Radiobiological Experiments with Heavy Ions at the Bevatron

Penetration of heavy charged particles can be characterized by three basic physical facts: 1) almost no scattering; 2) abrupt increase of linear energy deposition (LET) close to the point where the particles stop; and 3) exact range-energy relationship. These facts constitute the basis of very favorable depth-dose characteristics for heavy ions to be used in radiotherapy. Availability of accelerated high LET heavy ions in the Laboratory, though a recent development, has already provided us with much useful information in physics, chemistry, and radiobiology. Fast heavy ions could previously be studied only in outer space where they form important components of primary cosmic rays. Heavy ions in the Laboratory became available as early as 1957 at the Berkeley HILAC, but at very low energies. In August, 1971, two accelerators, the Princeton Synchrotron1 and the Berkeley Bevatron produced penetrating deflected beams of nitrogen nuclei. Since that time, carbon, oxygen and neon, as well as a few oxygen particles at Princeton, have been accelerated. Stopping-power curves as a function of range for various ions in water, as calculated theoretically, are shown in Figure 1. Various ion energies in units of Mev/nucleon are designated on each curve. The uniformly shaded area represents the stopping-power and associated ions accelerated at the HILAC and cyclotron. The BEVALAC (a compound accelerator formed from the HILAC and Bevatron) adds a new dimension, and initially it will be able to accelerate i ons wi th atomi c no. up to that of iron (Z = 26) to considerable energies.