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00:00
Transcriber: Andrea McDonough
Reviewer: Bedirhan Cinar
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翻译人员: Yin Zhu
校对人员: Jenny Yang
00:13
It is only in the last 100 or so years that human kind has understood
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近一百年来,人们渐渐明白
00:18
that the nucleus of the chemical elements is not always fixed.
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化学元素的原子核并不总是稳定的。
00:22
It can change spontaneously from one element to another.
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它可以瞬间变成另一个化学元素。
00:26
The name for this process is radioactivity.
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这种演变的过程叫放射。
00:29
You probably already know something about the nucleus:
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你可能对原子核已经有一定了解:
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it's much tinier than the atom,
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比如它比原子小,
00:35
it's made of particles called protons and neutrons,
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再比如原子核是由质子和中子构成的,
00:38
there are electrons orbiting around it.
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原子核周围还有电子围绕。
00:41
And though the atoms can share or swap electrons when they bond together,
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虽然原子结合时,他们可以共享或交换电子,
00:45
the nuclei themselves never change.
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但原子核保持不变。
00:47
Right?
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对吗?
00:48
Well, no.
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不对。
00:50
Certain nuclei are not stable in that way.
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某些原子核并不稳定。
00:52
This means they may change suddenly,
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这意味着,他们可能
00:55
spontaneously.
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瞬间改变。
00:57
The radioactive nucleus flings out a small particle
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有放射性的原子核会甩出小物质
01:00
and transforms into another element, just like that.
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并同时变成另外一个化学元素。
01:03
For example,
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比如,
01:04
the carbon nucleus can eject a fast-moving electron
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碳元素的原子核可以甩出一个高速移动的电子,
01:07
and turn into a nitrogen nucleus.
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并变成氮元素的原子核。
01:09
There are two different particles that can be emitted from radioactive nuclei,
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有放射性的原子核可以甩出两种物质,
01:13
but never together.
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但不能同时甩出这两种物质。
01:14
The very fast electron is known as a beta particle.
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移动速度快的电子叫β粒子.
01:18
If you know a little bit about electrons, you may be thinking,
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如果你对电子有了解,有可能会想,
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"What was the electron doing in the nucleus in the first place?"
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“为什么原子核里会有电子?”
01:25
The answer is there is a neutron in nucleus
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答案是因为核中的中子,
01:27
spontaneously changed into a proton,
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自发瞬间变成质子,
01:30
which stayed behind,
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质子留在原子核内,
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and the electron flew out as a beta particle.
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但一个电子以贝塔粒子形式弹射出来。
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This is not what chemistry has taught us to expect.
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这不是传统化学所教的。
01:37
The nucleus is supposed to be stable.
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原子核应该是稳定的。
01:40
Neutrons don't change into protons.
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中子不会自发变成质子。
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Except, sometimes they do!
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但是,有时候他们却会!
01:44
The other particle it emits spontaneously
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另一个从不稳定的原子核自发射出的粒子
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from an unstable nucleus is alpha.
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叫阿尔法粒子。
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An alpha particle is 8,000 times more massive than beta,
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阿尔法粒子是贝塔粒子质量的8,000倍,
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and it's a bit slower.
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而速度要慢些。
01:54
Alpha is made from two protons and two neutrons.
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阿尔法粒子是由两个质子和两个中子构成的。
01:58
If we trap all those alpha particles together,
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如果我们把弹射出的阿尔法粒子搜集起来,
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we get helium gas.
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我们将获得氦气。
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Alpha is a helium nucleus.
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阿尔法粒子是氦气的原子核。
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Like the beta particle, you would not have expected
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很难想象原子核会弹射出贝塔粒子,
02:08
a heavier nucleus to throw out helium.
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更不要说重很多的氦气了。
02:11
But again, it happens,
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但事实再一次超出我们想象,
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and the nucleus becomes a new element.
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弹射出的原子核变成一个新的元素。
02:16
So, is radioactivity useful or just dangerous?
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那放射是有益还是有危害呢?
02:20
Wherever you are sitting,
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其实无论你坐在哪里,
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it is quite likely that there is a device nearby
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周围很有可能有一件
02:24
which contains a source of alpha particles:
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有阿尔法粒子的电器:
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a smoke detector.
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比如烟雾探测器。
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The source is radioactive Americium.
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里面就有带放射性的銤元素。
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You are totally safe from these alpha particles,
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这些阿尔法粒子对你完全无害,
02:33
which cannot travel more than a few centimeters in air.
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因为它们只能在空气中传播几厘米。
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Beta particles penetrate much farther
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较阿尔法粒子而言,贝塔粒子穿透力
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through materials than alpha can.
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强很多。
02:41
Radioactive atoms are used in medicine as traces,
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放射性元素在医学上被用做示踪剂,
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to show where chemicals travel in the patient.
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来显示药物到达了人体的哪个部位。
02:46
Beta particles are emitted and have enough energy
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被释放的贝塔粒子有足够的能量
02:49
to emerge from the body and be detected.
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穿透人体并被测量到。
02:51
There is a third type of nuclear radiation:
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除了阿尔法和贝塔粒子外,还有第三种辐射:
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gamma, which is not a particle at all.
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伽马其实不是粒子。
02:56
It is an electromagnetic wave,
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它是一种电磁波,
02:58
like microwaves, or light,
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像微波或是光,
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but it is actually 1,000 times more energentic than visible light.
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但它是可见光能量的1,000倍。
03:05
Gamma rays may pass right through your body.
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伽马射线可以轻松穿透你的身体。
03:08
Gamma is used to zap the bacteria in fruit to increase its shelf life,
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伽马射线被用于农副产品的消毒,从而增加保质期,
03:12
or in radiotherapy to kill cancer cells.
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或被用于放射疗法来杀死癌细胞。
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Radioactive substances get hot,
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放射性元素会发热,
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and this heat can be used to generate power.
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而热能可以用于发电。
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This heat has been brought to you since space probes,
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这种热能被用在空间探测仪上,
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and, in the past, in pacemakers for hearts.
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而过去被用在心率起搏器上。
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The more abruptly nuclear radiation is slowed down,
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核辐射减速越快,
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the more damage it does to the atoms it hits.
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对被击中的原子伤害就越大。
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This is called ionization.
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这被称之为电离。
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Alpha causes the most ionization as it crashes into other atoms
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阿尔法粒子撞击其他原子时造成的电离最大,
03:39
and gamma the least.
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而伽马造成的电离最小。
03:41
In humans, the most serious effect of radiation
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对人类而言,发射对人体最大的危害
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is the damage that it can cause to our DNA.
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在于其对DNA的伤害。
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Although alpha cannot penetrate your skin,
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尽管阿尔法粒子不能穿透皮肤,
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if you inhale or injest a radioactive nucleus,
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如果你误吸食了放射性原子核,
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the health consequences can be severe.
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对健康影响会严重。
03:55
Radioactivity is both useful and deadly,
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发射既有用又有危险,
03:59
but it is all around us as a background to the natural world.
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但它就在我们身边,像自然世界的背景一样。
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