放射化學(xué)第四講放射性衰變-資料下載頁
2025-09-25 21:43本頁面
【正文】 體的 ?射線用于治療癌癥； ? 222Rn是唯一的氣體。 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(dá)到平衡的時(shí)間： 7t1/2(231Pa)??105a。 2022/10/23 釷系： 4n 2022/10/23 镎系： 4n+1 2022/10/23 镎系： 4n+1 ? 自然界中存在 4n、 4n+ 4n+3,3個(gè)放射系，缺少4n+1放射系。 ? 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(shí)間未被發(fā)現(xiàn) 。 2022/10/23 Branching Decay(分支衰變 ) ? The parent nuclide(A) decays by two (or possibly more) separated decay modes (?B, ?C,…). ? Each decay branch is independent of the other, and will have its own characteristic halflife and decay constant. These are called partial decay constant. ? Total activity of A is equal to the sum of the activity of each branch. The overall decay constant is the sum of the individual decay constant. TACBATACABTNNANNA??????????)(2022/10/23 Experimental Methods for Determination of HalfLife ? t1/2 , an important characteristic of a nuclide。 ? t1/2 , a very large range 1018 s to 1022 years。 ? No single technique is sufficient to measure all of them. Divided them into four groups: ?Long (several years and up)。 ?Medium (seconds to a few years)。 ?Short (seconds to milliseconds)。 ?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(duì)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。 ?Count only the desired activity。 ?Use absorbers。 teAA ??? 0tAA ??? 0lnln2022/10/23 ? Mixture of two radionuclides ? The activity of mixture measured and plotted, as done in single radionuclide。 ? Resolved into two ponents。 2022/10/23 The end of Chapter 4 2022/10/23 )2(211221)2(2121)1(21)1(21)2(212112tMmMmtNNtttNN???????????2022/10/23 Atomic bomb 2022/10/23 簡單推導(dǎo) 111011111021 1 2 2 1 1 2 2022 2 1 1tttN N edNNdtdNN N N e NdtdNN N edt????? ? ? ?????????? ? ? ??? CeNeNdeNeNddteNdteNdNedtetttttttt???????????)(011212)(011212)(01122212212212222)(???????????????????得兩邊同乘上)(0,021011212120112tt eeNNNCNt?????????? ????????初始條件
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