Attenuation length

In physics, the attenuation length or absorption length is the distance ${\displaystyle \lambda }$ into a material when the probability has dropped to ${\displaystyle 1/e}$ that a particle has not been absorbed. Alternatively, if there is a beam of particles incident on the material, the attenuation length is the distance where the intensity of the beam has dropped to ${\displaystyle 1/e}$, or about 63% of the particles have been stopped.

Mathematically, the probability of finding a particle at depth x into the material is calculated by Beer-Lambert law:

...
Wikipedia