Lecture 18: Nuclear Physics

Outline the structure and properties of the nucleus

Nucleus: densely packed centre of atom composed of (protons and neutrons)

• neutrons and protons are collectively called nucleons

• number of protons are called atomic number

• total number of nucleons (neutrons + protons) is called the atomic mass number

What is nuclear mass?

Measured using mass spectrometer and expressed with reference to the mass of carbon-12 atom, which is assigned a mass of exactly 12u, where u is unified atomic mass unit

What is the energy equivalence of mass?

Masses can also be expressed in terms of their energy equivalent

E = mc²

What is binding energy?

• the total mass of a nucleus is always less than the mass of protons and neutrons in it

• this mass difference is a measure of the BE that holds the nucleus together

What holds the nucleons together?

Two opposing forces act between nucleons: Nuclear and Coulomb

For a nucleus to be stable, attractive force between nucleons must overcome the repulsive force between protons

What is nuclear force?

A force that binds nucleons together

• attractive (between p-p, p-n, n-n)

• short range (only attracts nearest neighbours appx = 10^-15

What is coulomb force?

A force that breaks protons apart

• repulsive (p-p)

• long range (acts across the nucleus)

outline radioactivity

• late 19th century minerals were found that would darken a photographic plate even in the absence of light

• these minerals emit radiation

• this phenomenon is called radioactivity

what 2 highly radioactive elements did marie and pierre curie isolate

polonium and radium

Describe radiation

• radioactivity is the result of the disintegration or decay of an unstable nucleus

• certain isotopes aren’t stable and decay with emission of radiation

• they found rays could be classified into 3 distinct types according to their penetrating power (alpha, beta and gamma)

  • alpha rays are positively charged

  • beta rays are negatively charged

  • gamma rays are neutral

What is radioactivity?

Radioactivity: the spontaneous emission of radiation from unstable nuclei

• alpha, beta and gamma emission

• alpha = nuclei of helium atoms

• beta = electrons

• gamma = high energy photons

outline how a smoke detector works

• it contains about 0.2mg of the radioactive americium isotope Am-241 in the form of AmO2 (1.0 µCi)

• alpha particles from Am-241 continually ionizes the air molecules in the air space between the two oppositely charged plates

• the positive and negative ions are attracted by the charged plates which results in a current flow through the circuit

• when smoke enters between the plates, the ions become attached to the smoke particles, thus resulting in a drop of current

• the current drop is detected by the electronic circuitry and sets off the alarm

outline alpha decay

outline beta decay

outline gamma decay

photon is emitted from an excited nucleus