NUCLEAR ENERGY

The daughter nuclei in the reaction above are highly unstable. They decay by beta emission until they reach stable nuclei.

NUCLEAR ENERGY

When slow neutrons are captured they excite the compound nucleus formed by the amount of their binding energy in the compound nucleus. For a nucleus to fission, the fragments must overcome the potential barrier that keeps the whole nucleus together. Thermal neutron fission occurs only if the binding energy is greater than the barrier.

NUCLEAR ENERGY This is the principle underlining nuclear reactors for providing electric power. The energy liberated is converted to heat, which is extracted to a turn a turbine that generates electricity. It is also the principle behind nuclear bomb. In this case the fission process is allowed to proceed uncontrolled, whereas in the case of power reactors, the process is controlled.

NUCLEAR ENERGY The commonest fuel in reactors is the U-235. Naturally its abundance is just 0.7%. In nuclear bomb case, the fissions have to occur almost at the same time. For this to happen, the abundance of U-235 needs to be increased to about 21% - enrichment process. In order for a reactor to be useful the fission process has to form a chain reaction, neutron from one reaction initiating another set.

NUCLEAR ENERGY

Problems that can affect successful chain reaction 1. Neutron leakage at the surface of the reactor – make the volume of the reactor such that volume/surface area is maximum. 2. Most of the neutron produced in the fission process have kinetic energy about 2 MeV, fission is most efficient with neutrons of about 0.04 eV kinetic energy – use moderator (water often used).

NUCLEAR ENERGY 3. Neutron absorption – as neutrons are slowed down by moderator, they are absorbed by U-238 when their kinetic energy is between 1 and 100 eV – arrange fuel rods and moderator to minimize. 4. Neutron multiplication factor, k, - ratio of the number of neutrons at the end of one generation to the number at the beginning of the generation. For steady power k = 1.

NUCLEAR ENERGY k = 1 - reactor is critical k > 1 – reactor is super-critical k < 1 – reactor is sub-critical Use neutron absorbers rods to control the criticality of the reactor.

NUCLEAR ENERGY

PARTICLE PHYSICS Scientists at the close of 19 th century and early 20 th century thought that the elementary particles making the universe are electrons, protons and neutrons. Experiments in nuclear laboratories have discovered several particles with various properties. Scientists believe that the number of building blocks should be few.

PARTICLE PHYSICS

Fermions obey Pauli exclusion principle – no two particles can occupy quantum states having the same quantum numbers. They are described by Fermi-Dirac statistics. Bosons do not obey Pauli exclusion principle. They are described by Bose- Einstein statistics. Any number of bosons can occupy the same quantum state.

PARTICLE PHYSICS Another classification is Leptons/Hadrons. This classification depends on the dominant force acting on the particle 1. Gravitational force acts on all particles, but it is so weak – can be ignored in particle interactions. 2. Electromagnetic force acts between all charged particles – ignore in classification.

PARTICLE PHYSICS 3. Weak force results in beta decay 4. Strong force binds particles within the nucleus Leptons are particles are not acted upon by strong force e.g. electrons, muons, tau particle, their neutrinos and anti-neutrinos Hadrons are particles that strong force acts on e.g. protons, neutrons, pions, kaons, etc.

PARTICLE PHYSICS Hadrons can further be divided into those that are bosons (mesons e.g. pions, kaons) and those that are fermions (baryons e.g. protons, neutrons) To every particle, there is a corresponding anti-particle. The anti-particle has the same mass as the particle, but opposite electric charge. We sometimes write anti- particle with the particle symbol + a bar

PARTICLE PHYSICS

LEPTONS

HADRONS

Consider the quantities that must be conserved: 1. Energy conservation 2. Momentum conservation 3. Electric charge conservation 4. Conservation of spin 5. Conservation of lepton number 6. Conservation of baryon number. For the reaction given, all conservation laws are obeyed except (4) and (6).

QUARK MODEL