Neutron detectors and spectrometers

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: 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 Bonner spheres at NPL (Great Britain)

Medipix-2

Usage of neutronography

Used reactions: neutron + nucleus → reflected nucleus proton deuteron triton alpha particle fission products Very strong dependency of cross section on energy Compound detectors: 1) Convertor – creation of charged particles 2) Detector of charged particles Requierements on material of convertor and detector: 1) Large cross section of used reaction 2) High released energy (for detection of low energy neutrons) Complicated structures of convertor and detector ITEP CTU 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)

Detectors of slow neutrons

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 1) Detectors based on reactions with boron: A) BF 3 proportional chambers BF 3 serve as neutron convertor and also as gas filling of proportional counter High enrichment by 10 B isotope Low efficiency to gamma rays B) Boron on walls and alternative gas filling C) Scintillators with boron contents Pulse height H 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

Crystal diffraction spectrometers and interferometers

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 Monochromators utilizing reflection

Mechanical monochromators

rotated absorption discs – properly placed holes very accurate measurement of energy of low energy neutrons

Detectors of fast neutrons

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

Detectors and spectrometers based on neutron elastic scattering

Scintillation (for example NE213): Response L:

L



k



E

3 2

From that we obtain:

dL dE

 3 2

k



E

1 2

Energy derived from response: If:

dN dE



konst

then:

E

 1

L

3

k

2

dN dN dL



dE dL dE



konst

3 2

kE

2 1 

kons t



L

 1 3 

(for neutron scattering with E < 10 MeV) on protons Other factors: 1) influence of edges 2) multiple scattering 3) scattering on carbon 4) detector resolution 5) competitive reactions for higher E n Dependency of response on energy Dependency of response change with energy on energy Energy distribution of reflected nuclei (protons) Distribution of response at detectors

Neutron spectrometer based on reflected protons

1) Detection and determination of reflected proton energy E p .

2) Usage of reflection angle ψ knowledge Wide set of used detectors Problems: 1) Proper target size 2) Accuracy of angle determination target with high content of hydrogen ψ Detector of protons

TOF spectrometers

The most accurate determination of neutron energy

E KIN  E 0   1 1   2  1   β  v c  L tc σ E KIN  β 2 1  β 2 (E KIN  E 0 ) σ L L 2  σ t t 2

Problem of interaction point and detector thickness d = 4,3 m Δd = 0,25 m, Δt = 350 ps



Usage of inorganic scintillators for detection of relativistic neutrons: E[GeV] ΔE/E 0,1 0,02 1.5 0.15

TOF neutron spectrum from Bi + Pb collision (E = 1 GeV/A)

 (

E

)   0 (

E

)

e

  (

E

)

L THR

Response of BaF 2 neutrons detector on relativistic Dependency of BaF 2 efficiency on neutron energy for different thresholds Comparison of elmg a hadron showers

Activation detectors of neutrons

Sandwiches of foils from different materials (mostly monoisotopic) Usage of different threshold reactions → determination of neutron spectra 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

Induced fission & emulsion

Combination of 235 U, 238 U, 208 Pb Counting of ionization tracks number produced by fission fragments