放射化学第3讲(放射性)-05

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1、2021/8/17C.L.Liu放射化学Radiochemistry2021/8/17C.L.Liu复习与回顾核素？具有相同的质子数Z、相同的中子数N、处于相同的能态且寿命可测的一类原子称为核素(Nuclide)。238U，铀-2382021/8/17C.L.Liu复习与回顾同位素：质子数相同、中子数不同的两个或多个核素。2021/8/17C.L.Liu复习与回顾同中子异荷素:中子数相同、质子数不同的核素。2021/8/17C.L.Liu复习与回顾同质异位素:质量数相同、质子数不同的核素。2021/8/17C.L.Liu复习与回顾同质异能素：处于不同的能量状态且其寿命可以

2、用仪器测量的同一种原子核2021/8/17C.L.Liu复习与回顾等超额中子素：核中超额中子数(N-Z)相同的核素。2021/8/17C.L.Liu复习与回顾镜像素2021/8/17C.L.Liu复习与回顾27102853000Stabilityline2021/8/17C.L.Liu复习与回顾质量亏损？m(Z,A)=Zmp+(A-Z)mn-m(Z,A)=ZMH+(A-Z)mn-M(Z,A)质量过剩？=M(Z,A)-A；以原子质量单位表示的原子质量M(Z,A)与原子核的质量数A之差称为质量过剩2021/8/17C.L.Liu放射性衰变(RadioactiveDeca

3、y)2021/8/17C.L.LiuRadioactiveDecay不稳定原子核自发地放出粒子或电磁辐射变成另一种原子核的过程称为核衰变；核衰变是放射性核素的特征核性质，一般情况下不受外界条件，诸如温度、压力、电磁场、化学形态、物理状态等的影响；核衰变总是伴随着能量的释放（放热过程）2021/8/17C.L.LiuTheEnergyUnitofRadioactiveDecayElectronvolt(电子伏特，eV).1eV=1.60219×10-12erg=1.60219×10-19JOthercommonunitskeV(103eV),MeV(106eV),andGe

4、V(109eV).1MeV=1.60219×10-6erg=1.60219×10-13J2021/8/17C.L.LiuTypesofRadioactiveDecayNaturaldecaymodesAlphadecay(adecay)Betadecay(bdecay)(+,-,EC)Gammadecay(gdecay)SpontaneousfissionDelayedneutronandprotonemissionTwo-protondecayCompositeparticleemissionDoublebetadecay2021/8/17C.L.LiuTypeso

5、fRadioactiveDecayAlphadecay(adecay)Betadecay(bdecay)Gammadecay(gdecay)2021/8/17C.L.LiuTypesofRadioactiveDecay2021/8/17C.L.LiuAlpha-ParticleDecay2021/8/17C.L.LiuAlpha-ParticleDecayTypicalheavyelementdecayenergy:Q=49MeV2021/8/17C.L.LiuDecay2021/8/17C.L.LiuDecay,Q=E+Erecoil(Y)2021/8/1

6、7C.L.Liudecayenergy2021/8/17C.L.LiuDecayExampleCalculatetherecoilenergyandtotaltransitionenergyfora5.00-MeVaemitterwithadaughterofmassnumberequalsto200.2021/8/17C.L.Liudecayemittersseldomemitsingleenergyparticles,Inmanycases,particleswithdifferentenergiesareemitted:210Po()206Pb:E=

7、5.3045MeV,4.52MeV238U()234Th:E=4.20MeV,4.15MeV,4.04MeV235U()231Th:E(10kinds)2021/8/17C.L.LiuFinestructureofdecayspectrum2021/8/17C.L.LiuDecayschemefor226Th（衰变纲图）0+226Th30.6minQ=6.436(6.434)MeV2021/8/17C.L.LiuAlpha-ParticleDecayTheoreticallyunstabletodecay:A>1