Bremsstrahlung - Radioactivity

The phenomenon of bremsstrahlung (from "braking radiation" in German) applies mostly to particles with an electrical charge whose velocity is close to the ... EN|FR BremsstrahlungArelativisticphenomenonthatappliestoelectronsandpositrons... TheBremsstrahlungprocessAnultrarelativisticelectronpassingnearanucleusisinfluencedbyitsstrongelectricfield.Theinteractionbetweenelectricchargesisexplainedinmodernphysicsbytheexchangeofphotons.In(A),theelectronexchangesaphoton(notobserved)withthenucleus.In(B),itemitsaphotontobeobserved,the“braking”photon.Theelectronhaslostenergy,itstrajectoryhasbeendeflected.Iftheelectrondoesnotpassverynearthenucleus,theexchangeissoft,sothatthebremsstrahlungphotonenergyislow(infrared,visible,UV...).Onthecontraryiftheexchangeis'hard',thebremsstrahlungproducesanXorgammaray.IN2P3 Thephenomenonofbremsstrahlung(from"brakingradiation"inGerman)appliesmostlytoparticleswithanelectricalchargewhosevelocityisclosetothespeedoflight.Itoccurswhenthisultrarelativisticparticleinteractswithastrongelectricormagneticfield,whichcanbenatural(theelectricfieldofanucleus)orproducedbyman(thefieldofmagnetsinanaccelerator).Theelectronsandpositronseasilyreachingspeedsclosetothatoflightbecauseoftheirverylowmassarethefirstconcernedbythephenomenon,andtheonlyonesinthefieldofradioactivity. Bremsstrahlungplaysamarginalroleinthefieldofradioactivity,sincebetadecayelectronsdonotoftentravelfastenough.Onthecontrary,itplaysanimportantroleincosmicraysandinparticleaccelerators. Whileinteractingwithastrongelectromagneticfield,theelectronorpositronemitsaphotonthatcarriesashareofitsenergy.Theparticleslowsdownanditstrajectoryisdeflected.Thebremsstrahlungiscausingenergylossesinbigparticleacceleratorssuchascolliderswheretheparticlesaresubjecttotheactionofpowerfulmagnetsthatbendtheirtrajectories.Acceleratorengineersmustconstantlycompensatetheselosses. Theselossesthatannoysacceleratorengeneers,areviewedasabenedictionbyphysiciststhatneedintensesourcesofradiation,rangingfrominfraredtogammarays.Specialsynchrotronsacceleratorsaredesignedtoprovideabundantbremsstrahlungphotons.Physicistscallthemsynchrotronradiation. Amarginalprocessforradioactivity'sbetasComparisonofbremsstrahlungandionizationelectronsenergylossratesinamediumsuchascopper(Z=29).Thethicknessofanabsorberisusuallyexpressedingramspercm2.Forbetaelectronsofradioactovitywhoseenergyaregenerallylessthan1MeV,thecontributionofbremsstrahlung-thatincreaseswithenergy-isstillsmallormarginal.Thisenergylosswouldbemultipliedby8withheavyabsorberssuchaslead(Z=82).IN2P3 Synchrotronshavebeenbuiltspeciallyforthispurposefor20years.EachbeamlineoftheSynchrotronSOLEILnearOrsayconstituteafulllaboratoryforanalysisinbiology,chemistryorearthscience. Thebremsstrahlungisstillusedinradiotherapy,wheresmalllinearacceleratorsproduceelectronbeamsthatcanbeeitheruseddirectlyfortreatmentatashallowdepth,ortransformedintogammaraysusingalternatingmagneticfields. ThusX-raybeamsareobtainedwhichcanbeadjustedatwillindirectionandenergy.Thebremsstrahlungoccursquiterarelyinradioactivity,whenbetaelectronstravelnearthestrongelectromaneticfieldsclosetoatomicnuclei. TheSOLEILsynchrotronInafacilitysuchasSOLEIL,electronsofveryhighenergy(theirnominalenergyisof2750MeV)rotateatnearlythespeedoflightina354mcircumferencestoragering.Theemissionofradiationistangenttothetrajectoryinanextremelythin“light”line,whentheelectronstrajectoryiscurvedwithamagneticfieldthatplaystheroleofthenucleielectricfield.Thesynchrotronradiation,producedinbendingmagnetsandWigglersinvertersusedforinsertions),isfilteredinenergyorwavelengthanddirectedbyopticalsystemsinbeamlinestowarsexperimentalstations.Synchrotron-SOLEIL ThevastmajorityofbetaelectronsenergiesarebelowoneMeV,wellbelowtheenergiesofelectronscirculatinginanaccelerator.ItwouldrequireelectronsofseveralMeVforthebremsstrahlungenergylosstooutweighthelossbyionization. However,“radiationbraking”increasesasthesquareoftheelectricchargeZofnucleipresentinthematerial.Whenthebetaelectrongoesthroughamaterialmadeofrelativelylightatomssuchasaluminum,plasticorwater,thebremsstrahlungcontributiontotheslowdownisminimal. HowevertherearecaseswhereashieldingbecomesnecessaryagainstsecondaryXorgammaraysradiations.Thisisthecaseforexampleofphosphorus-32,aradioisotopeusedinmedicineforwhichthemaximumbetaelectronenergyof1.7MeVislarge.Conventionalshieldingswithheavymaterialssuchaslead(Z=82)arecontraindicatedbecausetheypromotebremsstrahlungXorgamarays.Low-densitymaterialsshouldbeusedinsteadforscreeningsuchasplexiglas,plastic,woodorwaterwithatomicnucleithatwoulddeceleratelesstheelectronandgeneratelessenergeticandpenetratingbremsstrahlungXorgammarays. ALSO:PhotoelectricEffect(gamma/electron) ALSO:ComptonEffect(gamma/electron)) Accesstopageinfrench. Learnmore:SOLEILsynchrotroncenterRadiotherapies Aboutthesite|Glossary|SiteMap|Contact