What is true about protons interacting with matter?

The statement that the rate of ionization increases as protons are slowed in matter is accurate because as protons pass through materials, they lose energy through collisions with electrons, leading to increased ionization events. When protons slow down, their interactions become more frequent due to the reduced speed, resulting in a higher probability of ionizing surrounding atoms. This phenomenon is characterized by the Bragg peak, where protons deposit a significant amount of energy near the end of their path, maximizing ionization in that region.

The other statements do not hold true in this context. For instance, the energies of protons do change as they interact with matter due to energy loss mechanisms such as ionization and excitation. While protons can interact with atomic nuclei, this typically happens at very high energies and is not the main interaction mechanism in most clinical contexts. Lastly, while bremsstrahlung can occur, it is more relevant for charged particles like electrons rather than protons, which primarily ionize matter directly instead of producing significant bremsstrahlung radiation.