Topic 6: Radioactivity

Question 1

What is the particle theory?

Answer 1

The first theory used by scientists. It’s helps explain the properties of liquids, solids and gases.

Question 2

Who invented the plum pudding model and what is it?

Answer 2

JJ Thomson carried out experiments and found atoms contain much smaller subatomic particles - electrons with negative charge and hardly any mass.

Question 3

Who was Ernest Rutherford and what did he discover and how?

Answer 3

Carried out the gold foil experiment and discovered atoms were mostly empty space with most mass in nucleus with electrons moving round it.

He fired alpha particles straight through the gold foil however some emerged from foil at different angles while some were reflected back by a tiny concentration of positively charged atoms. Model was replaced by the nuclear model.

Question 4

Why are atoms originally electrically neutral?

Answer 4

Neutrons have no charge and so the positive proton charge and negative electron charge cancel each other out.

Question 5

What is an isotope and how is different?

Answer 5

Isotopes are atoms of single elements that have a different neutron number.

This means their atomic number will always be the same but their mass number could change.

Question 6

what is the size difference between an atom and nucleus?

Answer 6

The atom is 100,000 bigger than the nucleus in the centre of it.

Question 7

What is ionisation?

Answer 7

Sometimes an atom gains so much energy that one of the electrons can escape from the atom altogether. An atom that has lost or gained an electron is called an ion.

Radiation that causes an electron to escape is called ionising radiation.

Question 8

What is back ground radiation and does it affect us?

Answer 8

Is Ionising radiation at a low level from space and from naturally radioactive substances in the environment.
We are constantly be exposed to it.

Question 9

What are some of the main sources of background radiation?

Answer 9

Main source is radon gas which is produced by rocks that contain small amounts of uranium. Radon then diffuses into the air and can vary throughout country.

49% - radon gas
1% - nuclear & other
10%food
15% - medical 
13% - ground and buildings 
12% cosmic rays

Question 10

How to measure radioactivity?

Answer 10

• using a photographic film, which becomes darker as more radiation reaches it. However it has to de developed in order to measure it.
• Dosimeter. This uses materials without needing to be measured.
• A Geiger Muller tube can also be used. Radiation passes through the tube, ionising gas inside it and allows a short pulse of current to flow through it.

Question 11

How does a Geiger Muller measure the count rate?

Answer 11

A GM tube can be connected to a counter to count the pulses of current or the tube may give clicks each time radiation is detected

Count rate = number of clicks per second/minute

Question 12

What do scientist need to do first before measuring the radioactivity of a source?

Answer 12

They need to measure the background radiation first by taking several measurements and working out the mean.

Question 13

How is radiation emitted?

Answer 13

Nucleus of a radioactive substance is unusable which means it can easily change or decay. When decay occurs radiation is emitted which cause nucleus to loose energy and become more stable.
YOU CANNOT PREDICT WHEN A NUCLEUS DECAYS. ITS A RANDOM PROCESS.

Question 14

What sorts of particles do nucleus emit?

Answer 14

Alpha particles
Beta particles
Gamma particles 
Positrons 
Neutrons

Question 15

What are alpha particles and what are their properties?

Answer 15

They contain 2 protons and 2 neutrons, just like a helium atom. Has a relative mass of four and no electrons so has a positive charge.

• they are usually produced when large atoms decay
• a thin piece of paper will stop it.

Question 16

Why do alpha particles only travel short distances?

Answer 16

The6 are emitted at high speeds and have a high relative mass so transfer a lot of energy. However each time they ionise an atom they loose energy. So they produce many ions in a short distance loose energy and therefore only a travel a few cm in air.

Question 17

Properties of Beta particles…

Answer 17

• They are high energy high speed electrons.
• have almost no relative mass and a charge of minus 1.
• travel a few meters in air.
• moderately ionising
• can be stopped by 3mm thick aluminium

Question 18

Properties of gamma rays…

Answer 18

• high frequency electromagnetic waves
• travel at speed of light
• have no electric charge
• travel a few km in air
• weakly ionising (10x less than beta)
• needs thick lead or several meters of concrete to stop it.

Question 19

Explain what happens during beta minus decay…

Answer 19

A neutron changed into a proton and an electron. The electron is ejected from the atom. The atomic number INCREASES BY 1 but there is no change to the overall mass number.

Question 20

What during alpha decay?

Answer 20

When an alpha particle is emitted the mass number of the nucleus goes down by 4 and the atomic number goes down by 2.

Question 21

What happened during beta plus decay?

Answer 21

A proton becomes a neutron and a positron. The atomic number DECREASES BY 1. The mass number does not change.

Question 22

What is another way they can become more stable?

Answer 22

They ma6 loose energy as gamma radiation when the subatomic particles in the nucleus are rearranged.

Question 23

What is a nuclear equation and what does it do?

Answer 23

Shows what is happening during radioactive decay. The equation must be balanced - the total mass number must be the same on each side and the total charges must be the same.

Question 24

What is a half life?

Answer 24

When an unstable nucleus undergoes radioactive decay its nucleus changes to become more stable.

THE TIME TAKEN FOR THE ACTIVITY OF A SAMPLE HALVE.

(TASH)

Question 25

How is half life measured?

Answer 25

The number of nuclear decay per second, measured in bequerels. (Bq)
1Bq = 1 nuclear decay per second
It can be referred to as the ‘count’ for a sample.

Question 26

How is radio activity used in killing microbes?

Answer 26

Foods contain microbes that cause them to decompose, however some bacteria cause food poisoning. Food can be irradiated with gamma rays to kill bacteria this does not make to food radioactive. It makes it safe to eat and it can be stored for longer without expiring.

Surgical equipment can be irradiated with gamma rays to kill microbes.

Question 27

How is radioactive detecting used?

Answer 27

Radioactive isotopes can be used at tracers. E.g. a gamma ray source is added to water and is used to detect leaks in pipes underground. A GM tube follows path of pipe and then detects higher levels of radiation.

Question 28

How can radioactivity be used to check paper thickness?

Answer 28

Paper is made by squeezing the wood pulp between rollers. Paper can be made in different thickness and rollers need to squeeze the write amount. The detector counts the rate at which beta particles get through the paper from a source of one side. When paper is too thin more beta particles penetrate it and detector records a higher count rate, computer senses this and changes it. When paper is too think the opposite happens.

Question 29

How is radioactivity used in smoke alarms?

Answer 29

• contains a source of alpha particles, usually americium-241.
• the detector has an electric circuit with an air gap between 2 electrically charge plates. The americium releases alpha particles which ionise molecules in the air. The ions attach to plates with an opposite charge and so allow a small electrical current to flow.
• as long as the current flows that alarm will not sound. When smoke gets into the air gaps it slows down the ions and current decreases setting off the alarm.

Question 30

What is a PET scanner and how is it used?

Answer 30

The tracer emits a positron, which then interacts with an electron. They annihilate each other and their mass is released as 2 gamma rays in opposite directions. the gamma rays are detected by PET scanners.
The scanner scans live areas of the organ that take up more of the tracer and this shows up brighter on the computer.

Question 31

What are the dangers of radioactivity?

Answer 31

• A large amount of ionising radiation can cause tissue damage such as reddened skin (radiation burns) and other unseen affects.
• small amounts of ionising radiation over long perished of time can cause damage to DNA inside cells. This can cause mutations.

Question 32

How are mutations caused from radiation?

Answer 32

DNA contains instructions controlling the cell so some mutations can cause the cell to malfunction and may cause cancers. Gene mutations that occur in gametes can be passed on to the next generation. However not all mutations are harmful and cells can often repair damage from radiation if it is a low dose.

Question 33

What precautions should be taken when dealing with radioactive sources?

Answer 33

Intensity of radiation decreases with distance so handle with tongs and don’t point it at people. Keeping them in lead lined containers means the lead can absorb the particles.

Question 34

What does irradiated and contaminated mean?

Answer 34

Irradiated: exposure to radiation.
Contamination: radiation and to make contact with an object.

Question 35

What are some of the medical uses for tracers?

Answer 35

Tracers contain radioactive isotopes that attach to molecules an will be taken up by particular organs in the body that will be shown on a gamma camera.

Tracers can be injected into the blood to find sources of internal bleeding.

Also used to detect tumours. Tracer is made up using radioactive glucose molecules that cancer cells absorb quickly.

Question 36

What do radioactive isotopes need to have in medical tracers.

Answer 36

A short half life so other parts of the body are affected as little as possible.

Question 37

What is the difference between internal and external radiotherapy?

Answer 37

Cancer cells divide more rapidly than most other cells in the body so are more susceptible to being killed by radiation.

Internal: uses a beta emitter, iodine-131 is placed inside the body within or close to a tumour.

External: uses beams of gamma rays, x rays or protons directed at the tumour from outside the body. Beams are directed from different directions.

Question 38

What is the difference between nuclear fission and nuclear fusion?

Answer 38

Nuclear fission: large nuclei (such as uranium-235) break up to form smaller nuclei and release energy. They are used in nuclear power stations.

Nuclear fusion: two smaller nuclei join together (fuse) to form a larger nucleus. Fusion reactions occur in stars.

Question 39

Why are nuclear fuels more effective than any other typer of fuel?

Answer 39

They store a lot more energy per kilogram per kilogram than any other type of fuel.
Nuclear fuels do not burn so they do not need air to allow them to release energy and so don’t produce carbon dioxide harmful gases or contribute to global warming.

Question 40

What are the negatives about nuclear power stations.

Answer 40

Expensive to build and dismantle safely. They produce radioactive waste that stays radioactive for millions of years and is expensive to treat safely. Normally buried underground in concrete.

Question 41

What processes occur in nuclear fission?

Answer 41

When uranium-235 nucleus absorbs a neutron it immediately splits in 2 smaller daughter nuclei, which are also radioactive. 2-3 more neutrons are released.

Question 42

How is nuclear fission used?

Answer 42

It is an uncontrolled nuclear chain reaction however it is used in nuclear power stations and reactions such as these occur in atomic bombs.

Question 43

What are reactors, fuel rods and moderator?

Answer 43

In a nuclear reactor the fuel is made into fuel rods. As the reaction occur neutrons leave the fuel rods at high speed. Inside the reactor core, fuel rods are inserted into the holes in a material called a moderator to slow down neutrons,

Question 44

What are control rods?

Answer 44

The reaction can be controlled using control rods which contain elements that absorb neutrons. These rods are placed between fuel rods in the reactor core. If rate of fission needs to increase control rods are moved out (up) so few neutrons are absorbed and vice versa.

Question 45

what process occurs in nuclear fusion?

Answer 45

Two smaller nuclei form to make a larger one. The mass of the new nucleus formed is less than the total of the two smaller nuclei as energy has been released.

Hydrogen nuclei combine to form a helium. This is the main energy source for starts. The protons in the nuclei are positively charged and like charges repel.

Question 46

How to nuclei fuse during fusion?

Answer 46

They need to get extremely close to each other (about 10 to the -15 m apart) the sun has a very strong gravitational field, which creates extremely high pressure at the centre. This forces nuclei to be very close to each other so they are more likely to collide.

Question 47

What will happen is a stars temperature is not hot enough during fusion?

Answer 47

In a star if the temperature is not hot enough the particles will simply collide and rebound due to electrostatic repulsion.

Temperature needs to be high so particles travel very fast in close approximately so they can overcome the electrostatic repulsion?

Question 48

Why is fusion only able to happen in starts?

Answer 48

Strong gravitational fields of stars means they have a high density nuclei at their cores. High density makes collisions more likely.

Not able to be produced on earth because so far none of the experimental reactors have produced more energy than that has been put in getting temperatures high enough with enough pressure is very hard to achieve on earth so it is still a far way off.