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
Medipix-2
Bonner spheres at NPL (Great Britain)
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
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)
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
Pulse height H
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
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
Monochromators utilizing reflection
neutron diffractometer of NPI CAS
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
Advantages: simplicity, wide energy range
Disadvantages: Very small energy resolution
Bonner spheres at NPL (England)
their usage at spectrometry
Detectors and spectrometers based on neutron elastic scattering
Scintillation (for example NE213):
Response L:
L  kE
3
2
From that we obtain:
Energy derived from response:
If:
dN
 konst
dE
2
1
E  L3
k
then:
1
dL 3
 kE 2
dE 2
dN
1

dN dE konst
3




kons
t

L

dL dL 3 12
kE
dE 2
(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 En
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 Ep.
ψ
2) Usage of reflection angle ψ knowledge
target with high
content of
hydrogen
Wide set of used detectors
Problems:
1) Proper target size
2) Accuracy of angle determination
Detector of
protons
TOF spectrometers
The most accurate determination of neutron energy
E KIN
 1

 E0 
 1
 1  2



v L
β 
c tc
2
σ E KIN
β2
σ  σ 

(E KIN  E 0 )  L    t 
2
1 β
 L   t 
2
Problem of interaction point and detector thickness
E[GeV] ΔE/E
d = 4,3 m Δd = 0,25 m, Δt = 350 ps  0,1
0,02
0.15
Usage of inorganic scintillators for detection 1.5
of relativistic neutrons:
TOF neutron spectrum from Bi
+ Pb collision (E = 1 GeV/A)
 (E)   0 (E)e ( E ) L
THR
Response of BaF2 detector on relativistic Dependency of BaF2 efficiency on
neutrons
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 235U,
238U, 208Pb
Counting of ionization tracks
number produced by fission
fragments