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1
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
1) Complicated reactions ? strong dependency of
efficiency on energy
Consequence
2) Small efficiency ? necessity of large volumes
3) Only part of energy is loosed ? complicated
energy determination ? common usage of TOF
Usage of neutronography
Bonner spheres at NPL (Great Britain)
2
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
Complicated structures of convertor and
detector ITEP CTU
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)
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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) BF3 proportional chambers
BF3 serve as neutron convertor and also as gas
filling of proportional counter
High enrichment by 10B isotope
Low efficiency to gamma rays
B) Boron on walls and alternative gas filling
C) Scintillators with boron contents
Usage of possibility to distinguish neutrons and
photons by pulse shape
2) Detectors based on 6Li reactions
3) Detectors based on 3He reactions
proportional counters convertor is also filling
4) Detectors based on fission
4
Crystal diffraction spectrometers and
interferometers
1) Determination of neutron energy 2)
Determination of crystal structure
Usage of diffraction
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
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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
Bonner spheres at NPL (England) their usage at
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
Advantages simplicity, wide energy range
Disadvantages Very small energy resolution
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Detectors and spectrometers based on neutron
elastic scattering
Scintillation (for example NE213)
Response L
From that we obtain
Energy derived from response
If
then
(for neutron scattering with E lt 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 En
Dependency of response on energy
Distribution of response at detectors
Energy distribution of reflected nuclei (protons)
Dependency of response change with energy on
energy
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Neutron spectrometer based on reflected protons
1) Detection and determination of reflected
proton energy Ep.
2) Usage of reflection angle ? knowledge
Wide set of used detectors

1. Proper target size
2. Accuracy of angle determination

Problems
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TOF spectrometers
The most accurate determination of neutron energy
Problem of interaction point and detector
thickness
TOF neutron spectrum from Bi Pb collision (E
1 GeV/A)
EGeV ?E/E 0,1 0,02 1.5 0.15
d 4,3 m ?d 0,25 m, ?t 350 ps ?
Usage of inorganic scintillators for detection of
relativistic neutrons
Response of BaF2 detector on relativistic
neutrons
Dependency of BaF2 efficiency on neutron energy
for different thresholds
Comparison of elmg a hadron showers
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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,?) 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 235U, 238U, 208Pb
Counting of ionization tracks number produced by
fission fragments