Electron string ion sources for carbon ion cancer therapy accelerators

The type of the Electron String Ion Sources (ESIS) is considered to be the appropriate one to produce pulsed C⁴⁺ and C⁶⁺ ion beams for cancer therapy accelerators. In fact, the new test ESIS Krion-6T already now provides more than 10¹⁰ C⁴⁺ ions per pulse and about 5 × 10⁹ C⁶⁺ ions per pulse. Such ion sources could be suitable to apply at synchrotrons. It has also been found that Krion-6T can provide more than 10¹¹ C⁶⁺ ions per second at the 100 Hz repetition rate, and the repetition rate can be increased at the same or larger ion output per second. This makes ESIS applicable at cyclotrons as well. ESIS can be also a suitable type of ion source to produce the ¹¹C radioactive ion beams. A specialized cryogenic cell was experimentally tested at the Krion-2M ESIS for pulse injection of gaseous species into the electron string. It has been shown in experiments with stable methane that the total conversion efficiency of methane molecules to C⁴⁺ ions reached 5%÷10%. For cancer therapy with simultaneous irradiation and precise dose control (positron emission tomography) by means of ¹¹C, transporting to the tumor with the primary accelerated ¹¹C⁴⁺ beam, this efficiency is preliminarily considered to be large enough to produce the ¹¹C⁴⁺ beam from radioactive methane and to inject this beam into synchrotrons.