# Interaction of Thermal Neutrons and Photons with Matter

Chapter

## Abstract

Spectroscopy is concerned with measurement of the structure and dynamics of the ground state or low lying excited states of a condensed matter. A radiation is useful as a tool for spectroscopy if it couples weakly (in a sense to be discussed later) to the many body system. Because in this case the double differential cross-section (per unit solid angle, per unit energy transfer) for the radiation scattering can be written schematically as (see Figure 1): where the first factor (dσ/dΩ)

$$\frac{{{d^2}}}{{d\Omega d\omega }} \sim {\left( {\frac{{d\sigma }}{{d\Omega }}} \right)_0}\;{\sum\limits_{i,f} {{P_i}\left| { < f\left| {\sum\limits_{\ell = 1}^n {{e^{i\left( {{{\underline k }_1} - {{\underline k }_2}} \right) \cdot {{\underline r }_\ell }}}} } \right|i > } \right|} ^2}\delta \left( {\hbar \omega - E{}_i + {E_f}} \right)$$

(1)

_{Q}refers to differential scattering cross-section from the basic unit of scattering in the system and the second factor, usually called the dynamic structure factor, represents the time dependent structure of the system as seen by the radiation. This clear separation of the basic scattering problem, as embodies in the first factor, from the dynamic structure of the system itself, is only possible when the radiation couples weakly to the system and therefore the use of Born-approximation in deriving Equation (1) is valid. Both thermal neutrons and photons with energy up to x-ray region satisfy this criterion and thus are useful as probes for condensed matter structures.## Keywords

Thermal Neutron Compton Scattering Rayleigh Scattering Random Walk Model Dynamic Structure Factor## Preview

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## References

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© Plenum Press, New York 1981