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, a greater charge of radiation will ionise more

Penetrating power

ability to pass through matter, a greater mass of radiation 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

Nuclear fuel

uranium/ plutonium isotopes that undergo nuclear fission

Chain reaction

When the neutrons released from a nucleus go on to cause the splitting of further nuclei

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

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 reaction

rate of reaction is limited to prevent it from 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

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 materials and leading to cancer

• large amounts of energy can damage or completely destroy cells

Background radiation

Radiation that is around us all of the time e.g rocks, cosmic rays and nuclear fallout

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

J J Thomson

Discovered the electron and plum pudding model

Plum Pudding model

sphere of positive charge, with negatively charged electrons in it

Alpha particle scattering experiment

two students 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

Results 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

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