P7 - Radioactivity

Question 1

What is nuclear fission?

Answer 1

The splitting of a large, unstable nucleus (uranium or plutonium) which requires a slow moving neutron to be absorbed.
Spontaneous fission (fission that occurs without a neutron being absorbed) is very rare.

Question 2

What are the products of a fission reaction?

Answer 2

• 2 daughter nuclei (roughly equal in size)
• 2-3 neutrons at high speed (called fission neutrons)
• Gamma rays

All fission products have kinetic energy.

Question 3

What is a chain reaction?

Answer 3

Neutrons released by a fission event that are travelling slow enough can be absorbed by another nucleus that then undergoes fission.
Neutrons released by each fission event can cause more fission events, maintaining the chain reaction.

Question 4

What is used as fuel in a nuclear reactor?

Answer 4

Rods of uranium that are rich in Uranium-235 (fissionable uranium isotope).

Question 5

What does the reactor core of a nuclear reactor contain?

Answer 5

Fuel rods (spaced out evenly), control rods and water at high pressure.

Question 6

What is the moderator needed for in a nuclear reactor?

Answer 6

Fuel rods are placed in a moderator (usually water) that slows down neutrons as high-speed neutrons do not cause further fission. Fission neutrons are slowed down by collisions with atoms in the water molecules.

Question 7

What are the two roles of water in a fission reactor?

Answer 7

Moderator - slows down fission neutrons
Coolant - water absorbs and removes heat created by fission reactions, kinectic energy increases as energy is transferred from fuel rods and neutrons. Water is pumped through the core to and from a heat exchanger where steam is produced to drive an electricity turbine.

Question 8

What are control rods needed for in a nuclear reactor?

Answer 8

Control rods in the reactor core absorb surplus neutrons to keep the chain reaction under control, the depth of rods in the core can be adjusted to maintain a steady chain reaction. Made of materials that absorb neutrons, such as boron

Question 9

What is the heat created by a nuclear reaction used for?

Answer 9

The heat is used to make steam that drives an electricity generating turbine.

Question 10

What is the reactor core in a nuclear reactor surrounded by?

Answer 10

Steel and concrete which prevents radiation from escaping and can withstand the high temperatures in the reactor.

Question 11

How can a fission reactor be shut down quickly in an emergency?

Answer 11

In an emergency, fission reactions can be shut down automatically by the release of control rods into the reactor. They can be immersed fully into the moderator, slowing down the reaction as quickly as possible.

Question 12

What is nuclear fusion?

Answer 12

The joining of 2 light nuclei to form a heavier nucleus

Question 13

What are the advantages and disadvantages of nuclear fusion?

Answer 13

Advantages - vast energy potential, clean fuel and products (helium is not radioactive), safe, fuel is readily available as heavy hydrogen is found naturally in seawater.
Disadvantages - Massive cost, technical difficulties - very high temperatures and pressures required to overcome repulsion of nuclei

Question 14

How does nuclear fusion work?

Answer 14

• 2 protons fuse together to create a heavy hydrogen nucleus (1x proton and 1x neutron).
• Two more protons collide separately with 2 heavy hydrogen nuclei and turn them into heavier nuclei
• The two heavier nuclei collide to form a helium nucleus

At each stage of fusion, the products have slightly less mass, the lost mass turns into pure energy.

Question 15

What is in a fusion reactor?

Answer 15

• Plasma is heated by passing a large electric current through it
• The plasma is contained by a magnetic field so that it doesn’t touch the reactor walls. If it did, it would go cold and fusion would stop

Question 16

How large are atoms and their nuclei?

Answer 16

Atoms are very small, with a radius of about 1 x 10^-10m
The radius of the nucleus is about 1/10000 of the radius of an atom

Question 17

What is ionisation?

Answer 17

The process of an atom losing or gaining an electron to become a charged ion.

Question 18

What are isotopes?

Answer 18

Forms of an element that have the same amount of protons (and electrons), but a different number of neutrons.

Question 19

What are the types of radiation emitted from the nucleus?

Answer 19

• alpha particles (𝛂)
• beta particles (𝜷)
• gamma rays (𝜸)
• neutrons (n)

Question 20

What is an alpha particle?

Answer 20

An alpha particle consists of 2 protons and 2 neutrons, it is the same as a helium nucleus.
Alpha particles have a relative charge of 2+
The atomic notation is:
4 4
𝛂 or He
2 2

Question 21

What is a beta particle?

Answer 21

Beta particles are high speed electrons
A neutron turns into a proton and an electron which is instantly emitted - n^0 → p+ + e-
The atomic notation is:
0 0
𝜷 or e
-1 -1

Question 22

What are gamma rays?

Answer 22

Gamma rays are electromagnetic radiation from the nucleus.
They are uncharged and have no mass.
The atomic notation is:
0
𝜸
0
Atomic mass of the nucleus that emits a gamma ray stays the same.

Question 23

What are nuclear equations used for

Answer 23

They are used to represent radioactive decay.

Question 24

Why do radioactive nuclei decay?

Answer 24

They are unstable due to having to much energy, weight or the wrong balance of protons to neutrons. Nuclei decay to become more stable.

Question 25

How has the model of the atom developed?

Answer 25

* 1804 (Dalton) - atoms are tiny spheres that cannot be broken down any further * 1897 (JJ Thompson) - **plum pudding model**, atom is a ball of positive charge with negative electrons embedded in it. * 1908 (Rutherford) - **Rutherford/nuclear model**, nucleus in centre of atom (concentrated mass and charge) surrounded by randomly moving electrons * 1913 (Niels Bohr) - **Bohr Model**, electrons orbit the nucleus at specific distances in shells/energy levels * Positive charge of any nucleus can be subdivided into a whole number of smaller particles (protons discovered) * 1932 (James Chadwick) - experimental work proved the existence of neutrons in the nucleus.

Question 26

What was Rutherford's experiment? What did it prove about atoms?

Answer 26

Rutherford fired a beam of alpha particles at a thin sheet of gold foil, most passed **straight through the gold foil** proving the atom is **mostly empty space**, but some (about 1 in 10,000) were deflected back violently (more than 90°) proving there is a dense positively charged nucleus where mass is concentrated in the centre of the atom.

Question 27

How is radiation detected? What is count-rate?

Answer 27

Radiation can be detected using an instrument called a **Geiger Counter**. **Count-rate** - the number of decays recorded each second by a detector (e.g. Geiger Counter)

Question 28

What is background radiation? What are the different sources?

Answer 28

**Background radiation** - low-level radiation that is around us all of the time **Different sources:** Radon Gas (50%), Rocks and Building Materials (15%), Medical testing (13%), Food (11%), Cosmic rays (10%) and other (including nuclear fallout) (1%)

Question 29

What are the different penetrating powers of alpha, beta and gamma radiation?

Answer 29

**Alpha** - low penetrating power (range of 1-2cm in air), stopped by paper **Beta** - medium penetrating power (range of 1m in air), stopped by 5mm aluminium sheet **Gamma** - high penetrating power (infinite range in air), stopped by thick lead sheet (several cms) or 1m of concrete

Question 30

What are the different ionising powers of alpha, beta and gamma radiation?

Answer 30

**Alpha** - high ionising power **Beta** - medium ionising power **Gamma** - low ionising power

Question 31

What do different doses of ionising radiation do to living cells?

Answer 31

**Low doses** of ionising radiation **damage cells** which can lead to the formation of a tumor. **High doses** of ionising radiation leads to **cellular death.**

Question 32

What is radiation dose measured in?

Answer 32

Sieverts - Sv

Question 33

How damaging are different types of radiation inside the body?

Answer 33

**Alpha radiation** is the most dangerous - **easily absorbed by cells** and has **high ionising power**. **Beta and gamma radiation** are less dangerous - **less likely to be absorbed** by a cell, will usually **pass right through**.

Question 34

How damaging are different types of radiation outside the body?

Answer 34

**Alpha radiation** is not as dangerous - **absorbed by the skin or air** so unlikely to reach living cells inside the body **Beta and gamma radiation** are the most dangerous - can penetrate skin and reach living cells inside the body

Question 35

What is radioactive contamination?

Answer 35

The unwanted presence of materials containing radioactive atoms on or in other materials

Question 36

What is irradiation?

Answer 36

The process of exposing an object to nuclear radiation * An irradiated object does not become radioactive. However, irradiation of human cells may cause cell damage or cell death

Question 37

What is the hazard from contamination?

Answer 37

The decay of the contaminating atoms, the type of radiation emitted affects the level of hazard.

Question 38

What is peer review?

Answer 38

Scientists publish their results for others to review and re-run their experiments.

Question 39

Why can we not predict when a radioactive nucleus will decay?

Answer 39

Because radioactive decay is a random process

Question 40

What is activity?

Answer 40

The rate at which a source of unstable nuclei decays, measured in becquerels (Bq). 1 Becquerel is 1 decay per second.

Question 41

What is a half-life?

Answer 41

The time it takes for the number of radioactive nuclei in a sample to half. OR The time it takes for the count-rate (or activity) from a sample containing the isotope to fall to half its initial level.

Question 42

What can irradiation be used for?

Answer 42

To kill bacteria on objects and to sterilise them

Question 43

What are radioisotopes used for in medicine?

Answer 43

They have many uses: **gamma knife**, **radiotherapy**, **medical tracers**, **gamma camera**

Question 44

What is nuclear radiation used for in medicine?

Answer 44

* The exploration of internal organs (tracers) * Control or destruction of unwanted tissue (e.g. tumors)

Question 45

What is usually used for radioactive tracers? Why?

Answer 45

**Gamma emitters** are usually used. This is because gamma rays are: * **Highly penetrating** - able to pass through the body and be detected * **Low ionising power** - harm to the patients cells is minimised Amount of isotope used is kept to a minimum to reduce exposure (low dose). Isotopes are chosen that have short half lives (a few hours) - long enough to carry out procedure, but will not cause long term exposure.