Glück, Ferenc

Klein, Manuel

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KATRIN Collaboration

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Journal of Physics: Conference SeriesPAPER • OPEN ACCESSTritium ion blocking and detection in the KATRINexperimentTo cite this article: Manuel Klein et al 2017 J. Phys.: Conf. Ser. 888 012073 View the article online for updates and enhancements.Related contentEM field, tracking and scatteringsimulations for the KATRIN experimentFerenc Glück and the KATRINCollaboration-The Windowless Gaseous Tritium Source(WGTS) of the KATRIN experimentFlorian Heizmann, Hendrik Seitz-Moskaliuk and KATRIN collaboration-The KATRIN pre-spectrometer at reducedfilter energyM Prall, P Renschler, F Glück et al.-This content was downloaded from IP address 129.13.72.197 on 07/11/2017 at 15:201Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd1234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012073  doi :10.1088/1742-6596/888/1/012073Tritium ion blocking and detection in the KATRINexperimentManuel Klein and Ferenc Glück, for the KATRIN collaborationKarlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344Eggenstein-LeopoldshafenE-mail: manuel.klein@kit.edu, ferenc.glueck@kit.eduAbstract.The aim of the KATRIN experiment is to determine the absolute neutrino mass scale in amodel independent way by measuring the electron energy spectrum shape near the endpoint ofmolecular tritium beta decay. The beta electrons are guided from the source to the spectrometersby a high magnetic field. The beta decays and ionisations of the beta electrons in the sourceproduce a variety of positive tritium ions that are not allowed to reach the spectrometers.Therefore they will be blocked by a positive potential and removed by dipole electrodes in thebeam transport system. Various ion detection methods (Faraday cup, FT-ICR and ionisation ofthe residual gas in the spectrometers) will be used to test the ion blocking and removal efficiency.Figure 1. The KATRIN beamline. Ring electrodes are used for ion blocking and dipoleelectrodes are used to remove ions.1. The KATRIN experimentThe KATRIN experiment is designed to determine the absolute neutrino mass scale with asensitivity of 200 meV (90% CL) by measuring the integral electron energy spectrum close tothe endpoint (∼ 18.6 keV) of molecular tritium β decay. For this purpose, KATRIN (Fig. 1)uses a high resolution MAC-E filter [1] and a high intensity Windowless Gaseous Tritium Source(WGTS). The beta electrons from the WGTS are guided to the spectrometers by a high (upto 5.7 T) magnetic field. The neutral tritium gas will be prevented to reach the spectrometersby the differential and cryogenic pumping sections (DPS and CPS). The energy analysis takesplace at the central plane of the main spectrometer (MS), where there is a small magnetic fieldon the order of 1 µT. The purpose of the pre-spectrometer (PS) is to reduce the backgroundfrom ionisation by beta electrons. The planned background level of the KATRIN experiment is10 mcps. For further details see references [1, 2, 3].21234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012073  doi :10.1088/1742-6596/888/1/0120732. Ion production in the Windowless Gaseous Tritium Source (WGTS)About 2·1012 tritium ions / s are produced in the WGTS by beta decays of tritium molecules andionisation of the tritium gas by the beta electrons. The ions include 3HeT+, 3He+, T+ and T+2 .The T+ and T+2 ions are transformed quickly to T+3 ions due to exchange reactions in collisionswith T2 molecules, e.g.: T2 +T+2 → T+3 +T. The T+3 ions can also be transformed to T+5 clusterions by the reaction: 2T2 + T+3 → T+5 + T2. Even larger cluster ions like T+7 , T+9 , etc. can beproduced by collisions. Those positive ions can recombine with thermalised secondary electronsthat are created from ionisations of the beta electrons. The dissociative recombination reactionof the T+3 ions is: T+3 + e− → T2 + T. The cluster ions have large dissociative recombinationcoefficients. According to calculations, the expected flux from the WGTS towards the transportsection is about 2 · 1011 positive tritium ions / s. In addition, a rate of 2 · 1010 T− ions / s isexpected to be created in the WGTS by associative attachment.3. Impact of ions on the KATRIN neutrino mass measurementsIn contrast to neutral tritium, ions are not pumped off the beamline because they follow themagnetic guiding field in the transport system, similar to the beta electrons. Without blockingthe positive ions, they would enter the spectrometers (PS and MS). There they would causea tritium contamination that is a million times higher than the acceptable level. In addition,ionisation by the positive ions in the residual gas of PS and MS would produce an extremelylarge background on the order of 10 kcps in a 10−11 mbar vacuum. This is also a million timeshigher than the background specifications.The positive ions can be blocked in the transport system by applying a positive blockingpotential. In this case, however, these ions will be stored between the blocking potential andthe gas flow at the detector end of the WGTS. The resulting high-density ion plasma couldincrease the space charge potential in the WGTS. Possible plasma instabilities could change theenergy of the beta electrons moving through this plasma. Also, the end point energies of betadecays of the various tritium ions is different from neutral tritium molecules. Hence, the storedions could cause non-negligible systematic effects for the KATRIN neutrino mass measurement.Therefore, the ions need to be removed from the transport system by E ×B-drift, as describedin the following section.4. Ion blocking and removal in the transport sectionPositive ions can be blocked due to their thermal energies of about 10 meV with two ringelectrodes (Fig. 2) in the transport section. Set to +100 V, each ring electrode forms a potentialwall for positive ions and a potential well for the measured > 18 keV electrons which remainunaffected. One of these ring electrodes is designed to serve in the experiments with the FT-ICRunit (Fourier Transform Ion Cyclotron Resonance, see section 5.2). Three more ring electrodesare placed in the spectrometer section (see Fig. 1).There are three dipole electrodes (Fig. 3) in the differential pumping section (DPS) betweenthe WGTS and the ion-blocking ring electrodes. One electrode of each dipole will be set to−100 V while the other electrode remains grounded. This causes an electric dipole field ~Etransverse to the magnetic field ~B and an ion drift with velocity ~v = ~E × ~B/| ~B|2. The blockedions leave the flux tube due to this drift after less than one second: after moving a few timesthrough the dipole electrodes, they are neutralised when they hit the dipole electrode surfacesor the beam-tube walls. The neutralised tritium atoms and molecules will be pumped off.5. Ion detection in the KATRIN beamlineSuccessful ion blocking is confirmed by detecting the residual ion flux behind the transportsection with Faraday cups and via ionisation in the spectrometers. Analysis of the ioncomposition is possible with a FT-ICR unit in the transport section.31234567890Neutrino2016 IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 888 (2017) 012073  doi :10.1088/1742-6596/888/1/012073Figure 2. Ring electrodes block ions. Figure 3. Dipole electrodes drift out ions.5.1. Faraday cups in the transport section (CPS)Two Faraday cups at the Forward Beam Monitor (FBM) in the CPS can measure an ion fluxof at least 107 ions / s and 108 ions / s. The larger Faraday cup is optimised on flux sensitivity.The smaller measures the radial ion flux distribution to validate the ion source model. TheFaraday cups will be used only during KATRIN commissioning since their detector board needsto be exchanged for the monitoring board of the FBM.5.2. Fourier Transform Ion Cyclotron Resonance (FT-ICR) in the transport section (DPS)The FT-ICR unit [4] also has an ion sensitivity of 107 ions / s. The FT-ICR method can beused to further distinguish between ion species, e.g. T+, T+3 and T+5 . Due to its location in thetransport section, the FT-ICR unit can be used to investigate the ion composition in the WGTSand yield important information about the physical processes in the source.5.3. Ionisation in the spectrometers (PS or MS)An ion flux of 500 ions / s can be measured via ionisation of residual gas in the spectrometersaccording to simulation (see section 3). Secondary electrons from ionisation are detected wheneither the MS or the PS is set to −18.6 kV while the other remains grounded. For ion detection,the spectrometers will be filled with He gas at 10−7 mbar. Setting the PS to a high negativepotential has the advantage that a possible flux of tritium ions will not reach the MS and willthus not contaminate it. These measurements with the spectrometers will only start when theFaraday cups and the FT-ICR unit measure no residual ion flux downstream from the ion-blocking ring electrodes. If either measurement with the spectrometers then shows a significantion flux, the valve between CPS and PS will be closed on a time scale of 10 s.6. Outlook: First tests of ion blocking and detectionFirst tests with ions in the KATRIN beamline are scheduled for November 2016. A pencil beamof 5 mm diameter and up to 109 deuterium ions / s will be sent through the KATRIN beamlinewith the ELIOTT ion source [5]. This allows a proof of principle of the ion blocking and drifting(section 4) and of ion detection with the spectrometers (section 5.3).References[1] J. Angrik et al. (KATRIN Collaboration), KATRIN Design Report 2004, Wissenschaftliche Berichte FZKA7090, http://bibliothek.fzk.de/zb/berichte/FZKA7090.pdf[2] G. Drexlin et al., Current Direct Neutrino Mass Experiments, Advances in High Energy Physics, Vol. 2013(2013) Article ID 293986.[3] M. Arenz et al., Commissioning of the vacuum system of the KATRIN Main Spectrometer, J. Instrum. 11,P04011 (2016).[4] M. Ubieto-Diaz et al., A broad-band FT-ICR Penning trap system for KATRIN, International Journal of MassSpectrometry 288.1 (2009): 1-5.[5] S. Lukic et al., Ion source for tests of ion behavior in the Karlsruhe tritium neutrino experiment beam line,Review of Scientific Instruments 82, 013303 (2011).

Tritium ion blocking and detection in the KATRIN experiment

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Tritium ion blocking and detection in the KATRIN experiment

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