What is the range in water for 20 MeV electron beams?

The range of electron beams in water is primarily determined by their energy. For 20 MeV electron beams, the general rule is that the range in centimeters is approximately equal to the energy in MeV divided by 2. This leads to a calculated range of about 10 cm for 20 MeV electrons.

However, in practice, the range can be somewhat adjusted based on various factors, including tissue density and the specific characteristics of the beam, often resulting in a slightly lower effective range. In this context, the most accurate and widely accepted measurement for the range of 20 MeV electron beams in water is around 9 cm, making it the most correct choice.

Understanding the physical principles of electron interactions with matter can further clarify this. As electrons penetrate tissues, they lose energy primarily through ionization and excitation of atoms, leading to a fairly predictable range in water equivalent materials, such as tissue. Thus, while the theoretical value could suggest a slightly larger range, clinical practice and measurements substantiate that around 9 cm provides a practical estimate for treatment planning in radiation oncology.