P2.5 What Happens When Radioactive
Substances Decay, And The Uses And
Dangers Of Their Emissions
1 P2.5.1 Atomic Structure
1.1 The basic structure of an atom is a small
central nucleus composed of protons and
neutrons surrounded by electrons. The
number of electrons is equal to the number
of protons in the nucleus. The atom has no
overall electrical charge.
1.1.1 Atoms may
lose or gain
184.108.40.206 The atoms of an element always have
the same number of protons, but have a
different number of neutrons for each
isotope. The total number of protons in an
atom is called its atomic number. The total
number of protons and neutrons in an
atom is called its mass number.
2 P2.5.2 Atoms And Radiation
2.1 Some substances give out radiation from the
nuclei of their atoms all the time, whatever is
done to them; they are radioactive.
2.1.1 Background radiation is all around us. Some of it comes from natural sources and some comes from artificial sources. Natural background
radiation includes: cosmic rays - radiation that reaches the Earth from space; rocks and soil - some rocks are radioactive and give off
radioactive radon gas; living things - plants absorb radioactive materials from the soil and these pass up the food chain. For most people,
natural sources contribute the most to their background radiation dose. However, human activity has added to background radiation by
creating and using artificial sources of radiation. These include radioactive waste from nuclear power stations, radioactive fallout from
nuclear weapons testing and medical x-rays. Artificial sources account for about 15 per cent of the average background radiation dose.
220.127.116.11 Photographic film goes darker when it absorbs radiation. The more radiation the film absorbs, the darker it is
when it is developed. The Geiger-Muller tube detects radiation. Each time it absorbs radiation, it transmits an
electrical pulse to a counting machine. This makes a clicking sound or displays the count rate.
18.104.22.168.1 There are three main types of radiation emitted from
radioactive atoms. These are alpha, beta and gamma
radiation. An alpha particle is identical to the nucleus of
a helium atom, which comprises two protons and two
neutrons. Beta radiation consists of high energy
electrons emitted from the nucleus. These electrons
have not come from the electron shells or energy levels
around the nucleus. Instead, they form when a neutron
splits into a proton and an electron. The electron then
shoots out of the nucleus at high speed. Gamma
radiation is very short wavelength - high frequency -
electromagnetic radiation. This is similar to other types
of electromagnetic radiation such as visible light and
x-rays, which can travel long distances.
22.214.171.124.1.1 Alpha radiation is the least penetrating. It can be stopped - or absorbed - by
just a sheet of paper. Beta radiation can penetrate air and paper. It can be
stopped by a thin sheet of aluminium. Gamma radiation is the most penetrating.
Even small levels can penetrate air, paper or thin metal. Higher levels can only
be stopped by many centimetres of lead or many metres of concrete.
126.96.36.199.1.1.1 Alpha particles are positively charged, beta particles are
negatively charged and gamma radiation is electrically
neutral. This means that alpha radiation and beta radiation
can be deflected by magnetic and electric fields, but
gamma radiation is not deflected. Beta particles are
negatively charged so they will be attracted towards a
positively charged plate. Positive alpha particles will be
attracted towards a negatively charged plate.
188.8.131.52.184.108.40.206 When radiation collides with molecules in living cells it can damage them. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. The cell
then divides rapidly and causes serious health problems. The greater the dose of radiation, the greater the chance the cell will become cancerous. However,
very high doses of radiation can kill the cell completely. We use this property of radiation to kill cancer cells, and also harmful bacteria and other micro-organisms.
220.127.116.11.18.104.22.168.1 If the radioactive source is inside the body, perhaps after being swallowed or breathed in alpha
radiation is the most dangerous because it is easily absorbed by cells and beta and gamma radiation
are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass
right through it. If the radioactive source is outside the body alpha radiation is not as dangerous
because it is unlikely to reach living cells inside the body and beta and gamma radiation are the most
dangerous sources because they can penetrate the skin and damage the cells inside.
22.214.171.124.126.96.36.199.1.1 The nuclei of radioactive atoms are unstable. They
break down and change into a completely different
type of atom. This is called radioactive decay.
188.8.131.52.184.108.40.206.1.1.1 A half-life is the time it takes for the number of nuclei of the
isotope in a sample to halve. Different radioactive isotopes
have different half-lives. The half-life of carbon-14 is 5,715
years, but the half-life of francium-223 is just 20 minutes.
220.127.116.11.18.104.22.168.22.214.171.124 Two protons and two neutrons are lost from a nucleus when it
emits an alpha particle. The atomic mass number decreases by 4.
The atomic number decreases by 2. A new element is formed that is
two places lower in the Periodic Table than the original element.
126.96.36.199.188.8.131.52.184.108.40.206.1 In beta decay, a neutron changes into a proton plus an electron. The
proton stays in the nucleus. The electron leaves the atom with high
energy as a beta particle. The nucleus has one more proton and one less
neutron when it emits a beta particle. The atomic mass number stays the
same. The atomic number increases by 1. A new element is formed that
is one place higher in the Periodic Table than the original table.