Depth-dose relations for heavy ion beams

Radiation transport of heavy ions in matter is of interest to radiological protection in space and high-altitude aircraft. In addition, heavy ion beams are expected to be of advantage in radiotherapy since their characteristic Bragg curve allows a relative reduction of the dose in reaching a tumor site and the near elimination of exposure beyond the tumor region as the beam exits the body. Furthermore, the radioresistance of tumorous cells due to their hypoxic state may be reduced or eliminated by the high specific ionization of heavy ion beams. The depth-dose distribution of heavy ion beams consists of energy deposited by the attenuated primary beam with its characteristic Bragg curve and a relatively unstructured background due to secondary radiations produced in nuclear reactions. As the ion mass increases, the secondary contribution becomes more structured and may add significantly to the Bragg peak of the primary ions. The result for heavy ions (z greater than 20) is a greatly broadened Bragg peak region, especially in comparison to straggling effects, which may prove to be of importance in radiotherapy and biomedical research.