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平衡重量数主要射线能量及分支比/keV(%)238U4.468109a4.91610-181234Th24.1d3.32910-71.45210-1192.6(5.41),63.3(3.812)234mPa1.17min9.87410-34.89510-16234U2.445105a8.98310-145.38010-5230Th7.7104a2.85310-131.66510-5226Ra1.6103a1.37310-113.40110-7186.2(3.28)222Rn3.823d2.09910-62.18510-12218Po3.05min3.78810-31.18910-15214Pb26.8min4.31110-41.02510-14351.9(37.21),295.2(19.25),77.1(10.5)214Bi19.9min5.80510-47.61410-15609.3(46.3),1764.5(15.8),1120 (15.1)214Po1.63710-4s4.2341031.04410-21210Pb22.26a9.86710-104.39610-9210Bi5.013d1.60010-62.71010-12210Po138.378d5.79810-87.48210-11206Pb稳定核素衰变类型半衰期衰变常数s-1平衡重量数主要射线能量及分支比/keV(%)232Th1.4051010a1.56310-181228Ra5.75a3.82010-94.02210-10228Ac6.13h3.14110-54.89110-14911.1(27.7),969.1(16.62),338.3(11.4)228Th1.913a1.14810-81.33810-10224Ra3.62d2.21610-66.81110-13240.98(3.95)220Rn55.61s1.24610-21.18910-16216Po0.146s4.7483.06610-19212Pb10.64h1.80910-57.89710-14238.6(44.65),77.1(18)212Bi,60.55min1.90810-47.48710-15727.2(11.83)212Po2.9810-7s2.3261063.93710-25208Tl3.053min3.78410-31.33010-162614.7(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%的核发生衰变成208Tl,其余发生衰变成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1.310-31.210-33.410-5混合蔬菜8.310-34.910-21.810-3草 类2.310-28.010-21.110-22