Chapter 11 Particle Physics

Question 1

Rutherford Scattering

Answer 1

• Evidence for the structure of the atom was discovered by Ernest Rutherford in the beginning of the 20th century from the study of α-particle scattering
• The experimental setup consists of alpha particles fired at thin gold foil and a detector on the other side to detect how many particles deflected at different angles

Question 2

α-particle scattering experiment set up

Answer 2

Question 3

α-particles are what?

Answer 3

the nucleus of a helium atom and are positively charged

Question 4

From this experiment, Rutherford results were: (alpha scattering)

Answer 4

• The majority of α-particles went straight through (A)
  
  • This suggested the atom is mainly empty space
• Some α-particles deflected through small angles of < 10^o
  
  • This suggested there is a positive nucleus at the centre (since two positive charges would repel)
• Only a small number of α-particles deflected straight back at angles of > 90^o (C)
  
  • This suggested the nucleus is extremely small and this is where the mass and charge of the atom is concentrated
  • It was therefore concluded that atoms consist of small dense positively charged nuclei, surrounded by negatively charged electrons

Question 5

Atomic Structure

Answer 5

• The atoms of all elements are made up of three types of particles: protons, neutrons and electrons.
• A stable atom is neutral (it has no charge)
• Since protons and electrons have the same charge, but opposite signs, a stable atom has an equal number of both for the overall charge to remain neutral

Question 6

Antimatter

Answer 6

• All matter particles have antimatter counterparts
  
  • Antimatter particles are identical to their matter counterpart but with the opposite charge

Question 7

Atomic Mass Unit (u)

Answer 7

• The unified atomic mass unit (u) is roughly equal to the mass of one proton or neutron:
  
  • 1 u = 1.66 × 10⁻²⁷ kg
• It is sometimes abbreviated to a.m.u

Question 8

The term nuclide

Answer 8

is used to refer to a nucleus with a specific combination of protons and neutrons

Question 9

The term nucleon

Answer 9

is the used to mean a particle in the nucleus – i.e. a proton or neutron

Question 10

Isotopes

Answer 10

• An isotope is an atom (of the same element) that has an equal number of protons but different number of neutrons
• Remember, the neutron number of an atom is found by subtracting the proton number from the nucleon number
• Since nucleon number includes the number of neutrons, an isotope of an element will also have a different nucleon/mass number
• Since isotopes have an imbalance of neutrons and protons, they are unstable. This means they constantly decay and emit radiation to achieve a more stable form

Question 11

AZX Notation

Answer 11

• The top number A represents the nucleon number or the mass number
  
  • Nucleon number (A) = total number of protons and neutrons in the nucleus
• The lower number Z represents the proton or atomic number
  
  • Proton number (Z) = total number of protons in the nucleus

Question 12

Conservation of Nucleon Number & Charge

Answer 12

• Nuclear processes such as fission and fusion are represented using nuclear equations (similar to chemical reactions in chemistry)
• The number of protons and neutrons in atom is known as its constituents

Question 13

what does this equation represent?

Answer 13

• The above equation represents a fission reaction in which a Uranium nucleus is hit with a neutron and splits into two smaller nuclei – a Strontium nucleus and Xenon nucleus, releasing two neutrons in the process
• In nuclear equations, the nucleon number and charge are always conserved
• This means that the sum of the nucleons and charge on the left hand side must equal the sum of the number of nucleons and charge on the right hand side
• In the above equation, the sum of the nucleon (top) numbers on both sides are equal

235 + 1 = 236 = 90 + 144 + 2 × 1

• The same is true for the proton (bottom) numbers

92 + 0 = 92 = 38 + 54 + 2 × 0

• By balancing equations in this way, you can determine the nucleon, proton number or the number of missing elements

Question 14

Balancing the number of nucleons shows that 3 neutrons must be released in the reaction

Answer 14

Question 15

Alpha, Beta & Gamma Particles

Answer 15

• Some elements have nuclei that are unstable
  
  • This tends to be when the number of nucleons does not balance
• In order to become more stable, they emit particles and/or electromagnetic radiation
  
  • These nuclei are said to be radioactive

Question 16

Alpha (α) particles

Answer 16

• are high energy particles made up of 2 protons and 2 neutrons (the same as a helium nucleus)
• They are usually emitted from nuclei that are too large

Question 17

Beta (β⁻) particles

Answer 17

• are high energy electrons emitted from the nucleus
• Beta (β⁺) particles are high energy positrons (antimatter of electrons) also emitted from the nucleus
  
  • β⁻ particles are emitted by nuclei that have too many neutrons
  • β⁺ particles are emitted by nuclei that have too many protons

Question 18

Gamma (γ) rays

Answer 18

• are high energy electromagnetic waves
• They are emitted by nuclei that need to lose some energy

Question 19

When radiation passes close to atoms, it can knock out electrons, ionising the atom

Answer 19

Question 20

The properties of the different types of radiation are summarised

Answer 20

• u is the atomic mass unit (see “Atomic Mass Unit (u)”)
• e is the charge of the electron: 1.60 × 10^-19 C
• c is the speed of light: 3 × 10⁸ m s^-1

Question 21

Neutrino Emission

Answer 21

• An electron neutrino is a type of subatomic particle with no charge and negligible mass which is also emitted from the nucleus
• The anti-neutrino is the antiparticle of a neutrino
  
  • Electron anti-neutrinos are produced during β– decay
  • Electron neutrinos are produced during β+ decay

Question 22

Alpha particles have discrete energy levels whilst beta particles have a continuous range of energies

Answer 22

Question 23

When the number of β particles is plotted against kinetic energy, the graph shows

Answer 23

• a curve
• This demonstrates that beta particles (electrons or positrons) have a continuous range of energies
• This is because the energy released in beta decay is shared between the beta particles (electrons or positrons) and neutrinos (or anti-neutrinos)
• This was one of the first clues of the neutrino’s existence
• The principle of conservation of momentum and energy applies in both alpha and beta emission

Question 24

Alpha Decay

Answer 24

• Alpha decay is common in large, unstable nuclei with too many protons
• The decay involves a nucleus emitting an alpha particle and decaying into a different nucleus
• An alpha particle consists of 2 protons and 2 neutrons (the nucleus of a Helium atom)
• When an unstable nucleus (the parent nucleus) emits radiation, the constitution of its nucleus changes
• As a result, the isotope will change into a different element (the daughter nucleus)

Question 25

***Alpha decay equation***

Answer 25

* When an alpha particle is emitted from a nucleus: * The nucleus loses 2 protons: **proton number decreases by 2** * The nucleus loses 4 nucleons: **nucleon number decreases by 4**

Question 26

***Alpha decay produces a daughter nucleus and an alpha particle (helium nucleus)***

Answer 26

Question 27

β^– decay

Answer 27

* A β^– particle is a high energy electron emitted from the nucleus * β^– decay is when a **neutron turns into a proton emitting an electron and an anti-electron neutrino** * When a β^– is emitted from a nucleus: * The number of protons increases by 1: **proton number increases by 1** * The total number of nucleons stays the same: **nucleon number remains the same**

Question 28

***Equation for beta minus emission***

Answer 28

Question 29

β⁺ decay

Answer 29

* ***A β⁺ particle is a high energy positron emitted from the nucleus*** * ***β⁺ decay is when a proton turns into a neutron emitting a positron (anti-electron) and an electron neutrino*** * ***A β⁺ particle is a high energy positron emitted from the nucleus*** * ***β⁺ decay is when a proton turns into a neutron emitting a positron (anti-electron) and an electron neutrino***

Question 30

***Equation for beta plus emission***

Answer 30

* When a β⁺ is emitted from a nucleus: * The number of protons decreases by 1: **proton number decreases by 1** * The total number of nucleons stays the same: **nucleon number remains the same**

Question 31

**Fundamental Particles: Quarks**

Answer 31

* Quarks are **fundamental particles** that make up other subatomic particles such as **protons and neutrons** * Protons and neutrons are in a category of particles called **hadrons** * Hadrons are defined as any particle made up of quarks * **Fundamental** means that quarks are not made up of any other particles. Another example is electrons * Quarks have never been observed on their own, they’re either in pairs or groups of three * There are six flavours (types) of quarks that exist

Question 32

***The six flavours of quarks***

Answer 32

Question 33

**Properties of Quarks**

Answer 33

* The charge of a hadron is determined by the sum of the charges of its quarks * Each flavour of quark has a certain relative charge:

Question 34

***Each flavour of anti-quark has a charge of either -⅔e or +⅓e. The quark composition of anti-protons and anti-neutrons changes to anti-quarks***

Answer 34

Question 35

**Fundamental Particles: Leptons**

Answer 35

* **Leptons** are a group of **fundamental** (elementary) particles * This means they are not made up of any other particles (no quarks) * There are six leptons altogether: * The muon and tau particle are very similar to the electron but with slightly larger mass * Electrons, muon and tau particles **all** have a charge of -1e and a mass of 0.0005u * There are three **flavours** (types) of neutrinos **(electron, muon, tau)** * Neutrinos are the most abundant leptons in the universe * They have **no charge** and **negligible** mass (almost 0) * Leptons interact with the weak interaction, electromagnetic and gravitational forces * However, they do **not** interact with the strong force * Although quarks are fundamental particles too, they are not classed as leptons * Leptons do **not** interact with the strong force, whilst quarks do

Question 36

**Quark Composition: Protons & Neutrons**

Answer 36

* Protons and neutrons are **not** fundamental particles. They are each made up of three quarks * **Protons** are made up of **two up quarks and a down quark** * **Neutrons** are made up of **two down quarks and an up quark**

Question 37

**Baryons & Mesons**

Answer 37

* **Hadrons** are the group of subatomic particles that are made up of quarks * These may be either a: * **Baryon (3 quarks)** * **Meson (quark and anti-quark pair)**

Question 38

Anti-hadrons can be either

Answer 38

* **Anti-baryons (3 anti-quarks)** * **Anti-meson (quark and anti-quark pair)**

Question 39

Note that all baryons or mesons have

Answer 39

* integer (whole number) charges eg. +1e, -2e etc. * This means quarks in a baryon are either all quarks or all anti-quarks. Combination of quarks and anti-quarks don’t exist in a baryon * The anti-particle of a meson is still a quark-antiquark pair. The difference being the quark becomes the anti-quark and vice versa

Question 40

**Quark Composition: β– & β+ decay**

Answer 40

* Beta decay happens via the **weak interaction** * This is one of the four fundamental forces and it’s responsible for radioactive decays

Question 41

Quark Composition: β^– decay

Answer 41

* Recall that **β^– decay is when a neutron turns into a proton emitting an electron and anti-electron neutrino** * More specifically, a neutron turns into a proton because a **down quark turning into an up quark**

Question 42

Quark Composition: β⁺ decay

Answer 42

* Recall that **β⁺ decay is when a proton turns into a neutron emitting an positron and an electron neutrino** * More specifically, a proton turns into a neutron because an **up quark turns into a down quark**