中子屏蔽

DOI10.1140/epjp/i2013-13077-1RegularArticle

Eur.Phys.J.Plus(2013)128:77THEEUROPEAN

PHYSICALJOURNALPLUS

ConcreteshieldingofneutronradiationsofplasmafocusanddoseexaminationbyFLUKA

M.J.Nematia,R.Amrollahi,andM.HabibiNuclearEngineeringandPhysicsDepartment,AmirkabirUniversityofTechnology,Tehran,IranReceived:30January2013/Revised:26May2013Publishedonline:22July2013–c󰁬Societ`aItalianadiFisica/Springer-Verlag2013

Abstract.PlasmaFocus(PF)isamongthosedeviceswhichareusedinplasmainvestigations,butthisdeviceproducessomedangerousradiationsaftereachshot,whichgenerateahazardousareafortheop-eratorsofthisdevice;therefore,itisbetterfortheoperatorstostayawayasmuchaspossiblefromthearea,whereplasmafocushasbeenplaced.InthispaperFLUKAMonteCarlosimulationhasbeenusedtocalculateradiationsproducedbya4kJAmirkabirplasmafocusdevicethroughdifferentconcreteshieldingconceptswithvariousthicknesses(square,labyrinthandcaveconcepts).TheneutronyieldofAmirkabirplasmafocusatvaryingdeuteriumpressure(3–9torr)andtwochargingvoltages(11.5and13.5kV)is(2.25±0.2)×108neutrons/shotand(2.88±0.29)×108neutrons/shotof2.45MeV,respectively.Themost

influentialshieldfortheplasmafocusdeviceamongthesegeometriesisthelabyrinthconceptonfoursidesandthetopwith20cmconcrete.

1IntroductionFLUKAisageneralpurposetoolforcalculationsofparticletransportandinteractionswithmatter,coveringanextendedrangeofapplicationsspanningfromprotonandelectronacceleratorshieldingtotargetdesign,calorimetry,activation,dosimetry,detectordesign,AcceleratorDrivenSystems,cosmicrays,neutrinophysics,andradiotherapy[1].ThePlasmaFocus(PF)isacoaxialdischargedevicethatcangenerateshort-lived(10–100ns)buthigh-temperature(0.1–2.0keV)andhigh-density(1018–1020particles/cm3)plasmawhichgivesfastneutronpulses(whenusingdeuterium

gas),softaswellashardX-raysandhighlyenergeticionandrelativisticelectronbeams[2].ThePFfirstwasdiscoveredindependentlybyMather(intheUSA)andFilippov(intheformerSovietUnion)[3].TheMatherandFilippovconfigurationsaredifferentintheelectrodedimensionsandtheaspectratio(diameter/length)oftheinnerelectrode.TheaspectratioislowerandhigherthanonefortheMather-andtheFilippov-typePFs,respectively[4].Acapacitorbank,afastsparkgapswitch,coaxialelectrodes,andavacuumchamberarethefundamentalelementsofaPFdevice[5].Overtheyears,manyplasmafocusdeviceswithstorebankenergyrangingfrom1kJto1MJhavebeenbuilt[6].Recently,someexperimentshavebeencarriedoutonthesmallPFdevicesintherangeoftenstohundredsofjoulesofcapacitorenergy[7].Whenusingdeuteriumgas,plasmafocusdevicesproducefusionD–Dreactionsgeneratingfastneutronpulseswithenergiesaround2.45MeV.Theneutronburstsusuallylastforabouttenstohundredsofnanoseconds.Lee[8]proposedthescalinglawwhichrelatesthestoredenergytothephysicalsizesofthePF.Hereportedthatbychangingintherangeofbankenergyfrom1kJto1MJneutronemissionspanstheintervalfrom107to1012neutronsperpulse[9].NeutronscalinglawofYn=3.2×1011×I4.5pinch;Yn=1.8×1010×I3.8peak;

Ipeak(0.3to5.7),Ipinch(0.2to2.4)inMAfromnumericalexperimentswaspresentedbyS.Lee[10],whereYnisthe

neutronyield.AlthoughtheneutronproductionmechanismofPFshasnotbeenclearlydeterminedyet,thethermonuclearfusionandbeamtargetmodelsaretwowidelyacceptedmechanismstobeinchargeofneutronproduction[11].Antanasijevi´cetal.andCastilloetal.[12,13]haveshownthat,whentheworkinggasispuredeuterium,theneutronangulardistributionhasaGaussianfunction.ExperimentalresultsofA.RosselandChoiproposedthat,whenadopinggasisadded,thereisasignificantincreaseintheneutronanisotropy[14].AccordingtoZakaullah

ae-mail:arashnemati@aut.ac.irPage2of9Eur.Phys.J.Plus(2013)128:77Fig.1.Conceptualdrawingoftheplasmafocus:(1)electricalbreakdownalongtheinsulatorsurface,(2)acceleratingcurrentlayerduetotheLorentzforce,and(3)denseplasmacolumnformation.

Fig.2.Innerelectrode(anode),outerelectrode(cathode)andinsulatorarrangementofthedevice.thedeuteriumfillingpressurehasanimportanteffectonthemagnitudeoftheanisotropicfactorinthewaythattheanisotropicfactorhasatendencytoincreasewithpressure[15].Infig.1aconceptualdrawingoftheplasmafocusisillustrated.Inthisstudytheplasmafocushasbeenconsideredasaneutronsourcewhichwillpresentaradiologicalhazardtopersonnel.Therefore,atotaleliminationofthedosetothepersonnelwhoisoperatingthedevicecannotbeachievedwithoutplacingthedeviceinaveryremotelocation.Additionally,thedevicemustbeaccessibleforbothexperimentaluseandmaintenance,sowemustprovideenoughshieldingtoreducetheradiationdosetothepersonneltoasafelevelwhilestillretainingfunctionalityofthedevice.Also,bothshieldthicknessesandmaterialsareimportantfordoseattenuation[16].Inthisstudytheinfluenceofeachshieldingwillberatedbythenumberoftimestheplasmafocuscanbefiredbeforethepersonnelreceive20mSvattallypointsthatareconsideredoutoftheshields(fig.5).Theyearlydoseof20mSvisaregulationlimitinIran,whichshouldaccountforadditionalradiationsourceslocatedinthelaboratoryoftheAmirkabirUniversity.Theplasmafocushasbeenplacedinanareaofabout900m2,inorderto