Thermalization path length

In the problem of retardation of subexcitation electrons, the two important characteristics are the thermalization time and the thermalization path length. In condensed media the key role is played by thermalization path length, which determines how far can an electron travel away from its parent ion when it is thermalized. The thermalization path length determines the probability of formation of a free ion. 

The thermalization path length of subexcitation electrons has been the object of many discussions from the time the first track models appeared up to this day. The reason is that for quite a long time there were no direct methods of measuring the path lengths of slow electrons, while the corresponding theoretical analysis is very difficult owing to the need to take into account all the processes relevant to retardation of subexcitation electrons. 

However, we have no way of knowing the form of t](E), to say nothing of g(E, r). So in order to determine the thermalization path length from radiation-chemical experiments, one usually performs the following procedure. One introduces the function F(r) according to the equation... 

So, as one can see, this method of determining the thermalization path length is not straightforward and involves many assumptions and suppositions. First, the value of r0 depends on the specific choice of the function F r) (see, e.g., Ref. 269) and is too uncertain to enable us to determine the processes that are responsible for retardation of subexcitation electrons in a medium (see the discussion in Ref. 271). However, by comparing the values of r0 for different substances we are able to determine some of the factors that affect the path length of slow electrons. Since the value of r0 depends on the density p of the liquid, it is reasonable to compare the products prQ rather than the values of r0 themselves. 

It is practically impossible to reconstruct the form of F(x) in detail by measuring the photocurrent. However, its first two moments can be measured in photoemission experiments. The first moment of the function F(x) is the projection of the average thermalization path length of electrons on the normal to the surface of the cathode ... 

In recent years the energy dependence of thermalization path lengths has been studied experimentally using a method based on photoionization by light of different wavelengths of a molecule-additive introduced into the matrix being studied. The molecule-additives often used are, for instance, those of pyren or of /V,/V,./V, ./V -tetramethyl-p-phenylene-... 

Thermalization Path Length of Slow Electrons in Water Electrolytic Solutions292... 

In the case of isotropic scattering, the thermalization path length is r0 - V3jc0. 

The core is surrounded by a graphite reflector (i.e., normal graphite, not graphite foam). The fuel is to be loaded into the foam s pores, thus minimizing the thermal path length between the fuel and the heat sink. The uranium-impregnated foam would be encased in a superalloy steel. The reflector/clad interface temperature is expected to be limited to 900 K. The reflector thickness is approximately 30 cm. 

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