Compact Neutron Generator Development and Applications

The Plasma and Ion Source Technology Group at the Lawrence Berkeley National Laboratory has been engaging in the development of high yield compact neutron generators for the last ten years. Because neutrons in these generators are formed by using either D-D, T-T or DT fusion reaction, one can produce either mono-energetic (2.4 MeV or 14 MeV) or white neutrons. All the neutron generators being developed by our group utilize 13.5 MHz RF induction discharge to produce a pure deuterium or a mixture of deuterium-tritium plasma. As a result, ion beams with high current density and almost pure atomic ions can be extracted from the plasma source. The ion beams are accelerated to ~100 keV and neutrons are produced when the beams impinge on a titanium target. Neutron generators with different configurations and sizes have been designed and tested at LBNL. Their applications include neutron activation analysis, oilwell logging, boron neutron capture therapy, brachytherapy, cargo and luggage screening. A novel small point neutron source has recently been developed for radiography application. The source size can be 2 mm or less, making it possible to examine objects with sharper images. The performance of these neutron generators will be described in this paper. Introduction: The RF-driven multicusp ion source developed at Lawrence Berkeley National Laboratory (LBNL) has found numerous applications ranging from neutral beam injection systems for fusion reactors to particle accelerators, proton therapy machines and ion implantation systems. Such sources are simple to operate; have long lifetimes, high gas efficiencies and provide high-density plasmas with high monatomic species yields. These characteristics make the RF-driven ion source a viable candidate for the next generation of compact, high-output, sealedtube neutron generators, utilizing the D-D, T-T or D-T fusion reactions. Recently, LBNL has developed compact, sealed-accelerator-tube neutron generators capable of producing 10 – 10 D-D neutrons per second. There are several enabling technologies contributing to a higher neutron yield in these neutron generators: (a) D yields over 90% have been achieved using 13.5 MHz RF-driven multicusp sources. High monatomic yields are essential for high neutron outputs in low energy accelerator; (b) The source could be operated at low gas pressure (~ 2 mTorr). Low gas pressure operation is necessary to reduce both charge exchange processes and high-voltage breakdown in the accelerator column. These experimental findings will enable one to develop a new generation of compact, high-output, sealed-tube 14 MeV neutron generators based on the D-T fusion reaction. Described below are three different types of neutron sources developed at LBNL for luggage and cargo container inspection. Results: I. Compact Axial Extraction Neutron Generator The compact axial extraction neutron generator is approximately 40 cm in length and 15 cm in diameter. The ion source is a quartz-tube incorporating external antenna. The back plate has the deuterium gas and pressure read-out feed-throughs. The target is housed in an aluminium vacuum vessel and it is insulated from the ground potential with an HV insulator. Figure 1 shows a drawing of the axial extraction neutron tube. The ion beam is extracted from a 3-mm-diameter aperture and accelerated to a target at 100 kV. The target is water-cooled and is made of explosive-bonded titanium-on-aluminum material