表1 天然放射系铀镭系核素衰变数核素衰变类型半衰期衰变常数s-1平衡重量数主要γ射线能量及分支比/keV(%)238Uα4.468×109a4.916×10-181234Thβ24.1d3.329×10-71.452×10-1192.6(5.41),63.3(3.812)234mPaβ1.17min9.874×10-34.895×10-16234Uα2.445×105a8.983×10-145.380×10-5230Thα7.7×104a2.853×10-131.665×10-5226Raα1.6×103a1.373×10-113.401×10-7186.2(3.28)222Rnα3.823d2.099×10-62.185×10-12218Poα3.05min3.788×10-31.189×10-15214Pbβ26.8min4.311×10-41.025×10-14351.9(37.21),295.2(19.25),77.1(10.5)214Biβ19.9min5.805×10-47.614×10-15609.3(46.3),1764.5(15.8),1120 (15.1)214Poα1.637×10-4s4.234×1031.044×10-21210Pbβ22.26a9.867×10-104.396×10-9210Biβ5.013d1.600×10-62.710×10-12210Poα138.378d5.798×10-87.482×10-11206Pb稳定核素衰变类型半衰期衰变常数s-1平衡重量数主要γ射线能量及分支比/keV(%)232Thα1.405×1010a1.563×10-181228Raβ5.75a3.820×10-94.022×10-10228Acβ6.13h3.141×10-54.891×10-14911.1(27.7),969.1(16.62),338.3(11.4)228Thα1.913a1.148×10-81.338×10-10224Raα3.62d2.216×10-66.811×10-13240.98(3.95)220Rnα55.61s1.246×10-21.189×10-16216Poα0.146s4.7483.066×10-19212Pbβ10.64h1.809×10-57.897×10-14238.6(44.65),77.1(18)212Biα,β60.55min1.908×10-47.487×10-15727.2(11.83)212Poα2.98×10-7s2.326×1063.937×10-25208Tlβ3.053min3.784×10-31.330×10-162614.7(99.8),583.1(84.2),510.8(21.6)208Pb稳定表2 天然放射系钍系核素衰变数*衰变的212Bi中有35.9%的核发生α衰变成208Tl,其余发生β衰变成212Po.表1 IAEA给出的U、226Ra和Th从土壤到植物的转移参数Bv值植 物Bv期望值[(mg/kg鲜重)/(mg/kg干土)]U226RaTh谷 物1.3×10-31.2×10-33.4×10-5混合蔬菜8.3×10-34.9×10-21.8×10-3草 类2.3×10-28.0×10-21.1×10-22。