- Email: [email protected]
Development of a Time-tagged Neutron Source for SNM Detection
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 66 (2015) 105 – 110
C 23rd Conference on Application of Accelerators in Research and Industry, CAARI 2014
Development of a Time-Tagged Neutron Source for SNM Detection Qing Jia,*, Bernhard Ludewigta, Joe Walliga, Will Waldrona, Jim Tinsleyb a
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, U.S. A. b Special Technology Laboratory, Santa Barbara, CA 93111, U. S. A.
Abstract Associated particle imaging (API) is a powerful technique for special nuclear material (SNM) detection and characterization of fissile material configurations. A sealed-tube neutron generator has been under development by Lawrence Berkeley National Laboratory to reduce the beam spot size on the neutron production target to 1 mm in diameter for a several-fold increase in directional resolution and simultaneously increases the maximum attainable neutron flux. A permanent magnet 2.45 GHz microwave-driven ion source has been adopted in this time-tagged neutron source. This type of ion source provides a high plasma density that allows the use of a sub-millimeter aperture for the extraction of a sufficient ion beam current and lets us achieve a much reduced beam spot size on target without employing active focusing. The design of this API generator uses a custom-made radial high voltage insulator to minimize source to neutron production target distance and to provide for a simple ion source cooling arrangement. Preliminary experimental results showed that more than 100 PA of deuterium ions have been extracted, and the beam diameter on the neutron production target is around 1 mm. © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014. Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 Keywords: neutron generator, associated particle imaging, SNM detection
1. Introduction Associated particle imaging (API) uses the deuterium-tritium fusion reaction 2
D3T o4He(3.5MeV ) n(14.1MeV )
* Corresponding author. Tel.: +1-510-486-4802; fax: +1-510-486-4888. E-mail address: [email protected]
1875-3892 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 doi:10.1016/j.phpro.2015.05.015
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Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
that produces an alpha particle in coincidence with the neutron. Both particles are kinematically correlated and emitted in nearly opposite directions. By detecting the alpha particle with a position sensitive detector the neutron’s time of emission and direction can be determined (See work by Beyerle et al (1990)). This information can be used in various ways for 2D or 3D imaging. API applications such as detection of mines, Improvised Explosive Devices, and explosives and other contraband in cargo, vehicles, etc., have been extensively studied by Beyerle et al (1990), Pesente et al (2009), and Chichester et al (2009). Using the timing and position information obtained to enhance the signal of interest in the presence of background, API offers new capabilities for enhanced SNM detection and for the characterization of fissile material configurations. Prompt neutron and gamma rays emitted in induced fission events can be correlated in time with the tagged primary neutron to greatly enhance the fission signal. Similarly, the known direction of the emitted neutrons can be used to eliminate the background generated by those neutrons that do not intercept a selected region of the interrogation space. Both, induced fission imaging and neutron transmission imaging methods developed by Hausladen (2007) and Cates (2012) for use in proliferation detection and nuclear safeguards, will benefit from much improved directional resolution and higher neutron rates. The goal of the time-tagged neutron source development effort reported here is to build a sealed-tube API generator that reduces the beam spot size on the neutron production target to 1 mm or less in diameter for a roughly five-fold increase in directional resolution and simultaneously increases the maximum attainable neutron yield (up to 109 neutrons/sec) by approximately one order of magnitude over existing sources. This article describes the design of the generator and first experimental results. 2. Approach An advanced ion source is the key for achieving the performance goals of the API generator. The ion source must produce a sufficiently high beam current density and operate at a sufficiently low gas pressure for sealed-tube operation. After researching Penning-discharge (Sy et al (2012)), RF-driven (Wu et al (2009)) and microwavedriven ion sources (Ji (2011)), the physics design of the API generator was established based on a microwave-driven ion source. This type of ion source has been selected because of its high current density that allows us to use a submillimeter aperture for the ion extraction and to achieve a much reduced beam spot size on target without employing active focusing. Ion source and beam optics studies will be reported on in another article. Mixed deuterium and tritium ion beams are extracted from the ion source through a sub-millimeter aperture and accelerated up to 100 keV when reaching the target. Neutrons are produced in a titanium layer, which is loaded by the beam with a deuterium-tritium mixture. For system compactness and simplicity, no active focusing elements are employed. The alpha-particle detector, a scintillation detector with a position sensitive readout, views the target at a 90 degree angle to the beam at a minimum distance of 10 cm. The high-voltage design must allow for an acceleration voltage between ion source and target of up to 100 kV. In order to avoid secondary electrons being accelerated into the alpha detector, which is on ground potential, the target is at ground potential and the microwavedriven ion source of the API system is at positive high voltage. This is the opposite of how the acceleration voltage is normally applied and poses significant design challenges. 3. Engineering Design The design of the API generator with its four key components – ion source, high voltage (HV) insulator, neutron production target, and alpha detector – must satisfy several basic engineering requirements. Since a deuterium-tritium gas mixture is used in the generator, the generator must be constructed as a sealed-tube with ultrahigh vacuum (UHV) and tritium-compatible materials. Given that microwave power of over 200 W is dissipated in the ion source, liquid cooling is needed to keep the permanent magnets surrounding the plasma chamber from getting too hot. Figure 1 shows a cross-sectional view of the API generator. The custom-made radial HV insulator was chosen because this configuration allows for the neutron production target to be positioned at the minimum distance from the ion source while allowing for relative ease of construction and a rather simple ion source cooling arrangement.
Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
The source-target distance of ~ 12 cm leaves just enough room for the alpha-particle detector and minimizes the beam spreading on the target.
Fig. 1. Cross sectional view of the API generator.
Fig. 2. Cross sectional view of the permanent magnet microwave-driven ion source.
3.1 Ion Source Figure 2 shows the engineering design of the compact, permanent-magnet microwave source. The source chamber is made of stainless steel, 4 cm in diameter and 5 cm in length. An aluminium liner is press-fitted inside the stainless steel cylinder to provide an aluminium surface for optimal ion source operation and atomic ion fraction. The three ring-shaped permanent magnets generate the required axial magnetic field. A stack of alumina discs, which are 3 cm in diameter, constitute the microwave window. The first layer of alumina is brazed onto a stainless steel metal ring to provide the vacuum seal. The alumina window, shown in Figure 2, has a combined length of 3 cm, which is equivalent to ¾ of the wavelength in the dielectric material. 3.2 High voltage design In order to avoid secondary electrons being accelerated into the alpha detector, which is on ground potential, the microwave-driven ion source of the API system is at positive high voltage and the target is at ground potential. Putting the ion source at high voltage poses a design challenge. Our generator design, shown in Figure 1, adopts a customized radial insulator. This unconventional design transforms the generator shape from a “tube” to a shorter but much larger radius cylinder. It allows attachment of the ion source on the outside of the vacuum vessel, which makes the cooling arrangement simpler. As the ion source is biased at high voltage and the microwave generator is operated at ground potential to avoid the use of a transformer, a high-voltage break is needed in the microwave wave-guide to electrically isolate the ion source from the microwave generator. A Teflon layer sandwiched between the two microwave waveguide sections serves for this purpose while transmitting microwave power with minimal losses. The microwave high voltage break design has been scaled up from an existing one to a thicker, 9 mm Teflon insulator to withhold 100 kV. The HV break has been tested for both low and high power transmission. The maximum power loss is less than 10%. The API generator is operated at a pressure of several mTorr. To avoid HV breakdown, the criteria of HV design is to keep the electric field below 65 kV/cm in the vacuum regions. The same limit has been used for the Flurinert (used as coolant) filled region surrounding the ion source. Results of the electric field analysis are shown in Figure 3. There are several high electric field regions that needed to be designed carefully. One is located at the electron suppressor, from which the plasma electrode, biased at up to 100 kV, is only 3 cm away. Choosing a
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suppressor with a larger curvature lowers the electric fields and mitigates the risk. The joint of the insulator and brazed metal ring, forming a triple point, is another hot spot. Field shaping structures (Figure 3a) have been added to reduce the electrical field at the joint. The electric field distributions for both in vacuum and microwave HV break are shown in Figure 3(a) and (b), respectively.
(a)
(b) Fig. 3. Electric field distribution in (a) vacuum, and (b) the microwave high-voltage break.
3.3 Electron suppressor and neutron production target The electron suppressor (shown in Figure 1) is biased at -2 kV relative to the grounded target to prevent secondary electrons from the target to stream back into the ion source. The neutron production target consists of a 140 Pm thick titanium layer on the end of a molybdenum rod. The surface of the neutron production target is 45 degree to the incident beam, facing the alpha detector. The target is liquid-cooled so that the temperature of the hot spot on the target surface is kept low enough to prevent desorption of deuterium/tritium atoms. 3.4 Gas pressure control A SAES getter will be used as a tritium/deuterium reservoir. This type of getter can serve as a temperature controlled reversible pump, which pump tritium/deuterium at low temperature and release it at high temperature.
Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
109
The pressure in the API generator can therefore be regulated by an electrical current that keeps the getter’s temperature and therefore the pressure in the tube at the desired values. The getter material does not absorb noble gases, so that helium does not get pumped. The getter pump is located on the outside of the prototype tube for convenience, and can be valved off. 3.5 Alpha detector The alpha detector design is based on the concept developed by Hausladen and Neal (2005). It consists of three parts: a solid YAP scintillator crystal, a fiber-optic face plat (FOFP), and a position-sensitive photomultiplier. Details of the alpha detector and its performance will be reported separately. 4. Preliminary test results Preliminary testing of ion source and beam optics at 3mTorr (the pressure anticipated for sealed operation) has been carried out using deuterium only. Figure 4 shows the measured deuterium beam on target as a function of the extraction voltage. The input microwave power is around 300 W. At 90 kV, more than 100 PA of beam has been achieved.
Fig. 4. At 90 kV of acceleration voltage, more than 100 PA of beam has been extracted. At the lower voltage range not all beam reaches the target because beam optics was optimized for 90 kV.
Fig. 5. Beam diameter on the titanium target is around 1 mm.
Figure 5 shows a picture of the titanium target after hours of beam bombardment. Liquid cooling was intentionally turned off to accelerate the surface erosion process. The diameter of the dip seen in figure 5 indicates that the beam diameter on the neutron production target, or the spot size, is around 1 mm. As shown in Figure 6, the DD neutron yield as measured with the available 3He neutron detector reached a nominal 3.7E+06 n/s after beam loading. However, there is reason to believe that the neutron detector was not properly calibrated for 2.5-MeV neutrons and the experimental setup. The actual yield is likely a factor of two higher than the values given in figure 6. This is still about a factor of two lower than the expected yield for a beamloaded Ti-target at 90 kV and 160 PA. The lower than expected yield may be due to the target surface temperature possibly being higher than optimal.
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Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
5. Summary A time-tagged neutron generator has been under development at the Lawrence Berkeley National Laboratory. A permanent magnet, 2.45 GHz microwave-driven ion source has been chosen for the generator because this type of ion source provides a high plasma density that allows the use of a sub-millimeter diameter aperture to achieve a much reduced beam spot size on target without employing active focusing and still extract a sufficiently high ion beam current for generating an order of magnitude higher yield than existing devices. First experimental results in DD operation indicate that the ion source is capable of delivering Fig. 6. DD neutron yield measurement over 100 PA of beam current onto a small, 1 mm diameter beam spot. In DT operation, which is the ultimate goal, this will produce a yield of up to 109 n/s. The API generator uses a custom radial high-voltage insulator to minimize source to neutron production target distance and to provide for a simple ion source cooling arrangement. Acknowledgements This work was performed under the auspices of the U. S. Department of Energy, NNSA Office of Nonproliferation and Verification Research and Development by the Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231. References A. Beyerle, J. P. Hurley, and L. Tunnell, 1990. Design of an associated particle imaging system. Nuclear Instruments and Methods in Physics Research A299, 458-462. S. Pesente, G. Nebbia, M. Lunardon, G. Viesti, S. Blagus, K. Nad, D. Sudac, V. Valkovic, I. Lefesvre, M. J. Lopez-Jimenez, 1990. Tagged neutron inspection system (TNIS) based on portable sealed generators. Nuclear Instruments and Methods in Physics Research B 241, 743747. D. L. Chichester, M. Lemchak, J. D. Simpson, 2005. The API 120: A portable neutron generator for the associated particle technique”, Nuclear Instruments and Methods in Physics Research B 241, 753–758. P.A. Hausladen, P. R. Bingham, J. S. Neal, J. A. Mullens, J. T. Mihalczo, 2007. Portable fast neutron radiography with the nuclear materials identification system for fissile material transfers”, Nucl. Instr. Meth. in Phys. Res., B 261, 387-390. J. W. Cates and J. P. Hayward and X. Zhang and P. A. Hausladen and B. Dabbs, 2012. Timing Resolution Study of an Associated Particle Detector for Fast Neutron Imaging. IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, 1750-1756. A. Sy, Q. Ji, A. Persaud, O. Waldmann, T. Schenkel, 2012. Novel methods for improvement of a Penning ion source for neutron generator applications. Review of Scientific Instruments, 83, 02B309. Y. Wu, J. P. Hurley, Q. Ji, J. Kwan, K.-N. Leung, 2009. Characteristics of a RF-driven ion source for a neutron generator used for associated particle imaging. AIP Conference Proceedings, Vol. 1099 Issue 1, p614. Q. Ji, 2011. Compact permanent magnet microwave-driven neutron generator. AIP Conf. Proc. 1336, 528-532. P. Hausladen and J. S. Neal, 2005. An alpha particle detector for a portable neutron generator for the Nuclear Materials Identification System (NMIS),” Nucl. Instr. Meth. in Phys. Res. B, vol. 241, pp. 835–838.
ScienceDirect Physics Procedia 66 (2015) 105 – 110
C 23rd Conference on Application of Accelerators in Research and Industry, CAARI 2014
Development of a Time-Tagged Neutron Source for SNM Detection Qing Jia,*, Bernhard Ludewigta, Joe Walliga, Will Waldrona, Jim Tinsleyb a
Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, U.S. A. b Special Technology Laboratory, Santa Barbara, CA 93111, U. S. A.
Abstract Associated particle imaging (API) is a powerful technique for special nuclear material (SNM) detection and characterization of fissile material configurations. A sealed-tube neutron generator has been under development by Lawrence Berkeley National Laboratory to reduce the beam spot size on the neutron production target to 1 mm in diameter for a several-fold increase in directional resolution and simultaneously increases the maximum attainable neutron flux. A permanent magnet 2.45 GHz microwave-driven ion source has been adopted in this time-tagged neutron source. This type of ion source provides a high plasma density that allows the use of a sub-millimeter aperture for the extraction of a sufficient ion beam current and lets us achieve a much reduced beam spot size on target without employing active focusing. The design of this API generator uses a custom-made radial high voltage insulator to minimize source to neutron production target distance and to provide for a simple ion source cooling arrangement. Preliminary experimental results showed that more than 100 PA of deuterium ions have been extracted, and the beam diameter on the neutron production target is around 1 mm. © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014. Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 Keywords: neutron generator, associated particle imaging, SNM detection
1. Introduction Associated particle imaging (API) uses the deuterium-tritium fusion reaction 2
D3T o4He(3.5MeV ) n(14.1MeV )
* Corresponding author. Tel.: +1-510-486-4802; fax: +1-510-486-4888. E-mail address: [email protected]
1875-3892 © 2015 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 doi:10.1016/j.phpro.2015.05.015
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Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
that produces an alpha particle in coincidence with the neutron. Both particles are kinematically correlated and emitted in nearly opposite directions. By detecting the alpha particle with a position sensitive detector the neutron’s time of emission and direction can be determined (See work by Beyerle et al (1990)). This information can be used in various ways for 2D or 3D imaging. API applications such as detection of mines, Improvised Explosive Devices, and explosives and other contraband in cargo, vehicles, etc., have been extensively studied by Beyerle et al (1990), Pesente et al (2009), and Chichester et al (2009). Using the timing and position information obtained to enhance the signal of interest in the presence of background, API offers new capabilities for enhanced SNM detection and for the characterization of fissile material configurations. Prompt neutron and gamma rays emitted in induced fission events can be correlated in time with the tagged primary neutron to greatly enhance the fission signal. Similarly, the known direction of the emitted neutrons can be used to eliminate the background generated by those neutrons that do not intercept a selected region of the interrogation space. Both, induced fission imaging and neutron transmission imaging methods developed by Hausladen (2007) and Cates (2012) for use in proliferation detection and nuclear safeguards, will benefit from much improved directional resolution and higher neutron rates. The goal of the time-tagged neutron source development effort reported here is to build a sealed-tube API generator that reduces the beam spot size on the neutron production target to 1 mm or less in diameter for a roughly five-fold increase in directional resolution and simultaneously increases the maximum attainable neutron yield (up to 109 neutrons/sec) by approximately one order of magnitude over existing sources. This article describes the design of the generator and first experimental results. 2. Approach An advanced ion source is the key for achieving the performance goals of the API generator. The ion source must produce a sufficiently high beam current density and operate at a sufficiently low gas pressure for sealed-tube operation. After researching Penning-discharge (Sy et al (2012)), RF-driven (Wu et al (2009)) and microwavedriven ion sources (Ji (2011)), the physics design of the API generator was established based on a microwave-driven ion source. This type of ion source has been selected because of its high current density that allows us to use a submillimeter aperture for the ion extraction and to achieve a much reduced beam spot size on target without employing active focusing. Ion source and beam optics studies will be reported on in another article. Mixed deuterium and tritium ion beams are extracted from the ion source through a sub-millimeter aperture and accelerated up to 100 keV when reaching the target. Neutrons are produced in a titanium layer, which is loaded by the beam with a deuterium-tritium mixture. For system compactness and simplicity, no active focusing elements are employed. The alpha-particle detector, a scintillation detector with a position sensitive readout, views the target at a 90 degree angle to the beam at a minimum distance of 10 cm. The high-voltage design must allow for an acceleration voltage between ion source and target of up to 100 kV. In order to avoid secondary electrons being accelerated into the alpha detector, which is on ground potential, the target is at ground potential and the microwavedriven ion source of the API system is at positive high voltage. This is the opposite of how the acceleration voltage is normally applied and poses significant design challenges. 3. Engineering Design The design of the API generator with its four key components – ion source, high voltage (HV) insulator, neutron production target, and alpha detector – must satisfy several basic engineering requirements. Since a deuterium-tritium gas mixture is used in the generator, the generator must be constructed as a sealed-tube with ultrahigh vacuum (UHV) and tritium-compatible materials. Given that microwave power of over 200 W is dissipated in the ion source, liquid cooling is needed to keep the permanent magnets surrounding the plasma chamber from getting too hot. Figure 1 shows a cross-sectional view of the API generator. The custom-made radial HV insulator was chosen because this configuration allows for the neutron production target to be positioned at the minimum distance from the ion source while allowing for relative ease of construction and a rather simple ion source cooling arrangement.
Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
The source-target distance of ~ 12 cm leaves just enough room for the alpha-particle detector and minimizes the beam spreading on the target.
Fig. 1. Cross sectional view of the API generator.
Fig. 2. Cross sectional view of the permanent magnet microwave-driven ion source.
3.1 Ion Source Figure 2 shows the engineering design of the compact, permanent-magnet microwave source. The source chamber is made of stainless steel, 4 cm in diameter and 5 cm in length. An aluminium liner is press-fitted inside the stainless steel cylinder to provide an aluminium surface for optimal ion source operation and atomic ion fraction. The three ring-shaped permanent magnets generate the required axial magnetic field. A stack of alumina discs, which are 3 cm in diameter, constitute the microwave window. The first layer of alumina is brazed onto a stainless steel metal ring to provide the vacuum seal. The alumina window, shown in Figure 2, has a combined length of 3 cm, which is equivalent to ¾ of the wavelength in the dielectric material. 3.2 High voltage design In order to avoid secondary electrons being accelerated into the alpha detector, which is on ground potential, the microwave-driven ion source of the API system is at positive high voltage and the target is at ground potential. Putting the ion source at high voltage poses a design challenge. Our generator design, shown in Figure 1, adopts a customized radial insulator. This unconventional design transforms the generator shape from a “tube” to a shorter but much larger radius cylinder. It allows attachment of the ion source on the outside of the vacuum vessel, which makes the cooling arrangement simpler. As the ion source is biased at high voltage and the microwave generator is operated at ground potential to avoid the use of a transformer, a high-voltage break is needed in the microwave wave-guide to electrically isolate the ion source from the microwave generator. A Teflon layer sandwiched between the two microwave waveguide sections serves for this purpose while transmitting microwave power with minimal losses. The microwave high voltage break design has been scaled up from an existing one to a thicker, 9 mm Teflon insulator to withhold 100 kV. The HV break has been tested for both low and high power transmission. The maximum power loss is less than 10%. The API generator is operated at a pressure of several mTorr. To avoid HV breakdown, the criteria of HV design is to keep the electric field below 65 kV/cm in the vacuum regions. The same limit has been used for the Flurinert (used as coolant) filled region surrounding the ion source. Results of the electric field analysis are shown in Figure 3. There are several high electric field regions that needed to be designed carefully. One is located at the electron suppressor, from which the plasma electrode, biased at up to 100 kV, is only 3 cm away. Choosing a
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Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
suppressor with a larger curvature lowers the electric fields and mitigates the risk. The joint of the insulator and brazed metal ring, forming a triple point, is another hot spot. Field shaping structures (Figure 3a) have been added to reduce the electrical field at the joint. The electric field distributions for both in vacuum and microwave HV break are shown in Figure 3(a) and (b), respectively.
(a)
(b) Fig. 3. Electric field distribution in (a) vacuum, and (b) the microwave high-voltage break.
3.3 Electron suppressor and neutron production target The electron suppressor (shown in Figure 1) is biased at -2 kV relative to the grounded target to prevent secondary electrons from the target to stream back into the ion source. The neutron production target consists of a 140 Pm thick titanium layer on the end of a molybdenum rod. The surface of the neutron production target is 45 degree to the incident beam, facing the alpha detector. The target is liquid-cooled so that the temperature of the hot spot on the target surface is kept low enough to prevent desorption of deuterium/tritium atoms. 3.4 Gas pressure control A SAES getter will be used as a tritium/deuterium reservoir. This type of getter can serve as a temperature controlled reversible pump, which pump tritium/deuterium at low temperature and release it at high temperature.
Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
109
The pressure in the API generator can therefore be regulated by an electrical current that keeps the getter’s temperature and therefore the pressure in the tube at the desired values. The getter material does not absorb noble gases, so that helium does not get pumped. The getter pump is located on the outside of the prototype tube for convenience, and can be valved off. 3.5 Alpha detector The alpha detector design is based on the concept developed by Hausladen and Neal (2005). It consists of three parts: a solid YAP scintillator crystal, a fiber-optic face plat (FOFP), and a position-sensitive photomultiplier. Details of the alpha detector and its performance will be reported separately. 4. Preliminary test results Preliminary testing of ion source and beam optics at 3mTorr (the pressure anticipated for sealed operation) has been carried out using deuterium only. Figure 4 shows the measured deuterium beam on target as a function of the extraction voltage. The input microwave power is around 300 W. At 90 kV, more than 100 PA of beam has been achieved.
Fig. 4. At 90 kV of acceleration voltage, more than 100 PA of beam has been extracted. At the lower voltage range not all beam reaches the target because beam optics was optimized for 90 kV.
Fig. 5. Beam diameter on the titanium target is around 1 mm.
Figure 5 shows a picture of the titanium target after hours of beam bombardment. Liquid cooling was intentionally turned off to accelerate the surface erosion process. The diameter of the dip seen in figure 5 indicates that the beam diameter on the neutron production target, or the spot size, is around 1 mm. As shown in Figure 6, the DD neutron yield as measured with the available 3He neutron detector reached a nominal 3.7E+06 n/s after beam loading. However, there is reason to believe that the neutron detector was not properly calibrated for 2.5-MeV neutrons and the experimental setup. The actual yield is likely a factor of two higher than the values given in figure 6. This is still about a factor of two lower than the expected yield for a beamloaded Ti-target at 90 kV and 160 PA. The lower than expected yield may be due to the target surface temperature possibly being higher than optimal.
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Qing Ji et al. / Physics Procedia 66 (2015) 105 – 110
5. Summary A time-tagged neutron generator has been under development at the Lawrence Berkeley National Laboratory. A permanent magnet, 2.45 GHz microwave-driven ion source has been chosen for the generator because this type of ion source provides a high plasma density that allows the use of a sub-millimeter diameter aperture to achieve a much reduced beam spot size on target without employing active focusing and still extract a sufficiently high ion beam current for generating an order of magnitude higher yield than existing devices. First experimental results in DD operation indicate that the ion source is capable of delivering Fig. 6. DD neutron yield measurement over 100 PA of beam current onto a small, 1 mm diameter beam spot. In DT operation, which is the ultimate goal, this will produce a yield of up to 109 n/s. The API generator uses a custom radial high-voltage insulator to minimize source to neutron production target distance and to provide for a simple ion source cooling arrangement. Acknowledgements This work was performed under the auspices of the U. S. Department of Energy, NNSA Office of Nonproliferation and Verification Research and Development by the Lawrence Berkeley National Laboratory under Contract No. DE-AC02-05CH11231. References A. Beyerle, J. P. Hurley, and L. Tunnell, 1990. Design of an associated particle imaging system. Nuclear Instruments and Methods in Physics Research A299, 458-462. S. Pesente, G. Nebbia, M. Lunardon, G. Viesti, S. Blagus, K. Nad, D. Sudac, V. Valkovic, I. Lefesvre, M. J. Lopez-Jimenez, 1990. Tagged neutron inspection system (TNIS) based on portable sealed generators. Nuclear Instruments and Methods in Physics Research B 241, 743747. D. L. Chichester, M. Lemchak, J. D. Simpson, 2005. The API 120: A portable neutron generator for the associated particle technique”, Nuclear Instruments and Methods in Physics Research B 241, 753–758. P.A. Hausladen, P. R. Bingham, J. S. Neal, J. A. Mullens, J. T. Mihalczo, 2007. Portable fast neutron radiography with the nuclear materials identification system for fissile material transfers”, Nucl. Instr. Meth. in Phys. Res., B 261, 387-390. J. W. Cates and J. P. Hayward and X. Zhang and P. A. Hausladen and B. Dabbs, 2012. Timing Resolution Study of an Associated Particle Detector for Fast Neutron Imaging. IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 59, 1750-1756. A. Sy, Q. Ji, A. Persaud, O. Waldmann, T. Schenkel, 2012. Novel methods for improvement of a Penning ion source for neutron generator applications. Review of Scientific Instruments, 83, 02B309. Y. Wu, J. P. Hurley, Q. Ji, J. Kwan, K.-N. Leung, 2009. Characteristics of a RF-driven ion source for a neutron generator used for associated particle imaging. AIP Conference Proceedings, Vol. 1099 Issue 1, p614. Q. Ji, 2011. Compact permanent magnet microwave-driven neutron generator. AIP Conf. Proc. 1336, 528-532. P. Hausladen and J. S. Neal, 2005. An alpha particle detector for a portable neutron generator for the Nuclear Materials Identification System (NMIS),” Nucl. Instr. Meth. in Phys. Res. B, vol. 241, pp. 835–838.