Download Ch-1-PPT

BASIC CONCEPTS
 Summary-1
 The net nuclear charge in a nuclear species is equal to + Ze, where Z
is the atomic number and e is the magnitude of the electronic charge.
 The fundamental positively charged particle in the nucleus is the proton
 The mass number of a nuclear species, indicated by the symbol A, is
the integer nearest to the ratio between the nuclear mass and the
fundamental mass unit, defined so that the proton has a mass of nearly
one unit.
 the mass of the electrons is negligible compared with the proton mass
mP ~ 2000me
 For nearly all nuclei, A is greater than Z, in most cases by a factor of
two or more. Thus there must be other massive components in the
nucleus.
 The presence of electrons within the nucleus could not be proven due to
following reasons:
i)
If electrons existed inside the nucleus, then it should be bound inside
with a force stronger than coulomb force.
ii) If electrons existed inside the nucleus, due to uncertainty principle
electrons should have an energy of 20 MeV but the electrons emitted
during -decay have energies less than 1 MeV.
iii) the proton and electron each have angular momentum (spin) of (1/2).
Quantum mechanically spin of the protons and electrons do not add
together to give observed spin of nuclei. three spins of 1/2 in
deuterium combine to a total spin of either 3/2 or ½ . Yet the
 Summary-2
 Neutron is proposed as building block of nucleus.
 The neutron is eclectically neutral and has a mass about equal to the proton
mass (actually about 0.1% larger).
 Thus a nucleus with Z protons and A - Z neutrons has the proper total mass
and charge
 we define a specific nuclear species, or nuclide,
by symbol
where X is the chemical symbol and N is the neutron number, A - Z.
 Neutrons and protons are the two members of the family of nucleons.
 Nuclides with the same proton number but different neutron numbers are
called isotopes; for example 35Cl and 37Cl.
 Nuclides with the same N but different z· these are called isotones.
 Nuclides with the same mass number A are known as isobars.
 Nuclear properties are mass, radius, relative abundance (for stable
nuclides), decay modes and half-lives (for radioactive nuclides), reaction
modes and cross sections, spin, magnetic dipole and electric quadrupole
moments, and excited states.
 In nuclear physics lengths are of the order of 10 -15 m, which is one
femtometer (fm), also called one fermi. Nuclear sizes range from about 1
fm for a single nucleon to about 7 fm for the heaviest nuclei.
 The time scale of nuclear phenomena has an enormous range from 10-20 s to
10-9 s (5He or 8Be, break apart in times of the order of 10-20 s),  decays
of nuclei occur within lifetimes of the order of 10-9 s to 10-12 s , but many
 and  decays occur with much shorter or longer lifetimes ( from minutes