Transcript Radioactivity - Lockland Schools

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
• 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
92

234Th
90
+ 4He
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
6
 14N + 0e
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 halflife graph for Americium242
• It can be used to find the
half-life of a radioisotope
• The heaviest naturallyoccurring radioisotope is
Uranium, which has a halflife 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