P7 - Radioactivity

Question 1

What is a radioactive substance?

Answer 1

a substance that contains unstable nuclei that emit radiation to become more stable, by the random process of radioactive decay

Question 2

What is meant by the term “random nature of nuclear decay”?

Answer 2

you can’t tell when the nucleus will decay

Question 3

Why is rolling dice and removing all those with a 6 a good model for radioactive decay?

Answer 3

-can’t predict which dice will “decay”
-can’t predict when a dice will “decay”

Question 4

What is the difference between activity and count rate?

Answer 4

-activity (Bq) is the rate at which a source of unstable nuclei decays

-count rate (Bq) is the rate of number of decays recorded by a detector (GM tube)

Question 5

What is absorbed dose?

Answer 5

a measure (in Sv) of the risk of harm resulting from an exposure of the body to radiation

Question 6

Describe the 4 types of radiation:

Answer 6

α - 2 neutrons + 2 protons (He nucleus)

β - neutron becomes proton and high speed e⁻ ejected from nucleus

γ - EM radiation from nucleus

n⁰ - just a neutron

Think of β as converting a neutral charge into +1 and -1, then ejecting the -1 so the nucleus is left as +1

Question 7

Compare the range in air of the 3 main types of radiation:

Answer 7

α - a few cm
β - a few metres
γ - infinite

Question 8

Compare the penetration power and the ionising power of the 3 main types of radiation:

Answer 8

α - stopped by paper/skin, highly ionising
β - thin aluminium, less ionising
γ - thick lead, weakest ionisation power

Question 9

What are nuclear equations for? How do you write one?

Answer 9

-used to represent radioactive decay

-mass number and atomic numbers must sum to the same thing on the “reactants” and the products side

Question 10

How does an element change if it emits the 3 types of radiation individually? Show them as a nuclear equation:

Answer 10

make the corresponding atomic/mass numbers add up to the original element’s atomic/mass number

Question 11

What is a half-life?

Answer 11

-time taken for the number of nuclei of a radioactive isotope in a sample to halve

-or for the count rate/activity from a sample of the isotope to fall to half its initial level

Question 12

What is radioactive contamination?

Answer 12

the unwanted presence of radioactive atoms on other materials

Question 13

Why might a physicist wear polythene gloves when handling radioactive material, even if they don’t prevent irradiation?

Answer 13

prevents contamination of their hands with radioactive material which would cause damage/irradiation over a longer time period

Question 14

Define irradiation:

Answer 14

the process of exposing an object to nuclear radiation

The irradiated object does NOT become radioactive.

Question 15

What is background radiation?

Answer 15

the radiation present in a location not due to the deliberate introduction of radioactive sources

Always subtract BGR from a GM tube reading

Question 16

Where can background radiation originate from?

Answer 16

-cosmic rays
-rocks
-nuclear fallout from bomb testing/accidents

Question 17

Give some medical uses of nuclear radiation:

Answer 17

-exploration of internal organs
-control/destruction of unwanted tissues (eg tumours)

Question 18

What are the properties to evaluate when choosing a radioactive source to use?

Answer 18

-the type of radiation it emits
-length of its half-life
-if it decays into a stable atom

Question 19

What is enriching?

Answer 19

removing most of the U-238 from a sample to get a higher proportion of the unstable isotope, U-235

Question 20

Describe the nuclear process of nuclear fission:

Answer 20

-splitting of large + unstable nucleus (U/Pu)

-unstable nucleus absorbs neutron to start fission reaction, splits into two smaller nuclei, emitting two or three neutrons + gamma

Question 21

How is nuclear fission used to generate energy?

Describe the processes happening inside the reactor core, and not the rest of the plant

Answer 21

-neutrons absorbed by large nuclei start fission chain reactions

-all fission products have kinetic energy which is converted to heat

Nuclear weapons use uncontrolled chain reactions

Question 22

How is the fission chain reaction controlled in the reactor core?

Answer 22

-moveable carbon control rods

-lowering them deeper into the core increases the number of neutrons absorbed
-this lowers the energy released from further fission reactions

Question 23

Draw a diagram explaining how nuclear fission causes a chain reaction:

[3]

Answer 23

must have:

-neutron absorbed by nucleus
-nucleus splitting in 2 and releasing 2/3 neutrons, where 1 is shown to be absorbed by another nucleus
-causes fission of this nucleus, and 2/3 more neutrons released from secondary fission reaction

Question 24

What is nuclear fusion? How does it release energy?

Answer 24

-joining 2 lighter nuclei to form a heavier nucleus
-the total mass of the particles DECREASES, as the mass may be converted into radiation energy

The lost mass is from the exchanging of gluons within the nucleons that releases energy, and the energy released is equal to the difference in mass between the old and new nuclei (by E=mc²)

Question 25

What type of nuclear radiation is deflected most in an electric field?

Answer 25

-beta
-even though it has less charge than alpha, it has far less mass

Question 26

Describe a use for alpha radiation:

Answer 26

-smoke detectors
-radioactive material emits alpha particles across a gap, and if smoke gets in the way, it triggers an alarm

Question 27

Explain why it is dangerous to be contaminated with a radioactive substance that emits alpha radiation:

Answer 27

-alpha radiation is highly ionising
-causes genetic mutations and increases the risk of cancer
-until the material is removed from the body