physics exam 2

a proton is how much heavier than an electron

2000

what is a nucleon?

protons and neutrons

what is the most powerful force?

strong nuclear

Mass of an intact nucleus is always ___ than the sum of the

masses of its individual free protons and neutrons

less

Strong nuclear force holds _____ together

nucleons

___ Z nuclei tend to have equal number of protons and neutrons

Low

Key causes of nuclear instability:

Imbalanced neutron-to-proton ratio

• If an atom has too many or too few neutrons relative to its protons, the balance fails.

Excessive atomic size

• Large nuclei (Z>83)

Odd nucleon numbers

How unstable nuclei fix themselves?

To gain stability, an unstable nucleus spontaneously emit energy or particles (radiation)

Beta Particles

Positive Beta Particles

Alpha Particles

Neutrons

Decay Law

The change in the number of atoms per unit time (∆N/∆T) is

proportional to the number of radioactive atoms (N) present:

What is the SI unit of activity?

Becquerel or Curie

Parent & Daughter Nuclei

An Unstable Nucleus (PARENT) —→ undergoes spontaneous emission of Energy

to become —→ A More Stable Nucleus (DAUGHTER)

Transient Equilibrium:

The half-life of the parent nuclide is only slightly greater than the half-life of the daughter nuclide

• The activity of the daughter nuclide increases as the parent nuclide decays

• After a certain point, the activity of the daughter nuclide matchesSecular Equilibrium: the rate of decay of the parent nuclide

Secular Equilibrium:

The half-life of the parent nuclide is MUCH GREATER than the half-life of the daughter nuclide

• The activity of the daughter nuclide increases as the parent nuclide decays

• The daughter nuclide eventually reaches a state of EQUAL ACTIVITY equilibrium with the parent nuclide

Beta-minus (B−) Decay

Undergone by radionuclides with an excess of neutrons (“neutron-rich”)

i.e. n/p ratio is too large for stability

• A neutron is lost & a proton is gained

Beta-plus (B+) Decay

Undergone by radionuclides with an excess of protons (“proton-rich”)

i.e. n/p ratio is too small for stability

• A proton is lost & a neutron is gained

What is emitted in beta minus decay?

electron and antineutrino

what is emitted in beta plus decay?

positron and neutrino

Electron Capture competes with:

Beta plus decay

Electron Capture vs. Beta Plus Decay

• Proton-rich nuclides (i.e. nuclides with an excess of protons)

• Want to improve stability by lowering their n/p ratio

• Electron Capture (EC) & Beta-plus decay are both decay modes that accomplish this

Electron Capture (EC):

• An orbital electron is captured by the nucleus

• The electron combines with a proton within the nucleus —> the pair becomes a neutron

• The nucleus gains a neutron & loses a proton

The minimum threshold energy for Beta plus decay to occur =

1.022 MeV

What are the two unique outcomes after electron capture?

• Fluorescence (Characteristic X-rays (photons)

• Auger Electrons

Characteristic X-rays: Key Idea

• Binding energy levels for each electron shell: discrete & unique to each atom

• The energies of the emitted photons from electrons transitioning to fill lower shell vacancies are unique to each atom

Auger Electrons

• characteristic x-ray does not leave the atom

• photon crashes into an outer shell electron

• The photon gives up all its energy

• The electron is freed from the atom

• The photon is entirely gone

Electromagnetic (EM) Decay:

is a means of an excited nuclide decaying to its ground state

3 Types of EM Decay:

• Gamma decay (Nuclear de-excitation)

• Internal Conversion (IC)

• Isomeric transition

Gamma decay (Nuclear de-excitation):

• Transition from excited state to ground state —→ emission of a gamma ray

• The nuclide does NOT change ——> A & Z values stay the same

Internal Conversion

• The energy is transferred directly from the excited nucleus to an orbiting electron

• Usually a K-shell electron, though L & M are both possible

• The electron leaves the atom —→ vacancy must be filled —→ fluorescence or Auger electrons

• Competes with Gamma Decay

Isomeric Transition

• Eventually, the excited nuclear isomer will emit a gamma & decay to its ground state

• Isomeric Transition is different from Gamma Emission because the excited state nuclide can exist for longer than a small fraction of a second

Nuclear Isomer:

a nucleus that got “stuck” in a metastable state, & has not decayed to its ground state