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 meV to (say) 20 MeV

3. Neutron reactions at low energies A neutron is absorbed to form a “compound nucleus”: n + A Z  A+1 Z* which lives for a short time and decays: elastic A+1 Z*  n + A Z (elastic) inelastic A+1 Z*  n + A Z* (inelastic) radiative capture A+1 Z*  A+1 Z*’ +  (radiative capture) fission A+1 Z*  A1 Z 1 * + A2 Z 2 * + xn (fission) n multiplication A+1 Z*  A+1-x Z* + xn (n multiplication) … Other contributions:

4. The CN formation probability is higher for certain neutron energies E n corresponding to quasi-bound or virtual states: resonances E R = S n + E n S n : neutron separation energy of CN ( <10MeV )  level separation D 0 ~ 1 eV – 100 keV A A+1  =  n +   +  f + … (  =  n +   +  f +...) Life-time  Energy-width:   ~ 1 meV – 100 keV

5. log E n 1/vresolvedunresolvedoverlapping log  resolution  > D 0 : overlapping resonances   E: resolved resonance region (RRR)  < D0,  <  E: unresolved resonance region (URR) 1/v: thermal Shape of neutron cross-section 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. Single Level Breit-Wigner Formalism: (n,  )   (E) =   2 g J  n   (E-E R ’) 2 + ¼  2 For -capture into an isolated spin J resonance at E R :  =  /  2  E (neutron wavelength) g J = 2J + 1 2(2I + 1) (spin stat. factor; |I - ± ½|  J  |I - + ½| )  (E) =  n (E) +  + …; (FWHM)  n (E) =  E/E R  n (E R ), =0 … and the channel radius R c n+ 197 Au, =0, J=2 E R = 4.9 eV  n = 15.2 meV   = 122.5 meV E n (eV)   (barn) R-Matrix

7. SLBW formalism: elastic and capture cross sections P 0 =  0 = k R C = , S 0 = 0 P =  2 P -1 / (( - S -1 ) 2 + P -1 2 ) S =  2 ( -S -1 ) / (( - S -1 ) 2 + P -1 2 ) -  =  -1 – tan(P -1 / ( -S -1 )) E R ’ = E R +  n (E R ) S (E R ) - S (E) 2P (E R )  = (E-E R ’) 2   n =  n (E R ) P (E) P (E R )   nn 1 +  2 1   = g J 44 k2k2 k =  2  E /   = g J [ sin 2  + cos 2  + sin 2  ] nn  1 +  2 1 nn   44 k2k2 g J = 2J + 1 2(2I + 1) ELASTICELASTIC CAPTURECAPTURE potentialresonantinterference (n,  ) (n,n) 238U, I, J, E R,  n (E R ),  , R c

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

9. 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) Neutron Reaction Data

10. Measurement of cross-sections (energy differential)  r (E) = Number of reactions Number of target nucleus per unit area  Number of neutrons of energy E NnNn nTnT NrNr 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  10 15 n/cm2/s

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

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

16. … GELINA U-TARGET & H 2 O MODER. NEUTRON SPECTRA H2O MODERATOR n yield vs. E e

17. CERN neutron Time Of Flight n_TOF p-spallation on Pb target E p = 20GeV, I p = 7x10 12 ppp Rate: 2.4s -1,  t = 14ns

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

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

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. TARGET: 80x80x60 cm 3 Pb + 5cm H 2 O SPALLATION PROCESS: ~600 n/p MODERATION:  =  ln(E i /E f )   1+ ln (  (H)=1,  (Pb)=0.01 ) A-1 A+1 (A-1) 2 2A Intensity distribution … n_TOF

21. 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 RF  t beam Beam energy- resolution time vs. energy RF @ 5eV RF @ 180keV … n_TOF

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

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

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

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

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

27. SAMPLE+FILTERS GELINA 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 T 1/2 Measurement of characteristic  -ray of known I  with Ge detector FZK Karlsruhe

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

32. n_TOF Total Absorption Calorimeter 40 BaF 2 crystals  /4  = 95%  E  6% … n_TOF

33. (from Karlsruhe 4  BaF 2 detector) BaF 2 (n,  ) contamination  = 95% BACKGROUND REDUCTION

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

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

36. Detectors: C 6 D 6 liquid scintillators Advantage: low neutron sensitivity Also detector dead material is important … Optimized BICRON HOME MADE: C-fibre cell No cell window No PMT housing … and surrounding materials   = 3-5%

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

38. … n_TOF PPAC assembly

39. Ionization chamber Gas used:Ar (90 %) CF 4 (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) … n_TOF Fission Ionization Chamber Assembly

40. PPAC FIC … n_TOF

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

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

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

45. Angular distribution effects have to be corrected

46. FZK ORELA (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

47. Acquiring the data: full train of detector pulses Digitizer: 8bit-500MS/s FADC + 8MB memory On-line “zero” suppression TOF & E C6D6 from pulse-shape fit of PMT anode signal  t = 2ns E n down to 0.6eV 0.5 MB/pulse/detector Dead-time and pile-up corrections

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

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

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

51. Slowing Down Spectrometer: LSDS (LANL-Los Alamos) Time-energy relation MC simulation Lead block + sample+counters

52. Pile oscillator method OAK RIDGE The oscillating motion of a sample in the neutron field of a reactor induces oscillations in the field (global and local) whose amplitude is proportional to the absorption cross section Needs correction for scattering

53. Bibliography: 1.The Elements of Nuclear Interaction Theory, A. Foderaro, MIT Press, 1971 2.Neutron Sources for Basic Physics and Applications, Ed. S. Cierjacks, NEA/OECD, Pergamon Press, 1983 3.Neutron Radiative Capture, Ed. R.E. Chrien et al., NEA/OECD, Pergamon Press, 1984 4.Evaluation and Analysis of Nuclear Resonance Data, F.H. Froehner, NEA-OECD, JEFF Report 18, 2000