Spring 2010 Radiation Detection & Measurements 1
XI. Detection of Neutrons
• Remarks
• Slow neutron detection
• Fast neutron detection
Spring 2010 Radiation Detection & Measurements 2
Reminder: Interactions of Neutrons
• As charge-neutral particles, neutrons can only interact via
strong interactions and ionize via secondary reactions
• Most neutron detectors consist of a material that converts
neutrons into charged particles within a conventional radiation
detector
• We have to distinguish two classes of interactions:
– Slow neutrons (thermal and epithermal, E < 1 keV)
• Radiative capture (n,γ)
• Charged particle production reaction (n,p), (n,α), …
• Neutron-capture induced fission (235U, 239Pu, …)
– Fast neutrons (E > 1 keV)
• Elastic scattering (n,n)
• Inelastic scattering (n,n’)
• Charged particle production (n,xn), (n,xpn), fission, …
Spring 2010 Radiation Detection & Measurements 3
Neutron- Energies – I.
Neutron energies form 1 MW research reactor
Spring 2010 Radiation Detection & Measurements 4
Neutron- Energies – II.
Thermal neutrons at room temperature:
1/40 eV = 25 meV ~ 2200 m/s
Spring 2010 Radiation Detection & Measurements 5
Compound Nucleus Formation
• Most neutron induced reactions proceed in two steps:
– Neutron-capture into compound nucleus
– Compound nucleus may decay in different ways
(dependent on Q-value and n-energy):
– Resonances:
• Compound nucleus formed in excited state
56Fe + n → (57Fe)*
56Fe + n (elastic scattering)
56Fe + n’ (inelastic scattering)
57Fe + γ (radiative capture)
55Fe + 2n (n,2n reaction)