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

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 structuresof convertor and detectorITEP CTU

Detectors of slow neutrons

1) Detectors based on reactions with boron:

High enrichment by 10B isotope

BF3 serve as neutron convertor and also as gas filling of proportional counter

A) BF3 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 raysExoenergy 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 6Li reactions

3) Detectors based on 3He reactions – proportional counters – convertor is also filling4) Detectors based on fission

Pulse height H

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

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

Detectors and spectrometers based on neutron elastic scattering

Scintillation (for example NE213):

Response L: 23EkL

21

2

3Ek

dE

dL

konstdE

dN

3

21Lk

E

From that we obtain:

Dependency of response on energy

Energy derived from response:

If: then:

(for neutron scattering with E < 10 MeV) on protons

3

1

2

1

2

3Ltkons

kE

konst

dE

dLdE

dN

dL

dN

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 En

1) Detection and determination of reflected proton energy Ep.

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 size2) Accuracy of angle determination

TOF spectrometers

The most accurate determination of neutron energy

1

1

1EE

20KIN tc

L

c

vβ

2

t

2

L0KIN2

2

E t

σ

L

σ)E(E

β1

βσ

KIN

Response of BaF2 detector on relativistic neutrons

Dependency of BaF2 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/E0,1 0,021.5 0.15

THRLEeEE )(0 )()(

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 235U, 238U, 208Pb

Counting of ionization tracks number produced by fission fragments