Lecture 27 Nuclear Reactions

1
Lecture 27Nuclear Reactions
Chapter 29.5 ? 29.8
Outline

• Radioactive Decay
• Binding Energy
• Nuclear Reactions

2
Half-Life
The half-life of a radionuclide is the period of
time needed for half of any its initial amount to
decay.
Radionuclides have unchanging half-life. This is
the base for archaeological dating. Half-life for
14C is 5730 years.
146C ? 147N e? ?
? ? antineutrino
Natural abundance ratio of 14C and 12C is
1.310?12 Creation of 14C is due to cosmic rays
After death, organisms stop absorbing 14C from
the atmosphere and the ratio decreases with time.
3
Half-Life
4
Problem with Carbon Isotopes
Problem A 1-gram sample of wood is taken from an
ancient site. If the 14C activity of the sample
is 12.5 that of the present-day organic
material, what is the age of the wood?
Solution Half-time of 14C 5730 years. 12.5
1/8 3 half-times of 14C. The sample age is
35730 years 17200 years.
5
Binding Energy
Hydrogen has 3 isotopes protium, deuterium, and
tritium.
The mass of 11H is 1.0078 u , the mass of a
neutron is 1.0087 u . The sum is 2.0165 u .
The mass of a deuterium atom is 2.0141 u 0.0024
u 2.2 MeV less than the combined mass of pn.
All atoms have smaller masses than the sum of
masses of their nucleons. Some mass and energy is
given off due to action of forces that hold the
protons and neutrons together. This energy is
called binding energy.
6
Binding Energy
Binding energy of a deuterium atom is 2.2 MeV
(1.1 MeV per nucleon) , of a 20983Bi is 1640 MeV
(7.8 MeV per nucleon).
A typical binding energy is 800 billion kJ/kg To
boil water it takes 2260 kJ/kg
The most stable element is iron (5626Fe).
7
Problem with Binding Energy
What is the binding energy per nucleon of
19779Au?
Masses 19779Au 196.967 u 11H 1.0078 10n
1.0087
1u 931.5 MeV/c2
Solution there are 79 p and 118 n in an Au
atom. ?mass(79p118n) 198.643 u Difference
between the actual mass and ? 1.676 u E mc2
1.676 u 931.5 (MeV/u) /197 7.92MeV
8
Nuclear Reactions
Nuclear reactions are changes of elemental
composition in atomic interactions. In chemical
reactions atomic composition does not change.
Elements lighter than Fe can be produced by
fusion of 2 smaller nuclei. This is possible,
because binding energy increases with atomic
number.
Fusion requires extremely high temperatures and
pressures ? fusing nuclei come very close
together to overcome Coulomb repulsion forces.
9
Fusion
10
Fission
Fission is splitting of a large nucleus into a
few lighter nuclei. It is possible for atoms
heavier than Fe because the binding energy
decreases with the atomic number.
Fission does not require extreme conditions and
could be easily controlled. The first controlled
fission reaction was achieved in 1939 .
Fission animation
11
Summary
There are two basic types of nuclear reactions
fusion and fission. Fusion is creation heavier
elements from lighter ones. It works from
hydrogen to iron. Fission is a decay of heavy
elements into lighter parts. It works in elements
heavier than iron. Fusion is much more energy
efficient than fission.