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Neutron detectors and spectrometers 1) Complicated reactions → strong dependency of efficiency on energy 2) Small efficiency → necessity of large volumes.

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Presentation on theme: "Neutron detectors and spectrometers 1) Complicated reactions → strong dependency of efficiency on energy 2) Small efficiency → necessity of large volumes."— Presentation transcript:

1 Neutron detectors and spectrometers 1) Complicated reactions → strong dependency of efficiency on energy 2) Small efficiency → necessity of large volumes 3) Only part of energy is loosed → complicated energy determination → common usage of TOF 1) Introduction and basic principles 2) Detectors of slow neutrons (thermal, epithermal, resonance) 3) Detectors of fast neutrons 4) Detectors of relativistic and ultrarelativistic neutrons Detection of neutrons – by means of nuclear reactions where energy is transformed to charged particles or such particles are created Consequence: Medipix-2 Bonner spheres at NPL (Great Britain) Usage of neutronography

2 Compound detectors: 1) Convertor – creation of charged particles 2) Detector of charged particles Used reactions: neutron + nucleus → reflected nucleus proton deuteron triton alpha particle fission products Very strong dependency of cross section on energy Requierements on material of convertor and detector: 1) Large cross section of used reaction 2) High released energy (for detection of low energy neutrons) or high conversion of kinetic energy 3) Possibility of discrimination between photons and neutrons 4) Price of material production as cheap as possible A) Neutron counters – proportional counters, convertor is directly at working gas or as admixture, eventually as part of walls B) Scintillators – organic (reflected proton and carbon), dopey by convertor liquid (NE213) or plastic (NE102A) Complicated structures of convertor and detector ITEP CTU

3 Detectors of slow neutrons 1) Detectors based on reactions with boron: High enrichment by 10 B isotope BF 3 serve as neutron convertor and also as gas filling of proportional counter A) BF 3 proportional chambers B) Boron on walls and alternative gas filling C) Scintillators with boron contents Low efficiency to gamma rays Choice of material with large cross section for thermal and resonance neutrons Importance of low efficiency to gamma rays Exoenergy reactions → energy released at detector is given by reaction energy Energy is determined for example by time of flight Usage of possibility to distinguish neutrons and photons by pulse shape 2) Detectors based on 6 Li reactions 3) Detectors based on 3 He reactions – proportional counters – convertor is also filling 4) Detectors based on fission Pulse height H

4 Crystal diffraction spectrometers and interferometers Mechanical monochromators rotated absorption discs – properly placed holes Usage of diffraction: 1) Determination of neutron energy 2) Determination of crystal structure Usage of crystal bend for measured energy change neutron diffractometer of NPI CAS very accurate measurement of energy of low energy neutrons Monochromators utilizing reflection

5 Detectors of fast neutrons Usage of moderation to slow neutrons Plastic and liquid scintillators – simultaneously detection and moderation Bonner spheres: Bonner spheres at NPL (England) their usage at spectrometry organic moderator around neutron detector of thermal neutrons Different diameter – moderation of neutrons with different maximal energy Spectrometry: Reconstruction of spectrum from measured count rates from spheres with different diameters Advantages: simplicity, wide energy range Disadvantages: Very small energy resolution Simulation of response by means of Monte Carlo codes

6 Detectors and spectrometers based on neutron elastic scattering Scintillation (for example NE213): Response L:From that we obtain: Dependency of response on energy Energy derived from response: If:then: (for neutron scattering with E < 10 MeV) on protons Energy distribution of reflected nuclei (protons) Distribution of response at detectors Dependency of response change with energy on energy Other factors: 1) influence of edges 2) multiple scattering 3) scattering on carbon 4) detector resolution 5) competitive reactions for higher E n

7 1) Detection and determination of reflected proton energy E p. 2) Usage of reflection angle ψ knowledge ψ target with high content of hydrogen Detector of protons Neutron spectrometer based on reflected protons Wide set of used detectors Problems: 1)Proper target size 2)Accuracy of angle determination

8 TOF spectrometers The most accurate determination of neutron energy Response of BaF 2 detector on relativistic neutrons Dependency of BaF 2 efficiency on neutron energy for different thresholds TOF neutron spectrum from Bi + Pb collision (E = 1 GeV/A) Usage of inorganic scintillators for detection of relativistic neutrons: Comparison of elmg a hadron showers Problem of interaction point and detector thickness d = 4,3 m Δd = 0,25 m, Δt = 350 ps  E[GeV] ΔE/E 0,1 0,

9 Activation detectors of neutrons Sandwiches of foils from different materials (mostly monoisotopic) Usage of different threshold reactions → determination of neutron spectra Induced fission & emulsion Measurement of resonance neutrons for different (n,γ) reactions (attention: influence of neutron absorption at foil) Problem with spectrum reconstruction → possibility of direct comparison of activated nuclei numbers Advantages: simplicity, small sizes, possible put to small space Disadvantages: complicated interpretation Combination of 235 U, 238 U, 208 Pb Counting of ionization tracks number produced by fission fragments

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