Neutron Tube Design Study for Boron Neutron Capture Therapy Application - eScholarship

NEUTRON TUBE DESIGN STUDY FOR BORON NEUTRON CAPTURE THERAPY APPLICATION J. M. Verbeke #a,b , Y. Lee a , K. N. Leung a , J. Vujic b , M. D. Williams a , L. K. Wu a and N. Zahir a,b a Lawrence Berkeley National Laboratory b Nuclear Engineering Department, University of California, Berkeley Berkeley, CA 94720 USA Abstract Radio-frequency (RF) driven ion sources are being developed in Lawrence Berkeley National Laboratory (LBNL) for sealed-accelerator-tube neutron generator application. By using a 5-cm-diameter RF-driven multicusp source H + yields over 95% have been achieved. These experimental findings will enable one to develop compact neutron generators based on the D-D or D-T fusion reactions. In this new neutron generator, the ion source, the accelerator and the target are all housed in a sealed metal container without external pumping. Recent moderator design simulation studies have shown that 14 MeV neutrons could be moderated to therapeutically useful energy ranges for boron neutron capture therapy (BNCT). The dose near the center of the brain with optimized moderators is about 65% higher than the dose obtained from a typical neutron spectrum produced by the Brookhaven Medical Research Reactor (BMRR), and is comparable to the dose obtained by other accelerator-based neutron sources. With a 120 keV and 1 A deuteron beam, a treatment time of ~35 minutes is estimated for BNCT. Recently, LBNL has developed a compact, sealed- accelerator-tube neutron generator capable of producing a neutron flux in the range of 10 9 to 10 10 D-T neutrons per second [2-3]. The ion source, a miniaturized variation of earlier RF-driven multicusp ion sources, is designed to fit within a 5-cm-diameter borehole. Typical operating parameters include repetition rates up to 100 pps, with pulse widths between 10 and 80 µs (limited only by the available RF power supply) and source pressure as low as 5 mTorr. In this configuration, peak extractable hydrogen current densities exceeding 1 A/cm 2 with H + ion yields over 94% have been achieved. From this output, a D-T neutron yield of 10 9 neutrons per second can be projected. Simple scaling of the ion source and extraction aperture size could bring the neutron output even higher. These experimental findings together with recent ion source testing and moderator design [4] will enable one to develop compact 14 MeV neutron generators based on the D-T fusion reaction. In this new system, the ion source, the accelerator and the target are all housed in a sealed metal container without pumping. With a 120 keV and 1 A average D + beam current, it is estimated that a treatment time of ~35 minutes is needed for boron neutron capture therapy (10 14 neutrons/sec). This article describes the design and characteristics of the new neutron generator. 1 INTRODUCTION The RF-driven multicusp ion source developed at Lawrence Berkeley National Laboratory has found numerous applications ranging from neutral beam injection systems for fusion reactors to particle accelerators, proton therapy machines and ion implantation systems [1]. 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, sealed-tube neutron generators, utilizing the fusion of deuterium and tritium, or deuterium and deuterium. 2 NEUTRON TUBE DESIGN In order to achieve a neutron yield of 10 14 neutrons/sec, a large multicusp source together with a multi-aperture extraction system to produce an ion beam current of 1 A, accelerated to 120 kV, and impinging on a well-cooled target is required. The main components of the sealed D-T neutron tube are the ion source, the 120 kV accelerator column, the water-cooled target and the vacuum system. Figure 1 shows a schematic diagram of the sealed D-T neutron generator. It is a scale up version of the compact neutron tube that LBNL has developed. The characteristics of this neutron generator are as follows. This work is supported by Sandia National Laboratory and the US Email: jmverbeke@lbl.gov Department of Energy under contract No. DE-AC03-76F00098.