Detection 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
Nội dung trích xuất từ tài liệu:
Detection of Neutrons XI. Detection of Neutrons•  Remarks•  Slow neutron detection•  Fast neutron detectionSpring 2010 Radiation Detection & Measurements 1 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 2 Neutron- Energies – I. Neutron energies form 1 MW research reactorSpring 2010 Radiation Detection & Measurements 3 Neutron- Energies – II. Thermal neutrons at room temperature: 1/40 eV = 25 meV ~ 2200 m/sSpring 2010 Radiation Detection & Measurements 4 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): 56Fe + n (elastic scattering) 56Fe + n’ (inelastic scattering) 56Fe +n→ (57Fe)* 57Fe + γ (radiative capture) 55Fe + 2n (n,2n reaction) –  Resonances: •  Compound nucleus formed in excited state Spring 2010 Radiation Detection & Measurements 5 Slow Neutron Detection•  Cross-section for elastic (potential) scattering : σe = 4πR2•  Cross-section for capture reaction follows characteristic 1/v dependence for low neutron energies•  The form can be derived from Breit-Wigner resonance lineshape (single level resonance formula), e.g. neutron capture and capture-independent gamma-ray emission (radiative capture): 197Au + n → 198Au + γE Commonly Used Neutron Reactions n + 3He → (4He)* → p + 3H, Q = 0.765 MeV, target abundance ~ 1.4x10-4 % (5.3 kb) (n,p) n + 6Li → (7Li)* → 4He + 3H, Q = 4.78 MeV, target abundance ~ 7.5% (940 b) (n,α) n + 10B → (11B)* → 7Li* + 4He, Q = 2.31 MeV, 94% branch, nat. abund. ~20 % (3.8kb) (n,α) → 7Li + 4He, Q = 2.79 MeV, 6% branch n + 113Cd → (114Cd)* → 114Cd + γ, Q ~ 8 MeV, target abundance ~ 12% (21 kb) (n,γ) n + 157Gd → (158Gd)* → 158Gd + γ, Q ~ 8 MeV, target abundance ~ 16% (255 kb) (n,γ) n + 235U → (236U)* → (fission fragments), Q ~ 200 MeV, target abundance ~ 0.7% (n,f) 5500 barns ! ~ 1/v ~ E-1/2Spring 2010 Radiation Detection & Measurements 7 Gas-Filled Detectors•  Common use: Gas-proportional counters 3He gas: W ~ 33 eV Typical 25 mm diameter tube, 50 µm anode P=5-10 bar (!) V ~ 1.5 kV… M ~ 20 … C ~ 20 pF Tcollection ~ 50 µs (due to slow ions)Spring 2010 Radiation Detection & Measurements 8 The 3He Proportional Counter The wall effect•  n + 3He → p + 3H, Q = 764 keV (3H = triton (t)) Thermal peak Lost p Lost 3H•  Assume En The BF3 slow neutron detectorn + 10B → (11B)* 7Li* + 4He, Q = 2.31 MeV [94%] “Ideal” response: large tube, all reaction products absorbed in gas volume.K.E. 0.84 MeV + 1.47 MeV 7Li + 4He, Q = 2.79 MeV [6%]•  BF3 gas, enriched to >90% of 10B•  Operated as proportional or G-M counter•  However, recombination and formation of negative ions require lower pressure P < 1atm –  Range of α-particles ~ 10 mm Obs. response due to partial energy loss in tube walls –  Pronounced wall effect•  As in 3He tube, spectrum reflects response of detector, NOT neutron energy BF3 counters: P ~ 0.5 – 1 atm 2000 – 3000 V M ~ 100-500 Spring 2010 Radiation Detection & Measurements 10 Fission Countersn + 235U (236U)* (fission fragments),Q ~ 200 MeV, Total kinetic energy (TKE) ~ 160 MeV• 235U coated ionization chamber or proportional counter• Large energy deposition per captured neutron –  Efficient discrimination between neutrons and backgrounds –  Non-linearities due to high ionization density limit ...