Chapter 25 - Radioactivity

Question 1

Define ionising power

Answer 1

The ability of radiation to be able to cause damage to other particles, and become ions.

Question 2

Alpha particles definition/ description

Answer 2

Positively charged particles.

A Helium atom- 2 particles and 2 protons.

Question 3

Beta particles definition/ description

Answer 3

FAST moving electrons (B-)

Deflected by E & B fields.

Question 4

Gamma particles definition/ description

Answer 4

High energy photons with no charge and they travel at the speed of light.

Question 5

Absorption of alpha particles

Answer 5

They have a large mass and charge so they can interact with many things and hence can ionise easily. So they have a short range.

Question 6

How can alpha particles be stopped?

Answer 6

A few cm of air or a thin sheet of paper.

They are deflected by E & B fields.

Question 7

Absorption of beta particles

Answer 7

They have a smaller mass and charge, so a larger range in air. 
Fast travelling (due to smaller mass)

Question 8

How can beta particles be stopped?

Answer 8

Stopped by 1-3mm of aluminium to stop.

Deflected by E & B Fields.

Question 9

Absorption of gamma particles

Answer 9

No charge, and so even less ionising.

They become more ionising the further they travel.

Question 10

How can gamma particles be stopped?

Answer 10

With thick lead.

Not deflected by E & B Fields.

Question 11

What is the nucleus before the decay called?

Answer 11

Parent nucleus.

Question 12

What is an alpha particle?

Answer 12

4 2 He.

Question 13

What is a beta minus

Answer 13

0 -1 e.

Question 14

What is a beta plus

Answer 14

0 +1 e.

Question 15

What is a gamma?

Answer 15

Not a particle.

Question 16

What causes beta decay

Answer 16

The weak nuclear force.
They generally have too many neutrons for stability (beta- minus).
So a neutron decays into a particle, an electron and n anti electron neutrino.

Question 17

When does gamma decay happen

Answer 17

Emitted when a nucleus has surplus energy after an alpha/ beta emission.

Question 18

Gamma decay equation

Answer 18

It stays the same, but it just releases the gamma wave.

Question 19

What does “natural radioactivity” mean

Answer 19

Spontaneous and random.

Question 20

Define the half life

Answer 20

Half life of an isotope is the average time it takes for half the number of an active nuclei in the sample to decay.

Question 21

Define activity

Answer 21

The rate at which the nuclei decay.

Question 22

Activity equation

Answer 22

A = λ N
A- activity
N- Number of undecayed nuclei N left in the source.
λ- Decay constant.

Question 23

Define the decay constant

Answer 23

The probability of decay of an individual nucleus per unit time.

Question 24

Radioactive decay calculations

for the number of undecayed nuclei

Answer 24

N =N0 e ^(-λt)

N = number of undecayed nuclei.

Question 25

Radioactive decay calculations for the activity

Answer 25

A = A0 e ^(-λt)

Question 26

Decay constant and half life equation

Answer 26

λ t1/2 = ln(2)

Question 27

What is the half life of C-14

Answer 27

5730 years.

The ratio of C-14/ C-12 is atmospheric carbon= and it is almost constant at 1.3 x 10^-12.

Question 28

Limitations of Carbon-dating

Answer 28

• You are assuming that the ration of C-14/ C-12 has remained constant over the years.
  But, it is often affected by volcanic eruptions, burning fossil fuels.
• and affected by solar flares and nuclear bombs.

Question 29

Note about conventional current

Answer 29

In magnetic fields, the conventional current flow ISNT the flow of electrons. The electrons travel in the opposite way to the conventional current.

Question 30

Dangers of radiation

Answer 30

1. Ionising radiation is dangerous because it damages living cells.
2. Cell DNA is damaged either directly or by creating free radicals that react with DNA.
3. Damaged DNA may cause cells to divide and grow uncontrollably, causing a tumour, which can be cancerous.
4. High doses of radiation can kill living cells.

Question 31

Precautions of radiation

Answer 31

1. No source should come into contact with skin.
2. Solid sources must be transferred using tongs, robots or glove boxes.
3. Liquid, gas and solids in powder form must be in a sealed container so they aren’t inhaled.
4. Only use when necessary.

Question 32

What are the 2 methods of detection ?

Answer 32

Gas chamber and a geiger tube

Question 33

Gas Chamber

Answer 33

It contains air saturated with water at a low temperature. The alpha and beta particles will ionise the air and the water molecules will be attracted to the ions, creating a track of minute, condensed water tracks.

Question 34

What do alpha particle tracks in gas chambers look like?

Answer 34

Straight, radiating from the source.
The same isotope gives the same length.
The same range or ionising power.

Question 35

What do beta particles look like in gas chambers?

Answer 35

Wispy tracks and so B-particles are easily deflected by air molecules.
Less easy to easy because they are less ionising.

Question 36

How does a geiger-muller tube work?

Answer 36

• It contains of a sealed tube with argon gas at a low pressure.
• The thin mica window allows the alpha and beta particles inside. The radiation ionises the gas particles in its track. The negative ions are attracted to the rod, and the positive ions are attracted to the walls.
• The ions collide with each other more often.
• As the particles touch the rod or the walls, they discharge, creating a pulse of charge to travel around the circuit and a voltage pulse across R, which is recorded as a click.

Question 37

Absorption- the Geiger tube

Answer 37

Before the count rate is examined, you must calculate the background radiation first. Then, subtract the background radiation from the count rate examined.