Radioactivity and Particles

Question 1

What is an alpha particle?

Describe Alpha emission.

Answer 1

• Alpha particles are Helium nuclei, so they have two protons and two neutrons.
• Mass number decreases by four. Atomic number decreases by two.

Question 2

What is a beta particle?

Describe beta emission.

Answer 2

• An electron that has been emitted from the nucleus of an atom when a neutron turns into a proton and electron
• The mass number stays the same
• Atomic number increases by one

Question 3

What is a gamma ray?

Describe gamma emission and gamma/alpha emission.

Answer 3

• They are very short EM waves. They have no mass, they are just energy.
• gamma emission: nothing happens. Mass and atomic number stays the same.
• Gamma/alpha emission: mass number decreases by, atomic number by two - basically just alpha emission.

Question 4

Give two uses of gamma radiation.

Answer 4

• Medical tracers

- Industrial tracers

Question 5

Describe how gamma radiation is used in medical tracers.

Answer 5

• Gamma source put in body
• Radiation penetrates body tissues and can be detected externally
• On-screen display is created to show where radiation is coming from and so doctors can use this method to check if organs are working as they should.

Question 6

Why would an alpha source be a bad choice for medical tracers?

Answer 6

• Because it does not have much penetrating power (so would be stopped by body tissues) and couldn’t be detected externally.
• It is strongly ionising so if it gets inside you it’s really harmful.

Question 7

Describe how gamma radiation is used in industrial tracers.

Why must the radiation be gamma?

Answer 7

• A gamma source is put into a pipe and allowed to flow along.
• If there’s a crack in the pipe the gamma radiation will build up in that area and so a detector will read extra higher radiation in that same area.
• Gamma radiation has a very high penetrating power so it can be detected through rocks and earth surrounding the pipes - alpha and beta would be too easily blocked.

Question 8

Describe how radioactive dating works.

Answer 8

• It allows scientists to accurately work out the age of rocks, fossils and archaeological experiments.
• It works on the principle of half-life. By measuring the amount of radioactive isotope left in a sample, and knowing its half-life, you can work out how long the thing has been left around.

Question 9

Define half-life.

Answer 9

The time taken for half of the radioactive atoms now present to decay.

Question 10

Define radioactive decay.

Answer 10

• The nuclei of unstable atoms break down at random, unaffected by physical conditions or any sort of chemical bonding.
• When the nucleus does decay, it releases one or more of the three types of radiation (alpha, beta or gamma)
• In the process, the nucleus often changes into a new element.

Question 11

Briefly describe how radiation can be used to treat cancer.

Answer 11

Once cancer has started, patients can be given radiotherapy to kill cancer cells and stop them dividing.
This involves high doses of gamma rays, carefully directed to zap cells in the tumour while minimising the dose to the rest of the body.
Because it is a high dose of radiation it kills cells completely without mutating them and causing more cancer.

Question 12

Describe how to use a graph to measure half-life.

Answer 12

• Take several readings from a radioactive source using a Geiger-Muller detector.
• Plot the time interval on the x-axis and the activity in becquerels on the y-axis.
• The half-life is found from finding the time interval that corresponds on the graph to a halving of activity on the vertical axis.
• Background radiation that has nothing to do with the source affects the results so you have to measure the level of background radiation first, and then subtract it from your activity readings to get an accurate value.

Question 13

Which sources of radiation are harmful outside the body? Why?

Answer 13

• Beta and Gamma
• They can penetrate soft tissues to reach delicate organs and do damage.
• If they get inside the body their radiation passes out without doing too much damage.

Question 14

Which source of radiation is harmful inside the body?

Answer 14

• Alpha
• It can’t penetrate the skin but if it gets inside the body it’s very dangerous because it will not pass quickly out again.
• Their damage is done in a very localised area.

Question 15

What is ionisation?

Why is it dangerous?

Answer 15

• When radiation enters your body it will collide with molecules in your body.
• These collisions cause ionisation. This means electrons are knocked from your molecules giving them a charge and making them ions.
• Ionisation damages or detroys molecules.

Question 16

How is ionisation related to cancer?

Answer 16

• Lower doses of ionising radiation tend to cause minor damage without killing the cell.
• This can lead to mutations in cells which then divide uncontrollably.
• This is cancer.

Question 17

Why is high-level radioactive waste difficult to dispose of safely?

Answer 17

• It has an extremely long half-life so it stays radioactive for tens of thousands of years. This means it has to be treated very carefully.
• It is often sealed in glass blocks which it then sealed in metal cannisters and buried deep underground.
• The site where it is buried MUST be geologically stable (no earthquakes) otherwise there is risk of a leak of radioactive material.

Question 18

Give some percautions to take when working with radioactive materials. (3)

Answer 18

• Never allow skin contact, hold it at arms length using tongs
• Keep source pointed away from you
• Store radioactive sources in sealed containers

Question 19

What are the extra percautions taken if you work regularly with? (3)

Answer 19

• Wear lead aprons and stand behind lead screens
• Industrial nuclear workers use full protective suits that prevent radioactive particles being inhaled or lodged in the skin
• They can use remote-control robot arms to carry highly radioactive sources.

Question 20

What is nuclear fission?

What is it used for?

Answer 20

• Nuclear fission is the splitting of the nucleus of an atom which results in the a small number of neutrons being released. This process releases a lot of kinetic energy.
• In a nuclear reactor nuclear fission is carried out in controlled chain reactions, releasing lots of energy.

Question 21

Give some sources of background radiation.

Answer 21

• substances on eath: air, food, soil e.c.t.
• radiation from space (cosmic rays)
• living things
• human activity, i.e. fallout from nuclear explosions or radioactive waste.

Question 22

What are the differences between rutherford’s nuclear model and the plum pudding model?

Answer 22

Plum pudding:
- Atoms were spheres of positive charge with tiny negative electrons stuck inside them.
Rutherford:
- Most of the mass is concentrated at the centre, and most of the atom is empty space
- The nucleus must be very small and positively charged

Question 23

What is a chain reaction?

Answer 23

When neutrons released from a split nucleus are absorbed by other atoms which then in turn release morre neutrons as their nuclei split. The process continues.

Question 24

What is the purpose of the moderator in nuclear reactors?

What could a moderator be made of?

Answer 24

• The moderator slows down neutrons so that they can succesfully collide with uranium nuclei and sustain a chain reaction
• They are normally made from graphite but sometimes take the form of a coolant (e.g. water)

Question 25

What is the purpose of the control rods in a nuclear reactor? What are they normally made of?

Answer 25

• They absorb some excess neutrons to limit the rate of the fission chain reactions
• Normally made of boron

Question 26

How is the kinetic energy generated from nuclear fission transfered into heat energy?

Answer 26

• A gas, typically carbon dioxide, is pumped through the reactor to carry away the heat generated.
• The gas is then passed through a heat exchanger, where it gives its energy to water. This water is heated and turned into steam, which turns a turbine (kinetic energy), which turn a generator (electrical energy), generating electricity.

Question 27

What happens when a uranium- 235 atom splits?

Answer 27

It forms two daughter nuclei. These daughter nuclei are lighter elements than uranium and they are normally radioactive because they have the ‘wrong’ number of neutrons.

Question 28

What is the main problem with generating energy by nuclear fission?

Answer 28

A large amount of radioactive waste is formed from the daughter nuclei of the split atoms. It is very difficult and expensive to dispose of safely.
It is often sealed into glass blocks which are then sealed in metal cannisters and buried deep underground.
Where it is buried MUST be geologically stable.

Question 29

What is the unit for radioactivity?

Answer 29

Becquerels - how many nuclei decay and give off radiation per minute.

Question 30

Describe how to use a geiger-muller detector to measure radiation. (6)

Answer 30

MP1 Use of ratemeter / scaler / counter;

MP2 Idea of measuring background radiation
e.g. background count / correction
/subtraction;

MP3 A safety precaution (based on distance or
absorption) e.g. use of tongs / shielding;

MP4 A controlled variable (time / distance /
positioning)

MP5 A practical consideration e.g. repeat /
average

MP6 Mention of becquerel / Bq