Continuous slowing down

The picture has not been confirmed experimentally because time scales of less than 10 ps are not accessible at present and there are difficulties envisaged In reducing this limit below 1 ps. However, as a theoretical model It fits much of the experimental data and Is of much greater value than that which uses the continuous slowing down approximation whereby energy Is assumed to be deposited continuously along the track. 

Figure 4 Continuous slowing down (CSDA) range and root mean square (rms) penetration in gaseous water according to LaVerne and Mozumder [36]. See text for details.

Figure 2-3. Stopping power (left) and continuous slowing down approximation (CSDA) range (right) of proton (p), a-particle (a) and electron (e) in polyethylene. Data were taken from [14] for proton and a-particle, and [5] for electron, respectively.

Eigure 2.3 is a graph of the range calculated by the continuous-slowing-down approximation (CSDA) of an alpha particle, in both air and aluminum. The data that generated the graph (ICRU 1993) indicate that a 5-MeV alpha particle has a... 

It is important to note that the concept of a slowing-down density is not limited to the continuous slowing-down model. This function has a much wider application, and it will be used later in connection with the analysis of the slowing-down process which allows for the step-function behavior illustrated in Fig. 4.10 (cf. Sec. 4.2c). 

S A point source in an infinite medium is emitting neutrons of zero lethargy isotropically at a constant rate in time. Assume that the slowing-down process may be described by means of the Fermi age model once the first collision has occurred. Thus at points of first collision, the neutrons enter a continuous slowing-down process. 

The symbol roiu) denotes the age baaed on the Fermi continuous slowing-down model. 

Apart from this limitation, the principal restriction in the use of the Fermi age model is due to the large number of conditions which must be satisfied by the physical system if the model is to be valid (see Sec. 7.3i). However, even with this shortcoming, the model is still very useful for preliminary studies and for gaining insight into the essential features of many reactor problems. The Fermi age or continuous-slowing-down diffusion theory, then, offers some generality without introducing excessive computational difficulties. 

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