Ch24: Particle Physics Flashcards
Briefly describe Rutherford’s alpha scattering experiment.
Narrow beam of positively charged alpha particles from a radioactive source were targeted at a thin piece of gold foil (few atomic layers thick). These were scattered and detected on a zinc sulfide screen.
What were the two main observations of the alpha scattering experiment?
- Most particles went straight through, only around 1 in 2000 scattered.
- Very few, 1 in 10,000 deflected by more than 90°.
What was the significance of the two observations of the alpha particle scattering experiment?
- Most went straight through: most of the atom is empty space with the mass concentrated in a small region.
- Very few deflected over 90°: Nucleus has a positive charge as it repelled the few particles that came near it.
How can a distance, d, of closest approach to the gold nuclei of an alpha particle be calculated?
initial ke of alpha = electrical potential energy at d
ke = Qq/4πε₀d
When an element is represented on the periodic table, what does the A number represent?
Nucleon number, combined number of neutrons and protons.
When an element is represented on the periodic table, what does the Z number represent?
Proton number (Atomic number), number of protons.
What does atomic number refer to in terms of an element?
Number of protons.
What is an ‘atomic mass unit’?
1/12 the mass of a carbon-12 atom.
What value is the radius of a nucleus proportional to?
A - nucleon number
What is the strong nuclear force?
Very short range force acting between all nucleons.
What are the 4 fundamental forces? What are their ranges?
- Strong nuclear, 10^-15m
- Gravitational, infinite
- Weak nuclear, 10^-18m
- Electromagnetic, infinite
What are fundamental particles?
Particles considered to have no internal structure so they cannot be divided into smaller bits.
What are some examples of fundamental particles?
Quarks, electrons, neutrinos.
What are the two categories of subatomic particles?
Hadrons and Leptons
What is a hadron?
Particles and antiparticles made up of quarks and affected by the strong nuclear force, and if charged the electromagnetic force. They decay by the weak nuclear force.
What are leptons?
Particles and antiparticles not affected by the strong nuclear force. If charged, will experience electromagnetic force.
What are some examples of hadrons?
Protons, neutrons, mesons.
What are some examples of leptons?
Electrons, neutrinos, muons.
What is a proton composed of?
uud, up + up + down
What is a neutron composed of?
udd, up + down + down
What are baryons and mesons?
Baryons: any hadrons made of 3 quarks
Mesons: hadrons made of a quark plus an anti quark
What are two features of a neutrino?
- Very small mass
- No charge
What happens in beta-minus decay?
A neutron decays into a proton, an electron, and an electron antineutrino.
What happens in beta-plus decay?
A proton decays into a neutron, a positron, and an electron neutrino
What happens in terms of quarks in beta-minus decay?
A down quark becomes an up quark (and an electron and an electron anti-neutrino).
What happens in terms of quarks in beta-plus decay?
An up quark becomes a down quark (and a positron and an electron neutrino).
