【DSE Physics 課題五】Radioactivity & Nuclear Energy 放射與核能｜αβγ｜Decay｜Fission & Fusion

Table of Contents

Radioactivity & Nuclear Energy 放射與核能

Chapter 25 Radiation and Radioactivity

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Radiation and Radioactivity

Ionizing radiation 致電離輻射

Ionizing radiation

ionizing radiation = radiation with energy high enough to knock electrons out of atoms (or molecules) and produce ion-pairs

致電離輻射的能量足以把電子從原子（ 或分子 ）移走，從而形成離子對

X-rays X 射線

X-rays

X-rays = high-frequency EM waves
X 射線為高頻電磁波

ionizing radiation with strong penetrating power
X 射線也是致電離輻射，穿透能力高

produced when fast electrons hit a heavy metal target
在高速電子和 X 射線管內的鎢靶碰撞時便會產生

Nuclear radiation 核輻射

Nuclear radiation

three types of nuclear radiation 有以下三種： 

ParticleChargeMass (a.m.u.)
α+2e≈ 4
β-e≈ 1/1800
γ00

emitted from unstable nuclei of atoms

核輻射從不穩定的原子核發出

Atomic structure 原子模型

consists of 包含：

ParticleMass (a.m.u.)Charge
Proton≈ 1+e
Neutron≈ 10
Electron≈ 1/1800-e

Symbolic notation 符號

Symbolic notation

$$\begin{aligned}&\text{name}&&\text{hydrogen-1}&&\text{helium-4}&&\text{lithium-7}\\&\text{symbol }\frac{A}{Z}X&&\frac{1}{1}\text{H}&&\frac{4}{2}\text{He}&&\frac{7}{3}\text{Li}\\&\text{mass no. }A\left(\text{proton neutron no.}\right)&&1&&4&&7\\&\text{atomic no. }Z\left(\text{protons no.}\right)&&1&&2&&3\\&\text{neutron no. }N=A-Z&&0&&2&&4\end{aligned}$$

Isotope 同位素

Isotope
  • same element (atomic no.); different neutron no.
    元素（ 原子序數 ）相同；中子數相異
  • same chemical properties; different physical properties
    化學性質相同；物理性質相異

Radioactive decay 放射衰變

spontaneous 自發反應

α decay α 衰變

α decay

α decay是指有α飛出來。由於α是He，因此mass number 要-4，atomic number 要-2

β decay β 衰變

β decay

β decay 是指有β飛出來。由於β是electron，因此mass number 要無需變動，但atomic number 要+1

Common Mistakes

β decay

尤其是讀chem的同學很容易出錯，他們會覺得這個是外層電子離開aton而已。

不過，當您細心一點睇時，您會發現proton +1了，連元素都轉變。

**注意：

β decay是來自nucleus，即是neutron分裂成proton + electron，然後這粒electron飛走。

γ decay γ 衰變

γ decay

γ decay是指由EM wave走出來，因此這個在formula上來説是最簡單的。mass number不用變，atomic number不用變。

唯一要注意的是要加*。您可以當作*是多能量的，然後經過γ decay失去部分能量，所以*消失了。

Decay series 衰變系

Decay series

Decay series沒有什麽特別，只是一連串的decay。

Ionizing power 致電離能力

Ionizing power

α > β > γ

必背：Cloud chamber tracks 擴散雲室的徑跡

Cloud chamber tracks

Penetrating power 穿透能力

penetrating power in air (range in air)

Penetrating power

γ > β > α

Penetrating power in materials 在物料中的穿透能力

Penetrating power in materials

要注意的是當γ穿過25mm lead，只是强度減半，不是完全擋住！

經典圖：Deflection in E-field 在電場中的偏轉

Deflection in E-field

要注意的是α比較難轉彎，因爲α比較重

經典圖：Deflection in B-field 在磁場中的偏轉

Deflection in B-field

要注意的是α比較難轉彎，因爲α比較重

必背答案：上述的lead shield有什麽作用

To produce a fine beam of radiation

To prevent light entering the badge to expose the film inside

必背答案：如何處理放射性物質

  1. Using special forceps to handle radioactive sources
  2. wearing protective coveralls
  3. working behind lead-glass windows

 

  1. 使用特殊鑷子處理放射源
  2. 穿著防護服
  3. 在鉛玻璃窗後面工作

必背答案：Application of radioisotope

  1. carbon-14 dating
  2. smoke detector
  3. sterilization of medical equipment

 

  1. 碳14測年
  2. 煙霧偵測器
  3. 醫療器械滅菌

Chapter 26 Rate of Decay and Uses of Radionuclides

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Rate of Decay and Uses of Radionuclides

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Radioactive decay 放射衰變

Radioactive decay 放射衰變是隨機過程，我們無法得知何時會進行衰變，但我們可以知道其概率。當nuclei越多，預測就會越準確。

Half-life 半衰期

Half-life

time for the no. of undecayed nuclei N (or activity A) to reduce by half

樣本內未衰變的原子核數目 N（ 或放射強度 A ）減少一半所需的時間

Activity 放射強度

Activity
  • no. of nuclei decayed in 1 s
    一秒內衰變的原子核數目
  • activity and no. of undecayed nuclei: A = kN
    放射強度及未衰變的原子核數目的關係：A = kN
  • decay constant k: probability that an undecayed nucleus decays in 1 s
    衰變常數 k：一顆原子核在一秒內衰變的概率

decay constant 定義要記

Exponential law of decay 放射衰變的指數定律

Exponential law of decay

Dose, equivalent dose and effective dose
劑量 、當量劑量及有效劑量

QuantityDefinitionUnit
DoseAmount of radiation absorbed per unit massGray (Gy)
Equivalent doseBiological effect on a tissue/organ by different types of radiationSievert (Sv)
Effective doseBiological effect on the entire human body exposed to radiationSievert (Sv)

Half-life and strength 半衰期與放射強度

Half-life and strength

Radiation with a longer half-life is not necessarily stronger than that of a shorter half-life.

半衰期較長 並不表示放射強度較強。

Finding half-life from count rates 從計數率計算半衰期

Take the background count rate into account when ϐinding the half-life from count rates.

如欲從計數率計算半衰期，必須考慮本底計數率。

必背：Background Radiation 背景輻射

cosmic rays from the Sun or outerspace

radioactive isotopes in the rock

來自太陽或外太空的宇宙射線

岩石中的放射性同位素

Chapter 27 Nuclear Energy

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Nuclear Energy

Nuclear fission 核裂變

Nuclear fission
  • a heavy nucleus split into two or more lighter nuclei
    一個質量較大的原子核分裂為兩個或多個質量較小的原子核
  • spontaneous reaction
    自發反應

例子：

$$_{92}^{235}\mathrm{U}+_{0}^{1}\mathrm{n}\longrightarrow_{56}^{141}\mathrm{Ba}+_{36}^{92}\mathrm{Kr}+3_{0}^{1}\mathrm{n}$$

尖嘴右手邊是daughter product

Chain reaction 連鎖反應

Chain reaction
  • neutrons emitted in a fission reaction initiate further reactions
    核裂變所釋放的中子引發更多核反應，使裂變過程不斷進行
  • conditions for chain reaction 產生連鎖反應的條件：
    • more than one neutron are emitted in the fission reaction
      在裂變過程中釋放兩個甚至更多中子
    • sufficiently high concentration of the reactant (e.g. U-235)
      反應物（ 例如鈾 235 ）的濃度要足夠高

For a chain reaction to occur, there must be at least two neutrons emitted in the ϐission reaction.

若要產生連鎖反應，在裂變過程中必須產生最少兩個中子。

Nuclear fusion 核聚變

Nuclear fusion

Mass–energy relation 質能關係

mass and energy are interchangeable

質量和能量的關係對等而且可以互換

$$\Delta E=\Delta mc^2$$

常見錯誤：Energy released in nuclear fission and fusion
核裂變及核聚變釋放的能量

Energy released in nuclear fission and fusion

The amount of energy released in the ϐission of U-235 is larger than that released in the fusion of H-2 and H-3. But for the same mass of fuel, more energy is released in the fusion reaction.

一個鈾 235 的分裂反應所釋放的能量比一個氫 2 及氫 3 的融合反應所釋放的能量較多。但對於相同質量的核燃料，則融合反應所釋放的能量較多。

必背答案：Similarity of Nuclear Fission and Fusion

The nuclides before and after the reaction are different in both fusion and fission.

聚變和裂變反應前後的核素都是不同的。

必背答案：Difference between Nuclear Fission and Fusion

In fission, a heavy nucleus splits into lighter nuclei. In fusion, two light nuclei fuse to form a heavier nucleus.

In fission, it is usually triggered by bombardments of neutrons while fusion us not.

The temperature at which fusion occurs is much higher than that of fission.

在裂變中，較重的原子核分裂成較輕的原子核。 在聚變中，兩個輕核融合形成一個較重的核。

在裂變中，它通常是由中子轟擊引發的，而聚變則不是。

聚變發生的溫度遠高於裂變的溫度。

Physics 必修課題

  1. Temperature and Thermometers
  2. Heat Capacity
  3. Change of State
  4. Gas laws and Kinetic Theory
  5. Motion
  6. Force
  7. More about Force
  8. Work Energy and Power
  9. Momentum
  10. Projectile Motion
  11. Uniform Circular Motion
  12. Gravitation
  13. Wave Motion
  14. Reflection Refraction and Diffraction
  15. Interference and Stationary Waves
  16. Light and Sound
  17. Reflection of Light
  18. Refraction of Light
  19. Lenses
  20. Electrostatics
  21. Circuit and Power
  22. AC and Domestic Electricity
  23. Electromagnetism
  24. Electromagnetic Induction
  25. Radiation and Radioactivity
  26. Rate of Decay and Uses of Radionuclides
  27. Nuclear Energy

Physics 選修課題 E1

  1. A Cosmic Journey
  2. Modelling the Universe
  3. Orbital Motions under Gravity
  4. Starlight: Messengers from the Stars

Physics 選修課題 E2

  1. Photoelectric Effect
  2. Atomic Models and Spectra
  3. Nanotechnology

Physics 選修課題 E3

  1. Lighting
  2. Cooking and Air-Conditioning
  3. Buildings and Transportation
  4. Different Sources of Energy

Physics 選修課題 E4

  1. Vision and Hearing
  2. Non-ionizing Medical Imaging
  3. Ionizing Medical Imaging

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