Radioactivity
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14.1 Types, Properties and Uses of Radioactivity
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Some substances give
out radiation all the time, whatever is done to them. These substances are
said to be radioactive. |
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There are three
types of radiation emitted by radioactive sources: |
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* alpha (*)
radiation - which is easily absorbed by a few centimetres of air or a thin
sheet of paper; |
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* beta (*) radiation
- which easily passes through air or paper but is mostly absorbed by a few
millimetres of metal; |
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* gamma (*)
radiation - which is very penetrating and requires many centimetres of
lead or metres of concrete to absorb most of it. |
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There are radioactive
substances all around us, including in the ground, in the air, in building
materials and in food. Radiation also reaches us from space. The radiation
from all these sources is called background radiation. |
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When radiation from
radioactive materials collides with neutral atoms or molecules these may
become charged (ionised). |
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When radiation ionises
molecules in living cells it can cause damage, including cancer. The
larger the dose of radiation the greater the risk of cancer. |
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Higher doses of
radiation can kill cells; they are used to kill cancer cells and harmful
micro-organisms. |
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As radiation passes
through a material it can be absorbed. The greater the thickness of a
material the greater the absorption. The absorption of radiation can be
used to monitor/control the thickness of materials. |
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When sources of
radiation are outside the body:* beta and gamma radiation are the most
dangerous because they can reach the cells of organs and may be absorbed
by them; |
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* alpha radiation is
least dangerous because it is unlikely to reach living cells. |
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When sources of
radiation are inside the body: |
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* alpha radiation is
the most dangerous because it is so strongly absorbed by cells; |
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* beta and gamma
radiation are less dangerous because cells are less likely to absorb the
radiation. |
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The half-life of a
radioactive substance: |
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* is the time it
takes for the number of parent atoms in a sample to halve. |
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* is the time it
takes for the count rate from the original substance to fall to half its
initial level. |
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Candidates should be
able to evaluate the appropriateness of radioactive sources for particular
uses, including as tracers, in terms of |
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* the types of
radiation emitted; |
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*
their half-lives. |
14.2 Atomic Structure and Nuclear Fission
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Radioactivity occurs as
a result of changes in the nuclei of atoms (nuclear changes). |
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Atoms have a small
central nucleus made up of protons and neutrons around which there are
electrons. |
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Candidates should be
able, when provided with appropriate information, to: |
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* describe the
"plum pudding" model of the atom which this model replaced; |
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* explain how the
scattering experiment by Rutherford's team provided evidence for their
model. |
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The relative masses of
protons, neutrons and electrons and their relative electric charges are as
shown: |
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|
Mass |
Charge |
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Proton
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1 |
+1 |
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Neutron
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1
|
0 |
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Electron
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negligible |
-1 |
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In an atom, the number
of electrons is equal to the number of protons in the nucleus. The atom as
a whole has no electrical charge. |
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All atoms of a
particular element have the same number of protons. Atoms of different
elements have different numbers of protons. The total number of protons
and neutrons (nucleons) in an atom is called its mass (nucleon) number. |
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Atoms of the same
element which have different numbers of neutrons are called isotopes. |
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Radioactive isotopes
are atoms with unstable nuclei. When an unstable nucleus splits up
(disintegrates): |
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* it emits
radiation; |
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* a different atom,
with a different number of protons, is formed. |
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The older a
radioactive material, the less radiation it emits. This idea can be used
to date materials. |
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Any isotope of an
element which is radioactive is called a radioisotope (radionuclide). |
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Alpha radiation
consists of helium nuclei, particles made up of two protons and two
neutrons. |
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Beta radiation
consists of electrons emitted from the nuclei of atoms. |
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For each electron
emitted, a neutron in the nucleus becomes a proton. |
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Gamma radiation is
very short wavelength electromagnetic radiation. |
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Nuclear reactors use
a process called nuclear fission. When an atom with a very large nucleus
is bombarded with neutrons: |
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* the nucleus splits
into two smaller nuclei; |
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* further neutrons
are released which may cause further nuclear fission resulting in a chain
reaction; |
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* the new atoms
which are formed are themselves radioactive. |
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[Details of nuclear
reactors are not required.] |
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The
energy released by an atom during
radioactive disintegration or
nuclear fission
is very
large compared to the energy released when a chemical bond is made
between two atoms. This idea can be used to date materials. |
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Uranium isotopes,
which have a very long half-life, decay via a series of relatively
short-lived radioisotopes to produce stable isotopes of lead. The relative
proportions of uranium and lead isotopes in a sample of igneous rock can,
therefore, be used to date the rock. |
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The proportions of
the radioisotope potassium-40 and its stable decay product argon can also
be used to date igneous rocks from which the gaseous argon has been unable
to escape. |
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Candidates should be
able to make such calculations when provided with appropriate data. |