5. define the term "mean free path" as applied to gamma rays.

The average free path λ is the average distance of  electron travel between events of a certain type, that is, the probability that a neutral causes an interaction traveling a "x" distance.

Explanation:

For densities of atoms and microscopic effective sections there are orders of magnitude of 10^23 cm^3 and 10^-23 cm^3  to 10^-24 cm^2 respectively, the typical macroscopic effective sections are of the order of 1 to 10^-1 cm^-1.

The inverse of the macroscopic effective section can be interpreted as the mean free path for neutrons that cause the interaction.

λ = A / N * p * Q

where:

A:  atomic weight (g/mol)

N: Avogadro's number (6.2 *10^23 atoms / mol)

p: density (g/cm^3)

The average free trajectory for all types of events is considered individual dispersions. It should be kept in mind that the average free trajectory is always smaller than the smallest average free trajectory of the events involved.