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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 MeV, nonrelativistic kinematics (B: 80% 𝐵, 20% 10𝐵 11 ( 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 7 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. 8 Scintillator: lithium iodide LiI (Eu) , Eu as an activator similar to NaI(Tl) 9 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 11 Detectors: Energy spectra of fission fragments emerging from flat U𝑶𝟐 deposits 12 13 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 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 16 𝒕𝟏 > 𝒕𝟎 17 The number of counts: neutron flux 18 19 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 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 22 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 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 θ𝑙𝑎𝑏 V velocity of CM system 𝐸𝐴 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 MeV) probability of scattering into a CMS solid angle Ω 28 General formula after n-scattering on hydrogen Lethargy u= ln 𝑬𝟎 − 𝒍𝒏 𝑬 ∝ average u(θ) θ≡𝜽𝒄𝒎𝒔 29 Average lethargy change after one scattering is constant ! Slowing down from energy 𝐸0 to 𝐸 ′ − 𝒉𝒐𝒘 𝒎𝒂𝒏𝒚 𝒄𝒐𝒍𝒍𝒊𝒔𝒊𝒐𝒏𝒔? 30 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 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 MeV, 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. MeV 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. NaI scintillator, which provides NaI 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 41 𝟒 𝑯𝒆 42 Detectors: 43 Recoil proton telescope: neutron scattering of hydrogen 𝜱𝒍𝒂𝒃 44 DETEKCE 45 46 47 48 Účinný resonanční průřez (n,γ) pomalých neutronů na rhodiu 49 Spektrometrie neutronů 50 51 52 53