National 5 Radiation

Question 1

1. State what is meant by an alpha particle.

Answer 1

An alpha particle is a Helium nucleus or two protons and two neutrons.

Question 2

1. State what is meant by a beta particle.

Answer 2

A beta particle is a fast moving electron.

Question 3

1. State what is meant by a gamma ray.

Answer 3

A gamma ray is an electromagnetic wave or pure energy.

Question 4

2. State for alpha particles

• the relative speed,
• the relative mass,
• the charge,
• the approximate range in air and
• what is the minimum needed to significantly absorb

Answer 4

Alpha particles

• are relatively slow moving
• are relatively heavy
• have +2 charge
• can only travel several cm in air
• are easily absorbed by something like a sheet of paper

Question 5

2. State for beta particles

• the relative speed,
• the relative mass,
• the charge,
• the approximate range in air and
• what is the minimum needed to significantly absorb

Answer 5

Beta particles

• are fast moving
• are relatively light
• have -1 charge
• can travel several 10s of cm in air
• are absorbed by a few mm of aluminium

Question 6

2. State for gamma rays

• the relative speed,
• the relative mass,
• the charge,
• the approximate range in air and
• what is the minimum needed to significantly absorb

Answer 6

Gamma rays

• travel at the speed of light
• have no mass
• have no charge
• they travel several metres in air
• are only absorbed by a few cm of lead or a few m of concrete

Question 7

3. Explain the term ionisation.

Answer 7

Ionisation occurs when radiation interacts with other atoms.

It is the removal of an electron from an atom to create a positively charged ion.

(There are other chemical processes that can result in an atom gaining an electron to create a negative ion.)

Question 8

4. State the relative density of ionization of alpha, beta and gamma radiation

Answer 8

• α particles: highly ionising
• β particles: ionising
• γ rays: very weakly ionising

Question 9

5. Define the Activity of a source and its unit of measurement.

Answer 9

Activity is the number of nuclei which decay each second.

1 Becquerel means 1 nucleus decays each second.

Question 10

5. What is meant by “An activity of 5 kBq”?

Answer 10

An activity of 5 kBq means that 5000 nuclei decay each second.

Question 11

6. A = N/t

(Define symbols and units)

Answer 11

A - Activity (Bq)

N - Number of Nuclei which decay (no units)

t - time (s)

Question 12

6. Example

A source has an activity of 20 MBq. How many nuclei will decay in 1 min?

Answer 12

A = 20 MBq = 20 x 10<sup>6</sup> Bq
t = 1 min = 60 s
N = ?

A = N/t
20 x 10⁶ = N/60
N = 20 x 10⁶ x 60
N = 1.2 x 10⁸ nuclei will decay in one minute

Question 13

7. Explain the term background radiation and describe two of its sources.

Answer 13

Background radiation is radiation which is always present but which is not due to the deliberate introduction of radiation sources.

Sources of background radiation
Natural
Radon Gas emitted by rocks in the ground
Cosmic Rays that reach the Earth from outer space
Soil, Rocks and Building Materials
All animals (including ourselves) emit natural levels of radiation
Our food and drink can contain natural levels of radiation
Artificial
Medical uses of radiation such as scans and treatments.

Question 14

8. Describe the dangers of ionising radiation to living cells and the need to
   measure exposure to radiation.

Answer 14

Since ionisation changes the chemical properties of materials, ionising radiation is dangerous to living cells which use chemical interactions to function. It can change their nature (e.g. make them cancerous) or kill them completely. Therefore, it is important that we can accurately measure exposure to radiation to minimise the risks.

Question 15

9. Define absorbed dose, radiation weighting factor and equivalent dose.

Answer 15

Absorbed dose, D, is defined as the energy absorbed per unit mass of the absorbing material.

Radiation Weighting Factor (w_R) is a number which indicates the biological effect of a particular type of radiation.

Equivalent Dose, H, is a measure of the potential harm that could be caused by a particular exposure to radiation

Question 16

10. D = E/m

(Define symbols and units)

Answer 16

D - Absorbed Dose (Gy - Grays)

E - Energy absorbed (J)

m - mass (kg)

Question 17

10. H = Dw_R

(Define symbols and units)

Answer 17

H - Equivalent Dose (Sv - Sieverts)

D - Absorbed Dose (Gy - Grays)

w_R - Radiation Weighting Factor (no units)

Question 18

10. Example

A tumour of mass 0.2 kg absorbs 20 µJ of energy during treatment. What is the absorbed dose?

Answer 18

m = 0.2 kg
E = 20 µJ = 20 x 10<sup>-6</sup> J
D = ?

D = E/m
D = 20 x 10<sup>-6</sup>/0.2
D = 1 x 10<sup>-4</sup> Gy

Question 19

10. Example

An exposure to alpha radiation results in an equivalent dose of 2 mSv. Calculate the absorbed dose.

Answer 19

H = 2 mSv = 2 x 10<sup>-3</sup> Sv
w<sub>R</sub> = 20 (from data sheet)
D = ?

H = Dw_R
2 x 10^-3 = D x 20
D = 2 x 10^-3/20
D = 1 x 10^-4 Gy

Question 20

What factors can affect the equivalent dose from an exposure to radiation?

Answer 20

1. The absorbed dose
2. The type of radiation involved
3. The part of the body affected

Question 21

What are the main ways of reducing the risk from a radioactive source?

Answer 21

1. Shielding - Place absorbing material between the person and the source.
2. Distance - Move the person and the source further apart.
3. Time - Decrease the time the person is exposed to the source.

Question 22

11. Define equivalent dose rate

Answer 22

Equivalent dose rate is the equivalent dose per unit of time.

(Equivalent dose rate can be quoted in a variety of units sieverts/millisieverts/microsieverts per second/minute/hour/year. Make sure that the units you use in any problem are consistent.)

Question 23

12. Ḣ = H/t

(Define symbols and units)

Answer 23

Ḣ - Equivalent Dose Rate (Sv/s) or (mSv/yr) or …

H - Equivalent Dose (Sv) or (mSv) or …

t - time (s) or (yr) or ….

Question 24

12. Example

Calculate the equivalent dose rate for an exposure of 2 mSv delivered over 8 hours.

Answer 24

H = 2 mSv
t = 8 h
Ḣ = ?

Ḣ = H/t
Ḣ = 2/8
Ḣ = 0.25 mSv/h

Question 25

13. Compare the equivalent dose rates due to a variety of natural and artificial sources.

Answer 25

Worldwide average values are given below. You are not expected to memorise these but have a rough idea of their order.

Natural
Radon Gas 1.2 mSv / yr
Rocks and Soils 0.5 mSv / yr
Cosmic Rays 0.4 mSv / yr
Food and Drink 0.3 mSv / yr
Artificial
Medical Uses 0.3 mSv / yr
Fallout from Nuclear Testing/Accidents 0.0072 mSv / yr

Question 26

14. State values for the
    a. Average annual background radiation in the UK.
    b. Annual effective dose limit for member of the public.
    c. Annual effective dose limit for radiation worker.

Answer 26

a. Average annual background radiation in the UK. 2.7 mSv / yr
b. Annual effective dose limit for member of the public. 1 mSv / yr (in addition to background)
c. Annual effective dose limit for radiation worker. 20 mSv / yr (in addition to background)

Question 27

15. Describe applications of nuclear radiation:

Radiotherapy

Answer 27

High doses can be used to treat malignant diseases (e.g cancer).
• outside the body (external radiotherapy) - Gamma rays or X-rays are fired at the tumour from many different angles so that the tumour receives a higher dose than the surrounding healthy tissue. (See Image)
• within the body (internal radiotherapy) - either by inserting radioactive material into, or close to, the tumour or by injecting or drinking a liquid that is absorbed by the cancerous cells - for example, radioiodine for thyroid cancer.

Question 28

15. Describe applications of nuclear radiation:

Tracers

Answer 28

This involves the introduction of radioactive sources in liquid form known as a tracer inside a patient. A gamma camera outside the body can then be used to detect the path of these tracers to observe how the body is functioning over time. (e.g. blood flow or the digestive system).

Gamma sources are needed so that the radiation is penetrating enough to make it out of the body without being absorbed. The sources will have a short lifetime to avoid exposure to radiation for longer than necessary

Question 29

15. Describe applications of nuclear radiation:

Sterilisation

Answer 29

Radiation is also used to sterilise hospital equipment, especially plastic syringes that would be damaged if heated. The material is sealed before it is irradiated with Gamma rays so that it remains sterile.

Question 30

15. Describe applications of nuclear radiation:

Industrial Quality Control

Answer 30

In industry, radiation is used in quality control of materials, monitoring the thickness of paper, for example. A radioactive source and detector is used. When the material between the radioactive source and the detector changes thickness, the level of radiation detected also changes. If the level of radiation decreases, this would indicate the paper is thicker and the process can be controlled to make it thinner again. If the level of radiation increases, this would indicate the paper is thinner and the process can be controlled to make it thicker again.

Question 31

16. Define the meaning of the term ‘half-life’

Answer 31

Half life is the time taken for the activity of a radioactive source to half.

Question 32

17. Example

The activity of a source falls from 80 MBq to 5 MBq in 8 days. Calculate its half-life.

Answer 32

80 MBq → 40 MBq → 20 MBq → 10 MBq → 5 MBq
1 2 3 4

4 Half Lives = 8 Days

1 Half Life = 8/4 = 2 Days

Question 33

17. Example

The half life of a source is 2 hours. It is prepared in a hospital at 8 am with an activity of 800 MBq. Calculate its activity at 6 pm.

Answer 33

8 am to 6 pm = 10 hours.

This is 10h/2h = 5 half lives.

800 MBq → 400 MBq → 200 MBq → 100 MBq → 50 MBq → 25 MBq
1 2 3 4 5

The activity at 6 pm will be 25 MBq

Question 34

17. Example

Calculate the half life from the following graph of corrected activity

Answer 34

Starting Point A = 480 Bq at t = 0
480/2 = 240
A = 240 Bq at t = 12 days
Half Life is 12 days

NB you can use other staring points which make reading the graph easier e.g.
Starting Point A = 400 Bq at t = 3 days
400/2 = 20
A = 200 Bq at t = 15 days
Half Life is 15 - 3 = 12 days

Question 35

18. Describe the principles of a method for measuring the half-life of a
    radioactive source.

Answer 35

First of all the background count rate is measured with no source present, using a Geiger Muller tube connected to a counter. The count rate from the source is then measured at regular fixed intervals over a period of time. Make sure the source is close enough to measure alpha radiation if relevant. The background count rate is subtracted from each measurement to get the corrected count rate of the source. A graph of the corrected count rate of the source against time is plotted. From the graph, the time taken for the corrected count rate to fall by half is measured. A number of measurements are made and an average value is calculated. The average value is the half-life of the radioactive source.

Question 36

19. Describe qualitatively what is meant by fission, including the difference between stimulated and spontaneous fission.

Answer 36

Nuclear fission is the process in which a nucleus splits into more than one lighter nuclei.

For example, when a neutron is fired into the nucleus of a uranium 235 atom, the atom will split into two new nuclei emitting further neutrons and releasing energy in the form of heat.

This is know as stimulated fission. Some heavy nuclei are not stable and undergo spontaneous fission to produce lighter nuclei without the need for a neutron to be fired into it.

Question 37

20. Describe what is meant by a fission chain reaction.

Answer 37

The neutrons produced by a fission reaction can go on to cause further fission reactions, producing more neutrons, which can cause further fission reactions, and so on. This is known as a chain reaction.

Question 38

21. Describe qualitatively how a fission reactor can be used to generate electricity.

Answer 38

In a nuclear reactor, the chain reaction is controlled by control rods. These absorb neutrons, allowing the reactor to produce energy at a steady rate. This steady production of heat energy is used to boil water. The steam produced drives turbines which turn the generators to produce electricity.

Question 39

22. Describe qualitatively what is meant by fusion.

Answer 39

Nuclear fusion is the process in which the nuclei of light elements combine, or fuse together, to give heavier nuclei.

Question 40

23. Describe what is meant by plasma containment.

Answer 40

The biggest issue faced by Fusion reactors is the extreme temperatures required for the fusion reactions to take place. In order for the nuclei to combine they have to overcome the natural repulsion of two positively charged objects. This can only occur when they are moving fast i.e. at extremely high temperatures. At these temperatures gases become a charged plasma. This plasma is hotter than the melting point of metals so we need to find a way to suspend the plasma so that it is not in contact with the walls. This is known as Plasma Containment.

Two possible ways of achieving Plasma Containment are:

A TOKAMAK device uses magnetic fields to contain the charged particles of the plasma in a doughnut shaped ring inside a vacuum chamber.

Inertial Confinement uses small fuel pellets. Intensely powerful LASERs are focused on the pellets, starting a fusion reaction but also holding the products in place.

Question 41

24. Describe qualitatively how a fusion reactor could be used to generate electricity.

Answer 41

Fusion reactors are still at the experimental stage and have yet to be introduced on a commercial basis. In theory, fusion reactions can produce heat which can be used to boil water. The steam produced drives turbines which turn the generators to produce electricity.