Transcript Radioactive Decay

Marie Curie
1867 - 1934
Radioactivity
Wilhelm Roentgen
1845 - 1923
Spontaneous
emission of radiation
from the nucleus of
an unstable isotope.
Antoine Henri Becquerel
1852 - 1908
Radioactive Decay
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Unstable nucleus emits particle and energy
Most common decays involve the emission of
 - particle ( Decay)
 - particle ( Decay)
 - particle ( Decay)
Alpha Decay
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Alpha decay occurs when there are too many protons in the
nucleus which cause excessive electrostatic repulsion.
An alpha particle is ejected from the nucleus.
An alpha particle is 2 protons and 2 neutrons.
An alpha particle is also a helium nucleus.
Alpha particle symbol: 4
2
He
The Numbers
Atomic mass number: number of nucleons
4
2
He
Atomic number: number of protons
Neutrons = number of nucleons – number of protons
Beta Decay
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Beta decay occurs when neutron to proton ratio is too big
A neutron is turned into a proton and electron and an
antineutrino
The electron and the antineutrino are emitted
Gamma Decay
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Gamma decay occurs when the nucleus is at too high an
energy
Nucleus falls down to a lower energy level
High energy photon – gamma ray - is emitted
Sources of Radiation
Penetrating Power
Shielding
Radioactive Decay
4.5 x 109 yr half-life
U
238
92
4
2
He
234

90
Th
24 day half-life
1.17 min half-life
Th  e 
0
1
234
90
234
91
Pa

0
1
e
234
91
234
92
Pa
250,000 yr half-life
U
U 238 Decay
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Decay Series
Half-Life
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Half-life is the time required for half of the element to
decay to another element.
For Thorium-234, the half-life is 24 days.
That means that every 24 days half of the thorium-234
in a sample has decayed.
If you begin with a 1 kg chunk of Thorium-234, after 24
days, you will have ½ kg of Thorium-234.
After 48 days?
Am ount of Th-234 vs Tim e
¼ kg
1.2
After 72 days?
1
0.8
1/8 kg
0.6
After 96 days?
0.4
1/16 kg
0.2
After 120 days?
0
0
50
100
150
200
1/32 kg
Mass (kg)
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Tim e (day)
Carbon Dating
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Cosmic rays cause transmutation of Nitrogen to
Carbon-14
1
0
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n N  H  C
14
7
1
1
14
6
C-14 is radioactive with a half-life of 5730 years
It decays back to Nitrogen by beta decay
C  e N
14
6
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0
1
14
7
The ratio of C-12 (stable) atoms to C-14 atoms in
our atmosphere is fairly constant – about 1012/1
This ratio is the same in living things that obtain
their carbon from the atmosphere
Carbon Dating
The radioactive carbon isotopes in the skeleton diminish by
one-half every 5730 years.