Penetration of electrons

Penetration of electrons in matter is directly proportional to the density of the material. In unit density material (water), penetration at 1 m.e.v. is of the order of 4.6 mm. The last 20% of this penetration is not generally useful since the total energy conveyed to that depth is but a fraction of that distributed at the surface. 

With a known specific gravity, one can predict the penetration for different thicknesses of material at various energies. The ultimate penetration of electrons with some specific energy is difficult to measure because... 

To allow electrons to transmit through the materials, samples have to be very thin (50-100 nm). The energy of the electrons in the TEM determines the relative degree of penetration of electrons in a specific sample. So energy of 400 kV provides high resolution and high penetration in samples of medium thickness. TEM resolution often exceeds 0.3 nm (Yada et al., 1995). TEM allows to obtain magnifications of 350,000 times and over (www.uq.edu.au/nanoworld/tem gen.html). 

Figure 62 The spatial distribution of the EL emission represented by the amount of emission from a block of 10 monolayers as a function of its distance from the A1 electrode in a PPP LED ITO/PPP (120nm)/Al. The experimental data are indicated by the bars. The curves are exponential functions illustrating an exponentially decreasing penetration of electrons injected from A1 into the PPP film for a set of fixed values of the penetration depths se = 10, 20, 30 nm. After Ref. 342. Copyright 1997 Wiley-VCH, with permission.

The greater the energy of the electrons of the beam and the smaller the mean atomic number of the elements making up the studied sample, the greater the depth of penetration of electrons into the sample. In the electron microscope, the penetration varies from a few tenths to several micrometres, and in p-XRE it is significantly greater, even of the order of millimetres. 

The authors also suggested that because of the limited penetration of electrons, it would be dif-hcult to irradiate samples thicker than 30 mm with electrons. However, nowadays 100-mm-thick samples of most polymers can be easily irradiated using a 10-MeV electron accelerator [Woods and Pikaev, 1994 Singh et al., 1996]. 

From the relation of the element in question to the inert gases, the value of n can thus normally be inferred. Comparison with the apparent value, in spectral terms, gives 8 and hence information about the penetration of electron paths into regions near the nucleus, as referred to earlier. 

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