Detekce a spektrometrie neutron neutron detection and spectroscopy

Detekce a spektrometrie neutronů neutron detection and spectroscopy 1. Slow neutrons 2. Fast neutrons 1

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 2

a) Active detectors Reactions E very small ~1 Me. V, nonrelativistic kinematics ( E is neglected, neutron velocity v is small ) Cross section: ~1/v, structureless, thermal cross section is ~3840 barns 3

Large tubes – α and Li fully absorbed 4

α particle 5

Anode diameter `0. 1 mm, operated voltage 2000 -3000 V anode 6

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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 8 reactions or the products are detected by semiconductor detectors in coincidences.

Scintillator: lithium iodide Li. I (Eu) , Eu as an activator similar to Na. I(Tl) 9

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Me. V Fission nuclei: almost all α radioactive the signal from α particles << signal from fission products good separation of both signals 11

Detectors: 12

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Fission cross section vs neutron energy 14

b) Passive detectors – activation foils The measured radioactivity ⟹ determination of the neutron flux and the energy spectrum 15

Rate R of neutron interactions in the foil (assumption: the neutron flux remains unperturbed, OK. for thin foils) 16

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The number of counts: neutron flux 18

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R depends on the cross sections 20

Decay constants ( ~half time) Nature of induced activity 2. 7 days γ decay Other materials : Mn, Ag, Cu, Co metallic foils or wires 21

Thermal neutrons: the cross section ~1/v but resonances at higher energies > 1 e. V 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 e. V, then the sharp decrease A thickness of 0, 5 mm act as a selective filter, i. e. it blocks thermal neutrons whereas the neutrons with E>0. 4 e. V passes the filter 22

c) Mechanical monochromators (mechanical selector) Princip: time of flight metods slit Neutron detector 23

2. Fast neutrons a) Detection using neutron moderation b) Direct detection of fast neutron reactions c) Detection using fast neutron scattering 24

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. 25

Slowing down of neutrons E neutron V velocity of CM system neutron nucleus(A) CM system: 26

E scattered neutron kinetic energy Scattering on protons, A=1 Recoil nucleus energy Slowing down is more efficient on light nuclei 27

Energy distribution of neutrons Assumption: isotropic angular distribution in CMS (valid for E< 15 Me. V) probability of scattering into a CMS solid angle Ω 28

General formula after n-scattering on hydrogen 29

Average lethargy change after one scattering is constant ! 30

moderator Fast neutron moderated and captured Fast neutron partly moderated and escaping without reaching the detector Neutron captured by the moderator 31

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 32

Detection: sum of energies = a peak Suitable for moderate energies, at higher energies a competing reaction for E> 2. 5 Me. V, detection: a continuum of deposited energy Detector: lithium sandwich spectrometer 33

Coincidence exists No coincidence 34

Competing reactions: simple elastic neutron scattering from helium nuclei cross section >> for (n, p) reaction (n. d) reaction for E >4. 3. Me. V 35

Fast neutrons which lost energies in the external materials Elastic scattering (n. p) reaction 36

Detectors: 37

Activation counters for fast neutrons a) slow neutron activation materials (Ag, Rh) inside a moderating structure The counter is placed within a polyethylene moderator 38

b) Use threshold activation materials and to rely on direct activation by the fast neutrons without moderation e. g. Na. I scintillator, which provides Na. I nuclei and detects β and γ from the F product 39

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) 40

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Detectors: 43

Recoil proton telescope: neutron scattering of hydrogen 44

DETEKCE 45

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Účinný resonanční průřez (n, γ) pomalých neutronů na rhodiu 49

Spektrometrie neutronů 50

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