Reflection shielding mechanism

As already mentioned and shown in Figure 9.1, the primary shielding mechanism is reflection for which shield should possess free charge carriers (electrons/holes) that can interact with incident EM field. Therefore, electrical properties of ICP-based blends and composites are extremely important and their detailed understanding is of paramount importance, for regulation of attenuation as well as shielding mechanism. 

In the search for better understanding of the heat transfer, we must realize that the use of the simple Stefan-Boltzmann law does not apply to the heat transfer across multiple radiation shields separated by fibrous fiber web. The layer of fine fibers between two reflective shields represents an absorbing medium, and thus the applicable heat transfer mechanism becomes complex, similar to that of heat transfer in a gray enclosure containing gray absorbing gas, discussed in simple form in classical textbooks. The mathematical solution of simultaneous complex equations is vastly more complex, and almost impossible without the help of a fast computer. 

It can be seen that, in all cases, relaxation rates are directly proportional to (Aa). Because Aa reflects the anisotropy of the shielding tensor and because the chemical shift originates from the shielding effect, the terminology Chemical Shift Anisotropy is used for denoting this relaxation mechanism. Dispersion may be disconcerting because of the presence of Bq (proportional to cOq) in the numerator of and R2 (Eq. (49)). Imagine that molecular reorientation is sufficiently slow so that coo 1 for all considered values of coo from (49), it can be seen that R is constant whereas R2 increases when Bq increases, a somewhat unusual behavior. 

A ideal Knudsen cell holds the source in an isothermal box with a relatively small exit orfice. This can be implemented by placing the crucible and filament inside a radiation shield without making contact between the three elements. The filament heats the crucible radiatively (there is no gas or direct mechanical connection to conduct heat), and this multiple reflection and conduction inside the crucible homogenizes the temperature along the crucible length. This configuration also increases the temperature that can be achieved for a given power input, since less heat is lost to the environment via radiation. The maximum temperature which can be achieved by a K-cell is limited by the metal elements used to fabricate the cell and the thermocouples, and is typically 1200-1400°C. 

Spinning at the magic angle also accomplishes line-width reduction arising by another mechanism. This is the chemical shift anisotropy (CSA), and is a reflection of the fact that in a solid, it is not possible to ignore the orientational dependance of nuclear shielding. 

Different modes of damage are possible as a result of the mechanical tests. It is necessary to consider the results of these modes for any analytical assessment to demonstrate compliance with the applicable requirements. The fracture of a critical component or the breach of the containment system may allow the escape of the radioactive material. Deformation may impair the function of radiation or thermal shields and may alter the configuration of fissile material and it should be reflected in the assumptions and predictions in the criticality assessment. Local damage to shielding may, as a result of the subsequent thermal test, give rise to deterioration of both thermal and radiation protection. Consequently, investigations should include stress, strain, instability and local effect for all attitudes of drop where symmetry does not prevail. 

The attenuation of an electromagnetic wave can occur by three mechanism absorption (A), reflection (R) and multiple reflection (B). Thus shielding effectiveness is the sum of three terms ... 

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