放射化學第四講放射性衰變(參考版)
2024-10-06 21:43本頁面
【正文】 ? Resolved into two ponents。 ?Use absorbers。 ?Very short (less than milliseconds). 2022/10/23 Specific activity method ? It is impossible to monitor the change of activity over time ? A quantity that can be directly determined and related to halflife is the specific activity (As, 比活度 ) of the substance. The As is the activity per unit mass: mAmdtdNA s ???2022/10/23 Specific activity method ? To determine the halflife, we need only measure the absolute activity(絕對活度 ) of the sample and its mass. MmAt??? ))(6 9 ( 232/1NtNdtdNA2/1???? ? )10( 6 . 0 2 3Mm 23???N2022/10/23 Specific activity method ? 4p counters 2022/10/23 Medium Halflives ? Simple decay curve method ? The Simple decay curve method gives best results if a pure radionuclide source is used. ?Chemical separation。 ?Medium (seconds to a few years)。 ? t1/2 , a very large range 1018 s to 1022 years。 ? 4n+1放射系是用人工方法得到的； ????? ??? PaNpAmPuU aaa 2 3 3)(2 3 7)(2 4 1)(2 4 122 4 1 6????? 在該衰變鏈中， 237Np的壽命最長，其他核素都衰變完了，只剩下 237Np； ? 除 209Bi外，其他子體在自然界中的存量很小，因此長時間未被發(fā)現(xiàn) 。 2022/10/23 錒系： 4n+3 2022/10/23 錒系： 4n+3 ? 7次 ?衰變， 4次 ?衰變； ? 235U與 231Pa達到平衡的時間： 7t1/2(231Pa)??105a。 2022/10/23 鈾系： 4n+2 2022/10/23 鈾系（ 4n+2) ? 8次 ?衰變， 6次 ?衰變； ? 238U與 234U達到平衡的時間： 7t1/2(234U)??106a。 ? Uraniumradium series( uranium family):A=4n+2。)(21)(212 1 42 1 8??PbPott2022/10/23 The change of total activity with different ?1/?2. 2022/10/23 Similar Halflives 1 / 21 / 2 1 / 2 1 / 2 1 / 2( 2 )1( 2 ) ( 1 ) ( 1 ) ( 2 )tt t tt t t t? ? ???????])21(1[)(1)2(21)1(21)2(212)1(21)2(21ttttNtttN????2022/10/23 Parent with Multiple Radioactive daughters ? There are many radionuclides that decay to produce a sequence of radioactive daughters. e t c .DCBA 4321 ? ??? ??? ??? ?? ????2022/10/23 多子體衰變 Parent with Multiple Radioactive daughters ... . . . . . . . . .λλ3322322112111NNdtdNNNdtdNNdtdN?????????When t=0 N1(0)=N1， 0 N2(0)=N3(0)=N4(0)=0 ? Bateman equation ?Two assumptions: ? Only the parent is present at t = 0。 ? Calculation of mass ratios of radionuclides that are in SRE。 5 times the half life ? In addition, the decay constant of the longlived parent will be very small pared to that of the daughter. Secular Radioactive Equilibrium 2022/10/23 Secular Radioactive Equilibrium )1(21)2(212112)2(211212)2(21)1(2112)(11212]1[。 計算經(jīng)過多長時間注射到病人體內(nèi)的放射性活度減少為開始注射時的 %？ A/A0= %=， = e()(t) ln() = ()t， t = h teAA ??? 02022/10/23 長期平衡 暫時平衡 不成平衡 放射性平衡 2022/10/23 Radioactive Equilibriums ? A parent will produce one or more radioactive daughters, which will themselves decay into stable or active “granddaughters.” ?A parent radionuclide with only one radioactive daughter。 其 ?衰變的半衰期是
化學相關(guān)推薦
鄂ICP備17016276號-1