Beam-induced back-streaming electron suppression analysis for an accelerator type neutron generator designed for 40Ar/39Ar geochronology

A facility based on a next-generation, high-flux D-D neutron generator has been commissioned and it is now operational at the University of California, Berkeley. The current generator designed for ⁴⁰Ar/³⁹Ar dating of geological materials produces nearly monoenergetic 2.45 MeV neutrons at outputs of 10⁸ n/s. The narrow energy range is advantageous relative to the ²³⁵U fission spectrum neutrons due to (i) reduced ³⁹Ar recoil energy, (ii) minimized production of interfering argon isotopes from K, Ca, and Cl, and (iii) reduced total activity for radiological safety and waste generation. Calculations provided show that future conditioning at higher currents and voltages will allow for a neutron output of over 10¹⁰ n/s, which is a necessary requirement for production of measurable quantities of ³⁹Ar through the reaction ³⁹K(n,p)³⁹Ar. A significant problem encountered with increasing deuteron current was beam-induced electron backstreaming. Two methods of suppressing secondary electrons resulting from the deuterium beam striking the target were tested: the application of static electric and magnetic fields. Computational simulations of both techniques were done using a finite element analysis in COMSOL Multiphysics^®. Experimental tests verified these simulations. The most reliable suppression was achieved via the implementation of an electrostatic shroud with a voltage offset of -800 V relative to the target.