Edexcel物理 放射性 半衰期 Alpha Beta Gamma

Edexcel物理 放射性 半衰期 Alpha Beta Gamma

放射性是 Edexcel GCSE Physics (1PH0) 的第六个主题，在 Paper 1 里常和原子模型、能量、核电站一起出题。这篇按对比的方式整理：三种射线怎么区分，两种 β 衰变方程怎么配平，裂变和聚变到底差在哪里。文中引用的条目编号都来自 Edexcel 官网公布的 1PH0 specification，方便学生对照原文复习。

Radioactivity is Topic 6 of the Edexcel GCSE Physics (1PH0) specification, and it usually appears in Paper 1 next to the atomic model and nuclear power. This article is organised as a set of comparisons, covering how the three radiations differ, how the two beta decay equations balance, and what separates fission from fusion. Specification references are given so that students can check each point against the published document.

原子核的尺度与质量分布

Edexcel 的 specification 第 6.1 条把原子写成一个带正电的核，外面绕着带负电的电子。核里只有质子和中子，原子序数是质子数，质量数是质子数加中子数。原子半径大约 10^-10 米，核半径只有 10^-14 米，两个数差了四个数量级。体积差得更夸张，核只占原子的极小一部分。可质量几乎全在核里，因为质子和中子的质量约为电子的一千八百四十倍。第 6.2 条把原子与分子的大小放在同一数量级，这类题只要求给量级，不要求精确到小数。很多学生以为电子多少也占点质量，答题时写成质量均匀分布就丢分了。阅卷时看到质量几乎全在核里这句话，通常就是这一分的判定点。

Most of an atom is empty space, yet nearly all of its mass is packed into a nucleus whose radius is about ten thousand times smaller than the atom itself. That is why nuclear matter is described as extraordinarily dense rather than merely heavy, and why questions about scale usually want an order of magnitude instead of a precise figure. A useful check is to remember that the outer edge of an atom is set by electron shells, not by the nucleus, so the two radii belong to completely different scales.

卢瑟福散射实验与玻尔模型

第 6.17 条要求用葡萄干布丁模型和卢瑟福散射实验说明原子模型怎么演变。汤姆孙的模型把正电荷摊成一团，电子像葡萄干一样嵌在里面。卢瑟福让 α 粒子去打金箔，多数粒子直穿过去，少数偏转很大，极少数几乎被弹回。只有把正电荷集中在很小的核里，才可能出现这种大角度反弹。玻尔后来把电子放进分层轨道，解释了电子为什么不直接掉进核里。模型的演变顺序是先有汤姆孙的葡萄干布丁模型，再有卢瑟福的核式模型，最后才是玻尔的轨道模型，顺序写错会扣叙述分。常见的错误是只写 α 粒子被弹回，不写这证明核很小且带正电，得分点就漏了一半。这道题在 Paper 1 里反复出现，答案的骨架几年都没变。

Alpha particles fired at thin gold foil mostly passed straight through, a small fraction were deflected sharply, and a handful returned towards the source. A spread-out positive charge could never produce that pattern, so the evidence forced the conclusion that charge and mass sit in a minute nucleus. Bohr then placed electrons in fixed shells, which answered why they do not simply spiral inwards. Examiners want the observation stated before the inference, so write the evidence first and the conclusion second.

同位素 质量数 原子序数的写法

第 6.3 与 6.4 条要求用原子序数和质量数描述同位素的核。同一种元素的质子数固定，中子数可以不同，这类原子就叫同位素。碳-14 与碳-12 都是碳，质子各六个，中子分别是八个与六个。规范写法是左上角写质量数，左下角写原子序数，比如 14 在上、6 在下。质量数变了，化学性质几乎不变，放射性却可能完全不同。碳-14 做 β− 衰变，碳-12 一直稳定。第 6.5 条还要求记住质子、中子、电子与正电子的相对质量与相对电荷，正电子质量与电子相同、电荷相反。很多学生以为同位素连化学性质都不一样，其实决定化学行为的是最外层电子数。

Isotopes of one element share the same proton count but differ in neutrons, which is why their chemical behaviour stays almost identical while their nuclei behave very differently. Carbon-14 decays by beta minus emission, whereas carbon-12 is stable, yet both form the same compounds in a living cell. Notation matters here: the mass number goes top left and the atomic number bottom left, and mixing the two up costs marks even when the physics is understood.

Alpha Beta Gamma 三种射线的本质与电荷

α 粒子就是氦核，两个质子加两个中子，电荷 +2。β− 是核里放出的电子，电荷 -1，质量小到可以忽略。β+ 是正电子，质量与电子相同，电荷 +1。γ 是电磁波，没有电荷，也没有静止质量。中子辐射不带电，但它能把别的核撞成不稳定核，所以在反应堆里也要防。这几种辐射都从 unstable nuclei 里随机放出，第 6.10 条把随机两个字写进了同一条。α 与 β 都带电，经过电场或磁场时会偏转，γ 不偏转，这是区分三种射线的经典实验方法。写法上最容易错的，是把 β+ 的电荷写成 -1。

Alpha, beta minus, beta plus and gamma radiation come from unstable nuclei in a random process, and the specification lists neutron emission alongside them. The distinguishing features are charge and mass: an alpha particle carries +2 and four nucleons, an electron or positron carries one unit of charge and almost no mass, and a gamma ray carries neither. Students who memorise only the penetration list tend to confuse beta plus with beta minus, so learn the charge first and the penetration follows.

穿透本领与电离本领正好相反

第 6.16 条要求比较三种射线的穿透与电离能力。α 电离最强，穿透最弱，一张纸或几厘米空气就能挡住。β 穿透中等，几毫米铝板可以挡住大部分。γ 穿透最强，要厚铅或混凝土才明显衰减。这几个性质是反着来的，电离越强，能量在短距离内损失得越快。检测 α 源要把源放进探测器，γ 则隔着金属外壳也能读数。第 6.14 条要求记住两种检测手段：照相底片记录累积剂量，盖革-米勒管给实时计数。常见的错误是把穿透强当成危害大，其实 α 被皮肤挡住，进入体内才真正危险。

Ionising power and penetrating power run in opposite directions, which is the single idea behind most comparison questions. Alpha radiation loses energy over a few centimetres of air because it ionises densely, while gamma rays pass through paper and skin with little interaction and need thick lead to be attenuated appreciably. A detector placed outside a sealed source therefore tends to register betas and gammas, not alphas, and that practical detail explains many experimental results.

α 衰变方程怎么写

α 衰变写成母核 -> 子核 + 氦核。质量数减 4，原子序数减 2，等号两边都要配平。铀-238 变成钍-234，就是 238 减 4 得 234，92 减 2 得 90。写方程时先核质量数，再核电荷，最后把两边总数各加一遍。电荷守恒这条，第 6.22 条明确要求用题目给出的数据来配平。配平之后最好把子核的元素符号也写出来，钍的原子序数是 90，写在空格里比只写数字清楚。很多学生以为质量数对上就够了，原子序数改错一个数整题就白做。这一步最容易在草稿上算对，抄到答题卡上抄错。

In alpha decay the parent nucleus loses two protons and two neutrons, so the mass number drops by four and the atomic number by two. Uranium-238 becoming thorium-234 is the standard example, and both totals must balance on either side of the arrow. The reliable routine is to balance mass first, then charge, then re-add both columns as a check, because a single miscopied digit invalidates the whole equation.

β− 与 β+ 衰变方程怎么写

β− 衰变的本质是中子变成质子加电子。中子不带电，变成带正电的质子，原子序数加 1，质量数不变。β+ 反过来，质子变成中子加正电子，原子序数减 1，质量数仍然不变。两种写法都要在核旁边保留电子或正电子的符号，质量数写 0。正电子遇到电子会湮灭，放出两个 γ 光子，PET 扫描就靠这一对光子定位。第 6.21 条提醒，衰变后的核常处在激发态，放出 γ 光子才回到低能态，所以 γ 往往跟在 α 或 β 之后。中子发射则是质量数减 1、原子序数不变，这一条最容易被忘掉。

Beta minus decay converts a neutron into a proton plus an electron, so the proton number rises by one while the mass number stays put. Beta plus decay runs the other way, turning a proton into a neutron plus a positron, and the proton number falls by one. Writing the emitted particle with mass number zero is what keeps the equation balanced, and positron annihilation into two gamma photons is the principle PET scanners rely on.

活度与半衰期的定义

活度的单位是贝可勒尔，1 Bq 表示每秒衰变一次。第 6.25 条给出两种等价说法：一半未衰变的核完成衰变，或者活度减半，所需的时间。答题时写其中一种就够，两种都写更稳。第 6.26 条考的是随机与统计的分界：单个核不可预测，大量核的活度可以按半衰期推算。单个核什么时候衰变无法预测。探测器读到的是计数率，要把计数除以测量时间才得到每秒数值，其中只有一部分来自样品本身。样品越大，统计越稳定，衰变曲线越平滑。把半衰期理解成样品寿命的一半，是最常见的误解。活度下降是指数式的，永远不会真正归零。

Activity is measured in becquerels, where one becquerel means one decay per second, and half-life is the time for half the undecayed nuclei to decay or for the activity to halve. The random nature of decay applies to individual nuclei, yet the behaviour of a large sample becomes predictable, which is why decay curves are smooth rather than jagged. Half-life is not the lifetime of the sample, and treating the two as the same thing is the error that appears most often in written answers.

半衰期计算与衰变曲线

题目通常给初始活度和半衰期，问若干时间之后的活度。做法是数经过了多少个半衰期，每经过一个就除以 2。2400 Bq 的源，半衰期 5 年，15 年后活度是 2400 除以 2 的三次方，得 300 Bq。反向题给出两个时刻的活度，先求比值，再换算成半衰期的个数。碳-14 半衰期约 5730 年，这个数字常用来估算考古样品的年代。衰变曲线读图时先找活度减半对应的横坐标，比反复试算快得多。画图时纵轴用活度或计数率，横轴用时间，曲线只会越来越贴近横轴，不会与横轴相交。很多学生以为活度会降到零。用核数减半代替活度减半来算结果相同，前提是样品里只有一种核在衰变。

A typical question gives the initial activity and the half-life, then asks for the activity after a stated time. Count how many half-lives have passed, halve the activity that many times, and check the arithmetic in the other direction by doubling back. Carbon-14, with a half-life of about 5730 years, is the number used in archaeological dating questions, and reading a decay curve is usually faster than repeated calculation.

背景辐射的来源与盖革计数器

背景辐射是环境里本来就存在的电离辐射。来源分两类，一类来自地面，比如岩石与土壤释放的氡气，另一类来自太空的宇宙射线。食物、建筑材料与医疗检查也会贡献一部分。测样品之前先测背景计数，再用总计数减去背景，才得到样品的净计数。盖革-米勒管是最常用的探测器，它把每次电离放大成一个脉冲。照相底片可以记录累积剂量，工作人员常把它戴在身上做剂量监测。第 6.13 条把来源限定为地球与太空两类，答题时不要写成核电站泄漏这类人为事故。不扣背景直接读数是实验题最常见的失分点。

Background radiation comes from the ground, mainly radon released by rock and soil, and from cosmic rays arriving from space, with food, building materials and medical procedures adding smaller amounts. Counting the background before measuring a sample, then subtracting it, is the only way to obtain a meaningful net count rate. A Geiger-Muller tube turns each ionisation event into a pulse, while film badges record accumulated dose for staff who work with sources.

辐照与污染是两件不同的事

辐照是被射线照到，人本身不带放射性，离开源就结束。污染是放射性物质留在皮肤、衣服或台面上，会持续照射。两者的防护措施不一样，辐照靠距离、屏蔽与缩短时间，污染还要加上清洗与更换衣物。α 污染进入体内最危险，因为它的电离密度最高。处理泄漏时先围出区域，再按剂量由低到高的顺序清理。防护的三条基本手段是时间、距离与屏蔽，距离加倍时剂量率降到四分之一，这条平方反比关系常在计算题里用到。第 6.32 条明确要求对比这两种危害，答题要写清差别，只写都有害拿不到分。剂量限制对病人是治疗收益与风险的权衡，对医护则是尽量少受照。

Irradiation means being exposed to radiation from a source outside the body, and it stops once the person moves away or the source is shielded. Contamination means radioactive material has been left on skin, clothing or a bench, so exposure continues until it is washed off or removed. The precautions differ accordingly, and alpha contamination inside the body is the case that causes the most damage per unit of energy deposited.

核裂变链式反应与核聚变

裂变是一个重核吸收中子后裂成两个较小的核，同时放出两到三个中子。铀-235 放出的中子如果再去打别的铀核，就形成链式反应。反应堆里用慢化剂把快中子减速，用控制棒吸收多余的中子，让每次裂变平均只留下一个中子。聚变是两个轻核合成一个较重的核，质量亏损变成能量释放出来。太阳的能量来自聚变，聚变燃料多、产物放射性低，但要先克服质子之间的静电斥力。第 6.45 条要求解释为什么低温低压下聚变不会发生。第 6.42 条要求记住裂变产物本身有放射性，废料要长期屏蔽存放，这也是核电争议的一个核心。聚变发电站迟迟做不成，主要卡在工程与成本上。

Fission splits a heavy nucleus into two smaller nuclei and releases two or more neutrons, which can trigger further fissions and produce a chain reaction. A reactor keeps that chain steady by slowing neutrons with a moderator and absorbing the surplus with control rods, so on average one neutron per fission goes on to cause another. Fusion joins light nuclei instead, converting a small mass difference into energy, and the electrostatic repulsion between protons is why it needs extreme temperature and pressure.

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