8.1 - Alpha, Beta and Gamma Decay

Question 1

What was the Rutherford scattering experiment?

Answer 1

The scattering of alpha particles on passage through thin metal foils in an angular distribution.

Question 2

What does the Rutherford scattering experiment show?

Answer 2

It was proof of a new atomic model (nucleus and electron orbits)
as it indicated a concentration of positive charge at the atomic nucleus. (Since some alpha particles which were also positive got repelled and flung in different directions).

Question 3

What are alpha particles?

Answer 3

High energy particles made up of 2 protons and 2 neutrons (helium nucleus).

Question 4

When are alpha particles often emitted?

Answer 4

When a nucleus becomes too large.

Question 5

What is the most ionising type of radiation? Why?

Answer 5

Alpha due to:
- highest charge +2e
- leads to greatest number of ion pairs per mm in air
- therefore can do more damage to cells

Question 6

Which type of radiation is the least penetrating?

Answer 6

Alpha . It can only travel through a couple cm of air (can be stopped by paper).

Question 7

What are beta minus particles?

Answer 7

High energy electrons that are emitted from the nucleus.

Question 8

What are beta plus particles?

Answer 8

High energy positrons emitted from the nucleus. (antimatter)

Question 9

When are beta minus particles emitted?

Answer 9

When a nucleus has too many neutrons.

Question 10

When are beta plus particles emitted?

Answer 10

When a nucleus has too many protons.

Question 11

How ionising is beta decay?

Answer 11

Moderately, due to it having +1e charge
(slight damage to cells).

Question 12

How penetrating is beta decay?

Answer 12

Moderately, 20cm-3m in air.
Can be stopped by a few mm of aluminium foil.

Question 13

What are gamma rays?

Answer 13

High energy electromagnetic waves.

Question 14

When are gamma rays emitted?

Answer 14

When a nucleus must lose some energy.

Question 15

What is ionisation?

Answer 15

The removal of electrons from atoms. If the type of radiation has enough energy, it can do this (mainly alpha).

Question 16

What type of radiation is the most penetrating?

Answer 16

Gamma rays as they can travel the furthest distance in air before being absorbed. (infinite range - follows inverse square law)
Can be stopped by a few cm of lead.

Question 17

What happens to the nucleons when a B minus particle is emitted?

Answer 17

A neutron turns into a proton

Question 18

What happens to the nucleons when a B plus particle is emitted?

Answer 18

A proton turns into a neutron.

Question 19

How do smoke detectors work?

Answer 19

They have Americium-241 stored in them (weak alpha source).
If smoke enters the alarm, it absorbs the alpha particles, hence reducing the current which causes the alarm to sound.

Question 20

How do thickness controls work?

Answer 20

Beta radiation can determine the thickness of aluminium foil, paper plastic and steel.
Beta because -
alpha = absorbed by materials
gamma = pass straight through

Geiger counter on the other side of material to detect how many make it through.

Question 21

What is the inverse square law?

Answer 21

When a quantity is inversely proportional to the square of the distance from the source because the surface area of a sphere increases with the square of the radius.

Question 22

What is the inverse square law equation?

Answer 22

I = k/x²
I = intensity of the gamma radiation (wm⁻²)
k = constant of proportionality
x = the distance from the source (m)

Question 23

What is the inverse square law equation for gamma radiation? (ratios)

Answer 23

I₁/I₂ = (x₁/x₂)²
I₁ = intensity of the gamma radiation at x₁
I₂ = intensity of the gamma radiation at x₂
x₁ = the initial distance from the source
x₂ = the subsequent distance from the source

Question 24

What are the 2 categories of background radiation?

Answer 24

1 - natural sources
2 - man made sources

Question 25

What are 4 examples of natural background radiation?

Answer 25

• Radon gas from rocks and soil (most of the background radiation)
• Cosmic rays from space
• Carbon-14 in biological material
• Radioactive material in food and drink

Question 26

What are 4 examples of man made background radiation?

Answer 26

• Medical sources
• Nuclear waste
• Nuclear fallout from nuclear weapons
• Nuclear accidents

Question 27

What is a corrected count rate?

Answer 27

Count rate found from source - background radiation = ccr

Question 28

What are the best characteristics of a source (if you have to handle it)? (2)

Answer 28

• Short lived isotopes
• The smaller amount of radioactive material the better

Question 29

What are 6 precautions to take when handling radioactive material?

Answer 29

• Lead lined box
• Protective clothing
• Keeping personal items far away
• Limited exposure (less time)
• Long tongs
• Monitoring the exposure of workers, (radiographers use detector badges for example)

Question 30

What are the 2 precautions taken by the patient before radiation therapy?

Answer 30

• Patient must be protected with lead to cover parts of the body not to be exposed to radiation
• Exact dose, accurately directed at cancerous tissue

Question 31

What are the 4 precautions taken by the radiography before radiation therapy?

Answer 31

• Handle source with tongs
• Protected by screen
• Far away from source while dose is given
• Source immediately stored in its lead case after dose is given.