Why does a pencil beam of electrons expand into a larger cross-sectional beam when incident on a foil?

When a pencil beam of electrons encounters a foil, the interaction with the foil can lead to scattering of the electrons. This scattering is primarily due to the Coulombic interactions between the electrons in the beam and the atomic nuclei present in the foil. As the incoming electrons pass through the foil, they experience deflection caused by the electric fields generated by the positively charged protons in the nuclei.

This interaction results in a spread of the electron paths, causing the initially narrow pencil beam to expand into a larger cross-sectional area. The degree of expansion depends on several factors, including the energy of the electrons and the thickness and material properties of the foil. The more significant the scattering, the larger the effective area of the electron beam becomes. This phenomenon underscores the importance of understanding scattering effects in dosimetry and treatment planning, as it influences the dose distribution in therapeutic applications.

The other options do not accurately represent the primary cause of beam expansion in this context. Energy loss, while related to interactions in the foil, does not directly account for the spatial spreading of the beam. Absorption largely refers to energy being taken out of the beam rather than affecting its size, and although energy loss can lead to scattering, it is not the main reason for beam