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

Abstract

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 Ar/Ar dating of geological materials produces nearly monoenergetic 2.45MeV neutrons at outputs of 10n/s. The narrow energy range is advantageous relative to the U fission spectrum neutrons due to (i) reducedAr 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 10n/s, which is a necessary requirement for production of measurable quantities of Ar through the reactionK(n,p)Ar. 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 -800V relative to the target.

Possible evidence of nonstatistical properties in the Cl35(n, p)S35

“…The HFNG (a cutaway illustration is shown in Fig. 3) [26][27][28] is a self-loading custom D + D (DD) neutron generator. A 100 kV deuterium beam is extracted from an RF-heated deuterium plasma through a nozzle, whose shape was designed to form a flat-profile beam, 5 mm in diameter.…”

“…Instead, the shroud causes the electrons to experience a force pushing them back towards the target. A more detailed explanation of this technique for secondary electron suppression is described in Ref [28].…”

Measurement of the 64Zn,47Ti(n,p) cross sections using a DD neutron generator for medical isotope studies

“…This generator extracts deuterium ions from an RF-heated deuterium plasma (using ion sources similar to designs from the Lawrence Berkeley National Laboratory [4]) through a nozzle, whose shape was designed to form a flat-profile beam, 5 mm in diameter. This deuterium beam is incident upon a water-cooled, self-loading titanium-coated copper target [2,3], where the titanium layer acts as a reaction surface for DD fusion, producing neutrons with a well-known energy distribution as a function of emission angle [23]. While the machine's design features two deuterium ion sources impinging from both sides of the target, only a single source was used in the present work.…”

“…This interest is due to the volumetric absorption of neutrons as compared to charged particle beams (ranges of g /cm 2 as compared to 10's of mg /cm 2 ), together with the fact that isotope production facilities often produce large secondary neutron fields. Particular interest has been paid to (n,p) and (n,α) charge-exchange reactions since these reactions produce high-specific activity radionuclide samples without the use of chemical carriers in the separation process.…”

“…The research group at UC Berkeley has developed a High Flux Neutron Generator (HFNG) that features an internal target where samples can be placed just several millimeters from the neutron producing surface in order to maximize the utilization of the neutron yield for the production of a desired radionuclide [2,3,4]. The HFNG uses the D(D,n) 3 He reaction to produce neutrons with energies near 2.…”