What is Radiation? The breaking down of unstable atomic nuclei As the nucleus of an unstable atom breaks down, it gives out rays and particles called emissions. The breaking down of unstable nuclei happens spontaneously – it is unaffected by heat, pressure, or whether the element is solid, liquid or gas.

What is Radiation? Isotopes (atoms of the same element with differing numbers of neutrons) whose nuclei break down at random are referred to as radioactive. They are also known as radioisotopes. There are three different types of atomic radiation – alpha (α), beta (β), or gamma (γ).

Atomic Structure The Nucleus – This is the centre of the atom. It contains protons and neutrons. The mass of the atom is concentrated in the nucleus. Protons – These have a positive charge and a mass of 1. Neutrons – These don’t have a charge and have a mass of 1. Electrons – These move around the nucleus. They have virtually no mass and have a negative charge.

Alpha Radiation 238U  234Th + 4He 92 90 2 2 protons and 2 neutrons (= helium nucleus) ejected from the nucleus Positive charge of +2 Very high ionising power – this means it collides with lots of atoms and knocks electrons off them, making them ions Short range in air – a few centimetres Stopped by a piece of paper. 238U  234Th + 4He 92 90 2

Beta Radiation An neutron that breaks down into a proton and an electron The electron is ejected from the nucleus; the atomic number of the atom changes because there is an extra proton in the nucleus Negative charge of –1 Low ionising power Stopped by a piece of aluminium foil Travels several metres in air 14C  14N + 0e 6 7 -1

Gamma Radiation Not made of protons or electrons A high-energy electromagnetic wave Emitted from nuclei changing from a high energy level to a lower one Frequently accompanies α and β emissions No charge, so very low ionisation power The most penetrating atomic radiation – can travel huge distance through air Stopped by several feet of lead

Important Experiments Wilhelm Conrad Roentgen(1845-1923) – On November 8th 1895 Roentgen discovered X-rays, a momentous event that instantly revolutionized the field of physics and medicine. He determined that a glowing fluorescent screen on a nearby table was caused by invisible rays originating from the partially evacuated glass Hittorf-Crookes tube he was using to study cathode rays. These rays penetrated the opaque black paper wrapped around the tube. For this discovery, Roentgen received the Nobel prize for physics in 1901.

Important Experiments Pierre Curie(1859-1906) Marie Curie(1867-1934) – In 1895, Marie and Pierre Curie were married and worked together on their research. The term ‘radioactivity’ was first coined by Marie. The Curies experimented with the chemical extraction of uranium from the ore. The conclusion was that the ore contained, in addition to uranium, new elements that were also radioactive. This led to their discoveries of the elements of polonium and radium. For their work the Curies were awarded the Nobel prize in physics and years later, Marie was awarded the Nobel prize in chemistry.

Important Experiments Antoine Henri Becquerel( 1852-1908) – Initially Becquerel believed that the sun’s energy was being absorbed by the uranium which then emitted X-rays. He found that uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity, the spontaneous emission of radiation by a material. For this discovery he was awarded the 1903 Nobel prize for physics.

Half-Life The time taken for the number of atoms in a sample of an element to decay by half Half-life is fixed – no matter how big the sample, what the temperature or pressure is, it is always the same length of time. A sample of a radioisotope will never completely disappear… …its radioactivity always disappears by half, even in the tiniest amounts.

Some Half-Life Examples This is an example of a half-life graph for Americium-242 It can be used to find the half-life of a radioisotope The heaviest naturally-occurring radioisotope is Uranium, which has a half-life of 4.5 x 109 years The more unstable a radioactive isotope is, the shorter its half-life

Dangers of Radiation The main danger from radioactivity is the damage it does to the cells in your body. Most of this damage is due to ionisation when the radiation passes. If levels of radiation are high there can be damage due to heating effects as your body absorbs the energy from the radiation, rather like heating food in a microwave oven. This is particularly true of gamma rays.

Dangers of Radiation Alpha Particles – These are slow and have a short range in air. These are the most dangerous type of particle and can turn cells cancerous. Beta Particles – These have a longer range than alpha particles, but ionise much less strongly. They have more penetrating power which means they can get through your skin and affect the cells inside you. Gamma Rays – Gamma rays hardly ionise at all, so do not cause damage directly in this way. They are very difficult to stop and when they are absorbed by an atom they can gain quite a bit of energy, and may then emit other particles.

Uses of Radioactivity - Alpha Smoke detectors: The contain a small amount of Americium-241, which emits α radiation. This ionises the air so a current flows. When smoke enters the detector it absorbs the α radiation and the circuit is broken. An alarm sounds.

Uses of Radioactivity - Beta Thickness testing: A radioactive source emitting β emissions, and a Geiger counter, are placed either side of the paper. The amount of β radiation reaching the counter through the paper is measured If too little or too much radiation gets through, the machine is automatically adjusted to make the paper thinner or thicker

Uses of Radioactivity - Gamma Tracers: Medical purposes-used to follow the route of substances through the body e.g. To detect a blocked kidney Civil purposes-used to detect leaks in pipes by putting γ source into pipe & measuring emissions using a Geiger counter

Uses of Radioactivity - Gamma Radiotherapy: Cancer cells are exposed to gamma rays which kills them off Makes the patient feel unwell Correct dose vital – too much can kill healthy cells, too little won’t prevent spread of cancer Sterilisation: Civil use – γ radiation used to kill bacteria on some foods. Prolongs shelf-life but may change the taste. Medical use – sterilises medical equipment that would be damaged by heat e.g. Syringes Welding Gamma emissions passed through metal onto photographic film, to check for bubbles

Uses of Radioactivity – Half-life Carbon-14 Dating: All living things contain a fixed proportion of radioisotope Carbon-14 (14C) When animals and plants die, the proportion of 14C starts to fall, because decaying 14C is no longer being replaced by 14C being taken in e.g. food 14C also has a known half-life, of 5700 years Scientists can work out the age of ancient organic substances e.g.. bones, by comparing the amount of 14C left to the proportion in living organisms, and using half-life