This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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The depth sensitivity of REELS can be varied by changing the primary-beam energy [1] or the incidence and collection angles.[2] The sampling depth probed in REELS depends on the primary electron beam energy, can be described based on the first-order term in the Poisson distribution, and is given by the probability of the occurrence of one inelastic collision,

          --------------------- [2301a]

where,
         d -- The depth;
         λ -- The inelastic mean free path, depending on the primary electron beam energy;
         p₁ -- The probability that one inelastic collision occurs in depth d.

The EELS sampling depth is given by,

          --------------------- [2301b]

where,
          α -- The incidence angle;
          β -- The collection angle.

In REELS measurements, d is normally defined as the depth from which 95% of the inelastically scattered electrons are collected. For many materials, the d is often in the range of less than 5 nm at the beam energies of less than 1 keV.

[1] D. F. Cox and G. B.Hoflund, Surf. Sci. 151,202 (1985).
[2] P. Sen, M. S. Hegde, and C. N. R. Rao, Appl. Surf. Sci. 10, 63 (1982).