Radiation
Atomic Anatomy
• Atoms
– electrons (e-)
– protons (p+)
– neutrons (n)
Atoms are electrically neutral with
no net charge.
Ions are atoms that have been
stripped of one
or more of their electrons and have a
net charge.
Isotopes
• Identical Chemical Properties, Different
Atomic Weight
• Difference = presence of # of neutrons in
the nucleus
Hydrogen = 1.0079 amu
A
ZX
X = Element Symbol
Z = Atomic Number (periodic table,
# protons)
A = Isotope Number (# neutrons)
Isotope Designations
Hydrogen
1
1 proton, 0
neutrons
Helium-4
4
2 protons, 2
neutrons
Uranium-235
235 U
92
1H
2He
92 protons,
143 neutrons
Radioactive Particles
• Alpha a
Ejection of 2 protons and 2 neutrons from an
4 He = a
unstable nucleus.
2
• Beta b
Ejection of an electron from an unstable nucleus
0 e = b
as part of the decay of a neutron.
-1
• Gamma g
Atomic nucleus transition, yielding high energy
photons.
226
88Ra
Nuclear Reactions
222 Rn + 4 He
86
2
Note that the numbers all add-up (conservation
of particles).
A X
Z
A=Protons + neutrons (Total particles in
nucleus) 226 = 222 + 4
Z protons Number of protons
88 = 86 + 2
Alpha Decay
• Alpha a =
4
2He
Parent
226
Ra
Radium
Daughter
222
Rn
Radon
88
86
Radiation 42He
226
88Ra
a
222
56Rn +
4
2
He
Most of the energy is with the lighter particle, in this case
the alpha particle.
Beta Decay
• Beta b =
1 n
0
0
-1e
1
0 e
p
+
1
-1
The decay of a neutron into a proton
and electron.
Beta Decay
• Beta b =
1
0
n
0
-1e
1
1
p + 0-1e
The decay of a neutron into a proton and electron.
14
6C
90
38
Sr
14
90
0 e
N
+
7
-1
39
Y+b
(Radioactive Carbon)
Gamma Radiation
• Gamma g
Very high energy photons are emitted from
the nucleus.
Excess radiation emitted from an excited
nucleus….
87 Sr*
87 Sr + g
38
38
Fission
The splitting of an unstable atomic nucleus into
two or more nuclei.
Fission occurs spontaneously, generally when a
nucleus has an excess of neutrons, resulting in
the inability of the strong force to bind the
protons and neutrons together.
The fission reaction used in many nuclear reactors and
bombs involves the absorption of neutrons by
uranium-235 nuclei, which immediately undergo
fission, releasing energy and fast neutrons.
Shielding
We can detect the radiation from a
radioactive source.
Say we get X counts/minute (cpm).
Geiger Counter
Shielding
We can shield the source with various
materials to test their usefulness in
protecting against the radiation.
Geiger Counter
Shielding Efficiency
a
b
g
Cotton Fabric
Wood
Lead
g’s are the most penetrating type of radiation.
Half Life
• The amount of time required for exactly 1/2 of the
original (No) sample of parent atoms to decay into
daughter products.
• After one half life, you have 1/2 No parent atoms,
and 1/2 No daughter atoms.
Half Life
• After two half lives, you have 1/4 No parent atoms,
and 3/4 No daughter atoms.
• After three half lives, you have 1/8 No parent atoms,
and 7/8 No daughter atoms.
Radioactive Decay
If you start out
with a sample of
parent atoms (No),
after some time
there will be fewer
because of
radioactive decay
into the daughter
atoms.
Radioactive Carbon Dating
14 C
14 N + b
Radioactive Carbon
6
7
Half-life = 5730 years
There is a certain amount of 146C occurring
naturally. Living things continually replenish
this by ingesting plants or drinking water with
some 146C in them.
When death occurs, no replenishment takes
place and the 146C decays into 147N.
By measuring the ratios of
14 C and 14 N, with the half6
7
14
life value of 6C an accurate
estimate of the age is
obtained.
Fusion
The joining together of atomic nuclei,
especially hydrogen or other light
nuclei, to form a heavier nucleus,
especially a helium nucleus.
Fusion occurs when plasmas are heated to extremely high temperatures,
forcing the nuclei to collide at great speed. The resulting unstable
nucleus emits one or more neutrons at very high speeds, releasing more
energy than was required to fuse the nuclei, thereby making
chain-reactions possible, since the reaction is exothermic.
Fusion reactions are the source of the energy in the Sun and in other
stars, and in hydrogen bombs.