a different wavelength?


I think it's to do with the Uncertainty Principle. You can't say for
sure that a particular X-ray photon has gone off in that direction (if
you could you would know both its position and momentum accurately which
is not allowed). If you integrated the momentum over all possible
outcomes I'm sure you would find that it's conserved (it has to be in an
elastic collision).

-- Ian

> -----Original Message-----
> From: owner-ccp4bb@jiscmail.ac.uk
> [mailto:owner-ccp4bb@jiscmail.ac.uk] On Behalf Of Murray, James W
> Sent: 31 May 2007 10:30
> To: CCP4BB@JISCMAIL.AC.UK
> Subject: RE: [ccp4bb] Is anomalous signal a different wavelength?
>
>
> Dear All,
>
> While we are talking about X-ray scattering, I have another
> question. If an X-ray is elastically scattered from an
> electron at an angle theta, its energy is the same is the
> incoming X-ray. However, the momentum is not the same, as it
> now has a component in a perpendicular direction (see fig
> below). As I don't believe that the conservation of momentum
> really is violated, what is the source of the discrepancy?
>
> Contrast this with most textbook descriptions of Compton
> scattering, where the X-ray loses energy and the electron
> gains kinetic energy.
>
> best wishes
>
> James
>
> X-ray --------> e-
>
>
>
>
>
> Dr. James Murray
> Biochemistry Building
> Department of Biological Sciences
> Imperial College London
> London, SW7 2AZ
> Tel: +44 (0)20 7594 5276
>
>
>
>
>
>


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CCP4bb <-- 2007 <-- June 2007 <-- 04 June 2007
Previous message:
Subject: ERROR: can't read "array(ef2ccp4)": no such element in array
From: Tiancen Hu tony_htc {- at -} 126 {- dot -} COM
Date: 2007-06-04 		Next message:
Subject: B-factor & Space gr questions!
From: "Ibrahim M {- dot -} Moustafa" I {- dot -} moustafa {- at -} PSU {- dot -} EDU
Date: 2007-06-04


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