Neutrons Flashcards
(43 cards)
Define Nuclear stability (include relationship to atomic number and line of stability)
Defined as the inherent ability of an atom to resist changing its atomic structure or energy. Line of stability is centered on the ratio of 1 neutron per proton. No known stable atom above an atomic number of 83.
Define Mass-energy equivalence
E = MC Squared.. Approximately 931.5 MeV/AMU
Define Mass Defect
When a nucleus is made up from its component parts (protons and neutrons), the mass of the nucleus is less than the mass of the individual protons and neutrons.
This mass difference is called the mass defect.
Define Binding Energy
It is the energy equivalent of the mass defect.
Binding energy represents that amount of energy that is released when an atom is formed from its component protons, neutrons, and electrons
Define Binding Energy per Nucleon
The average energy required to remove a nucleon from a given nucleus.
Found by dividing the total binding energy by the number of nucleons.
Given a plot of binding energy per nucleon, explain the changing slope of the curve.
The larger the mass number the greater number of protons and increasing electrostatic repulsion forces.
Define microscopic cross section.
σ is the probability that a given interaction will occur between a target nucleus and an incident neutron.
It is based on the makeup of the nucleus and, hence, on its “ability” to absorb the neutron, not on the “size” of the nucleus itself.
List each component of the total microscopic cross section.
For any given target (isotope):
- Radiative capture
- Fission
- Elastic scattering
- Inelastic scattering.
Classified as either absorption reactions (radiative capture/fission) or scattering reactions.
Describe radiative capture.
A neutron is absorbed by the target nucleus, resulting in an excited compound nucleus. The compound nucleus returns to ground state by emitting gamma rays
Describe fission.
Similar to radiative capture except that sufficient energy has been added to the target so that the target nucleus splits apart.
Describe elastic scattering.
Occurs when a nucleus deflects a neutron without absorbing the neutron.
Elastic scattering conserves kinetic energy and is often visualized as a “Billiard Ball” type of collision
Describe inelastic scattering.
Similar to elastic scattering, except that kinetic energy is not conserved.
The neutron is, for a short time, incorporated into the nucleus. The kinetic energy transferred from the neutron to the nucleus raises internal energy of target nucleus.
A neutron is subsequently released, with less kinetic energy than the incoming neutron had.
The target nucleus then gives up the energy it
received from the incident neutron and returns to ground state by emitting a gamma ray.
Define macroscopic cross section.
Σ is defined as the probability of an incident neutron interacting with a target nucleus per unit length of travel of the incident neutron. Σ=Nσ.
Where:
Σ = macroscopic cross section (cm-1) N = atomic density (atoms/cm<sup><span style="font-size: 13.5px; line-height: 0px;">3</span></sup>) σ = microscopic cross section (barns)
Explain how changing neutron energy will affect the magnitude of a cross section for a given isotope.
Increasing neutron energy results in a smaller magnitude of cross section.
List each component of the total macroscopic cross section.
It is the product of the atomic density and the microscopic cross section.
Σ=Nσ.
Where:
Σ = macroscopic cross section (cm-1)
N = atomic density (atoms/cm3)
σ = microscopic cross section (barns)
Define a fast neutron.
All fission neutrons are born as fast neutrons. They have a kinetic energy greater than 0.1 MeV (105 eV - 100,000 eV)
Define Critical Energy.
The minimum amount of energy required for fission to occur in a specific fuel type.
Define “Instantaneous” as it pertains to particle production from fission.
The immediate products of fission vice the delayed products.
83% of energy from kinetic energy of initial fission fragments and 6% from initial neutrons and gamma rays.
Define “delayed” as pertaining to particle production from fission.
Accounts for 7% of energy from fission and results from the decay of the initial fission product fragments.
Define fissile material.
A fuel type that will fission simply because of the binding energy of an incident neutron.
Define fissionable material.
A fuel type that requires kinetic energy in addition to binding energy of an incident neutron for fission to occur.
Given a graph of fission product yields, explain the shape of the distribution curve.
Maywest Curve
The curve shows that one of these fission fragments is lighter (A»95) while the other is heavier (A»139).
Splitting into two identical masses (A=118) is a low probability event.
Give a typical value for the energy in MeV (mega electron volts) released from an average fission event.
Approximately 200 MeV.
Given the origin, energy level, and production time of a neutron; classify the neutron.
Origin based:
- Fission Neutrons or Source Neutrons
Energy Level:
- Fast Neutrons are > 0.1 MeV,
- Intermediate (epithermal) are 0.1 MeV - 1 eV
- Slow Neutrons are <1 eV.
Production Time Classification
- Prompt Neutrons are born less than 10-14 seconds after fission
- Delayed Neutrons are born greater than 10-14 seconds after fission.
