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Production Yield of Muon-Induced Neutrons in Lead: Measured at the Modane Underground Laboratory PDF

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Springer Theses Recognizing Outstanding Ph.D. Research Holger Kluck Production Yield of Muon-Induced Neutrons in Lead Measured at the Modane Underground Laboratory Springer Theses Recognizing Outstanding Ph.D. Research Aims and Scope The series “Springer Theses” brings together a selection of the very best Ph.D. theses from around the world and across the physical sciences. Nominated and endorsed by two recognized specialists, each published volume has been selected foritsscientificexcellenceandthehighimpactofitscontentsforthepertinentfield of research. For greater accessibility to non-specialists, the published versions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthefield.Asawhole,theserieswill provide a valuable resource both for newcomers to the research fields described, and for other scientists seeking detailed background information on special questions. Finally, it provides an accredited documentation of the valuable contributions made by today’s younger generation of scientists. Theses are accepted into the series by invited nomination only and must fulfill all of the following criteria (cid:129) They must be written in good English. (cid:129) ThetopicshouldfallwithintheconfinesofChemistry,Physics,EarthSciences, Engineeringandrelatedinterdisciplinary fields such asMaterials,Nanoscience, Chemical Engineering, Complex Systems and Biophysics. (cid:129) The work reported in the thesis must represent a significant scientific advance. (cid:129) Ifthethesisincludespreviouslypublishedmaterial,permissiontoreproducethis must be gained from the respective copyright holder. (cid:129) They must have been examined and passed during the 12 months prior to nomination. (cid:129) Each thesis should include a foreword by the supervisor outlining the signifi- cance of its content. (cid:129) The theses should have a clearly defined structure including an introduction accessible to scientists not expert in that particular field. More information about this series at http://www.springer.com/series/8790 Holger Kluck Production Yield of Muon-Induced Neutrons in Lead Measured at the Modane Underground Laboratory Doctoral Thesis accepted by Karlsruhe Institute of Technology, Karlsruhe, Germany 123 Author Supervisor Dr. Holger Kluck Prof. JohannesBlümer Institute of Atomic andSubatomic Physics Institute for Nuclear Physics Vienna University of Technology Karlsruhe Institute of Technology Vienna Karlsruhe Austria Germany ISSN 2190-5053 ISSN 2190-5061 (electronic) SpringerTheses ISBN978-3-319-18526-2 ISBN978-3-319-18527-9 (eBook) DOI 10.1007/978-3-319-18527-9 LibraryofCongressControlNumber:2015942785 SpringerChamHeidelbergNewYorkDordrechtLondon ©SpringerInternationalPublishingSwitzerland2015 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpart of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission orinformationstorageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilar methodologynowknownorhereafterdeveloped. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publicationdoesnotimply,evenintheabsenceofaspecificstatement,thatsuchnamesareexemptfrom therelevantprotectivelawsandregulationsandthereforefreeforgeneraluse. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authorsortheeditorsgiveawarranty,expressorimplied,withrespecttothematerialcontainedhereinor foranyerrorsoromissionsthatmayhavebeenmade. Printedonacid-freepaper SpringerInternationalPublishingAGSwitzerlandispartofSpringerScience+BusinessMedia (www.springer.com) Seht ihr den Mond dort stehen? Er ist nur halb zu sehen, Und ist doch rund und schön! So sind wohl manche Sachen, Die wir getrost belachen, Weil unsre Augen sie nicht sehn. —Matthias Claudius ’ Supervisor s Foreword The Swiss physicist and astronomer Fritz Zwicky noted in 1933 that the Coma cluster of galaxies had a spread of individual velocities that was “too large”: the massoftheclusteritself—deducedfromthevisibleobjects—wastoosmalltokeep thegalaxiesboundtogether.Turningtheargumentaroundusingthevirialtheorem he found that the Coma Cluster must contain a large amount of matter not accounted for by the light of the stars. He called it “Dark Matter”. Since then astronomers, cosmologists and physicists have found more evidence for theexistence of Dark Matter.Galaxies rotate fasterthan Newtonian gravitation would suggest. Precision measurements of the cosmic microwave background radiationcanonlybeexplainedbyacertainmixtureofordinarymatter,DarkMatter and an even more mysterious substance called “Dark Energy”. Dark Matter is electricallyneutralandinteractsveryweaklywithordinarymatter.Aniceexample is the so called “bullet” cluster (1E 0657–558, Fig. 2.4 of this book). Here, galaxy clusters have penetrated each other. A superposition of optical, radio and x-ray images shows that gas clouds are interacting violently and are left behind their clusterswhilemostofthemassisatthecenterofthegalaxyclusters,measuredby the gravitational lensing effect. What is Dark Matter? We don’t know, but serious speculations say that new kinds of particles should exist,so-calledsupersymmetric particles that relate tothe knownzooofsubatomicparticles.AgenericnameisWIMPforWeaklyInteracting MassiveParticle.Ifthatmodeliscorrect,thenalsoourowngalaxyshouldbefilled with a cloud of WIMPs and the Sun and the Earth are moving through. WIMPs would gather almost everywhere, preferentially bound in gravitational fields. They wouldmoveratherslowlyduetotheirlargemass,rangingfromafewtothousands of proton masses. How can we find WIMPs? There are essentially three ways. First, WIMPs may scatteroffordinaryatomicnuclei,butveryrarelyso.Iftherecoilingnucleusispart of a very sensitive detector material, a tiny energy release might be detected. Second, WIMPs could annihilate with each other somewhere in the Universe, leadingtoacharacteristicradiation,whichinturncanbeobservedbyastronomers. vii viii Supervisor’sForeword Finally, WIMPs might be produced at accelerators like the Large Hadron Collider LHC at CERN, Geneva. They would leave the detectors without a trace, but the energy-momentum balance of the other, well-measured particles would indicate their creation and escape. My student Holger Kluck was part of a team— the EDELWEISS collaboration—that pursues the first kind of WIMP search. The difficulties in the three kinds of WIMP searches are different. For the third method,LHCdetectorsmustworkverywelltoaccountforso-calledmissingmass. For the so-called indirect searches astronomers must be sure that no ordinary radiation source is mistaken for WIMP annihilation. For the direct search with sensitivedetectors,theexpectedscatteringinanytargetmaterialisveryrareindeed. RadioactivityandotherparticlesmaymimicaWIMPsignal,e.g.fromcosmicrays that impinge all the time on Earth. Therefore, the EDELWEISS detector is well shieldedintheModaneUndergroundLaboratoryintheFrenchAlps,locatedinthe middle of the Frejus tunnel between France and Italy. A particularly nasty kind of backgroundsignalarisesfromneutrons,whichmaybeproducedbycosmicraysin thesurroundingmaterialclosetothedetector.Theymaywanderaroundunseenby veto detectors, hit a detector nucleus and release a tiny amount of energy just as a WIMP would. Holger’s task was to perform a detailed study of this background. In this thesis Holger Kluck presents a unique summary of the research on WIMPs emphasizing ontheorigin andthesuppressionofthisneutronbackground whichcouldcompromisetheWIMPsearch.Hedescribesthephysicalprocessesin great detail und offers a comprehensive account of all measurements to date. That wouldn’t have been possible without a dedicated interpretation of data and— importantly—of very detailed simulations. Holger’s own measurements using a neutron detector developed at the Karlsruhe Institute of Technology (KIT), his model and his extensions of the GEANT4 software package are finally consistent with each other and in accordance with newer results from other groups. Holger’sthesisinthisformhasbecomeahandbookofcosmic-inducedneutrons that is relevant for understanding and suppressing neutron reactions. The commu- nity of WIMP searchers is grateful for his effort, and so am I that Springer has agreed to make this research easily accessible. Karlsruhe Institute of Technology Prof. Johannes Blümer January 2015 Abstract Muon-inducedneutronsareanimportantbackgroundsourceforrareeventsearches such as Dark Matter searches looking for nuclear recoils induced by the elastic scattering of galactic WIMPs off nuclei. Due to a shielding of 4800 mwe against muonsattheLaboratoireSouterraindeModane(LSM),therateofmuon-induced neutronsinEDELWEISSistoolow,tobestudiedinsituwithsatisfyingstatistical accuracy. One thus relies on Monte Carlo (MC) modelling of the relevant pro- cesses,usinge.g.thepackageGeant4.However,thereliabilityofMCsimulationsis debatable, as the published differences between simulation and measurement is often larger than a factor two. ThelackofreliabledataontheneutronproductionyieldinleadatLSMandthe dubious accuracy of the MC simulations motivated this work and lead to the following results: A high statistics reference data set of muon-induced neutrons at LSM was collectedbyrunningadedicatedneutroncounterconsistingofaleadtargetbelowa neutron multiplicity meter based on 1000 l liquid scintillator loaded with gado- linium.Within alive-time of964.5dfrom2009to2012, asample of5583tagged muons were measured in coincidence with 313 candidates for muon-induced neutrons distributed over 181 neutron cascades. Using the modelling package Geant4, we propagated about 5:5(cid:1)107 muons (μþ=μ(cid:3) (cid:4)1:37) through a detailed three-dimensional geometry and tracked all electromagnetic and hadronic shower products. Albeit more than 95.5 % of all neutronswhichterminatedintheliquidscintillatorwereproducedwithinadistance of1.19maroundtheneutroncounter,only78.2%ofthemoriginatedfromthelead target. This highlights the importance of a detailed geometry implemented in simulation packages. Taking into account a calibrated detector response model on an event-by-event basis, the measured and simulated absolute integral rates of neutron candidates agree within the statistical, systematic, and theoretical uncertainties. The experi- mental value of ð3:2þ0:5Þ(cid:1)10(cid:3)1 neutrons per day is reproduced by MC to within (cid:3)0:3 ix x Abstract 15%.Alsothemeasuredabsolute multiplicity spectrumiswell reproducedbyour model. The neutron production yield in lead at LSM is, for the first time, derived to be hyi¼ð2:7(cid:3)þ01::70Þ(cid:1)10(cid:3)3 cm2 g−1 for muon energies hEμi¼267 GeV. This work demonstrates that Geant4 can reliably model the production and detection of muon-induced neutrons once all relevant production processes and a detailed description of the detector response and geometry are implemented in the model.Thus,oneofthemostprominentbackgroundsourcesforDarkMattersearch can be accurately modelled and eventually suppressed.

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