Phys102 Lecture 34/35 Nuclear Physics and Radioactivity

Phys102 Lecture 34/35
Nuclear Physics and Radioactivity
Key Points • Structure and Properties of the Nucleus • Alpha, Beta and Gamma Decays • Calculations Involving Decay Rates and Half-Life • Radioactive Dating
References SFU Ed: 42-1,2,3,4,5,6,7,8,9,10. 6th Ed: 30-1,2,3,4,5,6,7,8,9,10,11.

Atomic Structure
Nitrogen (N) Atom
The nucleus

neutron

proton

Structure and Properties of the Nucleus
A nucleus is made of protons and neutrons. A proton is positively charged. Its mass is:
A neutron is electrically neutral:

Structure and Properties of the Nucleus
Neutrons and protons are collectively called nucleons. The different nuclei are referred to as nuclides. Number of protons: atomic number, Z Number of nucleons: atomic mass number, A Neutron number: N = A – Z. Therefore, A and Z are sufficient to specify a nuclide.
Nuclides are symbolized as :
X is the chemical symbol for the element; it contains the same information as Z but in a more easily recognizable form.

Structure and Properties of the Nucleus
Nuclei with the same Z – so they are the same element – but different N are called isotopes. For many elements, several different isotopes exist in nature. Natural abundance is the percentage of a particular element that consists of a particular isotope in nature.

Structure and Properties of the Nucleus
Because of wave-particle duality, the size of the nucleus is somewhat fuzzy. Measurements of high-energy electron scattering yield:
(42-1)

Structure and Properties of the Nucleus
Masses of atoms are measured with reference to the carbon-12 atom, which is assigned a mass of exactly 12u. A u is a unified atomic mass unit.

Structure and Properties of the Nucleus
From the following table, you can see that the electron is considerably less massive than a nucleon.

Binding Energy and Nuclear Forces
The total mass of a stable nucleus is always less than the sum of the masses of its separate protons and neutrons.
Where has the mass gone?

Binding Energy and Nuclear Forces
It has become energy, such as radiation or kinetic energy, released during the formation of the nucleus.
This difference between the total mass of the constituents and the mass of the nucleus is called the total binding energy of the nucleus.