FLASH irradiator at the Radiological Research Accelerator Facility.

The Radiological Research Accelerator Facility has modified a decommissioned Varian Clinac to deliver FLASH dose rates: Operating in 9 MeV electron mode (FLASH mode), samples can be irradiated inside the Clinac head at average dose rates of up to 600 Gy/sec (3 Gy per 0.5 µsec pulse, 180 pulses per sec). In this mode multiple pulses are required for most irradiations. By modulating pulse repetition rate, dose rates below 1 Gy/min can be achieved, allowing comparison of FLASH and conventional irradiations with the same beam. Operating in 6 MV photon mode, with the conversion target removed (superFLASH mode), samples are irradiated at instantaneous dose rates of up to 1 GGy/sec (0.2-150 Gy per pulse, 360 pulses per sec) and most irradiations can be performed with a single very high dose rate pulse. In both modes we have seen the expected inverse relation between dose rate and irradiated area, with the highest dose rates obtained for beams with a FWHM of about 2 cm and ±10% uniformity over 1 cm diameter. As an example of operation of the FLASH irradiator, we present dose rate dependence of dicentric chromosome yields.