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放射性同位素——核辐射的主角

    ①同位素就是一种元素存在着质子数相同而中子数不同的几种原子。由于质子数相同,所以它们的核电荷和核外电子数都是相同的,并具有相同核外电子排布。由于最外层电子数相同,因此原子核的某些物理性质也有所不同,例如放射性,并不是所以同位素都具有放射性,有放射性的同位素称为“放射性同位素”,没有放射性的则成为“稳定同位素”。大多数的天然元素都是由几种同位素组成,目前已知的稳定同位素约300多种,而放射性同位素竟达1500种以上。
    ②一般来说,原子质量很大金属,像鈈、铀、镭等,都具有较强的放射性,在化学元素周期表中,锕系元素和镧系元素以及铀元素全部带有放射性。另外某些原子质量小的同位素也带有放射性,如碳14、钴60。
    ③放射性同位素的原子核很不稳定,全不间断地,自发地放射出射线,直至变成另一种稳定同位素,这就是所谓“衰变”,放射性同位素在进行衰变的时候,可放射α射线、β射线、γ射线对人体危害不大,而γ射线对人体有较大的伤害,会诱发人体基因突变。
    ④放射性元素的原子核有半数发生衰变时所需要的时间,叫半衰期。换言之,半衰期是指某个样品中一半的原子核发生衰变所需的时间,不同放射性同位素的半衰期差异很大,短的只有几天、几个小时、几分钟,甚至不到1秒钟,长的却达几千年、几万年,甚至是几亿年,几十亿年,例如,日本“3.11”地震及海啸引发的核辐射中的碘131的半衰期约为8天,铯137为30年,鈈239为24000年,铀238则为44.7亿年。半衰期越短,其原子越不稳定。
    ⑤经过连接的几个半衰期后,放射性同位素的活度会因衰变而减至初始活度的
1
2
1
4
1
8
,等等。这意味着我们可以预测任何时候的剩余活度。随着放射性同位素数量的减少,所发出的辐射也相应的减少。
    ⑥放射性同位素释放的放射性能够破坏活的细胞,对人体造成巨大的伤害,但在医疗上,可以用来杀菌消灭微生物,并且可以用来杀灭癌细胞等。放射线也具有很强的贯穿能力,它可以用来观察固体内部的目标,就像x射线那样用于病灶的检查。在工业上,放射性也很多应用,例如用β射线来测量纸的厚度,用γ射线照片来检查机器内部结构等。
    ⑦当然,如果应用不当,核辐射也会造成难以估计的损失。

(选自《中学科技》2011年第5期,有删改)

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In a Station of the Metro

    The apparition of these faces in the crowd;Petals on a wet, black bough.

    This is the only Ezra Pound poem that many people will read in their lives. Why? One obvious reason is that it's just two lines long. The poem, which can be understood as “A man sees a bunch of faces in the subway and thinks they look like flowers on a tree branch,” is an exercise in shortness. Pound wrote it after having a spiritual experience in a Paris metro (subway) station.

    In 1916, the US poet originally thought he could best describe his vision in a painting. Unfortunately, he wasn't a painter. So he wrote a thirty-line poem, which he didn't like. He dropped the long version in the waste bin. Six months later, he wrote a shorter poem, but didn't like that one either. Finally, a full year after the experience, he had been reading short Japanese poems called haikus, and he figured he would try this style. The result, which was published in 1913, is one of the most famous and influential works in modern poetry.

    This poem is one of the monuments (纪念物) of the 20th-century artistic movement known as “Imagism”. Basically, Pound and his friends thought that images (意象) weren't just decoration: they were the highest form of speech. By finding the right image, the poet can express the true, spiritual reality of a thing, which is more important than using a bunch of adjectives to describe its physical appearance. Thus, “In a Station of the Metro” is a poem that consists of one image expressed with absolute (绝对的) exactness and nothing else.

    To the imagists, the best way to describe an experience is not to use more and more words; the best way is to find exactly the right words. Have you ever told a beloved one that “words can't express” how much you love them? Well, Pound would say that you're just being lazy. In his view, words can express anything, even if it takes an entire year to find the right ones.