1.5 Atomic and Nuclear Physics ☢️

Alpha particle (α)

Helium nucleus of two protons/ neutrons

• strong ionising power

• weak penetrating power (paper/ skin)

• range of 5cm

Beta particle (β)

neutron splits into proton and high speed electron which is ejected from nucleus

• moderate ionising power

• moderate penetrating power (few mm aluminium)

• range of 1m

Gamma ray (γ)

Electromagnetic wave used to ‘relax’ an unstable nucleus

• weak ionising power

• strong penetrating power stopped by several cm lead or 1m concrete

• range of 1km

Ionising power

ability to ionise materials by displacing electrons, greater charge will ionise more

Penetrating power

ability to pass through matter, greater mass will penetrate less

Range (in air)

How far radiation travels in air before absorbed

Nuclear reactor

equipment in nuclear power station which fission (or fusion) takes place

How does nuclear reactor work

neutrons are absorbed by control rods, heating water which can be transferred into electricity

Nuclear fuel for fission

uranium/ plutonium isotopes obtained from mining ore

Nuclear fuel for fusion

hydrogen/ deuterium/ tritium found in oceans and seas

Unstable nuclei

has too few or too many neutrons

Radioactive decay

random process where an unstable nucleus emits radiation to become more stable

Types of nuclear radiation

Alpha particle, beta particle, gamma ray, neutron

Geiger-Muller (GM) tube

detects ionising radiation and measures activity of radioactive source

activity

rate which a source of unstable nuclei decays

unit for activity

Becquerels (Bq)

count-rate

number of decays recorded each second by a detector

Half life

the time it takes for half the atoms in a substance to decay

Nuclear fusion

joining of two light nuclei to form a heavier nucleus, releasing energy (gamma rays)

Spontaneous fusion

Occurs in stars when two hydrogen nuclei fuse to form helium nucleus

Conditions required for fusion

Very high temperatures and pressure to overcome repulsion

Fusion in the ITER project

1. deuterium and tritium are fired into plasma to overcome repulsion and fuses them together

2. releases four times the energy produced during conventional nuclear fission

Mass-energy conversion

extra mass of fusing particles is converted into energy

repulsion

two nuclei repel each other as they are both positive

Advantages of fusion

1. solve world’s energy needs

2. hydrogen/ deuterium are widely available and relatively cheap (seas/oceans)

3. does not emit any greenhouse gases or have radioactive waste

4. four million times more energy per kg than burning fossil fuels

Disadvantages of fusion

1. Temperatures approaching the the sun are required which is very difficult to reach/ contain

2. may take 50 years before it becomes commericially available

3. expensive to build and operate because of technology and conditions required

Nuclear fission

splitting of large and unstable nucleus, after absorbing slow moving neutron, producing two lighter nuclei which releases more neutrons and energy (gamma rays)

Fissile material

nuclei can be split by fission
e.g. uranium and plutonium

Spontaneous fission

spontaneous splitting of large and unstable nucleus (very rare)

Energy of fission products

products have kinetic energy so will move away

Chain reaction

neutrons released from nucleus cause splitting of further nuclei

Controlled chain reactions

rate of reaction limited to prevent getting out of control e.g nuclear reactors

Uncontrolled chain reaction

not limited and eventually leads to an explosion e.g atomic bomb

Advantages of fission

1. does not release the greenhouse gas carbon dioxide

2. releases one million times more energy per kg than burning fossil fuels

3. Modern reactor designs are extremely safe and create employment opportunities

Disadvantages of fission

1. incidents (Chernobyl) have caused huge damage to area surroundings

2. mining, transport and purification of fuels release lots of greenhouse gases and are non-renewable

3. radioactive waste is extremely dangerous and long lasting with fears of leaking out when stored

4. decommissioning is extremely expensive

Irradiation

object is exposed to radiation but doesn’t become radioactive

Sterilisation

using radiation to ensure a sample contains no living things

Using irradiation to sterilise food

gamma rays destroy any bacteria but do not affect the fruit itself, useful when not able to refrigerate

Using irradiation for medical purposes

sterilise surgical instruments without high temperatures and kill cancerous tumours using gamma rays (radiotherapy)

Radioactive contamination

unwanted radioactive atoms are mixed with other materials and they become contaminated

Why use tracers

find out what’s happening in an object without needing to break in

Using contamination for medical tracers

isotope is injected into body, then later passes out, where it is detected and used to form an image

Factors to consider when using medical tracers

• gamma source so it can pass through body and be detected

• short half life so no radioactive material is left

Using radiation for thickness control

• emitter is placed on one side of a sheet and a detector on the other

• if there is a change in thickness, the activity increases or decreases

factors to consider for thickness control

• beta source so it can penetrate and vary in activity

• long half-life so count rate remains constant and not often replaced

Using contamination to check for leaks in water pipes

• radioactive isotope is added to water and cracks cause contaminated water to leak

• leaks have a build up of radiation which can be detected

Factors to consider when checking for leaks

• gamma source so it can penetrate ground

• short half life so damaging effects don’t last long as water is poisonous

Using radiation for smoke detectors

• particles pass between two charged metal plates, and ionise air, creating a current

• if smoke enters particles are absorbed causing smaller currents to flow and alarm sounds

Factors to consider when using smoke detectors

• alpha source so it ionises in air and doesn‘t penetrate far

• long half-life so count rate remains constant and not often replaced

Dangers of irradiation and contamination

damages living cells and causes mutations
e.g radiation sickness or cancer

How to reduce irradiation

block with suitable shielding or as soon as source is removed

Why it is difficult to reduce contamination

radiation can’t be blocked and it is very difficult to remove it all

Precautions to take when using radioactive sources

• sources kept in lead-lined box

• wear protective clothing

• avoid contact with bare skin

• limit exposure time

• use tongs to handle sources

• monitor exposure

how can radiation be dangerous

• causes ionisation, damaging genetic material which can make cells cancerous

• large amounts of energy can damage or completely destroy cells

Background radiation

radiation around us all the time, even when no source is present e.g rocks or cosmic rays

Radiation in food chain

gas, living things and plants may absorb and pass it along

Human behaviour effect on background radiation

medical X-rays, radioactive waste power plants and radioactive fallout from nuclear weapons testing

Ways to natural background radiation

high levels of radon gas require homes to be well ventilated to remove it

Why activity is unsuitable to measure radiation exposure

activity could be the same, but different decay, so would have a different effect

Why alpha radiation is more dangerous inside the body

more ionising and won’t penetrate skin so can’t escape from the body

Why beta radiation is more dangerous outside the body

less ionising but will penetrate the skin so can pass into the body

Atom

Building block of matter

JJ Thomson

Discovered the electron and plum pudding model

Plum Pudding model (dough)

sphere of positive charge, with negatively charged electrons in it

Alpha particle scattering experiment

directed beam of alpha particles at very thin gold foil suspended in a vacuum, tiny flash of light is emitted when it hits the screen

Observations from experiment

most alpha particles passed straight through foil but small number were deflected by large angles or straight back

Conclusions from experiment

mass of atom is concentrated at the centre (nucleus) that had a positive charge

Nuclear model

atom is mostly empty space with positively charged centre containing most the mass and electrons orbiting

Ernst Rutherford

Tested plum pudding model to create nuclear model

Niels Bohr

Adapted nuclear model, suggesting electrons orbit at specific distances

James Chadwick

discovered the neutron

What happens to electrons when an atom absorbs energy

jump to higher levels (larger shells)

When electrons in an atom move to a lower energy level

atom emits energy as frequency of light

Structure of an atom

positively charged nucleus surrounded by electrons

Proton

positively charged particle found in the nucleus which defines the atom

+1, 1

Neutron

neutral particle found in the nucleus

0, 1

Electron, charge and mass

negatively charged particle found orbiting the nucleus in shells

-1, 1/1840

Ion

atom that has lost or gained electrons

Reason why atoms have no overall charge

number of electrons is equal to the number of protons so charges cancel out

Atomic number (Z)

number of protons in the nucleus

Mass number (A)

total number of protons and neutrons in the nucleus

Symbol (X)

represents what element it is

Isotope

atoms of same element with same number protons but different number of neutrons