Transcript 11.6 Nuclear Radiation

Physics
(14 - 16)
Nuclear Radiation - 2
Types of Radiation
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Radiation
The nucleus of most atoms is stable, however there are three
causes of instability of a nucleus:
 A nucleus with more than 82 protons
 A nucleus with an atomic mass of more than 209
 An unstable ratio of neutrons to protons - this can be caused
by too many or too few neutrons in the nucleus
Radioactive decay is a process by which the nucleus of an atom
becomes more stable.
There are three types of radiation:
 Alpha ()
 Beta ()
 Gamma ()
Alpha Radiation ()
Alpha Radiation ()
Alpha radiation is formed when an unstable nucleus decays,
emitting an alpha particle.
The particle emitted is a helium nucleus (a helium atom minus its
two orbiting electrons) composed of two protons and two neutrons.
Alpha particles have an
atomic mass of 4 and a
relative charge of +2.
This can be written in either of two ways:
4
or
2
He
4

2
Alpha Radiation ()
Alpha particles have about 8000 times the mass of an electron.
Alpha radiation is strongly ionising as the large, slow moving
alpha particles are very likely to collide with atoms as they pass
through a substance.
(An ion is a charged atom that has fewer or more electrons than
normal, so that the positive charge from the protons in the nucleus
is not exactly cancelled out.)
As an alpha particle collides with atoms it loses some of its energy.
After many thousands of collisions the energy of the alpha particle
is low enough for electrons to attach themselves.
When two electrons have attached themselves to an alpha particle,
one atom of helium is created.
Alpha Radiation ()
Alpha particles are commonly emitted from heavy nuclei which
have a large number of protons and neutrons, e.g.
226
88 Ra
Radium - unstable metal
222
86 Rn
+
Radon - more stable gas
Checking the atomic mass figures...
Checking the atomic number figures...
4
2
He
Alpha particle
226 = 222 + 4
88 = 86 + 2
When a radioactive atom emits an alpha
particle, its mass number decreases by four
and its atomic number decreases by two.
Alpha Radiation ()
Sometimes, a decay reaction produces another radioisotope, which
itself decays...
Thorium becomes radium:
228
90 Th
224
88 Ra
+
4
2
He
Radium becomes radon...
224
88 Ra
220
86 Rn
+
4
2
He
Each individual step is known as an alpha decay.
The changing of one radioactive element progressively into others,
is called a decay series. The series stops when a stable nucleus is
formed.
Beta Radiation ()
Beta Radiation ()
Beta radiation consists of free, high energy electrons.
There are protons and neutrons in the nucleus of an atom, but no
electrons. So it may seem strange that beta particles are emitted
from the nuclei of atoms.
Beta particles are made when a neutron changes
into a proton and an electron. The proton stays in the
nucleus and the electron is emitted as a beta particle.
The stability of a nucleus is determined by the relative proportion
of protons and neutrons. Radioactive decay changes this relative
proportion, to make the nucleus more stable.
Beta Radiation ()
The mass of a beta particle is negligible, so when a beta particle is
emitted, it does not alter the atomic mass of the nucleus.
It does however change the charge of the nucleus, because a beta
particle carries a relative charge of -1.
Beta particles have an atomic mass
of 0 and a relative charge of -1.
This can be written in either of the two forms below:
0
e
-1

-1
0
Beta Radiation ()
Beta radiation is moderately ionising as beta particles are less
likely than alpha particles to collide with the atoms in a substance.
Beta radiation ionises by transferring energy to outer electrons
which then leave their orbits.
When a radioactive atom emits a beta particle, its mass number
remains unchanged and its atomic number increases by one (as a
neutron changes into a proton).
Example:
Carbon-14 becomes nitrogen-14.
14
C
6
14
7
N
This decay reaction is known as beta decay.
+
0
-1
e
Gamma Rays ()
Gamma Rays ()
Gamma rays are one of the seven types of electromagnetic wave
which form the electromagnetic spectrum.
Gamma rays have the highest frequency and shortest wavelength
of all the seven types of electromagnetic wave.
Wavelength
Gamma Rays ()
As gamma rays are electromagnetic waves, they have zero mass
and zero charge. A gamma ray is written as:

0
0
When a gamma ray is emitted from an atom, it has no effect on
either the atomic mass or atomic number. It does however
increase the stability of the nucleus, as it reduces its energy level.
The gamma rays are weakly ionising, as they tend to pass through
a substance rather than collide with its atoms.
When gamma rays do collide with an atom, energy is transferred
to an electron which leaves its orbit.
Gamma Rays ()
Gamma rays mostly occur as a result of alpha or beta radiation,
although there are some elements that only emit gamma radiation,
such as protactinium...
234
Pa
91
234
Pa
91
Checking the atomic mass figures...
234 = 234 + 0
Checking the atomic number figures...
91 = 91 + 0
+

0
0
Gamma Rays ()
When a radioactive nucleus emits an alpha or beta particle, the
protons and neutrons in the newly formed nucleus are often in an
excited, unstable state.
In this case, a gamma ray is emitted from the newly formed
nucleus, reducing the energy level and increasing the stability of
the nucleus...
60
Co
27
60
Ni
28
+
0
e
-1
+

0
0
Cobalt-60 is commonly used as a gamma ray source for food
irradiation, radiotherapy and for general use in laboratories.
Penetration of Radiation
Press the button to see how different types of radiation penetrate...
Penetration of Radiation - Alpha ()
 As alpha particles have a relatively high mass and low speed,
they collide easily with atoms in a substance.
 When they collide with atoms, it
prevents them from passing through
materials, so alpha particles have a
very short range and low
penetration.
 They only travel around 6cm in air.
 They are stopped by skin or a piece of paper.
Penetration of Radiation - Beta ()
 Beta particles have a very low mass and high speed, so they
collide less easily than alpha particles with atoms in a substance.
 Beta particles have a greater range
than alpha particles, and are
considerably more penetrating.
 They can travel up to 10m through
the air, and through some obstacles,
such as pieces of paper.
 They are stopped by a 3mm thick layer of aluminium.
Penetration of Radiation - Gamma ()
 Gamma radiation has no mass and travels at the speed of light,
so gamma rays collide infrequently with atoms in a substance.
 Gamma rays have a greater range, and
are considerably more penetrating,
than alpha or beta particles.
 They can travel long distances through
the air, through pieces of paper and
aluminium foil.
 Several centimetres of lead or several metres of concrete are
needed to stop the passage of gamma rays.
The Effect of a Magnetic Field on Radiation
Press a button to see the effect of a magnetic field on different types
of radiation...
The Effect of an Electric Field on Radiation
Press a button to see the effect of an electric field on different types
of radiation...
Effects of Electric & Magnetic Fields - Summary
The effects of an electric field can be summarised as:
 Alpha particles are bent slightly towards the negative terminal
of an electric field.
 Beta particles are bent strongly to the positive terminal of an
electric field.
 Gamma rays are not affected by an electric field.
The effects of a magnetic field can be summarised as:
 Both Alpha and Beta particles are deflected at right angles to a
magnetic field. Alpha and Beta particles will move in opposite
directions according to Fleming’s left hand rule.
 Gamma rays are not affected by a magnetic field.
End of Show
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