Nuclear Physics
IGCSE Physics Topic 5 15:46 English narration · English + 中文 subtitles burned in
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Everything around you is made of atoms — yet an atom is almost entirely empty space.
你周围的一切都由原子组成——可原子几乎全是空的。
If the nucleus were the size of a marble, the whole atom would be the size of a sports stadium.
如果原子核只有一颗玻璃弹珠那么大，整个原子就有一座体育场那么大。
And packed inside that tiny nucleus is a vast store of energy: the energy of the stars, and of the power that can heal, or harm.
而在那个小小的原子核里，藏着巨大的能量：恒星的能量，以及既能治病、也能伤人的力量。
This is nuclear physics — the science of the atom's tiny heart, and the radiation it gives off.
这就是核物理——研究原子那颗小小核心、以及它放出的辐射的科学。
Let's go inside.
让我们进到里面去。
Let's begin.
让我们开始吧。
At the centre of every atom is a tiny nucleus, packed with positive protons and neutral neutrons.
每个原子的中心是一个小小的原子核，里面装满带正电的质子和不带电的中子。
Around it, negative electrons move in a cloud.
核外，带负电的电子在一片电子云中运动。
We describe a nucleus with two numbers: the proton number — how many protons — which decides the element; and the nucleon number — the protons and neutrons added together.
我们用两个数来描述原子核： 质子数——有多少质子——它决定了元素；以及核子数——质子和中子加在一起的总数。
An atom is neutral because its protons and electrons balance; lose or gain an electron and it becomes a charged ion.
原子是电中性的，因为质子和电子相互平衡；失去或得到一个电子，它就变成带电的离子。
The nucleus holds two kinds of particle, and together we call them nucleons.
原子核里有两种粒子，合起来叫做核子。
A proton has a relative charge of plus one, and a relative mass of one.
质子的相对电荷是正一，相对质量是一。
A neutron has no charge at all, and a relative mass of one as well.
中子完全不带电，相对质量也是一。
An electron in orbit around the nucleus carries a charge of minus one, but its mass is so tiny that we simply ignore it.
电子带负一的电荷，但它的质量小到可以直接忽略。
So almost all the mass of an atom is packed into that nucleus.
所以原子几乎全部的质量都集中在原子核里。
A complete atom is neutral, because it has as many electrons as protons.
完整的原子是电中性的，因为它的电子数和质子数一样多。
But electrons can be knocked off, or picked up.
但电子可以被打掉，也可以被得到。
Lose one electron and the protons now outnumber the electrons, so the atom is left with a positive charge: it has become a positive ion.
失去一个电子后，质子就比电子多，原子便带上正电荷：它变成了正离子。
Gain an extra electron instead, and it becomes a negative ion.
如果反过来得到一个电子，它就变成负离子。
Notice that the nucleus never changes — only electrons move.
注意，原子核始终没有变——移动的只是电子。
Here is gold, written in nuclide notation.
这是用核素符号写出的金。
The top number is the nucleon number: one hundred and ninety-seven.
上面的数是核子数：一百九十七。
The bottom number is the proton number, seventy-nine, so this nucleus holds seventy-nine protons.
下面的数是质子数，七十九， 所以这个核有七十九个质子。
To count the neutrons, subtract the bottom number from the top one.
要数中子，就用上面的数减去下面的数。
One hundred and ninety-seven take away seventy-nine leaves one hundred and eighteen neutrons.
一百九十七减去七十九，剩下一百一十八个中子。
And since only protons carry charge, the relative charge of this whole nucleus is plus seventy-nine.
又因为只有质子带电，整个原子核的相对电荷就是正七十九。
How do we know the nucleus is there?
我们怎么知道原子核在那里？
In a famous experiment, alpha particles were fired at a thin gold foil and their scattering was watched.
在一个著名的实验中，人们把阿尔法粒子射向一片很薄的金箔。
Most of them sailed straight through, showing that the atom is mostly empty space.
大多数粒子径直穿了过去，说明原子内部大部分是空的。
But a few bounced sharply back — as if they had struck something tiny, dense, and positively charged.
但有少数几个被猛地弹了回来——好像撞上了某个又小、又密、又带正电的东西。
That something is the nucleus.
那个东西就是原子核。
Three results, three conclusions.
三个结果，三个结论。
Almost every alpha particle went straight through the foil, so the atom must be mostly empty space.
几乎每一个阿尔法粒子都直接穿过了金箔，所以原子内部大部分一定是空的。
A few were deflected through small angles: the alpha particles are positive, so something positive inside the atom pushed them aside.
有少数几个被偏转了很小的角度：阿尔法粒子带正电，所以原子里有某个带正电的东西把它们推开了。
And a very few bounced almost straight back, which can only happen if the thing they hit is tiny and far heavier than they are.
还有极少数几个几乎被直接弹了回来，这只有在它们撞到的东西又小、又比它们重得多时才会发生。
So the nucleus holds nearly all the mass of the atom.
所以原子核几乎占了原子全部的质量。
Atoms of the same element always have the same number of protons — but they can have different numbers of neutrons.
同一种元素的原子，质子数总是相同的——但它们的中子数可以不同。
These are isotopes.
这些就是同位素。
Some are stable; some are not.
有些同位素稳定，有些不稳定。
Split a heavy nucleus into two, and it releases energy — that is nuclear fission, the power inside a reactor.
把一个重原子核分裂成两个，会放出能量——这就是核裂变， 反应堆里的能量之源。
Push two light nuclei together, and it releases even more — that is nuclear fusion, the power of the Sun.
把两个轻原子核挤到一起，放出的能量更多——这就是核聚变，太阳的能量。
Carbon has two well-known isotopes.
碳有两种常见的同位素。
Carbon twelve has six protons and six neutrons.
碳十二有六个质子和六个中子。
Carbon fourteen also has six protons, but eight neutrons, because fourteen take away six is eight.
碳十四也有六个质子，但有八个中子， 因为十四减六等于八。
The proton number is the same, so both of them are carbon, and both behave the same way in chemistry.
它们的质子数相同，所以两者都是碳，在化学上表现完全一样。
Their nuclei are not the same, though: carbon twelve is stable, while carbon fourteen is unstable, so it decays.
不过它们的原子核并不相同：碳十二是稳定的，而碳十四不稳定，会发生衰变。
Now fission as an equation.
现在把裂变写成方程。
A uranium two-three-five nucleus absorbs a slow neutron. It splits into barium and krypton, and throws out three more neutrons and a great deal of energy.
一个铀二三五的原子核吸收一个慢中子，然后分裂成钡和氪， 同时放出三个中子和大量能量。
Check the balance.
来检查平衡。
On top, two hundred and thirty-five plus one is two hundred and thirty-six, and one hundred and forty-one plus ninety-two plus three is two hundred and thirty-six as well.
上面：二百三十五加一等于二百三十六， 一百四十一加九十二加三也等于二百三十六。
On the bottom, ninety-two equals fifty-six plus thirty-six.
下面：九十二等于五十六加三十六。
If one number were missing, the balance would hand it to you.
如果少了一个数，平衡就会把它告诉你。
Fusion is the opposite move.
聚变正好相反。
Two heavy forms of hydrogen, deuterium and tritium, join to make helium plus one spare neutron.
两种较重的氢——氘和氚——结合成氦，再加上一个中子。
The numbers still balance: two plus three is five, and four plus one is five; one plus one is two, and two plus zero is two.
数字仍然平衡：二加三等于五，四加一也等于五；一加一等于二，二加零也等于二。
Inside the Sun, hydrogen nuclei fuse into helium in much the same way.
在太阳内部，氢核以差不多同样的方式聚变成氦。
In fission and in fusion alike, a little mass disappears, and that lost mass is what becomes the energy.
无论裂变还是聚变，都会有一点质量消失，而消失的质量就变成了能量。
An unstable nucleus does not last forever.
不稳定的原子核不会永远存在。
Sooner or later it decays, throwing out radiation to become more stable.
它迟早会衰变，放出辐射，变得更稳定。
This is radioactivity.
这就是放射性。
It is completely random — we can never predict which nucleus will decay next, or exactly when.
它完全是随机的——我们永远无法预测下一个衰变的是哪个核，也无法预测确切的时刻。
And it is all around us: background radiation comes from rocks, from outer space, and even from our own food.
而且它就在我们周围：本底辐射来自岩石、来自外太空，甚至来自我们自己的食物。
Why is a nucleus unstable in the first place?
原子核为什么会不稳定？
Either it has too many neutrons for its protons, or it is simply too heavy.
要么是它相对质子来说中子太多，要么就是它本身太重。
When it goes, the decay is spontaneous: it happens all by itself, and nothing you do will speed it up or slow it down.
衰变发生时，它是自发的：完全靠自己发生，你做什么都无法让它变快或变慢。
Heating it, squeezing it, or reacting it chemically changes nothing at all.
加热、加压、或者让它参加化学反应，都毫无影响。
And it is random: you cannot say which nucleus goes next, or exactly when.
而且它是随机的：你无法说出下一个衰变的是哪个核，也说不出确切的时刻。
Some radiation is around us all the time, and we call it background radiation.
有一些辐射一直存在于我们周围，我们把它叫做本底辐射。
For most people the biggest source is radon gas, which seeps out of the ground into the air.
对大多数人来说，最大的来源是氡气， 它从地下渗到空气中。
Then come the rocks and the bricks of the buildings we live in.
其次是岩石，以及我们所住建筑里的砖石。
Some of it comes from our food and drink, because living things take radioactive atoms in.
还有一部分来自我们的食物和饮水，因为生物会把放射性原子吸收进体内。
And some arrives as cosmic rays from space.
另有一部分则是来自太空的宇宙射线。
Background adds to every reading, so measure it and subtract it.
本底会加进每一次读数，所以要先测量再减去。
Radiation is measured with a detector joined to a counter.
辐射用探测器连上计数器来测量。
The count rate is just the number of counts each second, or each minute.
计数率就是每秒、或者每分钟记录到的计数个数。
But the counter picks up the background as well, so you have to take that away: the corrected count rate is the measured count rate minus the background count rate.
但计数器也会把本底一起记进去，所以必须把它减掉： 修正计数率等于测得的计数率减去本底计数率。
Try one.
来做一个。
A detector next to a source reads two hundred and fifty counts per minute.
放在放射源旁的探测器读数是每分钟二百五十次。
With the source removed, the background alone is thirty.
把放射源拿走后，只有本底时是三十。
Subtracting gives two hundred and twenty counts per minute.
相减就得到每分钟二百二十次。
There are three types of nuclear radiation.
核辐射有三种。
Alpha is a heavy, slow, positive particle — two protons and two neutrons.
阿尔法是又重、又慢、带正电的粒子——由两个质子和两个中子组成。
Beta is a fast, light, negative electron.
贝塔是又快、又轻、带负电的电子。
Gamma is a high-energy electromagnetic wave, with no mass and no charge.
伽马是高能的电磁波，没有质量，也没有电荷。
They differ in how far they travel: alpha is stopped by paper, beta by a few millimetres of aluminium, and gamma needs thick lead or concrete.
它们的穿透能力不同：阿尔法被一张纸挡住，贝塔被几毫米的铝挡住， 而伽马需要厚厚的铅或混凝土。
Alpha ionises the most, but penetrates the least — it is the most easily absorbed.
阿尔法电离能力最强，但穿透能力最弱。
Ionising means knocking electrons off the atoms in its path.
电离的意思是把路径上原子里的电子打掉。
Alpha does this best: explain it from charge and kinetic energy: it carries a double positive charge, and it is heavy and slow, so it tugs hard on every electron it passes — and it spends all its energy in a few centimetres of air.
阿尔法最擅长这件事： 它带两个正电荷，又重又慢，所以会强烈地拉扯它经过的每一个电子—— 而它也会在几厘米的空气里就把能量全部用完。
Beta has a smaller charge and moves much faster, so it ionises less but travels further.
贝塔电荷更小、速度快得多， 所以电离更弱，但能走得更远。
Gamma has no charge at all, so it ionises least, and thick lead only reduces it.
伽马完全不带电，所以电离最弱，厚铅也只能把它减弱。
Now deflection in fields. Alpha and beta particles are charged, so an electric field or a magnetic field will bend their paths.
阿尔法粒子和贝塔粒子都带电，所以电场或磁场都会让它们的路径弯曲。
Because their charges have opposite signs, they bend in opposite directions — the positive alpha towards the negative plate, and the negative beta the other way.
因为它们的电荷符号相反，它们弯曲的方向也相反——带正电的阿尔法偏向负极板， 带负电的贝塔则偏向另一边。
The beta particle bends much more, because it is thousands of times lighter.
贝塔粒子弯曲得多得多，因为它轻了几千倍。
Gamma radiation has no charge, so it goes straight on, undeflected.
伽马射线不带电，所以它笔直前进，完全不偏转。
When a nucleus decays, we can write it as an equation.
当原子核衰变时，我们可以把它写成方程。
In alpha decay, the nucleus loses two protons and two neutrons — the nucleon number drops by four, and the proton number by two.
在阿尔法衰变中，核失去两个质子和两个中子—— 核子数减少四，质子数减少二。
In beta decay, a neutron turns into a proton and fires out an electron — the proton number rises by one, while the nucleon number stays the same.
在贝塔衰变中，一个中子变成一个质子，并射出一个电子—— 质子数增加一，而核子数保持不变。
In every equation, the top numbers balance, and so do the bottom ones.
在每一个方程里，上面的数相等，下面的数也相等。
Here is the classic question.
这是最典型的题。
A radium two-two-six nucleus emits an alpha particle: find the nucleon number and the proton number of the new nucleus.
一个镭二二六的原子核放出一个阿尔法粒子： 求新原子核的核子数和质子数。
Use the rule.
用规则来做。
Alpha decay takes four off the top number and two off the bottom one.
阿尔法衰变让上面的数减四，下面的数减二。
So the top becomes two hundred and twenty-six minus four, which is two hundred and twenty-two.
所以上面变成二百二十六减四，等于二百二十二。
The bottom becomes eighty-eight minus two, which is eighty-six.
下面变成八十八减二，等于八十六。
Element eighty-six is radon, so radium has turned into radon.
八十六号元素是氡，所以镭变成了氡。
Beta decay starts inside the nucleus, where a neutron turns into a proton plus an electron.
贝塔衰变从原子核内部开始：一个中子变成一个质子加一个电子。
The proton stays behind, and the fast electron shoots out as the beta particle.
质子留在核里，而那个快速的电子飞出去，成为贝塔粒子。
Take sodium twenty-four.
以钠二十四为例。
It becomes magnesium twenty-four, plus a beta particle.
它变成镁二十四，再加一个贝塔粒子。
The nucleon number does not change, because a neutron was simply swapped for a proton.
核子数没有变，因为只是把一个中子换成了一个质子。
The proton number rises by one, from eleven to twelve — and a new proton number means a new element.
质子数增加一，从十一变成十二——而质子数变了，就意味着变成了新的元素。
Gamma emission is different again: the nucleus loses only energy, not particles.
伽马发射又不一样：原子核只失去能量，不失去粒子。
So both numbers stay unchanged — the same nucleon number, the same proton number, the same element.
所以两个数都保持不变——核子数一样，质子数一样，还是同一种元素。
Gamma is usually given out just after an alpha or a beta decay, to carry away the spare energy the nucleus is left with.
伽马通常在阿尔法衰变或贝塔衰变之后紧接着放出，用来带走原子核剩下的多余能量。
Every decay leaves the nucleus more stable, and that gamma ray is the last of the spare energy leaving.
每一次衰变都让原子核变得更稳定，而那道伽马射线就是最后离开的多余能量。
Because decay is random, we measure it with the half-life — the time for half of the nuclei to decay.
因为衰变是随机的，我们用半衰期来衡量它——也就是一半的原子核衰变所需的时间。
After one half-life, half remain; after two, a quarter; after three, an eighth.
经过一个半衰期，剩下一半；经过两个，剩下四分之一；经过三个，剩下八分之一。
The curve falls fast at first, then flattens, never quite reaching zero.
曲线一开始下降很快，然后变平，永远不会真正到达零。
So if a sample starts at eight hundred becquerels with a half-life of two hours, then after six hours — that is three half-lives — the activity has dropped to one hundred.
所以，如果一个样品从八百贝可勒尔开始， 半衰期是两小时，那么经过六小时——也就是三个半衰期——放射性活度就降到了一百。
Here is the definition to learn by heart: the half-life is the time taken for half the unstable nuclei in a sample to decay.
这个定义要背下来：半衰期是样品中一半的不稳定原子核发生衰变所需的时间。
Say a source starts at eight hundred counts per minute, with a half-life of three hours.
假设一个放射源一开始是每分钟八百次计数，半衰期是三小时。
After three hours it reads four hundred.
三小时后，它读数为四百。
After six hours, two hundred.
六小时后，二百。
After nine hours, one hundred.
九小时后，一百。
Look carefully at six hours: two half-lives have passed, so the count rate has halved twice, not once.
仔细看六小时这一点：已经过了两个半衰期， 所以计数率减半了两次，而不是一次。
In the exam you often have to read a half-life off a decay curve.
考试中常常要你从衰变曲线上读出半衰期。
Pick any count rate on the curve — it does not have to be the starting value.
在曲线上任选一个计数率——不一定要是起始值。
Find where the curve has dropped to half of it, and read the time between those two points: that time is the half-life.
找到曲线降到它的一半的地方，再读出这两点之间的时间：那段时间就是半衰期。
Start somewhere else and you get the same answer, because the fraction is always one half.
换一个地方开始，你会得到同样的答案，因为比例总是二分之一。
Notice too that the curve flattens but never quite reaches zero.
还要注意，曲线会变平，但永远不会真正到达零。
One more.
再来一题。
The activity of a source falls from eight hundred counts per minute to one hundred, and its half-life is five days.
一个放射源的活度从每分钟八百次降到一百次，它的半衰期是五天。
How long did that take?
这用了多长时间？
Do not guess from the numbers — count the halvings.
不要凭数字猜——去数减半的次数。
Eight hundred to four hundred is one.
八百到四百是第一次。
Four hundred to two hundred is two.
四百到二百是第二次。
Two hundred to one hundred is three.
二百到一百是第三次。
That is three half-lives, each of five days, so the answer is fifteen days.
也就是三个半衰期，每个五天，所以答案是十五天。
Radioactivity is useful.
放射性很有用。
Gamma rays sterilise medical tools and treat cancer.
伽马射线可以给医疗器械消毒、治疗癌症。
A tracer, followed by a detector, finds leaks in pipes and maps blood flow in the body.
一种示踪剂，配上探测器， 能找出管道的泄漏、绘出体内的血流。
And the steady tick of decay lets us date ancient rocks and fossils.
而衰变那稳定的滴答，让我们能测定古老岩石和化石的年代。
But radiation is dangerous — it damages living cells.
但辐射很危险——它会破坏活细胞。
So we keep our distance, limit our time near a source, and shield ourselves with lead, handling sources with tongs, never with bare hands.
所以我们要保持距离、缩短靠近放射源的时间、用铅来屏蔽， 并用夹子而不是徒手来拿放射源。
Which isotope you pick depends on two things: the type of radiation, and the half-life.
选哪种同位素取决于两点：辐射的类型，以及半衰期。
A smoke alarm uses an alpha source, because alpha is blocked by almost anything, so it is safe outside the alarm, and a long half-life keeps it working for years.
烟雾报警器用阿尔法源，因为阿尔法几乎被任何东西挡住，所以在报警器外面是安全的， 而长半衰期让它能用好多年。
Sterilising food or surgical tools needs gamma, because it has to pass through the sealed packet to kill the bacteria inside.
给食品或手术器械消毒要用伽马， 因为它必须穿过密封的包装，杀死里面的细菌。
Measuring the thickness of paper or metal uses a beta source: the amount getting through changes as the sheet changes.
测量纸张或金属的厚度用贝塔源：透过去的量会随着厚度改变。
And gamma is used to find and to treat cancer, because it can travel into and out of the body.
而伽马被用来发现和治疗癌症，因为它能进入身体、也能从身体里出来。
Ionising radiation damages living cells.
电离辐射会损伤活细胞。
It can kill a cell outright, cause a mutation — a change to the genes — or start a cancer.
它可能直接杀死细胞、引起突变——也就是基因的改变——或者引发癌症。
So all of radiation safety comes down to one idea: we keep the dose as low as we can, in three ways. Reduce the exposure time — spend less time near the source. Stand further away from it.
所以我们要尽量把剂量降到最低，有三个办法：缩短待在放射源附近的时间；离它站得更远； 以及在放射源和身体之间加上屏蔽，比如铅。
And put shielding, such as lead, between the source and your body. In the lab that means handling sources with tongs and never with bare hands, keeping them pointed away from people, and storing them in a lead-lined box.
在实验室里，这就意味着用夹子而不是徒手去拿放射源、让它始终指向没有人的方向， 并把它存放在衬铅的盒子里。
Three marks to lock in.
锁住三个分。
First, remember the penetration order: alpha stopped by paper, beta by aluminium, gamma by lead.
第一，记住穿透顺序：阿尔法被纸挡住，贝塔被铝挡住，伽马被铅挡住。
Second, in a decay equation the nucleon numbers on top must balance, and so must the proton numbers below.
第二，在衰变方程里，上面的核子数必须相等，下面的质子数也必须相等。
Third, radioactive decay is random and spontaneous — nothing you do, not heat, not pressure, can change a half-life.
第三，放射性衰变是随机而自发的——你做什么都没用，不管是加热还是加压，都无法改变半衰期。
Master these, and nuclear physics is yours.
把这些掌握好，核物理就是你的了。
Four more habits worth easy marks.
还有四个能拿分的习惯。
First, the number of neutrons is the nucleon number minus the proton number — top take away bottom, every time.
第一，中子数等于核子数减去质子数——每次都是上面减下面。
Second, isotopes have the same proton number but different nucleon numbers: the same element carrying extra neutrons.
第二，同位素的质子数相同，但核子数不同：同一种元素带着多出来的中子。
Third, always subtract the background count rate before you use a source's reading.
第三，在使用放射源的读数之前，一定要先减掉本底计数率。
And fourth, when a question gives you activities, count the halvings instead of dividing the total time.
第四，当题目给你活度时，要去数减半的次数，而不是拿总时间去除。