Detekce a spektrometrie neutronů. neutron detection and spectroscopy

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

2 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

3 a) Active detectors Reactions E very small ~1 MeV, nonrelativistic kinematics (B: 80% 11 B, 20% 10 B ( E is neglected, neutron velocity v is small ) Cross section: ~1/v, structureless, thermal cross section is ~3840 barns 3

4 Large tubes α and Li fully absorbed 4

5 α particle 5

6 Anode diameter `0.1 mm, operated voltage V anode 6

7 7

8 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

9 Scintillator: lithium iodide LiI (Eu), Eu as an activator similar to NaI(Tl) 9

10 Detectors: 3 He proportional tube 10

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

12 Detectors: Energy spectra of fission fragments emerging from flat UO 2 deposits 12

13 13

14 Fission cross section vs neutron energy 14

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

16 Rate R of neutron interactions in the foil microscopic cross section Σ act = σ act n, n number of foil nuclei in 1 cm 3 (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

17 t 1 > t 0 17

18 The number of counts: neutron flux 18

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20 R depends on the cross sections 20

21 Decay constants ( ~half time) Nature of induced activity 2.7 days Other materials : Mn, Ag, Cu, Co γ decay metallic foils or wires 21

22 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

23 c) Mechanical monochromators (mechanical selector) Princip: time of flight metods slit Neutron detector - Several wheels k 1, k n 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= mv 2 /2, in the detector- monochromatic beam 23

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

25 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 he 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

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

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

28 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

29 General formula after n-scattering on hydrogen Lethargy u= ln E 0 ln E average u(θ) θ θ cms 29

30 Average lethargy change after one scattering is constant! Slowing down from energy E 0 to E how many collisions? 30

31 Fast neutron moderated and captured moderator Thermal neutron detector BF 3 tube, LiI scintillators 3 He tubes Fast neutron partly moderated and escaping without reaching the detector Neutron captured by the moderator 31

32 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

33 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

34 Coincidence exists No coincidence 34

35 Competing reactions: simple elastic neutron scattering from helium nuclei cross section >> for (n,p) reaction (n.d) reaction for E >4.3. MeV 35

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

37 Detectors: 37

38 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

39 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

spectrum of the recoil nuclei is measured For fixed incident neutron

40 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 41

42 4 He 42

43 Detectors: 43

44 Recoil proton telescope: neutron scattering of hydrogen Φ lab 44

45 DETEKCE 45

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47 47

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

50 Spektrometrie neutronů 50

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