The basic principle used by EPT to measure the energy of the particles is quite intuitive:
Moreover, the penetration depth will also depend on the nature of this projectile: an electron, a proton and an alpha particle (Helium ion) with the same energy will penetrate in decreasing depths in a given material and the energy deposited by length unit will increase from the electron to the alpha.
That is why for a given particle, the measurement of the penetration distance x in the absorption material gives an indication on the initial energy of this particle.
In practice, the absorbing material is frequently chosen to be active, like a semiconductor (silicon) that gives an electric signal proportional to the energy deposition (detector).
To stop the spatial electrons, protons and alpha particles, depths about cm or more would be necessary, but industrial and physical building constraints of semiconductor detectors limit their actual widths from 0.05 to 0.3 mm.