Topic 6 - Radioactivity

Question 1

Describe an atom

Answer 1

Atoms are very small, they have a radius of around 1×10^-10 metres.

The modern view of the atom is of a positively-charged nucleus containing protons and neutrons with smaller electrons orbiting outside the nucleus.

Question 2

What are the charges of subatomic particles?

Answer 2

Proton - +1
Neutron - 0
Electron - -1

Question 3

What is the relative mass of the subatomic particles?

Answer 3

Proton - 1
Neutron - 1
Electron - 0.0005

Question 4

How has the atomic model changed over time?

Answer 4

J.J. Thomson discovered the electron. Atoms are neutral overall, so in Thomson’s ‘plum pudding model’:

• atoms are spheres of positive charge
• electrons are dotted around inside

Rutherford considered these observations and he concluded:

• The atom being mostly empty space.
• There is a concentration of positive charge in the atom. Like charges repel, so the positive alpha particles were being repelled by positive charges.
• The very small number of alpha particles coming straight back suggested that the positive charge and mass are concentrated in a tiny volume in the atom (the nucleus). The tiny number doing this means the chance of being on that exact collision course was very small, and so the ’target‘ being aimed at had to be equally tiny.

Niels Bohr improved Rutherford’s model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus.

Question 5

What are isotopes?

Answer 5

Isotopes are forms of an element that have the same number of protons but different numbers of neutrons.
Isotopes have the same atomic number as the original element but they have a different mass number as isotopes have different number of neutrons to the original.

Question 6

Why are atoms neutral?

Answer 6

This is because they have the number of electrons as protons so they are neutral.

Question 7

How are ions formed?

Answer 7

Atoms can lose or gain electrons due to collisions or other interactions. When they do, they form charged particles called ions:

• if the atom loses one or more electrons, it becomes a positively-charged ion
• if the atom gains one or more electrons, it becomes a negatively-charged ion

Question 8

How can an atom be represented using symbol notation?

Answer 8

Z
X
A
Where:

Z is the mass number
A is the atomic number
X is the symbol

Question 9

When is an atom stable?

Answer 9

An atom’s nucleus can only be stable if it has a certain number of neutrons for the number of protons it has.

Question 10

When do atoms emit radiation?

Answer 10

As the number of protons increases, more neutrons are needed to keep the nucleus stable, for example lead, lead-206 has 82 protons and has 124 neutrons.

Nuclei with too many, or too few, neutrons do exist naturally but are unstable and will decay, in a random process, emitting radiation.

Question 11

How can an unstable neutron decay?

Answer 11

An unstable nucleus can decay by emitting an alpha particle, a ß- (beta minus) particle, a ß+ (positron), a gamma ray or in some cases a single neutron.

Question 12

What is an alpha particle, when is it emitted and what impact does it have on the atom when emitted?

Answer 12

If the nucleus is unstably large, it will emit a ‘package’ of two protons and two neutrons called an alpha particle.

An alpha particle is also a helium-4 nucleus, so it is written as 42He

Question 13

What is an Beta minus particle, when is it emitted and what impact does it have on the atom when emitted?

Answer 13

If the nucleus has TOO MANY NEUTRONs, a NEUTRON will TURN INTO a PROTON and EMIT a FAST-MOVING ELECTRON . This electron is called a beta minus (β-) particle - this process is known as BETA RADIATION.

A beta particle has a RELATIVE MASS OF 0, so its mass number is zero. As the beta particle is an electron, it can be written as 0-1e.

Electrons are not found in the nucleus but neutrons can SPLIT INTO POSITIVE PROTON and an ELECTRON which is then ejected at high speed and carries away a lot of energy.
Causes the ATOMIC NUMBER of the NUCLEUS to INCREASE by ONE and the MASS NUMBER remains the SAME.

Question 14

What is an Beta plus particle, when is it emitted and what impact does it have on the atom when emitted?

Answer 14

If the nucleus has too few neutrons, a proton will turn into a neutron and emit a fast-moving positron. This positron can be called a beta plus (β+) particle - this process is known as positron emission.
It has the same relative mass of zero, so its mass number is zero, but a +1 relative charge. It can be written as 0+1e.
Causes the atomic number of the nucleus to decrease by one and the mass number remains the same.

Question 15

Describe gamma ray emission.

Answer 15

A re-arrangement of the particles in a nucleus can move the nucleus to a lower energy state. The difference in energy is emitted as a very high frequency electromagnetic wave called a gamma ray.

After emitting an alpha or beta particle, the nucleus will often still have excess energy and will again lose energy. A nuclear re-arrangement will emit the excess energy as a gamma ray.

Gamma ray emission causes no change in the number of particles in the nucleus meaning both the atomic number and mass number remain the same.

Question 16

Describe neutron emission.

Answer 16

This can occur naturally, ie absorption of cosmic rays high up in the atmosphere can result in neutron emission.
Or it can occur artificially, eg the work done by James Chadwick firing alpha particles at beryllium.
Neutron emission causes the mass number of the nucleus to decrease by one and the atomic number remains the same.

Question 17

Describe the penetrating power, ionising power and range in air for Alpha

Answer 17

Stopped by Skin/paper
Highly ionising
smaller than 5cm

Question 18

Describe the penetrating power, ionising power and range in air for Beta

Answer 18

Stopped by 3mm aluminium foil
Low ionising power
1m in air

Question 19

Describe the penetrating power, ionising power and range in air for Gamma

Answer 19

Stopped by Lead/concrete
Very low ionising power
Bigger than 1km

Question 20

What can all types of radioactive be detected by?

Answer 20

All types of radioactive decay can be detected by photographic film, or a Geiger-Muller tube (G-M tube). The photographic film is chemically changed by the radiations so it can be developed to see if there has been exposure. In a G-M tube, the radiations ionise the gas inside and the resulting charged particles move across the chamber and get counted as charges rather like an ammeter.

Question 21

What is the half-life of an nucleus?

Answer 21

Half-life is the time it takes for half of the unstable nuclei in a sample to decay or for the activity of the sample to halve or for the count rate to halve.

Question 22

What is the activity of a substance measured in?

Answer 22

The activity of a radioactive substance is measured in Becquerel (Bq). One Becquerel is equal to one nuclear decay per second.

Question 23

How could the amount of a sample after four half-lives be shown as?

Answer 23

the amount of a sample remaining after four half-lives could be expressed as:

a fraction - a ½ of a ½ of a ½ of a ½ remains which is ½ x ½ x ½ x ½ = 1/16 of the original sample
a decimal - 1/16 = 0.0625 of the original sample

Question 24

Do one NUCLEAR EQUATION question

Answer 24

Do one NUCLEAR EQUATION question

Question 25

What is background radiation?

Answer 25

The low-level radiation that’s around us all the time

Question 26

Where does background radiation come from? (4)

Answer 26

• Food
• Building materials
• Space (cosmic rays)
• fallout from nuclear explosions or waste

Question 27

What is irradiation?

Answer 27

When you are exposed to a radioactive source but you’re not radioactive.

Question 28

What is contamination?

Answer 28

If unwanted radioactive atoms get onto an object, the object is said to be contaminated. These contaminating atoms might then decay which can cause harm to you. Contamination is dangerous as the radioactive particles can get inside your body

Question 29

How can irradiation be reduced?

Answer 29

Keeping sources in a lead-lined box

Standing behind barriers or in a different room

Question 30

How can contamination be reduced?

Answer 30

Gloves and tongs should be used to handle sources.

Protective suits to prevent inhalation of radioactive particles.

Question 31

How is radioactivity used in smoke alarms?

Answer 31

Alpha particles are easily absorbed. A household smoke alarm measures the movement of alpha particles across a small gap. If smoke enters the detector, it will absorb the alphas and the detector will measure a drop in the number getting across the gap. This drop in measurement will trigger the alarm to sound.

Americium-241 is the most common material used as the alpha emitter in a domestic smoke alarm.

Question 32

How is radioactivity used in thickness monitoring?

Answer 32

Radiation is absorbed as it passes through materials. This allows it to be used to gauge the thickness of a material by measuring the amount of radiation that passes through.

This is commonly used with beta minus emitters in industry, for paper milling and the production of aluminium foil.

Question 33

How is radioactivity used in medicines?

Answer 33

Doctors also use radioactive sources for a number of reasons, eg:

• Sterilisation of surgical instruments.
• Beams of gamma rays, called a gamma knife, can be used to kill cancerous tumours deep inside the body. The beams are aimed at the tumour from many different directions to maximise the dose on the tumour but to minimise the dose on the surrounding soft tissue. This technique can damage healthy tissue, so careful calculations are done to establish the best dose - enough to kill the tumour but not so much so that the healthy tissue is damaged.

Question 34

What happens if the half-life of the radioactive substance used in medicine is too long?

Answer 34

If the half-life chosen is too long, the damaging effects of the radiation would last for too long and the dose received would continue to rise.

Question 35

How is radioactivity used for preservation?

Answer 35

Irradiation can be used to preserve fruit sold in supermarkets by exposing the fruit to a radioactive source - typically cobalt-60. The gamma rays emitted by the cobalt will destroy any bacteria on the fruit but will not change the fruit in any significant way. The process of irradiation does not cause the irradiated object to become radioactive.

Question 36

How can tumours be treated by contamination?

Answer 36

Cancer tumours can be treated by injecting them with a radioactive material. The cancer cells absorb radiation from the material and receive a high dose of energy. Doctors must work out the danger to nearby healthy tissue before giving this treatment.

Question 37

What do tracers do?

Answer 37

They make soft tissues such as blood vessel or kidneys to show up on medical imaging processes. An isotope emits gamma rays that easily pass through the body to a detector outside the body, for example an x-ray machine or a ‘gamma camera’. In this way, the radioactive isotope can be followed as it flows through a particular process in the body.

Question 38

How to PET Scanners work?

Answer 38

Positron emission tomography (PET) scans use a positron emitter as the contamination tracer. These tracer materials, such as fluorine-18, have very short half-lives, and need to be manufactured in the hospital and injected into the patient’s blood soon after manufacture.

The positrons emitted will react with electrons in the patient’s body and produce gamma rays which can be detected outside the body. Multiple detections can build up a picture of the movement of the tracer inside the body.

Question 39

Advantages of Irradiation

Answer 39

Sterilisation can be done without high temperatures.

It can be used to kill bacteria on things that would melt.

Question 41

What are the disadvantages of irradiation?

Answer 41

It may not kill all bacteria on an object.
It can be very harmful - standing in the environment where objects are being treated by irradiation could expose people’s cells to damage and mutation.

Question 42

Advantages of contamination

Answer 42

• Radioactive isotopes can be used as medical and industrial tracers
• Radioactive isotopes may not go where they are wanted
• Use of isotopes with a short half-life means exposure can be limited

Question 43

Why are isotopes used in PET Scanners produced nearby?

Answer 43

The half-lives of radioisotopes used in PET scans are very short which means that they could lose their power if they are made far away and are delivered.

Question 44

Describe nuclear power

Answer 44

The nuclei of atoms contain a large amount of energy. Releasing this energy would free the world from having to use fossil fuels.

Question 45

What is nuclear fission?

Answer 45

Nuclear fission is the splitting of a large atomic nucleus into smaller nuclei.

Question 46

Describe the process of nuclear fission.

Answer 46

In a nuclear reactor, a slow-moving neutron is absorbed into a nucleus (typically uranium-235). This causes the nucleus to become uranium-236, which is unstable.

The entire nucleus splits into two large fragments called ‘daughter nuclei’. In addition to the ‘daughter’ products, two or three neutrons also explode out of the fission reaction and these can collide with other uranium nuclei to cause further fission reactions. This is known as a chain reaction.

The fast moving neutrons carry most of the energy from the reaction with them but before the neutrons can collide with fresh uranium nuclei, they need to be slowed down.

Their energy is passed on to other components in the nuclear reactor, which is used to heat water to drive the turbines that turn the generators.

Question 47

What are the different parts of fission reactors?(5)

Answer 47

Nuclear fuel - the uranium or plutonium isotope that will split when triggered by an incoming neutron. The fuel is held in rods so that the neutrons released will fly out and cause nuclear fission in other rods.
Moderator - graphite core - a graphite core, for example, slows the neutrons down so that they are more likely to be absorbed into a nearby fuel rod.
Control rods - these are raised and lowered to stop neutrons from travelling between fuel rods and therefore change the speed of the chain reaction.
Coolant - this is heated up by the energy released from the fission reactions and is used to boil water to drive turbines in the power station.
Concrete shield - the daughter products of the fission reaction are radioactive and can be a hazard.

Question 48

Advantages of nuclear power stations (4)

Answer 48

Produces no polluting gases.
Does not contribute to global warming.
Very low fuel costs.
Low fuel quantity reduces mining and transportation effects on environment.
High technology research required benefits other industries.
Power station has very long lifetime.

Question 49

Disadvantages of nuclear power stations (5)

Answer 49

Waste is radioactive and safe disposal is very difficult and expensive.
Local thermal pollution from wastewater affects marine life.
Large-scale accidents can be catastrophic.
Public perception of nuclear power is negative.
Costs of building and safely decommissioning are very high.
Cannot react quickly to changes in electricity demand.

Question 50

What is nuclear fusion?

Answer 50

Nuclear fusion is when two small, light nuclei join together to make one heavier nucleus. Fusion reactions occur in stars where, for example, two hydrogen nuclei fuse together under high temperatures and pressure to form a nucleus of a helium isotope.

Question 51

What are the problems associated with fusion?

Answer 51

The issue with fusion is that it requires the fusing of nuclei, which are positive particles. As two nuclei approach each other, they will repel because they have the same charge. The fusion of the nuclei has to happen under intense pressure and very high temperatures in order to force the nuclei together and overcome this electrostatic repulsion.

This need for a very high temperature and pressure makes it very difficult to build a practical and economic fusion power station. For fusion to occur at the lower pressures in a reactor on Earth, the temperature would need to be between 100 and 200 million degrees. Fusion has been successfully achieved by scientists at the JET experiment near Oxford but so far they have been unable to create a financially viable reactor.

Question 52

What is the difference between fusion and fission?

Answer 52

Fission is the splitting of a heavy, unstable nucleus into two lighter nuclei, and fusion is the process where two light nuclei combine together releasing vast amounts of energy. Fission is used in nuclear power reactors since it can be controlled, while fusion is not utilized to produce power since the reaction is not easily controlled and is expensive to create the needed conditions for a fusion reaction.

Question 53

Disadvantages of contamination (3)

Answer 53

Radioactive isotopes may not go where they are wanted
It can be difficult to ensure that the contamination is fully removed so small amounts of radioisotope may still be left behind
Exposure to radioactive materials can potentially damage healthy cells