1 / 53

Low Energy Neutron Data for Nuclear Technology

Low Energy Neutron Data for Nuclear Technology. J.L. Tain Instituto de Física Corpuscular C.S.I.C - Univ. Valencia. IP EUROTRANS-ITC2 Santiago de Compostela, June 6-10, 2006. Layout of the lectures. Low energy neutron reactions: a catalogue

yuki
Download Presentation

Low Energy Neutron Data for Nuclear Technology

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Low Energy Neutron Data for Nuclear Technology J.L. Tain Instituto de Física Corpuscular C.S.I.C - Univ. Valencia IP EUROTRANS-ITC2 Santiago de Compostela, June 6-10, 2006

  2. Layout of the lectures • Low energy neutron reactions: a catalogue • R-Matrix formalism: short introduction. Data bases. • Cross section measurements: general ideas • Neutron beam production • Experimental techniques: • Total cross section • (n,) cross section • Fission cross section • Elastic cross section • Inelastic cross section • (n,xn) cross section • (n,p), (n,), … For energies going from meVto (say) 20 MeV

  3. Neutron reactions at low energies A neutron is absorbed to form a “compound nucleus”: n + AZ A+1Z* which lives for a short time and decays: A+1Z* n + AZ (elastic) Other contributions: A+1Z* A+1Z*’ + g (radiative capture) A+1Z* A1Z1* + A2Z2* + xn (fission) A+1Z* n + AZ* (inelastic) A+1Z* A+1-xZ* + xn (n multiplication) …

  4. The CN formation probability is higher for certain neutron energies En corresponding to quasi-bound or virtual states: resonances A Sn : neutron separation energy of CN ( <10MeV )  level separation D0~ 1 eV – 100 keV ER = Sn + En A+1 Life-time  Energy-width:   = n+ + f+ … ( = n +  + f + ...)  ~ 1 meV – 100 keV

  5. resolution log  1/v resolved unresolved overlapping log En Shape of neutron cross-section 1/v: thermal  < D0,  > E: resolved resonance region (RRR)  < D0,  < E: unresolved resonance region (URR)  > D0 : overlapping resonances In the RRR region,  is described using the R-Matrix formalism, in one of its usual approximations. In the URR region, average  are described by Hauser-Feshbach statistical theory At higher energies cross section are described using Optical Model and other reaction models It is a parametric approach since nuclear theory cannot predict the values. Experimental information is strictly necessary.

  6. R-Matrix n  (E) = 2 gJ n+197Au, =0, J=2 ER = 4.9 eV n = 15.2 meV  = 122.5 meV (E-ER’)2 + ¼2  (barn)  = / 2E 2J + 1 gJ = 2(2I + 1) En (eV) n(E) =  E/ERn(ER) , =0 Single Level Breit-Wigner Formalism: (n,) For -capture into an isolated spin J resonance at ER: (neutron wavelength) (spin stat. factor; |I -  ±½|  J  |I -  +½| ) (E) = n(E) +  + …; (FWHM) … and the channel radius Rc

  7. n n 4 1   = gJ[ sin2+ cos 2+ sin 2 ]  1 + 2  1 + 2 k2  n 4 1  = gJ  1 + 2 k2 238U (n,) 2  = (E-ER’)  2J + 1 gJ = 2(2I + 1) S(ER) - S(E) P(E) (n,n) ER’ = ER + n(ER) n = n(ER) 2P(ER) P(ER) k = 2E /  SLBW formalism: elastic and capture cross sections ELASTIC potential resonant interference CAPTURE P0 = 0 = k RC = , S0 = 0 P = 2 P-1 / ((- S-1 )2 + P-12) S = 2(-S-1) / ((- S-1 )2 + P-12) -   = -1 – tan(P-1/ ( -S-1)) , I, J,ER , n(ER),, Rc

  8. Hauser-Feshbach Cross sections are described by average strength functions (S) and level densities () Statistical Nuclear Model Porter-Thomas fluctuations: Wigner fluctuations:

  9. Neutron Reaction Data • From the analysis of experimental data on capture, total, fission, … cross-sections, resonance parameters are obtained for every nucleus. • All the information is combined, cross-checked for consistency, etc, in a process called “evaluation” until a recommended set of parameters is obtained. • This information is published in a Evaluated Nuclear Data File using an accepted standard format (ENDF-6) • There exist several files: • BROND-2.2 (1993, Russia) • CENDL-3 (2002, China) • ENDF/B-VI.8 (2002, US) • JEFF-3.0 (2002, NEA+EU) • JENDL-3.3 (2002, Japan)

  10. Nn nT Nr Measurement of cross-sections (energy differential) Number of reactions r(E) = Number of target nucleus per unit area  Number of neutrons of energy E Needs: • sample of known mass and dimensions • count the number of incident neutrons of energy E • count the number of reactions … but there are a number of experimental complications

  11. Cross sections can also be: • Reaction product angle differential • Reaction product energy differential • Reaction product multiplicity dependent • Energy weighted • …

  12. Neutron Beams • Need to span a huge energy range:1meV – 20MeV • Since neutrons cannot be accelerated, they have to be produced by nuclear reactions at certain energy and eventually decelerated by nuclear collisions (moderated) • Energy determination: • kinematics of two-body reaction • mechanical selection of velocities (“chopper”) • Time Of Flight measurement • Sources: • Radioactive • Nuclear detonations • Reactor • Light-ion accelerator • Electron LINACS • Spallation

  13. High Flux Reactor at the Institut Laue-Langevin (Grenoble) thermal  1015 n/cm2/s

  14. 30keV > Thresh. ForschungsZentrum Karlsruhe Van de Graaf 7Li(p,n)7Be, Q= -1.644MeV Ep~ 2MeV  En ~ 5-200keV Rate: 250kHz, t = 0.7ns

  15. Geel Electron LINear Accelerator (,n), (,f) on U (bremsstrahlung) Ee~ 100MeV, Ie ~ 10-100A Rate: 100-800Hz, t ~ 0.6-15ns

  16. … GELINA n yield vs. Ee NEUTRON SPECTRA U-TARGET & H2O MODER. H2O MODERATOR

  17. CERN neutron Time Of Flight n_TOF p-spallation on Pb target Ep = 20GeV, Ip = 7x1012 ppp Rate: 2.4s-1 , t = 14ns

  18. n_TOF Entrance to the target PROTONBEAMLINE p-intensity pickup p current monitor n beam tube NEUTRON BEAM LINE Water cooled Pb target shielding shielding

  19. … n_TOF 2nd collimator shielding Bending magnet BEAM DUMP n monitor Sample changer n monitor EXPERIMENTAL AREA

  20. The characteristics of the spallation-moderation process and the collimators in use determine the neutron beam parameters: intensity- energy distribution, energy resolution and spatial distribution. … n_TOF MODERATION: • =  ln(Ei/Ef)   1+ ln ( (H)=1, (Pb)=0.01 ) (A-1)2 A-1 2A A+1 TARGET: 80x80x60 cm3 Pb + 5cm H2O SPALLATION PROCESS: ~600 n/p Intensity distribution

  21. … n_TOF time vs. energy RF @ 5eV Beam energy-resolution RF @ 180keV RF t beam The statistical nature of the moderation process produces variations on the time that a neutron of a given energy exits the target assembly Resolution Function … also important: time spread of beam

  22. … n_TOF Beam profile 4cm  10-5 The collimation system determines the final number of neutrons arriving to the sample an its spatial distribution Neutrons on sample

  23. Counting neutrons Common reactions used for neutron detection: Elastic scattering: • n + 1H  n + 1H • n + 2H n + 2H (abund.=0.015%) Charged particle: • n + 3He3H + 1H + 0.764 MeV (abund.=0.00014%) • n + 6Li 4He + 3H + 4.79 MeV (abund.=7.5%) • n + 10B7Li* + 4He 7Li + 4He + 0.48 MeV  +2.3 MeV (abund.=19.9%, b.r.=93%) Radiative capture: • n + 155Gd156Gd* -ray + CE spectrum (abund.=14.8%) • n + 157Gd158Gd* -ray + CE spectrum (abund.=15.7%) Fission: • n + 235U fission fragments + ~160 MeV • n + 239Pu fission fragments + ~160 MeV • n + 238U fission fragments + ~160 MeV

  24. … n_TOF Si Si Si 200g/cm2 on 3m Mylar Si Counting neutrons Reaction: n + 6Li  t +  Si detectors Neutron Intensity Monitoring:

  25. 6Li(n,t) Scintillator + Photomultiplier 10B(n,)7Li Ionisation Chamber …GELINA Neutron Monitors

  26. Counting reactions Total reaction cross sections Transmission measurements: counting the neutrons disappearing from the beam

  27. GELINA SAMPLE+FILTERS NEUTRON MONITOR

  28. Background correction using black resonance filters Black Resonance Filters

  29. Techniques for radiative capture (n,)detection: • Detection of the capture nucleus • Activation measurements • Detection of -ray cascade • Total Absorption Spectrometers • Total Energy Detectors • Moxon-Rae Detectors • Pulse Height Weighting Technique • High Resolution Ge detectors n

  30. Activation measurement Irradiation: A(n,)A+1 A+1 radioactive with suitable T1/2 Measurement of characteristic -ray of known I with Ge detector FZK Karlsruhe

  31. Define: i EC : total efficiency for -ray of energy Ei E1 pi : peak efficiency for -ray of energy Ei E2 Then: total efficiency for cascade: E3 peak efficiency for cascade: m C = 1 -  (1 - i) i=1 If pi = 1, i  pC = C = 1 Total Absorption Spectrometer m pC =pi i=1 m m If i« 1 & i = kEi, i  C  i = k  Ei = k EC i=1 i=1 Total Energy Detector Detection of -ray cascade

  32. … n_TOF n_TOF Total Absorption Calorimeter 40 BaF2 crystals /4 = 95% E  6%

  33. (from Karlsruhe 4 BaF2 detector) BACKGROUND REDUCTION  = 95% BaF2(n,) contamination

  34. Bi-converter C-converter  / E Mo-converter Bi/C-converter E Total Energy Detectors • Moxon-Rae type detectors: The proportionality between efficiency and -ray energy is obtained by construction: (,e-) converter + thin scintillator + photomultiplier Maximum depth of escaping electrons increases with E …  e- But … proportionality only approximate (need corrections) Not much in use nowadays

  35. imax Rij =  j i=1 imax Wi Rij = Ej i=1 Total Energy Detectors Pulse Height Weighting Technique: The proportionality between efficiency and -ray energy is obtained by software manipulation of the detector response (Maier-Leibniz): If Rij represents the response distribution for a -ray of energy Ej: E = 2MeV Rij E = 9MeV it is possible to find a set of weighting factors Wi (dependent on energy deposited i) which fulfil the proportionality condition (setting k=1): Wi for every Ej

  36. Detectors:C6D6 liquid scintillators Advantage: low neutron sensitivity  = 3-5% Also detector dead material is important … Optimized BICRON … and surrounding materials HOME MADE: • C-fibre cell • No cell window • No PMT housing

  37. Fission cross section Detection of fission products Problems: • strong energy loss • angular distribution • often -decay contamination

  38. … n_TOF PPAC assembly

  39. … n_TOF Fission Ionization Chamber Assembly Ionization chamber Gas used: Ar (90 %) CF4 (10 %). Gas pressure: 720 mbar Electric field: 600 V/cm Gap pitch: 5 mm Deposit diameter: 5 cm Deposit thickness: 125 µg/cm Support thickness: 100 µm (Al) Deposits on both sides. Electrode diameter: 12 cm Electrode thick.: 15 µm (Al) Windows diameter: 12 cm (KAPTON 125 µm)

  40. … n_TOF PPAC FIC

  41. Elastic scattering measurements Detection of neutrons Inelastic scattering measurements Detection of neutrons Detection of -ray from de-excitation cascade Measurement of scattered neutron energies by TOF requires monochromatic beams

  42. Tohoku University 238U Kinematics determines neutron energy and resolution En=465 keV n-source: 7Li(p,n)

  43. Neutron multiplication (n,xn) cross sections Detection of the residual nucleus: activation measurements (see before) Detection of neutrons (ambiguity due to limited neutron efficiency) Detection of -ray from de-excitation cascade In order to transform -ray production cross section into reaction cross section it is required a complete knowledge of the level scheme

  44. Angular distribution effects have to be corrected

  45. (n,p), (n,), … cross sections Activation measurements Direct detection of p, , … At low energies thin samples windowless detectors (similar to fission) At high energies Si detectors FZK ORELA

  46. Acquiring the data: full train of detector pulses t = 2ns En down to 0.6eV 0.5 MB/pulse/detector Digitizer: 8bit-500MS/s FADC + 8MB memory On-line “zero” suppression TOF & EC6D6 from pulse-shape fit of PMT anode signal Dead-time and pile-up corrections

  47. shielded Single-scattering Analysing the data:  Yield: Sample effects: Self-shielding: Multiple scattering correction: elastic collision(-s) + reaction

  48. 58Ni(n,) ER =136.6keV T = 300K Thermal (-Doppler)broadening For a fix neutron energy center-of-mass energy varies because of thermal vibrations Beam effects: Resolution Function A given TOF corresponds to a distribution of neutron energies or Energy distribution of “monochromatic” neutron beam

  49. 56Fe(n,) ER= 1.15keV T broad. single-scattering double scattering RF 197Au(n,) ER= 4.9eV  Y =  Use a R-Matrix code as SAMMY to fit the data and extract the parameters: ER,  , n, …

More Related