What is the meaning of negative scattering length in neutron diffraction? Ex: Vanadium has negative scattering length for neutrons.
Positive neutron scattering length of a nucleus means repulsive potential the neutron is subject to as it approches the nuclus whereas negative scattering length means the neutron is subjected to a attractive potential of the nucleus. It is as simple as that. The existence of positive and negative scattering lengths for atomic nuclei is very useful for structural researcg. For example if you study manganites, like LaMnO3 (or its alkali doped varaiety) then you are pretty well off with neutron diffraction than with X-rays because Mn has a large negative scattering length whereas O has a large positive scattering length. So the contrast is very good and you can determine the atomic positions more accurately by neutron diffraction. Different isotopes of the same atom can have positive and negative scattering lengths. Then if you can do isotope substitution then you can determine the atom position very accurately. This is called isotope contrast and is very useful tool in neutron diffraction.
You can read the book 'Neutron Diffraction' by George Bacon (Oxford Clarendon Press) for details or come back to me with specific questions.
Positive neutron scattering length of a nucleus means repulsive potential the neutron is subject to as it approches the nuclus whereas negative scattering length means the neutron is subjected to a attractive potential of the nucleus. It is as simple as that. The existence of positive and negative scattering lengths for atomic nuclei is very useful for structural researcg. For example if you study manganites, like LaMnO3 (or its alkali doped varaiety) then you are pretty well off with neutron diffraction than with X-rays because Mn has a large negative scattering length whereas O has a large positive scattering length. So the contrast is very good and you can determine the atomic positions more accurately by neutron diffraction. Different isotopes of the same atom can have positive and negative scattering lengths. Then if you can do isotope substitution then you can determine the atom position very accurately. This is called isotope contrast and is very useful tool in neutron diffraction.
You can read the book 'Neutron Diffraction' by George Bacon (Oxford Clarendon Press) for details or come back to me with specific questions.
If you cannot get Bacon's book then try my book Magnetic Neutron Scattering, ed. Tapan Chatterji. You may have limited access to it in Google Books.
A negative scattering length indicates that the Fermi pseudopotential for the neutron scattering is attractive (the actual potential may have a very different form).
For cold and thermal neutron (low energy neutrons) scattering, you could generally say that positive scattering lengths result in negative phase shifts, and negative scattering lengths result in positive phase shifts.
I got this from looking at calculations of hard sphere elastic scattering. For wavelengths much larger than the dimension of the sphere, the scattered wave has phase shift (for a positive potential) -kr, where k is the wave number and r is the dimension of the sphere.
This is similar to the neutron nucleus interaction because the scattering potential "looks" very short ranged in comparison to the neutron's de Broglie wavelength. For higher energy neutrons (larger k ... k =2pi/wavelength), this is obviously not true.
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