Evaluation of stable W isotopes Evaluation of stable W isotopes in the resolved resonance region in the resolved resonance region Joint Research Centre Institute for Reference Materials and Measurements (IRMM) http:/ / irm m .jrc.ec.europa.eu
Outline • Motivation of the re-evaluation • Description of the GELINA facility • Transmission and capture experiments • Samples • Data reduction and background treatment • Starting values of Resonance Parameters and other input data • Results and conclusions
im portance of neutron cross section libraries for W Astrophysics Dosim etry � 186 W (n, γ ) Fission technology Fusion technology: structural material for PFCs P lasma F acing C omponent s : •divertor •limiter •first wall
Geel Electron LI Near Accelerator • Electrons accelerated up to 150 MeV • Repetition rate: 50 – 800 Hz FLI GHT PATHS NORD • Pulsed neutron source optimized for ELECTRON high resolution time of flight LI NAC measurements • Multi-user facility with 10 flight paths • Flight path lengths: 10 m - 400 m • The measurement stations with special TARGET HALL equipment for: • Total cross section measurements FLI GHT PATHS • Partial cross section measurements SOUTH
Neutron production Target • Rotating U-target (10% Mo alloy) • Cooled with liquid Hg Neutron production as a 2-steps process: • Electrons produce Bremsstrahlung inside the U-target ( γ ,n) and ( γ ,f) reactions in the U-target • Neutron beam characteristics: • Pulsed neutron beam with a white spectrum (10 meV < E n < 20 MeV) • Fast (not moderated) and slow components • Low energy part of the neutron spectrum enhanced by a water moderator canned in Be Neutron intensity: 1.6 × 10 12 n/ s – 2.5 × 10 13 n/ s •
Transm ission m easurem ents • Detector stations at FPL = 2 5 m , 5 0 m • Moderated neutron com ponent • Machine frequency: 5 0 and 8 0 0 Hz • 6 .3 5 cm thick 6 Li-loaded glass scintillators ( 9 5 % enriched to 6 Li) 6 Li( n,t) α detector • Sam ple changer rem otely controlled • Alternated sam ple-in and sam ple-out cycles ( ) ( ) − C t B t ( ) ( ) ( ) − = ↔ = n σ ⇒ Γ in in T t N T t e E , g tot ( ) ( ) exp T − theor r n C t B t out out sam ple and filters
Background in transm ission Condition of good transm ission geom etry: • all detected neutrons have passed through the sample • scattered neutrons are not detected Black resonance filter technique ( ) ( ) ( ) = + + B t B B t B t γ 0 n 2 .2 MeV γ -rays Scattered Am bient neutrons in the em itted in the com ponent detector station m oderator after neutron capture 7
Capture cross section m easurem ents •Detector stations at FPL = 12 m, 60 m C 6 D 6 liquid scintillators •Moderated neutron component •Machine frequency: 50 and 800 Hz •C 6 D 6 liquid scintillators •Total energy detection principle •Pulse hight weighting technique ∫ = R ( E , E ) W ( E ) dE kE γ γ d F d d ∫ = C ( T ) C ( T , E ) W ( E ) dE w n c n d F d d • 10 B Frish gridded ionization chamber for neutron flux (80 cm before the sample) 1 0 B Frish gridded σ Γ Γ − ( ) ionization cham ber g N C B − σ γ γ = ↔ − + ⇒ Γ n n C W W Y F Y 1 e ... ( E , , ) tot ϕ ϕ ε − σ Γ exp r C B ϕ ϕ tot
Background in capture ( ) ( ) ( ) ( ) ( ) ( ) = + + − ' ' ' B t b k C t k C t C t γ 0 1 SO 2 Pb SO
Sam ples Metallic Enrichm ent Thickness Area W eight Areal disk ( % ) ( m m ) ( m m 2 ) ( g) density ( at/ b) nat W -- 15 2828 821 0.095 nat W -- 0.4 6060 118 0.0064 nat W -- 0.22 5030 21 0.0013 182 W 94.50% 1.29 3421 48 0.0047 182 W 94.50% 3.87 3421 140 0.0136 183 W 83.75% 1.30 3959 46 0.0038 183 W 83.75% 2.85 3964 94 0.0078 184 W 94.50% 1.15 3920 45 0.0038 184 W 94.50% 2.25 3615 90 0.0081 186 W 94.50% 1.09 3802 45 0.0038 186 W 94.50% 2.18 3761 90 0.0078
Starting values: resonance param eters Cam arda et al., Phys. Rev. C , Vol. 8 num . 5 , 1 8 1 3 -1 8 2 6 ( 1 9 7 3 ) • Columbia Univ. Nevis synchrocyclotron g Γ n • Measurements on 182 W, 184 W, 186 W enriched samples and nat W • Transmission measurements (200 m and 40 m) • Self-indication measurements (40 m) < Γ γ > Macklin et al., Nucl. Sc. Eng. , Vol. 8 4 , 9 8 -1 1 9 ( 1 9 8 3 ) (53 ± 2) meV 182 W • Oak Ridge Electron Linear Accelerator (55 ± 1) meV 183 W • Measurements on 182 W, 184 W, 184 W, 186 W enriched samples (57 ± 4) meV 184 W (60 ± 3) meV • Capture measurements (40 m) 186 W
Starting values: scattering radius and negative resonances Coherent scattering lengths, thermal capture cross sections and negative resonance parameters for tungsten isotopes 0 [ b ] σ γ Isotope b coh [fm] Negative Resonance Parameters Γ n [eV] Γ γ [eV] E r [eV] 182 W 0.12×10 -2 0.53×10 -1 7.04 ± 0.04 19.9 ± 0.3 - 4.16 183 W 0.55×10 -1 6.59 ± 0.04 10.4 ± 0.2 - 46.0 0.13 184 W 0.12×10 -2 0.57×10 -1 7.55 ± 0.06 1.7 ± 0.1 - 200.0 186 W 0.47×10 -2 0.60×10 -1 -0.73 ± 0.04 38.1 ± 0.5 - 217.5 Knopf and Mughabghab average value W aschkow ski 2 0 0 6 Mackling et al. 1 9 8 7 1 9 8 3 + ⎛ ⎞ A 1 = + × ⎜ ⎟ b a Z b ' = ne ⎝ ⎠ A Γ + ⎛ ⎞ 0 A 1 ∑ R 9 . 4 fm = − × ⎜ ⎟ nj ( ) ' 3 a R 2 . 277 10 − = − ± × 3 ⎝ ⎠ b ne 1 . 38 0 . 03 10 fm A E j 0 j
Data reduction and data analysis Resonance parameters extracted Data reduction performed with AGS package: with REFI T2 • least-square fitting program • Normalization of spectra • Dead time correction • Reich-Moore approximation of R matrix theory • Background subtraction • Takes into account various • Full uncertainty propagation experimental effect: • Experimental Covariance Matrix •Response of the TOF spectrometer •Sample in-homogeneities •Self shielding •Multiple scattering •Doppler broadening •Neutron sensitivity for capture detector •Gamma ray attenuation in the sample
Results
Conclusions and im provem ents •Preliminary results •Validity of the start parameters file based on literature •Validity of the approach based on natural thick and thin samples •Even isotopes need a minor adjustment • 183 W shows the most severe problems •A new evaluation of 183 W is therefore claimed with priority •Spin adjustment for 183 W •Use of the scattering radius obtained with optical model calculation and suitable choice of negative resonances to match the coherent scattering length and thermal capture cross section.
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