Detection of Neutrons

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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

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Nội dung Text: Detection of Neutrons

XI. Detection of Neutrons •  Remarks •  Slow neutron detection •  Fast neutron detection Spring 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 reactor Spring 2010 Radiation Detection & Measurements 3
Neutron- Energies – II. Thermal neutrons at room temperature: 1/40 eV = 25 meV ~ 2200 m/s Spring 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 + γ E
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-1/2 Spring 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 detector n + 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 Counters n + 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 energy response reducing discrimination abilities –  Continuous background due to α-decay • Trade-off between efficiency (thick layer) and response/quantitative measurement (thin layer) Spring 2010 Radiation Detection & Measurements 11
Li-based scintillators n + 6Li (7Li)* 4He + 3H, Q = 4.78 MeV K.E. 2.05 MeV + 2.74 MeV •  6Li loaded materials: –  No stable lithium containing gas available  Li-loaded scintillators: –  Solid LiI(Eu) [similar to NaI(Tl)] •  470nm, 51k photons/ MeV •  No wall effects •  Small detectors with ~ 100% efficiency (En < 0.5eV) 6Li-glass:slow and fast neutron •  Single peak at Q-value with continuous detection… γ-background (Ee=4.1MeV ~ ECP=4.8) Glass-based scintillation detectors can be –  Liquids: n-γ pulse-shape implemented as bulk and as long fibers (~ meters)! discrimination possible! Spring 2010 Radiation Detection & Measurements 12
Efficiencies and more … Efficiency ε: Σ: Macroscopic cross section Mean free path λ=1/Σ Σtot = Σabs+Σscat [1/cm] Sensitivity S: x: thickness I=nv: beam intensity S = true net counting rate / (neutron flux) A: target area = r/φ [cts/s per neutron /(m2 s)] ρN: atom density of target (N) r = εp R = G – B Complementary definitions: Assume εp = 1, B = 0  R = G and σ(E)=σ0v0/v(E)=σ0√(E0/E) Number of collisions per second: S = R/φ = NVσ0v0/v R = σ I ρN A x; with ρNAx = # of nuclei in target (NVφσ0v0/v) S ~ density (pressure) and volume Collision density: F = I ρN σ = Ι Σ εp: charged particle detection efficiency Also in terms of neutron flux φ: R: reaction rate: NVφσ0v0/v F = φ Σ (= fission rate) G: Gross counting rate B: Background counting rate Power = energy/fission x fission rate x volume Spring 2010 Radiation Detection & Measurements 13
Fast Neutron Detection •  All neutron detection relies on observing a neutron- induced nuclear reaction •  The capture cross sections for fast-neutron induced reactions are small compared to those at low energies (σcap ~ 1/v) •  Two approaches to detect fast neutrons: –  Thermalize/ moderate & capture as before, only providing count rates (i.e. neutron flux) –  Elastic scattering from protons at high energy •  Protons are easy to detect in conventional detectors •  Observe recoils for time-of-flight (ToF) enabling neutron energy measurements by measuring the velocity through ToF. Spring 2010 Radiation Detection & Measurements 14
Counters Based on Neutron Moderation •  Moderate neutrons to increase efficiency in conventional slow-neutron detectors •  Moderation with hydrogenous materials such as polyethylene or paraffin •  Optimum thickness between few cm to tens of cm for energies of keV to MeV •  Trade-off between sufficient slow down and detection cross section Spring 2010 Radiation Detection & Measurements 15
The Bonner Sphere – Spherical Dosimeter •  A 12” diameter sphere has – coincidentally -similar response curve as neutron dose spectrum in tissue (e.g. with LiI(Eu) scint. in center) •  Determine dose equivalent due to neutrons with an unknown or variable neutron spectrum over large range of neutron energies Spring 2010 Radiation Detection & Measurements 16
The Long Counter •  Goal: –  Neutron-energy independent efficiency (flat-response detectors) •  Long-counter: –  Slow-neutron BF3 detector in center –  Paraffin (moderator) + B2O3 (absorber) “shield” –  Sensitive only to neutrons from one side (right) Spring 2010 Radiation Detection & Measurements 17
Fast Neutron Detection via Nuclear Reactions •  So far, “moderation” based detectors: – High efficiency but slow (due to moderation) and no energy information from neutron •  Fast neutron induced reaction can be fast and can provide energies assuming En is not small compared to Q (however, much smaller cross section at higher neutron energies) – 6Li(n,α) and 3He(n,p) reactions of interest Spring 2010 Radiation Detection & Measurements 18
3He Proportional Counter •  3He(n,p) reactions due to –  slow neutrons (moderated in detector environment) leading to epithermal peak Pulse-height spectrum by –  Fast reactions (no neglecting wall effects moderation) leading to full energy peak: En + Q •  Elastic scattering n → 3He leading to recoil distribution with maximum energy ER = 0.75 En –  Cross section is larger than (n,p) reaction Spring 2010 Radiation Detection & Measurements 19
Fast Neutron Scattering and Kinematics •  Most common detection method for fast neutrons is by elastic scattering of neutrons on light nuclei producing a recoiling nucleus that can be easily detected (hydrogen → proton recoil) •  Elastic scattering   Q-value not important   Target nuclei masses are important!   How much energy can be deposited per interaction ? Spring 2010 Radiation Detection & Measurements 20