Detekce a spektrometrie neutronů

neutron detection and spectroscopy

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1. Slow neutrons

2. Fast neutrons

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1. Slow neutrons neutron kinetic energy E

a)

b) passive detectors – activation foils

c) mechanical monochromators

charged particles are produced , protons, α particle, or heavy fragments

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a) Active detectors

Reactions

E very small ~1 MeV, nonrelativistic kinematics

( E is neglected, neutron velocity v is small )

Cross section: ~1/v, structureless, thermal cross section is ~3840 barns

(B: 80%

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Large tubes – α and Li fully absorbed

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α particle

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anodeAnode diameter `0.1 mm, operated voltage 2000-3000 V

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Final state nuclei are always in the ground states , the total energy sum of tricium and α particle will give a signal of the form of a peak.The scintillation process is used for the detection of the product of neutron induced reactions or the products are detected by semiconductor detectors in coincidences.

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Scintillator: lithium iodide LiI (Eu) , Eu as an activator similar to NaI(Tl)

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Detectors:

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MeV

Fission nuclei: almost all α radioactive the signal from α particles << signal from fission products

good separation of both signals

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Detectors:

Energy spectra of fission fragments emerging from flat U deposits

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14Fission cross section vs neutron energy

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b) Passive detectors – activation foils

The measured radioactivity ⟹ determination of the neutron flux and the energy spectrum

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Rate R of neutron interactions in the foil

(assumption: the neutron flux remains unperturbed, OK. for thin foils)

From R ⟹ information about Decays of produced neutron induced nuclei: the rate is λN N total number of present radioactive nuclei, λ decay constantThe rate of change of N is dN/dt

foil nuclei in 1

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The number of counts:

neutron flux

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R depends on the cross sections

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Decay constants ( ~half time)

Nature of induced activity

γ decay2.7 days Other materials : Mn, Ag, Cu, Co

metallic foils or wires

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Thermal neutrons: the cross section ~1/v but resonances at higher energies > 1 eV

Observed activity corresponds to the mixture of thermal neutrons and neutrons with higher energies

Separation: cadmium difference method (n +Cd) cross section large for E<0.4 eV, then the sharp decrease A thickness of 0,5 mm act as a selective filter, i.e. it blocks the thermal neutrons whereas the neutrons with E>0.4 eV passes the filter

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c) Mechanical monochromators (mechanical selector) Princip: time of flight metods

- Several wheels with Cd, same distances l, mounted on a common - drive shaft- In each wheel an empty slit , slits are regularly shifted by an angle φ - Rotation with angular frequency ω- Shift by φ in time t= φ/ω - In time t neutrons passes distance l with the velocity v= l/t

- they have energy E= m, in the detector- monochromatic beam

slitNeutron detector

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2. Fast neutrons

a) Detection using neutron moderation

b) Direct detection of fast neutron reactions

c) Detection using fast neutron scattering

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a) Detection using neutron moderation Reaction of fast neutrons which produce detectable charged secondary particles similarly as for slow neutrons could be used. But the cross sections for fast neutrons are very small detection efficiencies of corresponding detectors are small

The fast neutron can be detected by the devices developed for slow neutron, if they are surrounded by a moderator, where fast neutrons are slowed down to the energies of thermal neutrons.

This method can be used for the detection of fast neutrons, but cannot be used for an estimation of the incident energies of fast neutrons.

nucleusneutron

CM system:

V velocity of CM systemθ 𝑙𝑎𝑏

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Slowing down of neutrons

E

𝐸 𝐴

neutron

(A)

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E scattered neutron kinetic energy

Recoil nucleus energy

Scattering on protons, A=1

Slowing down is more efficient on light nuclei

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Energy distribution of neutrons

Assumption: isotropic angular distribution in CMS (valid for E< 15 MeV) probability of scattering into a CMS solid angle Ω

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General formula after n-scattering on hydrogen

Lethargy u= ln

average u(θ) θ≡

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Average lethargy change after one scattering is constant !

Slowing down from energy to

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Thermal neutron detector B tubes

moderator

Fast neutron moderated and captured

Fast neutron partly moderated and escaping without reaching the detector

Neutron captured by the moderator

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b) Direct detection of fast inelastic neutron reactions

Slowing down ⟹ eliminates all information on the original energy of the fast neutrons process is slow, no fast response of the detectorNo moderation ⟹ direct detection of the reaction products direct energy measurement of the product energies sum of energies = incident neutron energy fast signals but the cross section are orders of magnitude lower then for thermal neutronsTwo reaction of major importanceOther detectors: based on the activation methods

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Suitable for moderate energies, at higher energies a competing reaction

for E> 2.5 MeV, detection: a continuum of deposited energy

Detection: sum of energies = a peak

Detector: lithium sandwich spectrometer

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Coincidence exists

No coincidence

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Competing reactions: simple elastic neutron scattering from helium nuclei cross section >> for (n,p) reaction (n.d) reaction for E >4.3. MeV

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Elastic scattering(n.p) reaction

Fast neutrons which lost energies in the external materials

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Detectors:

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Activation counters for fast neutronsa) slow neutron activation materials (Ag, Rh) inside a moderating structure

The counter is placed within a polyethylene moderator

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b) Use threshold activation materials and to rely on direct activation by the fast neutrons without moderation

e.g. NaI scintillator, which provides NaI nuclei and detects β and γ from the F product

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c) Detection using fast neutron scattering

nucleusneutron

Φ (E) neutron flux, E primary neutron energyThe energy spectrum of the recoil nuclei is measured

energy of recoil nucleus

For fixed incident neutron energy

E

E is continuous:

Computer program which solves this equation for Φ (E)

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𝑯𝒆❑𝟒

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Detectors:

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Recoil proton telescope: neutron scattering of hydrogen

𝜱𝒍𝒂𝒃

DETEKCE

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Účinný resonanční průřez (n,γ) pomalých neutronů na rhodiu

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Spektrometrie neutronů

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