Evaluation of stable W isotopes Evaluation of stable W isotopes - - PowerPoint PPT Presentation

Joint Research Centre

Institute for Reference Materials and Measurements (IRMM) http:/ / irm m .jrc.ec.europa.eu

Evaluation of stable W isotopes in the resolved resonance region Evaluation of stable W isotopes in the resolved resonance region

• 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

Outline

im portance of neutron cross section libraries for W

Astrophysics Dosim etry 186W (n, γ) Fission technology Fusion technology: structural material for PFCs Plasma Facing Components :

• divertor
• limiter
• first wall

Geel Electron LI Near Accelerator

• Electrons accelerated up to 150 MeV
• Repetition rate: 50 – 800 Hz
• Pulsed neutron source optimized for

high resolution time of flight measurements

• Multi-user facility with 10 flight paths
• Flight path lengths: 10 m - 400 m
• The measurement stations with special

equipment for:

• Total cross section measurements
• Partial cross section measurements

FLI GHT PATHS SOUTH FLI GHT PATHS NORD ELECTRON LI NAC TARGET HALL

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 production

Neutron beam characteristics:

• Pulsed neutron beam with a white spectrum (10 meV < En < 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 × 1012 n/ s – 2.5 × 1013 n/ s

Transm ission m easurem ents

detector

sam ple and filters

• 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 6Li-loaded glass scintillators

( 9 5 % enriched to 6Li) 6Li( n,t) α

• Sam ple changer rem otely controlled
• Alternated sam ple-in and sam ple-out cycles

( ) ( ) ( ) ( ) ( ) ( ) ( )

n r nσ theor

• ut
• ut

in in T exp

g , E e t T t B t C t B t C N t T

tot

Γ ⇒ = ↔ − − =

−

Condition of good transm ission geom etry:

• all detected neutrons have passed

through the sample

• scattered neutrons are not detected

Black resonance filter technique

Background in transm ission

( ) ( ) ( )

t B t B B t B

n

+ + =

γ

Am bient com ponent 2 .2 MeV γ-rays em itted in the m oderator after neutron capture Scattered neutrons in the detector station

7

Capture cross section m easurem ents

• Detector stations at FPL = 12 m, 60 m
• Moderated neutron component
• Machine frequency: 50 and 800 Hz
• C6D6 liquid scintillators
• Total energy detection principle
• Pulse hight weighting technique
• 10B Frish gridded ionization chamber for

neutron flux (80 cm before the sample)

γ γ

kE dE E W E E R

d d F d

=

∫

) ( ) , (

d d F d n c n w

dE E W E T C T C ) ( ) , ( ) (

∫

=

( )

) , , ( ... 1 Γ Γ Γ ⇒ + − ↔ − =

− γ γ σ

σ

n n W W C

g E e Y F B C N Y

tot

exp

Γ −

ϕ ϕ ϕ ϕ

σ ε

r tot

B C

C6D 6 liquid scintillators

1 0B Frish gridded

ionization cham ber

( ) ( )

( )

( ) ( )

( )

t C t C k t C k b t B

SO Pb SO ' ' 2 ' 1

− + + =

γ

Background in capture

Sam ples

Metallic disk Enrichm ent ( % ) Thickness ( m m ) Area ( m m 2) W eight ( g) Areal density ( at/ b)

natW

• 15

2828 821 0.095

natW

• 0.4

6060 118 0.0064

natW

• 0.22

5030 21 0.0013

182W

94.50% 1.29 3421 48 0.0047

182W

94.50% 3.87 3421 140 0.0136

183W

83.75% 1.30 3959 46 0.0038

183W

83.75% 2.85 3964 94 0.0078

184W

94.50% 1.15 3920 45 0.0038

184W

94.50% 2.25 3615 90 0.0081

186W

94.50% 1.09 3802 45 0.0038

186W

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
• Measurements on 182W, 184W, 186W enriched samples and natW
• Transmission measurements (200 m and 40 m)
• Self-indication measurements (40 m)

Macklin et al., Nucl. Sc. Eng., Vol. 8 4 , 9 8 -1 1 9 ( 1 9 8 3 )

• Oak Ridge Electron Linear Accelerator
• Measurements on 182W, 184W, 184W, 186W enriched samples
• Capture measurements (40 m)

gΓn

< Γγ>

182W

(53 ± 2) meV

183W

(55 ± 1) meV

184W

(57 ± 4) meV

186W

(60 ± 3) meV

Starting values: scattering radius and negative resonances

Coherent scattering lengths, thermal capture cross sections and negative resonance parameters for tungsten isotopes

Isotope bcoh [fm] σγ

0 [b]

Negative Resonance Parameters Er [eV] Γn [eV] Γγ [eV]

182W

7.04 ± 0.04 19.9 ± 0.3

• 4.16

0.12×10-2 0.53×10-1

183W

6.59 ± 0.04 10.4 ± 0.2

• 46.0

0.13 0.55×10-1

184W

7.55 ± 0.06 1.7 ± 0.1

• 200.0

0.12×10-2 0.57×10-1

186W

• 0.73 ± 0.04

38.1 ± 0.5

• 217.5

0.47×10-2 0.60×10-1 Mughabghab 2 0 0 6 Knopf and W aschkow ski 1 9 8 7 average value Mackling et al. 1 9 8 3

∑

Γ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + × − =

j j nj

E A A R a

3 '

1 10 277 . 2

ne

b Z a A A b × + ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ + = 1

( )

fm bne

3

10 03 . 38 . 1

−

× ± − =

fm R 4 . 9

' =

Data reduction and data analysis

Data reduction performed with AGS package:

• Normalization of spectra
• Dead time correction
• Background subtraction
• Full uncertainty propagation
• Experimental Covariance Matrix

Resonance parameters extracted with REFI T2

• least-square fitting program
• Reich-Moore approximation of R

matrix theory

• Takes into account various

experimental effect:

• 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
• 183W shows the most severe problems
• A new evaluation of 183W is therefore claimed with priority
• Spin adjustment for 183W
• 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.