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Compact Neutron Generators for Medical, Home Land Security, and Planetary Exploration* J

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Compact Neutron Generators for Medical, Home Land Security, and Planetary Exploration* J Proceedings of 2005 Particle Accelerator Conference, Knoxville, Tennessee COMPACT NEUTRON GENERATORS FOR MEDICAL, HOME LAND SECURITY, AND PLANETARY EXPLORATION* J. Reijonen#, E. O. Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA Abstract years[8]. These applications include H- injectors for large The Plasma and Ion Source Technology Group at accelerator systems[9] and for tokamak-fusion Lawrence Berkeley National Laboratory has developed applications[10], positive ion sources for advanced various types of advanced D-D (neutron energy 2.5 MeV), focused ion beam systems[11] and for semiconductor D-T (14 MeV) and T-T (0 – 9 MeV) neutron generators applications[12]. This RF-technology with unique for wide range of applications. These applications include impedance matching circuitry and source designs capable medical (Boron Neutron Capture Therapy), homeland of high power operation is now being used in various security (Prompt Gamma Activation Analysis, Fast neutron generators. RF-induction discharge ion source has Neutron Activation Analysis and Pulsed Fast Neutron unique features that make it an excellent source type for Transmission Spectroscopy) and planetary exploration high power, high yield neutron generators. These features with a sub-surface material characterization on Mars. include the high plasma density generation for high beam These neutron generators utilize RF induction discharge current extraction, extremely high atomic ion generation to ionize the deuterium/tritium gas. This discharge method from molecular gases like hydrogen, and reliable, long provides high plasma density for high output current, high life operation. These ion sources are currently used in atomic species from molecular gases, long life operation three different types of neutron generators; axial, single and versatility for various discharge chamber geometries. ion beam neutron generator; co-axial, multi-beamlet, high Four main neutron generator developments are discussed current neutron generator; and point neutron generator here: high neutron output co-axial neutron generator for with high instantaneous neutron yields. The latest addition BNCT applications, point neutron generator for security to the line of neutron generators is a sub-compact, low applications, compact and sub-compact axial neutron yield neutron generator for sub-surface water/hydrogen generator for elemental analysis applications. Current analysis applications. These different types of neutron status of the neutron generator development with generators will be discussed in the following sections. experimental data will be presented. AXIAL, HIGH VOLTAGE SHIELDED INTRODUCTION NEUTRON GENERATOR Compact neutron generators are becoming an attractive The axial, single beam neutron generator is a alternative to nuclear reactors and radioactive neutron development from the sectioned insulator, single beam sources in variety of fields of neutron science[1], medical neutron source, which was developed mainly for pulsed research[2] and various material analysis applications, beam applications. It’s design and operation is described ranging from coal and cement analysis[3] to various elsewhere[13]. The sectioned insulator neutron generator explosive detection schemes, from home-land security has been operated successfully for more than 500 hours. applications[4-6] to other explosive detection The external RF-antenna ion source has thus proofed the applications, like land mine detection[7]. Traditionally ability to provide long-life time operation with extremely compact neutron generators have been used in oil well stable operation characteristics. logging industry, using D-T fusion reaction for high In some applications there is a need for a neutron energy 14 MeV neutron production. Currently the main generators that do not have exposed high voltage commercial manufacturers of compact neutron generators elements. Thus the neutron generator can be placed close are Thermo Electron, Sodern EADS and Sclumberger. to various moderator materials or experimental devices. These neutron generators generate D-T neutron yield in 8 10 The latest axial neutron generator development has all the the range of 10 to 10 n/s. high voltage elements shielded by a grounded The Plasma and Ion Source Technology Group at the E. vacuum/target chamber. Figure 1 shows for a schematic O. Lawrence Berkeley National Laboratory has been drawing of the fully high voltage shielded neutron developing high power D-D neutron generators for generator. various applications. These neutron generators are The main performance parameters of the shielded differentiated from the commercial manufacturers mainly neutron generator are the following; D-D operation with by the method the plasma is generated and up to now by vacuum pumping or semi-sealed operation, actively the use of D-D, instead of D-T, fusion reaction. The water-cooled target, external antenna configuration, few P&IST group has been pioneering in RF-induction milli-amperes of extracted current, accelerator voltage 80 discharge for different applications for more than twenty – 100 kV and estimated neutron yield of ~108 n/s. The * This Work was supported by Tensor Technology Inc., by EUROSIA, RF-induction ion source has an aluminum outer cylinder Italy and by Department of Energy under Contract No. DE-AC03- 76SF00098. for efficient RF-noise suppression. Ion current density and #Email: [email protected] 49 0-7803-8859-3/05/$20.00 c 2005 IEEE Proceedings of 2005 Particle Accelerator Conference, Knoxville, Tennessee hydrogen atomic species distribution for the metal shield cooled back-plate of the ion source. The target material is source design are shown in Figure 2. titanium-coated copper. This neutron generator is fabricated for the Nuclear Engineering Department of the University of California Actively water- Berkeley to replace the 252Cf radioactive neutron source. cooled target The enclosed neutron generator will be tested during HV feed- Summer 2005 and the performance will be evaluated in through and one the P&IST group’s neutron generator test stands. water-cooling HIGH POWER CO-AXIAL NEUTRON RF-induction GENERATOR FOR MEDICAL ion source APPLICATIONS As previously reported[15,16], studies has been made Target shroud in determining the neutron yield required to achieve for secondary reasonable treatment time for NCT (Neutron Capture electron filtering Therapy), either for brain or liver tumor treatments. It has been shown that a liver tumor treatment can be carried out in 45 minutes, if ~1013 n/s D-T neutron generator is used. For the first phase in developing a treatment system for Figure 1: Fully high voltage shielded and grounded liver cancer, a high power D-D neutron generator has neutron generator design for medium neutron yield. been designed, fabricated and delivered to an Italian university and hospital consortium in Turin, Italy, see Fig. 3. The purpose of the neutron generator is to demonstrate 100% 110 the feasibility of operating a neutron generator at the 95% 100 power level required for treatments, using D-D reaction. ] 11 2 The yield of this neutron generator is 10 n/s. This is 90 90% achieved, using 350 mA of beam current and 120 kV of 80 acceleration voltage. The ion source performance and the 85% neutron generator design is presented elsewhere[13]. 70 80% 60 75% 50 70% Beam Species Fraction 40 Hydrogen Atomic Species Current Density [mA/cm 65% 30 Current Density 60% 20 0 1000 2000 3000 4000 PRF [W] Figure 2: Hydrogen atomic species fraction and the extracted current density as a function of the RF-discharge power at 8 mTorr source gas pressure. The neutron generator has a single gap extraction system. The geometries of the extraction system and the target shroud are simulated by using an IGUN ion extraction and transport code[14]. The design goal is to 2 have a power load in the target of the order of 500 w/cm or less. The biasing voltage of the secondary electron filter 11 Figure 3: The 10 n/s D-D co-axial neutron generator shroud is generated by zener diodes. The biasing is mounted in one of the P&IST group neutron test stands. necessary to minimize the potentially destructive high The neutron generator is 80 cm tall with the pumping intensity secondary electron beam. These electrons are chamber and 40 cm in diameter. originating from the target surface and they can accelerate towards the ion source. The lower intensity electron beam The neutron generator was delivered to a test stand in formed in the gas at the accelerator and the beam transport the, Physics Department of the University of Turin, Italy section inside the filter shroud is dumped onto the water- in November 2004. Currently the neutron generator is 0-7803-8859-3/05/$20.00 c 2005 IEEE 50 Proceedings of 2005 Particle Accelerator Conference, Knoxville, Tennessee being operated for neutron production and the first antenna configuration, the required discharge power level experiments are being carried out. is fairly high. This is due to the size of the discharge vessel. Preliminary results of the 10 cm in diameter, 90 POINT NEUTRON GENERATOR cm long prototype ion source are shown in Fig. 6. Other types of ion source options will be explored during Point neutron generator is developed specifically for summer 2005. PFNTS (Pulsed Fast Neutron Transmission Spectroscopy), an explosive detection scheme, developed by Tensor Technology Inc. The detection method is described in detail in ref.[17]. The requirements for the neutron generator are fairly demanding. The detection
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