I want to understand those terms in practical means, what actually those terms represents practically. I have gone through many sources of information but I am unable to understand what actually those terms mean.
In neutron diffraction the structure factor F(hkl) is defined in the same as as you must have learned from X-ray diffraction book. The structure factor for X-ray diffraction is defined as
F(hkl) = \sum_j^N f_jexp (2\pi i rj.s (1)
In X-ray case f_j is atomic scattering factor of the scattering atoms. In X-ray scattering takes places from all the electrons of the atom and they are extended in space. the scattering factor of the atom f_j is the Fourier transform of the electron distribution. The scattering of X-ray from atoms are therefore angle dependent. It falls off with the scattering angle. Now in neutron case the scattering takes place from the nucleus (nuclear scattering) which can be considered point like. So we have different scattering factor for neutrons. It is constant with the scattering angle because it is the Fourier transform of a point like object. These are called scattering lengths b_j of atoms. These are tabulated in the International Tables of Crystallography C. So by replacing f_j in X-ray structure factor equation (1) by b_j (neutron scattering length of atom j) you get the structure factor equation for neutron diffraction. It is as simple as that.
Now Naziya please read the book Neutron Diffraction by G.E. Bacon Oxford (1975) and learn yourself. If you do not understand then do come back to me.
Thank U Dr. Tapan Chatterji, I am bit cleared with the help of ur concept. But still i have some doubts..
Sir, I read that "Structure factor is a mathematical description of how a material scatters incident radiation". Plz explain me this line,, Is it, more the value of structure factor more is the scattering cross section provided by the material to incident radiation???
And also i read that "The intensity of the diffracted beam is directly related to the amplitude of the structure factor". Now my question is, if structure factor is just a mathematical description then where come its amplitude??? In what sense amplitude is related to this structure factor???
I have doubt in scattering length as well, plz clearify..
I read that "scattering length of the nucleus measures the strength of the neutron-nucleus interaction". My question is, if it just measures the strength of interaction then why it is measured in units of LENGTH??? If it is measured in units of lengths then which length it actually measures???
Sir, I have the book, Neutron Diffraction by G.E. Bacon Oxford (1975). I read it but still i have doubts.. .
The structure factor has two parts: The first part consist of f_j which the atomic scattering factor (in X-ray diffraction) or b_j the neutron scattering length ( neutron diffraction) of the jth atom. The second part consists of purely geometry of the lattice and the atomic positions. The first part governs the strength of scattering and the second part tell you how the amplitudes of scattering are distributed for different reflections scattering from different lattice planes. The structure factor is the amplitude of the scattered wave from the crystal and can be a complex quantity in general. We measure however the intensity which is proportional to F\times F* or F^2 for simpler cases. You can express the scattering factor in different units. In case of X-rays it is express in the units of scattering amplitude of an electron. In case of neutron it is expressed in terms of scattering length in cm which is of the order of 10 ^-13 cm (typical diameter of nucleus). It is true that neutron scattering length is dependent on the scattering power of neutron from nucleus. But it has two main terms: potential scattering and spin scattering. The potential scattering increases slowly with the atomic number whereas the spin part varies quite irregularly from isotope to isotope. The nuclear theory is unable to calculate the scattering length of neutron from atoms. So we depend on the measured quantities. X-ray scattering factor on the other hand can be calculated quite accurately my quantum mechanics.
Now Naziya read a good text book on X-ray diffraction viz. B.E. Warren and of corse Becon for neutron diffraction. But if you are interested in neutron scattering from magnetic materials then you should consult "Neutron scattering from magnetic materials" ed. Tapan Chatterji, Elsevier 2006. I have not talked about magnetic neutron diffraction. But you can read all these in the above book. And do come back to me if you do not understand.
Thank u Sir for giving such a clear explanation.. I will go through those books and ur paper as well and if i find any doubt i will come back to u. .
Regards,
Naziya.
The structure factor is simply an interference pattern (sum) of all of the waves scattered (neutrons, x-rays etc..) by a material. It is manifest when the wavelength of the probe particles (neutrons, x-rays etc..) is similar to the interatomic distances within the material.
A scattering length (b) can be more easily thought of as a radius of a scattering cross-section (sigma). In other words sigma = 4 pi b^2
http://en.wikipedia.org/wiki/Scattering_length
The larger the cross-section, the more likely a probe particle is to interact with it.
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