Nuclear Physics

Question 1

What happened in the Rutherford scattering experiment?

Answer 1

1. Most alpha particles passed through gold leaf undeflected
2. Some were slightly deflected
3. A tiny proportion (1 in 8000) reflected

Question 2

What did Rutherford’s experiment reveal about the atom?

Answer 2

1. Atom is mostly empty space
2. Centre of atom is small, dense and positively charged (nucleus)

Question 3

Compare the ionising power of the three main types of radiation

Answer 3

• *Alpha most ionising**
• *Beta medium ionising**
• *Gamma least ionising**

Question 4

Compare the penetrative power of the three main types of radiation

Answer 4

• *Gamma most penetrating**
• *Beta medium penetrating**
• *Alpha least penetrating**

Question 5

Compare the range of the three main types of radiation

Answer 5

Alpha = 3-7cm
Beta = 0.2-3m
Gamma = Very long distance

Question 6

What materials can shield the three main types of radiation?

Answer 6

• *Alpha = Paper**
• *Beta = Aluminum**
• *Gamma = Lead or several meters of concrete**

Question 7

What are the main sources of background radiation?

Answer 7

Question 8

How does a Geiger-Muller tube detect radiation?

Answer 8

1. Radiation ionises gas in tube
2. Negative ions attracted to metal rod
3. Positive ions attracted to casing
4. Small current generated in circuit
   NOTE: The Geiger-Muller tube is not useful in areas with high levels of background radiation

Question 9

How is radiation used to control the thickness of paper?

Why can’t alpha radiation be used?

Answer 9

If detector count too low -> metal too thick -> Rollers move closer together

If detector count too high -> metal too thin -> Rollers move apart

Alpha won’t be detected

Question 10

How is radiation used in a smoke detector?

Answer 10

Alpha radiation ionizes air between detector
Ionized air causes current to flow
Smoke blocks ionization of air
Current stops
Alarm sounds

Question 11

If the detector is moved 3x further away what will happen to the count rate?

Answer 11

9x smaller

Gamma radiation follows inverse square law

Question 12

What are 5 safety precautions when using radiation?

Answer 12

1. Minimise exposure time
2. Maximize distance from source
3. Store in shielded containers
4. Don’t consume food or drink near source
5. Wear protective equipment

Question 13

How is radiation used for medical imaging?

Answer 13

Medical tracer with short half life injected

Tumors absorb radionuclides and emits gamma rays

Gamma rays detected outside body

Question 14

How is radiation used to destroy tumors?

Answer 14

Gamma radiation focused on tumor

High energy breaks apart tumor

Low levels through other tissue

NOTE: For safety, the radiation should have a short half life

Question 15

Sketch the graph of nuclear stability

Answer 15

Question 16

On this graph of nuclear stability highlight regions of the decays…

1. α
2. β^-
3. β⁺
4. Proton emission
5. Neutron emission

Answer 16

Question 17

What makes a nucleus unstable? (and radioactively decay)

Answer 17

1. An incorrect balance of protons and neutrons (off line of stability)
2. Too many nucleons
3. Nucleus in excited state

Question 18

What is electron capture and what is it’s equation?

Answer 18

Proton captures inner shell electron and becomes neutron

Question 19

What two forms of radiation are released after electron capture?

Answer 19

X-ray → electron de-excites to fill inner shell

γ → Nucleus reorders and de-excites

Question 20

How does distance of closest approach work?

Answer 20

KE at distance → PE closest

Use to get rough size of nucleus

Question 21

What two graphs could you plot to prove this relationship?

Answer 21

Question 22

What does r₀ represent?

Answer 22

Average radius of each nucleon

Question 23

How do you calculate the average density of a nucleus?

Answer 23

Question 24

Why is the average nucleus density so large? (∼2.3x10¹⁷kgm^-3)

Answer 24

Atom is mostly empty space

Question 25

How was **electron scattering** used to **determine nuclear diameter**?

Answer 25

Graph plotted First minima used to calculate diameter (Don't need to know equation)

Question 26

How is **electron scattering better than alpha recoil** to determine nuclear radius?

Answer 26

**Alpha Recoil** * Closest approach so only an estimate * Recoil of nucleus not considered * Effect of strong force not known **Electron Scattering:** * Not affected by strong force (leptons) * Electron **λ_db tunable**

Question 27

Define the **decay constant** **λ**

Answer 27

The **probability** that an unstable isotope **decays in one second**

Question 28

What's wrong with this calculation?

Answer 28

**Activity and time must be the same units**

Question 29

Define the **activity, A**, of a radioactive sample

Answer 29

The **total number** of unstable isotopes that **decay after one second**

Question 30

What does the activity, A, of a radioactive sample depend on?

Answer 30

1. The **decay constant** **λ** 2. The **number of unstable isotopes N**

Question 31

Define the **half life, T_½,** of a radioactive sample

Answer 31

Time taken for either… 1. Activity of sample to halve 2. **Number of unstable isotopes remaining to halve**

Question 32

How do you **derive the half life T_½** equation?

Answer 32

Set **N as 0.5N₀**

Question 33

What do the **gradient** and **y-intercept** of this graph represent?

Answer 33

where r₀ → **average radius of nucleon**

Question 34

1. Derive the equation of this graph 2. What is the **gradient** and **y-intercept**?

Answer 34

Question 35

How do you **prove this graph is exponential**?

Answer 35

Find **multiple T_½ and compare**

Question 36

What is the **gradient** of this graph?

Answer 36

r₀ → **average radius of nucleon**

Question 37

If this unstable isotope of caesium **decays by** **α** **emission** where does it end up on the graph?

Answer 37

N → neutron number Z → Proton Number

Question 38

If this unstable isotope of caesium **decays by** **β^-** **emission** where does it end up on the graph?

Answer 38

N → neutron number Z → Proton Number

Question 39

If this unstable isotope of caesium **decays by** **β⁺** **emission** where does it end up on the graph?

Answer 39

N → neutron number Z → Proton Number

Question 40

What is the corrected count rate?

Answer 40

The measured count rate minus background radiation

Question 41

What does radiation do to cells?

Answer 41

Ionises atoms in cells leading to dead or cancerous tissue

Question 42

What unit of time should you convert to when doing nuclear equations?

Answer 42

Seconds

Question 43

Why is half-life important?

Answer 43

Different uses will require differing half-lives - Medical uses require short half-lives - Smoke alarms and nuclear reactors require long half-lives Also, when disposing of nuclear waste it is important to know the half-life

Question 44

Describe carbon dating

Answer 44

If you are given the half-life of the subject, find the decay constant Use these values in the formula A = A₀e^-λt A - current count rate A₀ - initial count rate λ - decay constant t - time the subject has decayed

Question 45

Can the formula N = N₀ e^-λt be written in any other way?

Answer 45

Yes, can use A (activity) or C(Count rate) instead of N (A = A₀e^-λt) (C = C₀e^-λt) (M = M₀e^-λt)*

Question 46

For Nuclear questions, how do you work out the number of particles flowing through area A at distance D from a source of count rate C?

Answer 46

- Find the fraction of the area of a sphere that the cross-section represents (proportion of particles that will hit that area compared to the whole sphere) - Multiply this by the count rate of the source ( C * A/4πr²)