Download 6.3 Nuclear Reactions

Nuclear Reactions Fisson, Fusion
IB Physics Core Unit 6.3
Artificial Transmutation
• In artificial transmutations the nucleus is bombarded
with high-energy (kinetics energy) particles to induce
transmutation. There are two reactants or the nucleus that
is being bombarded and the high-energy particle.
• Note: High-energy particles are accelerated in accelerators,
by the application of electric and magnetic fields. Neutrons
cannot be accelerated in accelerators using electric and
magnetic fields because - WHY?
• their charge is 0.
Example
• The first artificial transmutation was accomplished in 1919
by Rutherford who bombarded nitrogen-14 with α
particles.
• Later, in 1930, Irene Curie and her husband Frederic
bombarded stable Al-27 with alpha particles.
Problem for You
• Find what X is in the following nuclear
reactions
Fisson
• A fission nuclear reaction is the splitting of a heavy
nucleus into two or more lighter nuclei.
• Example U-235 is bombarded with slow neutrons to
produce Ba-139, Kr-94, or other isotopes and 3 fast
moving neutrons
Fisson Chain Reactions
• A nuclear chain reaction is a reaction in which an initial
step, such as the reaction above, leads to a succession of
repeating steps that continues indefinitely. Nuclear chain
reactions are used in nuclear reactors and nuclear bombs.
Fusion
• A fusion nuclear reaction is the combination of very light
nuclei to make a heavier nucleus.
• Extremely high temperatures and pressures are required in
order to overcome the repulsive forces of two nuclei.
• Examples
Star Light Star Bright
•
Consider the fusion nuclear reaction taking place in stars.
•
Mass of reactants = 4 (mass of H-1) = 4 (1.00718 amu) = 4.02872 amu
•
Mass of products = 1 (mass of He-4) = 1 (4.00150amu) = 4.00150 amu
•
The difference between the mass of the reactants and products is 4.02872 amu
- 4.00150 amu or 0.02722 amu
•
This difference is called the mass defect and it is converted into energy
according to the formula E = mc2 (we will look at this later)
•
E = energy
•
m= mass
•
c = speed of light (3 x 108 m/s)
•
Since the speed of light is a large number a small mass change corresponds to
a large amount of energy.
Therefore
• Energy released during nuclear reactions
is much greater than the energy released
during chemical reactions.
• Energy released in a nuclear reaction
(fission or fusion) comes from the
fractional amount of mass converted into
energy.
• Nuclear changes convert matter into
energy.
There are benefits and risks associated with
fission and fusion reactions.
• Fission reactions
• Benefits:
• Large amount of energy is released
• Production of electricity in nuclear plants
• Development of nuclear weapons (Atomic bomb, depleted uranium
bullets)
• Creations of new elements in accelerators (Americium-241 used in
smoke detectors)
• Risks:
• Dangerous nuclear waste are produced
• Accidents can release radiation into the environment
• Fusion reactions
• Benefits:
• Release of larger amount of energy than fission. However,
they are not a practical source of energy because the
technical problems of high temperature, pressure, and
containment of reaction are enormous.
• Risks:
• Relatively low - (unless you are close when it blows!)
Unified Mass Unit
• The unified atomic mass unit (u), or Dalton (Da) or, sometimes,
universal mass unit, is a unit of mass used to express atomic and
molecular masses.
• It is the approximate mass of a hydrogen atom, a proton, or a neutron.
• The precise definition is that it is one twelfth of the mass of an
unbound atom of Carbon 12 (12C) at rest and in its ground state.
1 u = 1/NA gram = 1/ (1000 NA) kg (where NA is Avogadro’s No.)
1 u = 1.660538782(83)×10−27 kg = 931.494027(23) MeV
• The atomic mass unit is an older name for the same thing, which
differs slightly in definition, and differs in value by one part in 1000.
• 1 u = 1.000 317 9 amu (physical scale) = 1.000 043 amu (chemical
scale). Since 1961, by definition the unified atomic mass unit is equal
to one-twelfth of the mass of a carbon-12 atom.
Binding Energy
• Nuclear binding energy is derived from the strong nuclear
force and is the energy required to disassemble a nucleus
into free unbound neutrons and protons, strictly so that the
relative distances of the particles from each other are
infinite (essentially far enough so that the strong nuclear
force can no longer cause the particles to interact).
• A bound system has a lower potential energy Ep than its
constituent parts; this is what keeps the system together.
The usual convention is that this corresponds to a positive
binding energy.
Mass Defect
• Because a bound system is at a lower energy level than its
unbound constituents, its mass must be less than the total
mass of its unbound constituents.
• For systems with low binding energies, this "lost" mass
after binding may be fractionally small.
• For systems with high binding energies, however, the
missing mass may be an easily measurable fraction.
• The energy given off during either nuclear fission
or nuclear fusion is the difference between the
binding energies of the fuel and the fusion or
fission products.
• In practice, this energy may also be calculated
from the substantial mass differences between the
fuel and products, once evolved heat and radiation
have been removed.
• The measured mass deficits of isotopes are always listed as
mass deficits of the neutral atoms of that isotope, and
mostly in MeV.
• Of course, when nuclear decay happens to the nucleus, the
properties ascribed to the nucleus will change in the event.
• But for the following considerations and examples, you
should keep in mind that "mass deficit" as a measure for
"binding energy", and as listed in nuclear data tables,
means "mass deficit of the neutral atom" and is a measure
for stability of the whole atom.
Specific quantitative example: a deuteron
• A deuteron is the nucleus of a deuterium atom, and
consists of one proton and one neutron. The
experimentally-measured masses of the constituents as free
particles are
– mproton = 1.007825 u;
– mneutron= 1.008665 u;
– mproton + mneutron = 1.007825 + 1.008665 = 2.01649 u.
• The mass of the deuteron (also an experimentally
measured quantity) is
– Atomic mass 2H = 2.014102 u.
• The mass difference = 2.01649−2.014102 u
= 0.002388 u.
• Since the conversion between rest mass and
energy is 931.494MeV/u (See Appendix 1
sheet), a deuteron's binding energy is
calculated using E=mc2 to be:
• 0.002388 u × 931.494 MeV/u = 2.224 MeV.
• Thus, expressed in another way, the binding
energy is [0.002388/2.01649] x 100% =
about 0.1184% of the total energy
corresponding to the mass.
• In the periodic table of
elements, the series of light
elements from hydrogen up to
sodium is observed to exhibit
generally increasing binding
energy per nucleon as the
atomic mass increases.
• This increase is generated by
increasing forces per nucleon in
the nucleus, as each additional
nucleon is attracted by all of
the other nucleons, and thus
more tightly bound to the
whole.
• The region of increasing binding
energy is followed by a region of
relative stability (saturation) in the
sequence from magnesium through
xenon.
• In this region, the nucleus has
become large enough that nuclear
forces no longer completely extend
efficiently across its width.
• Attractive nuclear forces in this
region, as atomic mass increases,
are nearly balanced by repellent
electromagnetic forces between
protons, as atomic number
increases.
• Finally, in elements heavier than
xenon, there is a decrease in
binding energy per nucleon as
atomic number increases. In this
region of nuclear size,
electromagnetic repulsive forces
are beginning to gain against the
strong nuclear force.
• At the peak of binding energy,
nickel-62 is the most tightlybound nucleus, followed by iron58 and iron-56 (This is the basic
reason why iron and nickel are
very common metals in planetary
cores, since they are produced
profusely as end products in
supernovae).
• The most tightly bound isotopes are 62Ni,
58Fe, and 56Fe, which have binding energies
of 8.8 MeV per nucleon.
• Elements heavier than these isotopes can
yield energy by nuclear fission; lighter
isotopes can yield energy by fusion.