Neutrons Flashcards
Describe nucleons
Protons: positive 1 charge. Approx 1 amu.
Neutrons: neutral charge. Also approx 1 amu.
Located in nucleus
Describe electrons
Negative 1 charge.
Approx 1/1835 the mass of protons and neutrons.
Orbits around the nucleus.
What is an amu?
6.022x10^23 amu = 1 gram.
What is Atomic Number (Z)?
Number of protons in an atom.
Defines the specific element.
What is Neutron number (N)?
Number of neutrons in a nucleus.
Mass number - Atomic number
What is Mass Number (A)?
Total number of nucleons in a nucleus.
What is a nuclide?
All atoms containing unique combinations of protons and neutrons.
A specific nuclide has a specific number of neutrons as well as a specific number of protons.
What is an Isotope?
Varying nuclides of a particular element (same number of protons) with different numbers of neutrons.
3 forces that act on a nucleus
Electrostatic: strong (long range) repulsive between protons.
Gravitational: nucleons have mass and therefore gravity -> negligible.
Nuclear: strong (short range) attractive force. Holds nucleus together.
How does Neutron/Proton Ratio affect the stability of a nucleus?
Atoms with a higher Binding Energy per nucleons are more stable. As the Atomic number rises (above ~60) the BE/A decreases, and as such, larger (heavier) nuclei tend to be less stable. That also means that the Neutron/Proton ratio rises as more neutrons are required to overcome the repulsive force of the many protons in the nucleus.
Relationship between Mass Defect and Binding Energy.
Mass defect = mass lost to energy to “bind” an atom together.
Delta m=931.5 MeV.
Calculate Mass Defect
Delta m= {Z[m(p)+m(e)]+m(n)}-m(atom)
Calculate Binding Energy per Nucleon
BE/A
What is Elastic Scattering?
Collision in which KE is conserved.
KE in equals KE out.
Nucleus is not excited, just moves faster, and neutron loses KE equal to the amount transferred to the nucleus.
What is Inelastic Scattering?
KE is not conserved. Some KE of neutron is transferred to excitation energy of the nucleus. The nucleus then emits a neutron with lower KE and it also emits a gamma to lower back to ground state. KE out is lower than KE in due to emission of a gamma (energy).
What is Radioactive Capture?
A nucleus absorbs a neutron, becomes excited, and then emits a gamma to decay down to ground state.
What is Particle Ejection?
A nucleus absorbs a neutron and then becomes excited. The excited nucleus then emits a particle (alpha or proton) and a slightly smaller atom.
Ex: 5B10+n->(5B11)*->3Li7+2a4
(Can be looked at as the atom “splits” into smaller particles/atoms, however, this is not the same as fission)
Describe Fission
A heavy nucleus absorbs a neutron and splits into 2 smaller atoms [much larger than particles (alphas and protons)] in conjunction with 2-3 neutrons and gammas.
Define Excitation Energy
Energy that a nucleus contains above ground state energy.
Define Critical Energy
Energy required to be absorbed by a nucleus above ground state in order to cause that particular nucleus to fission.
Define Fissile Material
Material that will fission when absorbing neutrons of any energy level (BE)
Define Fissionable Material
Material that will fission if the nucleus absorbs a neutron with enough binding energy to over come the critical energy of the nucleus. Thermal neutrons (or neutrons of lower energy levels) may not have enough energy to cause the nucleus to reach critical energy.
List some Fissile Materials
Odd numbered mass numbers:
U233
U235
Pu239
List some Fissionable Materials
Even numbered mass numbers
U238
Pu240
Pu242
Is all Fissile Material also Fissionable?
Yes
Is all Fissionable Material also Fissile?
No
Describe average total energy released per fission event (excluding neutrinos)
Instantaneous:
KE of Fission Fragments-165 MeV
KE of Fission Neutrons-5 MeV
Instantaneous Gammas-7 MeV
Capture Gammas-10 MeV
Delayed:
KE of Betas-7 MeV
Decay Gammas-6 MeV
Total: 200 MeV
Which nuclides are most likely to result from fission?
Nuclides with ~140 and 95 mass numbers
Cs140 and Rb93 are most common.
Xe140 and Sr94 are also likely.
How is heat produced from fission?
Ionization and scattering interactions from fission fragments, gamma rays, beta particles and neutrons.
Describe Prompt Neutrons
Produces from fission within 10^-14 seconds of the fission event. (That’s freaky fast!)
~99.36% of neutrons released from fission.
Most probable energy of 1 MeV
Average energy of 2 MeV.
Describe Delayed Neutrons
Neutrons born indirectly from fission as fission fragments decay.
Ex: 35Br87->B- to 36Kr87->instantaneous->n+36Kr86.
Average time is 12.7 seconds after fission. (Any neutron >10^-14 sec).
Average energy of 0.5 MeV.
What is Delayed Neutron Fraction?
Fraction of Delayed Neutrons born from a particular nuclide during the average of all fission events from the same particular nuclide.
What is the Delayed Neutron fraction of U235 and Pu239?
U235: 0.0064. (Note: this is 1-.9936, which is the fraction of prompt neutrons for U235).
Pu239: 0.0021
What is a Neutron Lifetime?
Average lifetime from the time a neutron is born until it is lost from leakage or absorption.
Sum of Thermalization time and diffusion time. Diffusion time relates to Mean free path and neutron velocity.
What is Neutron Generation time?
Time from birth of a neutron to the birth of a next generation neutron.
Describe Prompt Neutron Generation Time
Prompt neutron lifetime plus the time required for a fissionable nucleus to emit a fast neutron after absorption.
10^-4 to 10^-5 seconds
Describe Delayed Neutron Generation Time.
Time from birth of delayed neutron from a delayed neutron precursor to the time of birth of a next gen neutron.
12.7 seconds.
Describe Thermalization
Reducing neutron energy to thermal range by elastic scattering.
What is a Moderator?
Material used to Thermalize neutrons.
Water is used because Hydrogen is similar in size to neutrons and efficiently transfers KE of neutrons to the water.
Describe Moderating Ratio.
Ratio of microscopic cross section for scatter to microscopic cross section for absorption for a particular material.
Benefits of using water as a moderator.
Plentiful
Cheap
Coolant
Properties of an ideal moderator.
Large scattering cross section.
Small absorption cross section.
Large energy loss per collision.
High atomic density.
Describe Moderator Density Effects.
Temp changes also change density of moderator which changes the number of molecules available per volume of moderator -> changes the amount of scattering interactions per volume.
Define Atomic Density.
Number of atoms per volume.
N=pN(a)/M
N=Atomic Density (atoms/cm^3)
p=density (g/cm^3)
N(a)=Avogadros number
M=gram atomic weight.
Define Microscopic Cross Section
Probability of a particular reaction between a neutron and a nucleus.
Define Barns.
Unit of microscopic cross section.
1 barn=10^-24 cm^2
Define Macroscopic Cross Section (Sigma)
Probability a neutron will interact per unit travel of the neutron within a certain material. Or in a certain volume.
Sigma=N•mac
N=atomic density
Mac=macroscopic cross section.
Define Mean Free Path (lambda).
Average distance traveled by a neutron in a particular material prior to interaction.
Inverse of macroscopic cross section.
Lambda=1/sigma.
Define Neutron Flux (phi).
Number of neutrons passing thru a unit area (cm^2) per second. n/cm^2•sec
Phi=n•v
n=neutron density
v=neutron velocity.
What is Thermal Neutron Flux?
Same thing as Neutron Flux but only pertaining to Thermal Neutrons.
This is the same concept for Fast Neutron Flux.
Define Fast Neutron
Fission neutrons are born fast.
>10^5 eV
Define Intermediate Neutrons
Neutrons with energy levels between 1 eV and .1 MeV (10^5 eV).
Define Thermal Neutrons
Neutrons with energy of 0.025 eV at 68F.
Note: Slow Neutrons have energy <1 eV. All Thermal Neutrons are considered Slow Neutrons.
Describe Neutron Energies vs Cross Sections.
Fast neutrons have low cross sections for absorption.
Resonance region (intermediate neutrons) cross sections gradually rise as neutron slow, with various high resonance peaks.
1/v region (slow neutrons) cross sections rise steadily as neutron slows.
How do changes in Macroscopic Cross section and Neutron Flux affect reaction rate?
Over core life fuel is depleted. As this occurs, the macroscopic cross section for fission lowers (due to lower fuel density) and so does reaction rate. Therefore in order to maintain the same reaction rate over core life, flux will have to be continually raised to compensate.
R=sigma•phi
Sigma=Mac•N
N=atomic density.
Relationship between neutron flux and reactor power.
Multiply reaction rate by total volume of the core to get reactor power.
P=[phi(th)•sigma(f)•V]/3.12x10^10 fissions per watt-sec
phi(th)=thermal neutron flux (n/cm^2•sec)
sigma(f)=Macroscopic cross section for fission (cm^-1)
V=volume of core (cm^3)
P=power (watts)
