Neutron Interactions with Matter

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Neutron Interactions with Matter

• Professor Jasmina Vujic
• Lecture 9 - Chapter 9
• Nuclear Engineering 162
• Department of Nuclear Engineering
• University of California, Berkeley

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NEUTRON INTERACTIONS WITH MATTER

• Neutrons are neutral and hence can travel
  appreciable distances in matter just like photons
  without interacting
• Neutrons are mostly exponentially by matter -
  (for small thicknesses and un-collided beams)
• Neutrons can also collide with atomic nucleus,
  which can scatter it, either elastically or
  inelastically
• Elastic Scattering - when the kinetic energy is
  conserved Inelastic Scattering (n, n') when
  the target nucleus absorbs some energy internally
  and acquire excited state

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NEUTRON INTERACTIONS WITH MATTER

• A neutron can be captured or absorbed by a
  nucleus leading to (n, p), (n, 2n), (n, a), (n,f)
  or (n, ?) Charged-Particle Reactions, Neutron
  Producing Reactions and Radiative Capture
• A fast neutron loses energy in matter by a series
  of mostly elastic scattering events. The slowing
  down process is called neutron moderation
• As the neutron slows down, the capture
  probability by nucleus increases
• As the neutron slows down to thermal energies, it
  will move about randomly by elastic scattering
  until absorbed by a nucleus

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ELASTIC SCATTERING

• Elastic scattering is most important process for
  slowing down neutrons the contribution by
  inelastic scattering is usually small in
  comparison
• The maximum energy that a neutron of mass m and
  kinetic energy can transfer to a nucleus M in a
  single (head-on) elastic collision is given by

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Maximum Fraction of Energy Lost, Qmax/En
byNeutron in a single collision
The average number of elastic scattering
collisions needed for 2 MeV neutron to slow down
to thermal energies 1H 18 collisions 2H 25
collisions 4He 40 collisions U gt 2000 collisions
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Neutron Interactions and Cross Sections

• Neutrons may interact with nuclei in one or more
  of the following ways
• Elastic Scattering, (n,n)
• Inelastic Scattering, (n,n)
• Radiative Capture (absorption) (n,?)
• Charged-Particle Reactions (absorption) (n,a)
  (n,p)
• Neutron-Producing Reactions (absorption) (n,2n)
• Fission (absorption) (n.f)

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NUCLEAR REACTIONS Cross Sections

• One projectile and one target
• n projectiles per cm3 and N targets per cm3

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NUCLEAR REACTIONS Reaction Rates

• The reaction rate is defined as the total number
  of collisions between an incoming particle
  (projectile) and nuclei (targets) per unit volume
  and unit time

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NUCLEAR REACTIONS Cross Sections

• Types of Cross Sections

Absorption cross section Scattering cross
section Total cross section
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Neutron Attenuation

• The intensity of the beam of uncollided neutrons
  after penetrating the distance x in the material
  with the total macroscopic cross section, Et

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Neutron Attenuation

• The average distance that a neutron moves between
  collisions is called the mean free path