【正文】 9Pu； 239Pu（屬 4n+3系）的半衰期為 10 4y。由于該半衰期比地球的年齡小，原生的 237Np在地球上已經(jīng)不存在了，因此，镎系不是天然存在的。v 第四個(gè)衰變系，镎系，由一組質(zhì)量數(shù)被 4除余 1的核素組成（ 4n+1系）。v基本概念鈾系：以 238U為起點(diǎn)，經(jīng)過 14次轉(zhuǎn)化（ 8次 ?衰變， 6次 ?衰變 ）終點(diǎn)為 206Pb；為 4n＋ 2系。操作鐳化合物應(yīng)該在防護(hù)下進(jìn)行以避免氡及其子體的輻射。鈾是巖石中的一個(gè)常見元素，也是建筑材料中的常見物質(zhì)。v 盡管鐳的化學(xué)相對(duì)簡單（類似鋇），但因鐳產(chǎn)生放射性氣體（氡）使它的處理變?yōu)閺?fù)雜。這些產(chǎn)物的多數(shù)核素都發(fā)射高能 γ 射線，因此 Ra用于醫(yī)治癌癥的 γ 源（放射治療）。v 鈾系供應(yīng)著元素鐳、氡和釙的最重要的同位素，它們可以從鈾礦處理過程中分離出來。 it undergoes αdecay with a halflife of 109y. The stable end product of the uranium series is 206Pb, which is reached after 8 α and 6 βdecay steps.v uranium decay series U系v 238U的比活度為 。他們主要來源于前面提到的三個(gè)衰變系。以鈾和釷同位素為起始核的重元素衰變系稱為 長衰變系 long decay series 。 KAr和 RbSr計(jì)年系統(tǒng)。這是 核計(jì)年 nuclear dating（也被稱為 “放射性時(shí)鐘 radioactive clock”）的基礎(chǔ)， 這個(gè)目的。隨著他們的衰變，衰變產(chǎn)物積聚在封閉的巖石環(huán)境中。 dpm/g C. Primordial radionuclides longlived nuclides lighter than leadv As the detection technique for radioactivity has been refined, a number of longlived radionuclides have been discovered in nature. The lightest have been mentioned. The heavier ones, not belonging to the natural radioactive decay series of uranium and thorium, are listed in Table . 50V is the nuclide of lowest elemental specific activity (~ Bq/g) while the highest are 87Rb and 187Re (each ~900 Bq/g). As our ability to make reliable measurements of low activities increases, the number of elements between potassium K and lead Pb with radioactive isotopes in nature can be expected to increase.Primordial radionuclidesv 由于這些核素的壽命特別長，他們肯定是在太陽系和地球形成時(shí)（甚至更早）形成的。 the global annual production rate is ~1 PBq, and global inventory ~8500 PBq (corresponding to ~75 tons). Of this amount ~140 PBq remain in the atmosphere while the rest is incorporated in terrestrial material. All living material (incl. body tissue) has a 14C concentration of ~227 Bq/kg. The halflife of 14C is 5715 y。 1020 Bq up to the end of the tests in 1962. This considerably exceeds the natural production inventory.v Before 1952 (first hydrogen bomb tests) the tritium content could be used to date water (. determine when it became isolated from contact with the atmosphere). This was very useful . for determining ice ages. However, due to the much larger content of anthropogenic tritium presently, this is no longer a useful technique for such dating.v Tritium in concentrations as low as 1 TU can be measured in low background proportional counters, and, after isotope enrichment (. by electrolysis of alkaline water, by which tritium is enriched in the remainder), down to TU. For very low concentrations mass spectrometry is preferred.14carbonv 14C is produced in the atmosphere by a variety of reactions, the most important being between thermalized neutrons from cosmic radiation and nitrogen atoms:v This reaction occurs with a yield of approximately 22 000 atoms 14C formed per s and m2 of the earth39。s andearly 196039。s surface。Chapter4 Radionuclides in nature第四章 自然界中的放射性核素楊金玲v Practically all natural materials contain radioactive nuclides, though usually in such low concentrations that they can be detected only by very sensitive analysis. This is true for water (rain water, rivers, lakes, sea), rocks and soil, and all living matter, as well as for structures on natural raw materials except where great care has resulted in use of anonradioactive material. The radionuclides in our environment can be divided into (i) those formed from cosmic radiation, (ii) those with lifetimes parable to the age of the earth, (iii) those that are part of the natural decay chains beginning with thorium and uranium, and (iv)those introduced in nature by modern techniques. v Sources can be categorized as: (i) cosmogenic宇生放射性的 , (ii) and (iii) primordial原生放射性的 , and (iv) anthropogenic 人類活動(dòng)產(chǎn)生的 .Cosmogenic ra