Detekce a spektrometrie
neutronů
neutron detection and
spectroscopy
1. Slow neutrons
2. Fast neutrons
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1. Slow neutrons
neutron kinetic energy E
a)
charged particles are produced , protons, α particle,
or heavy fragments
b) passive detectors – activation foils
c) mechanical monochromators
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a) Active detectors
Reactions
E very small ~1 MeV, nonrelativistic kinematics
(B: 80%
𝐵, 20% 10𝐵
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( E is neglected, neutron velocity v is small )
Cross section: ~1/v, structureless, thermal cross section is ~3840 barns
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Large tubes – α and Li fully
absorbed
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α particle
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Anode diameter `0.1 mm, operated voltage 2000-3000 V
anode
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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:
𝟑
𝑯𝒆 𝒑𝒓𝒐𝒑𝒐𝒓𝒕𝒊𝒐𝒏𝒂𝒍 𝒕𝒖𝒃𝒆
10
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 U𝑶𝟐 deposits
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Fission 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
𝐦𝐢𝐜𝐫𝐨𝐬𝐜𝐨𝐩𝐢𝐜 𝐜𝐫𝐨𝐬𝐬 𝐬𝐞𝐜𝐭𝐢𝐨𝐧 𝜮𝒂𝒄𝒕 = 𝝈𝒂𝒄𝒕 𝒏, 𝒏 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 foil nuclei in 1 𝒄𝒎𝟑
(assumption: the neutron flux remains unperturbed, OK. for thin foils)
From R ⟹ information about 𝐭𝐡𝐞 𝐟𝐥𝐮𝐱 𝝋
Decays of produced neutron induced nuclei: the rate is λN
N total number of present radioactive nuclei, λ decay constant
The rate of change of N is dN/dt
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𝒕𝟏 > 𝒕𝟎
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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
2.7 days
γ decay
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
slit
-
Neutron detector
Several wheels 𝒌𝟏 , … 𝒌𝒏 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= m𝒗𝟐 /𝟐, in the detector- monochromatic 23
beam
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
an estimation of the incident energies of fast neutrons.
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Slowing down of neutrons
E
neutron
θ𝑙𝑎𝑏
V velocity of
CM system
𝐸𝐴
neutron
nucleus(A)
CM system:
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E scattered neutron kinetic energy
Scattering on protons, A=1
Recoil nucleus energy
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 𝑬𝟎 − 𝒍𝒏 𝑬 ∝
average u(θ)
θ≡𝜽𝒄𝒎𝒔
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Average lethargy change after one scattering is constant !
Slowing down from energy 𝐸0 to 𝐸 ′ − 𝒉𝒐𝒘 𝒎𝒂𝒏𝒚 𝒄𝒐𝒍𝒍𝒊𝒔𝒊𝒐𝒏𝒔?
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moderator
Fast neutron moderated
and captured
Thermal neutron detector
B𝑭𝟑 𝒕𝒖𝒃𝒆, 𝑳𝒊𝑰 𝒔𝒄𝒊𝒏𝒕𝒊𝒍𝒍𝒂𝒕𝒐𝒓𝒔
𝟑
𝑯𝒆 tubes
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 detector
No 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 neutrons
Two reaction of major importance
Other detectors: based on the activation methods
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Detection: sum of energies = a peak
Suitable for moderate energies, at higher energies a competing reaction
for E> 2.5 MeV,
detection: a continuum of deposited energy
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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Fast neutrons which lost energies in the external materials
Elastic scattering
(n.p) reaction
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Detectors:
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Activation counters for fast neutrons
a) 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
energy of recoil nucleus
E
neutron
nucleus
Φ (E) neutron flux, E primary neutron energy
The energy spectrum of the recoil nuclei is measured
For fixed incident neutron energy
E is continuous:
Computer program which solves this equation for Φ (E)
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𝟒
𝑯𝒆
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Detectors:
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Recoil proton telescope: neutron scattering of hydrogen
𝜱𝒍𝒂𝒃
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DETEKCE
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Účinný resonanční průřez (n,γ) pomalých neutronů na rhodiu
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Spektrometrie neutronů
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