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 40Ar/39Ar dating of geological materials produces nearly monoenergetic 2.45MeV neutrons at outputs of 108n/s. The narrow energy range is advantageous relative to the 235U fission spectrum neutrons due to (i) reduced 39Ar 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 1010n/s, which is a necessary requirement for production of measurable quantities of 39Ar through the reaction 39K(n,p)39Ar. 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

Ayllon, Mauricio

Becker, Tim

Bernstein, Lee

Bibber, Karl Van

Kirsch, Leo

Leung, Ka-Ngo

Renne, Paul

Waltz, Cory

eScholarship - University of California

UC BerkeleyUC Berkeley Previously Published WorksTitleBeam-induced back-streaming electron suppression analysis for an accelerator type neutron generator designed for 40Ar/39Ar geochronology.Permalinkhttps://escholarship.org/uc/item/3392n7hnAuthorsWaltz, CoryAyllon, MauricioBecker, Timet al.Publication Date2017-07-01DOI10.1016/j.apradiso.2017.04.017 Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaContents lists available at ScienceDirectApplied Radiation and Isotopesjournal homepage: www.elsevier.com/locate/apradisoBeam-induced back-streaming electron suppression analysis for anaccelerator type neutron generator designed for 40Ar/39Ar geochronologyCory Waltza,b,⁎, Mauricio Ayllona, Tim Beckerc, Lee Bernsteina, Ka-Ngo Leunga, Leo Kirscha,Paul Rennec, Karl Van Bibberaa Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USAb Lawrence Livermore National Laboratory, Livermore, CA 94550, USAc Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94720, USAA R T I C L E I N F OKeywords:Neutron generatorBackstreaming electronsElectron suppressionElectrostatic shroudGeochronologyA B S T R A C TA facility based on a next-generation, high-flux D-D neutron generator has been commissioned and it is nowoperational at the University of California, Berkeley. The current generator designed for 40Ar/39Ar dating ofgeological materials produces nearly monoenergetic 2.45 MeV neutrons at outputs of 108 n/s. The narrowenergy range is advantageous relative to the 235U fission spectrum neutrons due to (i) reduced 39Ar recoil energy,(ii) minimized production of interfering argon isotopes from K, Ca, and Cl, and (iii) reduced total activity forradiological safety and waste generation. Calculations provided show that future conditioning at higher currentsand voltages will allow for a neutron output of over 1010 n/s, which is a necessary requirement for production ofmeasurable quantities of 39Ar through the reaction 39K(n,p)39Ar. A significant problem encountered withincreasing deuteron current was beam-induced electron backstreaming. Two methods of suppressing secondaryelectrons resulting from the deuterium beam striking the target were tested: the application of static electric andmagnetic fields. Computational simulations of both techniques were done using a finite element analysis inCOMSOL Multiphysics®. Experimental tests verified these simulations. The most reliable suppression wasachieved via the implementation of an electrostatic shroud with a voltage offset of −800 V relative to the target.1. IntroductionThe neutron energy spectrum from D-D neutron generators has beenshown by Renne et al. (2005) to be optimal for irradiation of samplesfor 40Ar/39Ar geochronology. The average energy and the narrow rangeof neutron energies from the 2H(d,n)3He fusion reaction allows for theproduction of 39Ar with significant advantages over irradiation of thesesamples in 235U fission reactors for reasons enumerated by Renne et al.(2005). Therefore, a customized neutron generator was designed andbuilt for such purpose.The High Flux Neutron Generator (HFNG) located at the Universityof California Berkeley is designed around two radio frequency-drivenmulti-cusp ion sources that straddle a titanium-coated water cooledcopper target, as shown in Fig. 1. Positively charged deuteron ions areaccelerated up to 125 keV from the ion sources and self-load into thetarget. Upon target saturation, neutron generation occurs through the d(d,n)3He fusion reaction. In order to take advantage of the fact that thehighest neutron flux is in the forward direction with respect to thebeam, a sample holder slot is located in the center of the target at adistance of 8 mm from the location where the deuterium ions strike thetarget and generate neutrons. A detailed description of the design andoperation of the HFNG is being prepared for publication.As deuterium ions extracted from the ion source strike the target,ionization occurs at the surface, releasing secondary electrons.According to a study done in Large and Whitlock (1962), approximately1.2 electrons are emitted per deuterium ion striking a titanium target at100 keV. Secondary electrons emitted from metal surfaces have en-ergies around 10 eV, and typically not more than 30 eV (Kaminsky,1965). Secondary electrons accelerate away from the target due to thenegative electric potential, resulting in an electron beam that strikes theextraction plate. The collision of this secondary electron beam with theextraction plate result in the emission of bremsstrahlung X-rays. If thecurrent density is large enough, melting can occur. Collision ofelectrons with any insulators inside the vacuum chamber will resultin charge build-up, which over time will discharge in the form of an arc.If arcing is frequent, damage to the high voltage power supply canoccur. Lastly, secondary electrons represent a leakage current to thehigh voltage power supply, making it difficult to accurately determinehttp://dx.doi.org/10.1016/j.apradiso.2017.04.017Received 1 February 2017; Accepted 11 April 2017⁎ Corresponding author at: Department of Nuclear Engineering, University of California, Berkeley, CA 94720, USA.E-mail address: waltz5@llnl.gov (C. Waltz).Applied Radiation and Isotopes 125 (2017) 124–128Available online 13 April 20170969-8043/ © 2017 Published by Elsevier Ltd.MARKthe contribution of current from deuterons compared to secondaryelectrons. Suppression of these electrons is vital for continuous opera-tion of a neutron generator. Two methods of suppressing electrons weretested on the HFNG: using magnetic fields produced by permanentmagnets and using an electric field created by installing an electrostaticshroud. Analysis results were first published in Waltz (2016).2. COMSOL multiphysics® Finite element simulation overviewCOMSOL Multiphysics® was used to model the electrical andmagnetic fields within the HFNG vacuum chamber (Comsol Multi-physics, 2013). COMSOL Multiphysics® uses the macroscopic form ofMaxwell's equations to determine the magnetic and electric fields.The fields are computed by using a finite element analysis over ameshed geometry. User entered boundary conditions include theelectric potentials at the surface of a material, and the remanent fluxdensity (Br) of magnets. Charged particle trajectories can then becomputed by COMSOL Multiphysics® by applying the forces on theparticles due to the calculated magnetic and electric fields. Correctionsto the electric field are made using an iterative process to fold in theeffect of space charge of the beam. The simulations were performedunder non-relativistic conditions since relativistic effects are negligiblewithin this energy range.3. Magnetic suppressionOne method of electron suppression involves the use of magneticfields. Fig. 2 shows the implementation of large neodymium magnets onthe HFNG target. The magnets are aligned with opposite poles facingeach other, creating magnetic field lines parallel with the target surface.This causes ejected electrons to spiral around the field lines and backtoward the target surface. It is worth noticing that the electron does notonly feel the force due to the magnetic field, but it also feels the force ofthe electric field used to accelerate the deuteron ions. This net forceresults in an E×B drift, which causes an orthogonal drift of the electronwith respect to the magnetic and electric fields.3.1. Electric and magnetic field simulation resultsA COMSOL Multiphysics® simulation shows the strength of themagnetic field across the surface of the target in Fig. 3a. Theneodymium magnets were grade N50, giving them a Br between14.1–14.5 kG. The black arrows represent the direction of the magneticfield lines in the respective arrow location. The magnetic field strengthat the center of the target is approximately 800 gauss. The field strengthalong the surface was verified using a Hall probe. The strength of theelectrical field can be seen in Fig. 3b, which shows a top view of thetarget assembly. The field is quite large near the corners of the magnetholders, approaching a value of 80 kV/cm. It is important to remainbelow the breakdown voltage in vacuum, which can be approximatedby the following empirical relation (Durand and Shroff, 1995):⎡⎣⎢⎤⎦⎥EkVcm V kV=8000[ ]max (1)where V is the extraction voltage and Emax is the maximum magnitudeof electric field allowed. At 100 kV, the breakdown E-field becomesapproximately 80 kV/cm.3.2. Simulation of electron trajectories using magnetic suppressionFig. 4 shows the COMSOL Multiphysics® simulated electron paths,assuming three stripes of deuteron beams striking the center of theangled target.The voltage of the target is −100 kV. The electrons liberated by thedeuteron beams spiral up the target due to the E×B drift. When theelectrons reach an area of weakened magnetic field, the electric fielddominates and the electrons accelerate to the vacuum chamber wall.Experimental tests confirmed the electrons were colliding with thevacuum chamber wall exactly in the location shown by the simulation.Further analyzing the simulation shows that when the electronsspiral back toward the target, they get close to the surface but do notcollide. The magnetic force a particle feels is proportional to itsvelocity. The electron slows down as it spirals back toward the surfacedue to the electric field, causing the magnetic force to weaken.Ultimately, this is a consequence of the fact that as magnetic fieldscan only change the path of the electron and do not add energy, it isdifficult for the electrons to overcome the electric field force andrecombine with the target.4. Electrostatic shroudAn electrostatic shroud is a device that suppresses back-streamingelectrons by introducing an opposing electric field near the targetsurface. This is done by installing a shroud around the target that is keptat a more negative potential. Locally, the resulting electric field isdirected away from the target surface, causing electrons to be repelledfrom the shroud and return to the target.The shroud design used for the HFNG is shown in Fig. 6. Theclamshell design allows for easy removal. Exchangeable face-platesallow for the ability to easily change the beam entrance windowgeometry. The voltage difference between the shroud and target iscreated through the use of multiple zener diodes in series, as shown inFig. 5. The total breakdown voltage of the zener diode chain was madeas high as 2400 V.Fig. 1. HFNG cross section exposing target, shroud, and ion sources. For scale, the ionsource is approximately 19 cm in diameter.Fig. 2. Neodymium magnet holders. (a) Photo of the magnet holders clamped to targetcooling tubes. (b) CAD drawing, top view, showing neodymium magnets.C. Waltz et al. Applied Radiation and Isotopes 125 (2017) 124–1281254.1. Electric field simulation resultsFig. 7 shows the simulation of the electric field with the installationof the shroud design shown in Fig. 6. In the simulation, the shroudvoltage is −102 kV, while the target is at −100 kV. Installation of theshroud reduced the distance between the ground and high voltage, butby designing the shroud to have a large radius of curvature everywhere,and installing insulating spacers to move the ion sources back, it waspossible to limit the electric field below the 80 kV/cm electricalbreakdown limit determined earlier. The maximum electric field isapproximately 47 kV/cm near the top of the shroud.The minimum voltage differential between the target and shroudrequired to suppress electrons was determined by analyzing the electricfield within the shroud. Fig. 8 shows the magnitude of the electric fieldalong the direction of the deuteron beam on the central plane withinthe shroud (same cross section plane shown in Fig. 1). Red areasFig. 3. Simulations of the (a) magnetic fields and (b) electric field.Fig. 4. COMSOL Multiphysics® electron path simulation.Fig. 5. Circuit schematic of the diode assembly showing the Ω16 k resistor that reducescurrent flow during arcing in order to protect the machine.Fig. 6. Electrostatic shroud: (a) assembled, (b) exploded view.Fig. 7. Electric field simulation outside shroud with target potential at −100 kV andshroud potential at −102 kV.C. Waltz et al. Applied Radiation and Isotopes 125 (2017) 124–128126indicate regions where E > 0z and blue areas indicate regions whereE < 0z (z is the direction along the deuteron beam). For a deuteronextracted from the top of Fig. 7, the electric field will accelerate thedeuteron downwards toward the target. When the deuteron enters theshroud and reaches the intersection of the red and blue region, it willhave an energy of approximately 102 keV. The red region will thendecelerate the deuteron to 100 keV. When the deuteron strikes thetarget surface and ejects electrons, the electrons will feel the electricforce pushing it back toward the target.When designing the shroud geometry, one must ensure that the redregion in the top of Fig. 8 does not touch the target (as it does inFig. 8a), in order to prevent an electron from being ejected into anelectric field that would accelerate the particle toward the vacuumchamber wall. Factors that affect the electric field within the shroudinclude the voltage differential and the distance between the shroudand target, as well as the window size.4.2. Simulation of electron trajectories using electrostatic shroudA simulation of the secondary electrons released into 2π from a1.3 mA deuterium beam striking the flat target surface is shown inFig. 9. Assuming 1.2 electrons released per deuteron ion striking thetarget, this results in 1.56 mA of electrons. As expected and describedpreviously due to the electric field shown in Fig. 8a, for a 400 V voltagedifferential between the shroud and the target, secondary electrons arenot suppressed and leave through the shroud window in a beam thateventually strikes the chamber with an energy of 100 keV. A shroudvoltage differential of 800 V results in the electrons returning to thesurface of the shroud, as seen in Fig. 9b. The maximum energy theelectron receives is equal to the energy at release.Testing of the shroud at 800 V voltage differential or greater on theHFNG has shown a large reduction in the amount of secondaryelectrons hitting the vacuum chamber. After implementing the shroud,no visual sign of chamber heating could be seen during machineoperation due to back-streaming electrons. Also, the dose of brems-strahlung X-rays detected by a Geiger counter in the vault decreased bya factor of more than 20 compared to the magnetic suppressiontechnique.5. ConclusionsTwo main methods of electron suppression were tested: usingpermanent magnets to bend electrons back to the target, and imple-menting an electrostatic shroud creating a suppressing electric field.Simulations of the permanent magnet design showed that even withmagnetic fields surpassing 1000 gauss, electrons would migrate alongthe target due to the E×B force without coming into contact with thesurface. When electrons would reach an area of lower magnetic fieldthey would escape the target area and collide with the vacuum chamberwall. Experimental tests confirmed the electrons were colliding with thevacuum chamber wall at the location shown in the COMSOLMultiphysics® simulation.Simulations of an electrostatic shroud showed full suppression ofelectrons emitted from the target surface. For the chosen shroudgeometry, a voltage differential of −800 V or greater compared tothe target resulted in successful electron suppression. Experimentalresults with the addition of a shroud reduced the back streamingelectrons significantly to undetectable levels. Furthermore, the dose dueto bremsstrahlung x-rays in the HFNG vault was reduced by a factor ofmore than 20. Analysis of both suppression techniques resulted in theconclusion that the use of electric fields for electron suppression is moreeffective due to the fact that electric fields add energy to the electronFig. 8. Electric field direction near shroud window at (a) 400 V, (b) 800 V, (c) and 1400 V voltage differential. Red indicates field along the direction of the deuteron beam (shown byarrow), blue indicates direction opposite the deuteron beam. T and S denote the target and shroud, respectively.Fig. 9. Cutaway view of secondary electron trajectories with shroud differential voltages of (a) 400 V and (b) 800 V at 100 kV target potential.C. Waltz et al. Applied Radiation and Isotopes 125 (2017) 124–128127while a magnetic field can only change its direction.AcknowledgmentsWork supported by NSF Grant No. EAR-0960138, U.S. DOE LBNLContract No. DE- AC02-05CH11231, and U.S. DOE LLNL Contract No.DE-AC52-07NA27344.We would like to thank Bernhard Ludewigt, Thomas Schenkel andQing Ji of LBNL for helpful guidance in designing the electronsuppression shroud.We also gratefully acknowledge a grant from the University ofCalifornia Office of the President.ReferencesComsol Multi-physics, 2013. AC/DC Module User Guide for Comsol Multi-physics (May).Durand, A., Shroff, A., 1995. High Voltage Vacuum Insulation, Latham, R.V. ed.,Academic Press, London.Kaminsky, M., 1965. Atomic and Ionic Impact Phenomena on Metal Surfaces, Academic,New York.Large, L., Whitlock, W.S., 1962. Secondary electron emission from clean metal surfacesbombarded by fast hydrogen atoms, Proceedings Phys. Soc. 79 148–157.Renne, P.R., Knight, K.B., Nomade, S., Leung, K.-N., Lou, T.-P., 2005. Application ofdeuterondeuteron (dd) fusion neutrons to 40ar/39ar geochronology. Appl. Radiat.Isot. 62 (1), 25–32. (〈http://dx.doi.org/http://dx.doi.org/10.1016/j.apradiso.2004.06.004〉URL 〈http://www.sciencedirect.com/science/article/pii/S0969804304003951〉).Waltz, C., 2016. Characterization of deuteron-deuteron neutron generators, (Ph.D.thesis), University of California Berkeley.C. Waltz et al. Applied Radiation and Isotopes 125 (2017) 124–128128

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

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

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