Recent progress in the design, construction, and operation of a neutron gas target source for cancer therapy

We have designed, constructed, and tested a gas target neutron source system at the University of Wisconsin. The system consists of a duo-plasmatron ion source, an extraction electrode, an electrostatic acceleration system, a quadrupole triplet lens, and a differentially pumped impedance system. The goal of our project is to develop a dt source which can deliver 10 rads per minute to a tumor 10cm below the surface of the patient at 120cm SSD. The differential pumping system currently consists of two mercury pumps Roots blower. The mercury pumps can operate into high (30T) backing pressure. The differentially pumped impedance system consists of four long narrow apertures. The angular acceptance of the impedance system is 2cm-m rad. Operational tests of the entire system have been carried out using the d,d reaction to reduce the radiation hazard. the d,d tests indicate that if we had tritium in the target we would have produced 4x1012n/s which corresponds to a dose rate of 8 rads per minute at 125cm SSD. The amount of current which can be focused into the gas target appears to be the present limitation on the source strength. We have redesigned and are constructing an improved pumping system to obtain higher deuteron beam currents in the gas target. The unit has been relocated in a shielded building where full scale output tests using tritium will begin in the near future