Radioactivity

Question 1

What is Rutherford scattering?

Answer 1

Experiment involved firing alpha particles in a thin stream through a thin layer of gold foil.
The results showed that the majority of particles passed straight through the foil- most of the atom is empty space.
Some were deflected slightly- atom has small amount of positive charge
A very small amount where deflected more than 90 degrees- the positive part must be very dense

Question 2

How was knowledge and understanding of the structure of the nucleus changed over time? (4 points)

Answer 2

1) Idea of atoms has been around sine the of the ancient Greeks
2) 1804, Dalton put froward a hypothesis that matter was made of tiny spheres ‘atoms’ that couldn’t be broken up. Each element made of different types of atom
3) J.J. Thompson discovered electrons could be removed from atoms. Made ‘plum-pudding model’. Atoms were spheres of positive charge with tiny negative electrons stuck in them
4) Rutherford’s experiment lead to proposal of nucleus

Question 3

How can you identify an unknown source of radiation?

Answer 3

1) Using a Geiger-Müller counter, measure the background count when the source is not present
2) Place the source of radiation close to the tube and measure the count rate
3) Place a piece of paper between the source and tube and measure count rate again. If it drops significantly then the source is an alpha emitter
4) Repeat with a piece of aluminium foil. If count rate drops significantly
then source is a beta emitter
5) Repeat with a block of lead. If count rate drops significantly then source is a gamma emitter

Question 4

Properties of alpha radiation

Answer 4

2 protons, 2 neutrons
Range in air of 2-10cm
Highly ionising
Absorbed by paper

Question 5

Properties of beta radiation

Answer 5

Electron/positron
Range in air of around 1m
Weakly ionising
Absorbed by Aluminium foil (around 3mm)

Question 6

Properties of gamma radiation

Answer 6

Electromagnetic radiation
Infinite range (follows inverse square law)
Very weakly ionising
Absorbed by several metres of concrete or several inches of lead

Question 7

How can radiation be used to monitor the thickness of certain materials whilst they are being produced?

Answer 7

In the production of aluminium foil, a beta source is placed on one side of the material, while a detector is placed on the other. If the material is too thick, less beta radiation will pass through the foil, the reading on the detector decreases causing the rollers to move closer together. Vice versa if material is too thin. Same can be done with alpha radiation for paper and gamma radiation for steel sheets.

Question 8

What are the relative hazards of the different types of radiation to humans?

Answer 8

Alpha radiation is highly ionising, so can be incredibly dangerous if inhaled or digested as it can ionise body tissues.
Beta particles are less ionising but can still cause damage to body tissue.
Prolonged exposure to gamma radiation can lead mutations and damage to cells

Question 9

How can you investigate the inverse square law for gamma radiation?

Answer 9

Measure the count rate of a gamma source at different distances from the GM tube​, making sure to adjust for the background radiation. Then you can plot a graph of corrected count against 1/x^2 , which will form a straight line verifying the law

Question 10

How can radioactive sources be handled safely?

Answer 10

● Using​ long handled tongs​ to move the source.
● Storing the source in a ​lead-lined container​ when not in use.
● Keeping the source as far away as possible​ from yourself and others.
● Never pointing the source towards others​.

Question 11

Sources of background radiation

Answer 11

Radon gas
Artificial sources- eg. nuclear fallout
Cosmic rays
Rocks containing naturally occurring isotopes

Question 12

Uses of gamma radiation in medicine

Answer 12

As a detector- a radioactive source with a short half life can be used to help diagnose patients
To sterilise medical equipment- gamma radiation will kill any bacteria present
In radiotherapy, gamma radiation can be used to kill cancerous cells

Question 13

Describe the random nature of radioactive decay

Answer 13

A decay can happen at any time, it is chance. So you can’t predict when a decay will happen.

Question 14

What is decay constant?

Answer 14

The probability of a nucleus decaying per unit time

Question 15

How is radioactive decay used to date objects?

Answer 15

​Nuclei with a long half-life such as ​carbon-14​ can be used to date ​organic​ objects, such as those found in archaeological sites. This is done by measuring the current amount of carbon-14 and comparing it to the initial amount, the percentage of which is approximately equal in all living things

Question 16

Key points when drawing line of stability

Answer 16

Check axis for whether neutrons or nucleons
For proton- neutron graph:
linear between 20 and 20
Then smooth curve
Max number of protons= 80
Mx no of neutrons= 120

Question 17

Why is the stability line not linear above 20 protons and 20 neutrons?

Answer 17

Because beyond this amount the ​electromagnetic force of repulsion becomes larger​ than the strong nuclear force keeping the nucleus together, and so more neutrons are needed to increase the distance between protons in order to decrease the magnitude of the electromagnetic force​ to keep the nucleus stable.

Question 18

What are the possible decay modes of unstable nuclei?

Answer 18

If the nucleus has too many neutrons- decays through beta minus emission
If the nucleus has too many protons- decays through beta-plus emission or electron capture
If it has too many nucleons it will decay via alpha emission
If it has too mush energy- decays via gamma

Question 19

What is an excited nuclear state?

Answer 19

Nuclei also have excited states. This is because of the different arrangements of protons and neutrons within a nucleus.
When the protons and neutrons rearrange to a lower energy state, the excess energy is released as gamma radiation.

Question 20

Why is technetium-99m used in medical diagnosis?

Answer 20

Technetium-99ms ideal for use in medical diagnosis as it is a pure gamma emitter
- gamma rays can be easily detected by a gamma camera and are only very weakly ionising, it has a half life of 6 hours, which is short enough to limit exposure but long enough for tests to be carried out, and it can be easily prepared on site.

Question 21

Typical values for nuclear radius?

Answer 21

around 5 fm