that the nucleus of the chemical elements is not always fixed.

人類才明白化學元素的 原子核並非永恆不變。

It can change spontaneously from one element to another.

它可以自發性地 從一種元素轉變成另一種元素。

The name for this process is radioactivity.

這就叫做放射性。

You probably already know something about the nucleus:

你可能已經認識原子核：

it's much tinier than the atom,

它比原子還要小的許多，

it's made of particles called protons and neutrons,

且由一些被稱為質子和中子的粒子組成，

there are electrons orbiting around it.

同時有電子圍繞運行。

And though the atoms can share or swap electrons when they bond together,

當原子們形成鍵結時 共享或交換電子，

the nuclei themselves never change.

但原子核本身並無改變。

Right?

對嗎？

Well, no.

嗯，不對。

Certain nuclei are not stable in that way.

有些原子核並不穩定。

This means they may change suddenly,

這表示它們會突然

spontaneously.

且自發性地改變。

The radioactive nucleus flings out a small particle

具放射性的原子核會射出粒子

and transforms into another element, just like that.

並轉化成另一種元素。就是這樣！

For example,

舉例而言：

the carbon nucleus can eject a fast-moving electron

碳原子核可以在激發出高速電子之後

and turn into a nitrogen nucleus.

轉變成氮原子核。

There are two different particles that can be emitted from radioactive nuclei,

有兩種粒子可以被放射性原子核所射出。

but never together.

但同個原子核不能同時射出兩種粒子。

The very fast electron is known as a beta particle.

高速電子被稱為貝塔粒子。

If you know a little bit about electrons, you may be thinking,

如果你對電子有一點點認識， 你可能會疑惑：

"What was the electron doing in the nucleus in the first place?"

「為什麼原子核裡會有電子呢？」

The answer is there is a neutron in nucleus

答案是因為原子核裡的

spontaneously changed into a proton,

中子自發性的轉變為質子，

which stayed behind,

並保留在原子核中，

and the electron flew out as a beta particle.

而電子就被射出成為貝塔粒子。

This is not what chemistry has taught us to expect.

這和我們化學課教的並不相同。

The nucleus is supposed to be stable.

原子核應該很穩定才對。

Neutrons don't change into protons.

中子應該不會變成質子。

Except, sometimes they do!

但有時就是發生了！

The other particle it emits spontaneously

另外一種會被自發性地

from an unstable nucleus is alpha.

從不穩定的原子核中射出的阿爾法粒子。

An alpha particle is 8,000 times more massive than beta,

阿爾法粒子的質量是貝塔粒子的8000倍，

and it's a bit slower.

而且它的速度也較慢。

Alpha is made from two protons and two neutrons.

阿爾法粒子由兩個質子和兩個中子組成。

If we trap all those alpha particles together,

如果我們蒐集到很多的阿爾法粒子，

we get helium gas.

我們就得到了氦氣。

Alpha is a helium nucleus.

阿爾法粒子就是氦氣的原子核。

Like the beta particle, you would not have expected

就像是貝塔粒子一般，你也不會預期

a heavier nucleus to throw out helium.

一個較重的原子核會丟出氦原子核。

But again, it happens,

但這仍然發生了！

and the nucleus becomes a new element.

而這原子核也從而變成另一種元素。

So, is radioactivity useful or just dangerous?

那麼，放射性元素到底是有用還是有害呢？

Wherever you are sitting,

在你現在坐著的地方

it is quite likely that there is a device nearby

就可能有一種裝置包含

which contains a source of alpha particles:

可以釋放出貝塔粒子的元素：

a smoke detector.

煙霧偵測器。

The source is radioactive Americium.

它含有放射性的鋂。

You are totally safe from these alpha particles,

但是阿爾法粒子對你完全無害，

which cannot travel more than a few centimeters in air.

它無法在空氣中前進超過數公分。

Beta particles penetrate much farther

貝塔粒子相較於阿爾法粒子

through materials than alpha can.

有更好的穿透力。

Radioactive atoms are used in medicine as traces,

放射性元素可以當作醫用追蹤劑，

to show where chemicals travel in the patient.

以顯示特定物質在人體內的移動。

Beta particles are emitted and have enough energy

被射出的貝塔粒子具有足夠的能量

to emerge from the body and be detected.

可以穿透身體而被偵測到。

There is a third type of nuclear radiation:

還有第三種放射形式：

gamma, which is not a particle at all.

伽馬，但它不是一種粒子。

It is an electromagnetic wave,

它是一種電磁波，

like microwaves, or light,

就像是微波或光一樣，

but it is actually 1,000 times more energentic than visible light.

但是它比可見光的能量高出一千倍。

Gamma rays may pass right through your body.

伽馬射線可以穿透人體。

Gamma is used to zap the bacteria in fruit to increase its shelf life,

伽馬射線可以用在水果消毒，以延長保存時間。

or in radiotherapy to kill cancer cells.

或是用在放射線治療上殺死癌細胞。

Radioactive substances get hot,

放射性元素會放熱，

and this heat can be used to generate power.

而這種熱能可以轉換成其他能量。

This heat has been brought to you since space probes,

這種熱能被應用在太空探測器上，

and, in the past, in pacemakers for hearts.

過去也能用在心臟起搏器中。

The more abruptly nuclear radiation is slowed down,

如果放射出的波或粒子 被停下來的速度越快，

the more damage it does to the atoms it hits.

對阻擋原子造成的損傷也越大。

This is called ionization.

這就是離子化。

Alpha causes the most ionization as it crashes into other atoms

阿爾法粒子撞擊其他原子 所造成的離子化程度最大，

and gamma the least.

而伽馬射線最小。

In humans, the most serious effect of radiation

放射線對人體最嚴重影響是

is the damage that it can cause to our DNA.

造成 DNA 損害。

Although alpha cannot penetrate your skin,

雖然阿爾法粒子並不足以穿透皮膚，

if you inhale or ingest a radioactive nucleus,

如果你吸入或吃入放射性元素，

the health consequences can be severe.

對健康會有嚴重影響。

Radioactivity is both useful and deadly,

放射性元素既有用且致命，

but it is all around us as a background to the natural world.

但是它會一直存在我們四周。