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1、Chapter4Radionuclidesinnature第四章自然界中的放射性核素杨金玲Practicallyallnaturalmaterialscontainradioactivenuclides,thoughusuallyinsuchlowconcentrationsthattheycanbedetectedonlybyverysensitiveanalysis.Thisistrueforwater(rainwater,rivers,lakes,sea),rocksandsoil,andalllivingmatter,aswellasforstr
2、ucturesonnaturalrawmaterialsexceptwheregreatcarehasresultedinuseofanon-radioactivematerial.Theradionuclidesinourenvironmentcanbedividedinto(i)thoseformedfromcosmicradiation,(ii)thosewithlifetimescomparabletotheageoftheearth,(iii)thosethatarepartofthenaturaldecaychainsbeginningwit
3、hthoriumanduranium,and(iv)thoseintroducedinnaturebymoderntechniques.Sourcescanbecategorizedas:(i)cosmogenic宇生放射性的,(ii)and(iii)primordial原生放射性的,and(iv)anthropogenic人类活动产生的.CosmogenicradionuclidesCosmicirradiationoftheatmosphereproducesneutronsandprotonswhichreactwithN2,O2,Ar,etc.r
4、esultingintheproductionofradioactivenuclides.Thesenuclidesareproducedatconstantratesandbroughttotheearthsurfacebyrainwater.Equilibriumisassumedtobeestablishedbetweentheproductionrateandthemeanresidencetimeoftheseradionuclidesinterrestrialreservoirs(theatmosphere,thesea,lakes,soil
5、,plants,etc)leadingtoconstantspecificradioactivitiesoftheelementsineachreservoir.CosmogenicradionuclidesIfareservoirisclosedfromtheenvironment,itsspecificradioactivitydecreases.Thiscanbeusedtodetermineexposuretimesofmeteoritestocosmicradiation(andtheconstancyofthecosmicradiationf
6、ield,using81Kr),datingmarinesediments(using10Be,26Al),groundwater(36Cl),glacialice(10Be),deadbiologicalmaterials(14C),etc.Theshorter-livedcosmogenicradionuclideshavebeenusedasnaturaltracersforatmosphericmixingandprecipitationprocesses(e.g.39Clor38S).OnlyTand14Careofsufficientimpo
7、rtancetodeservefurtherdiscussion.TritiumSatellitemeasurementshaveshownthattheearthreceivessomeofthetritiumejectedfromthesun.Muchlargeramountsareformedintheatmospherethroughnuclearreactions;e.g.,betweenfastneutronsandnitrogenatomsTheyieldforthisreactionisabout2500atomstritiumperse
8、condpersquaremeteroftheearth'ssurface;th
