Nuclear inelastic interaction

The organization of this chapter is as follows. In the following section, Sec. 4.2, the elastic and inelastic interaction cross sections necessary for simulating track structure (geometry) will be discussed. In the next section, ionization and excitation phenomena and some related processes will be taken up. The concept of track structure, from historical idea to modern track simulation methods, will be considered in Sec. 4.4, and Sec. 4.5 deals with nonhomogeneous kinetics and its application to radiation chemistry. The next section (Sec. 4.7) describes some application to high temperature nuclear reactors, followed by special applications in low permittivity systems in Sec. 4.8. This chapter ends with a personal perspective. For reasons of convenience and interconnection, it is recommended that appropriate sections of this chapter be read along with Chapters 1 (Mozumder and Hatano), 2 (Mozumder), 3 (Toburen), 9 (Bass and Sanche), 12 (Buxton), 14 (LaVerne), 17 (Nikjoo), and 23 (Katsumura). 

The possibility to finely tune the energy of the photons using high-resolution monochromators (HRMs) has resulted in a new technique for direct determination of the phonon density of states of the resonant element— the nuclear inelastic scattering (NIS) [23-25]. Thus, in the same experimental setup, one is able to probe simultaneously hyperfine interactions and lattice dynamics of the sample. 

Nuclear forward scattering (NFS) allows to study hyperline interactions, as obtained with conventional Mossbauer spectroscopy nuclear inelastic scattering (NIS) allows to investigate local phonon spectra (partial density of states, PDOS) at the Mossbauer probe nucleus. Compared, for instance, to Raman spectroscopy, NIS can achieve a higher resolution without perturbation of surrounding vibrations. Both synchrotron radiation techniques, NFS and NIS, are certainly on then-way to a great future. 

The problem of cross-section calculation for various inelastic collisions is mathematically equivalent to the solution of a set (in principle, infinite) of coupled wave equations for nuclear motion [1]. Machine calculations have been done recently to obtain information about nonadiabatic coupling in some representative processes. Although very successful, these calculations do not make it easy to interpret particular transitions in terms of a particular interaction. It is here that the relatively simple models of nonadiabatic coupling still play an important part in the detailed interpretation of a mechanism, thus contributing to our understanding of the dynamic interaction between electrons and nuclei in a collision complex. 

Because neutrons are electrically neutral, their interaction with electrons is very small and primary ionization by neutrons is negligible. The interaction of neutrons with matter is practically confined to the nuclei and comprises elastic and inelastic scattering and nuclear reactions. In elastic collisions the total kinetic energy remains constant, whereas in inelastic collisions part of the kinetic energy is given off as excitation energy. 

Different types of interaction are distinguished, as illustrated in Fig. 8.23. (The spherical form is a simplification which is only applicable for nuclei with nuclear spin / = 0.) On path 1 the nuclei are not touching each other elastic scattering and Coulomb excitation are expected. On path 2 the nuclei are coming into contact with each other and nuclear forces become effective inelastic scattering and transfer reactions... 

Inelastic neutron scattering spectroscopy is characterized by completely different intensities because the neutron scattering process is entirely attributable to nuclear interactions [110] Each atom features its nuclear cross section, which is independent of its chemical bonding. Then the intensity for any transition is simply related to the atomic displacements scaled by scattering cross sections. And because the cross section of the proton is about one order of magnitude greater than that for any other atom, the method is able to record details of quantum dynamics of proton transfer. 

From the comparison of the measured and calculated temperature dependences of the relaxation time (see Fig. 20), it follows that the inelastic phonon scattering is the most essential mechanism of the spin-lattice relaxation for Ge. It is evident that only at low temperatures T < 30K) some other mechanisms (the most probable one is the relaxation due to a small amount of paramagnetic impurities) become dominant. At T > 300/C some additional mechanism of relaxation may also exist. The interaction of the nuclear quadrupole moment with vibrations of the nearest four Ge atoms brings about the main contribution to the spin-lattice relaxation rate. The effective modulation of the EFG by the nearest bond charges is greatly reduced because of strong correlations between their displacements. As the main result of the present investigation of spin-lattice relaxation,... 

The transformed equation (22) would describe elastic molecular scattering processes only. The reason is that =8kJt- The inclusion of the nuclear part of the (Vu+Vrm) interaction operator permits describing inelastic scattering processes among rovibration states in the reactant molecular subspace. 

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