Radioactivity

5th Year Chemistry

Recap

Henri Bacquerel – uranium and photographic plate

Marie Curie – pitchblende, polonium and radium

Define Radioactivity?Spontaneous disintegration of certain

unstable nuclei accompanied by the emission of radiation

α-particles

Positively charged Consist of two

protons and two neutrons

The He nuclei Amercium-241 used

in smoke detectors is an α-emitter

Stopped by a sheet of paper

Β-particles

Formed by the disintegration of a neutron into a proton and electron.

Proton stays in nucleus – mass no. stays the same

Atomic number is increased by 1

Β-particles

Electrons emitted in a fast moving stream

Higher penetration than α-particles, 2-3mm of aluminium foil

Stopped by skin, harmful if they get inside the body

Carbon-14 used for age determination

γ-rays

Not a particle – radiationA Unstable nuclei can release photons of

high energy electromagnetic radiation called γ-rays

Fast moving and of the greatest penetrating power – stopped by thick lead or concrete

Cobalt 60 – cancer treatment and food irradiation

Chemical reaction Vs Nuclear reaction

Elements combine to form other compounds

Compounds break up into their constituent elements

Elements in compounds rearrange to form different compounds

Cause changes in the nucleus involving protons and neutrons

Cause elements to change into other elements

Chemical reactions

Nuclear Reaction

Radioisotopes

What is an isotope?Atoms of the same element, which differ

in the number of neutrons in the nucleusWhat is a radioisotope?Unstable radioactive isotopes

Half Life

The time taken for half of the radioactive atoms present to decay

Can vary from seconds to millions of years

E.g. Radium-214 has a half life of 20 minutes, Radium-226 has a half life of 1,620 years

Nuclear power waste – half life?

Half Life Graph

Background Radiation

Source of radiationSource of radiation PercentagePercentageRadon Gas (α-emitter) 60%

Rocks & Soil (γ-rays) 10%

Food & Drink 10%

Cosmic rays 10%

Medical e.g. Cobalt-60 9%

Weapons Tests 0.2%

Nuclear Waste <0.1%

Carbon Dating

Used to determine the age of carbon containing objects

When plant/animal is alive it will have C-12 and C-14 in ratio the same as air

When organism dies ratio changes with time due to decay of C-14.

Extent of the change allows age to be calculated

Cobalt- 60 Cancer Treatment

Gamma ray emitter Half life 5.23 years Radiotherapy – treatment of cancer with

radiation Cancerous cells more susceptible to damage

by ionising radiation than normal cells Ionising radiation is directed on to the tumour

from different directions Tumour dose is high but normal tissue

receives a much lower, less harmful dose

Cobalt - 60

Gamma rays kill micro-organisms in food

May also involve inhibiting sprouting, controlling ripening and pasteurising foods

Fears of genetic modification

Americium-241

Emitted α-particles ionise the air molecules, conduct an electric current between two terminals

Smoke clings to ionised air molecules and slows them down

Current decreases and a transistor switch activates the alarm

This type of alarm not used these days

Practice Nuclear reactions