n_TOF04: 186 Os, 187 Os, 188 Os capture cross sections

4 December 2006 n_TOF04: Os cross sections 1

n_TOF04: 186Os, 187Os, 188Os capture cross sections

Marita Mosconi1, Kaori Fujii5, Alberto Mengoni2, Michael Heil1,6, Ralf Plag1, Franz Käppeler1, Gaelle Aerts3, Rita Terlizzi4, Cesar Domingo Pardo1 and

the n_TOF collaboration1 Forschungszentrum Karlsruhe GmbH (FZK), Institut für Kernphysik, Germany2 CERN, Geneva, Switzerland; IAEA, Vienna, Austria3 CEA/Saclay, Gif-sur-Yvette, France4 INFN Bari, Italy5 INFN Trieste,Italy6 GSI, Darmstadt,Germany

MACS at 25 keV fundamental for the determination of the Re/Os cosmochronometer

Measured on August 2003 for a total of 2.3e18 protons

Neutron flux monitor : 6Li/silicon detector

-ray detection: C6D6 scintillators (PHWT)

4 December 2006 n_TOF04: Os cross sections 2

Samples measured• 186Os (2 g, 79 %) with and without filters (5.9e17 p)• 187Os (2 g, 70 %) with and without filters (3.6e17 p)• 188Os (2 g, 95 %) with and without filters (6.0e17 p)

• Al can background subtraction (7e16 p)

• 197Au (1.2g) with and without filters flux normalization (3.3e17 p)

• natPb (2 g) in-beam gamma background (1.4e17 p)

• natPb (2 g) with filters gamma background attenuation (1.6e17 p)

• natC (0.5 g) in-beam neutron background (1.1e16 p)

4 December 2006 n_TOF04: Os cross sections 3

Count rate

n_scatteringin-beam

Analysis split in two:

1.1. resonance analysis with resonance analysis with R-matrix code SAMMY R-matrix code SAMMY (Kaori Fujii)(Kaori Fujii)

2.2. unresolved resonance unresolved resonance regionregion

Background subtraction by decoupling the three components of the background

4 December 2006 n_TOF04: Os cross sections 4

Derivation of the cross sections in the unresolved region

• double counting rejection

• -response corrected by weighted functions (CEA/INFN)

• neutron flux determined by Au count rate 4.9 eV resonance for the resonance analysis, Ratynski experimental MACS for the continuum*

• background subtraction (next slide)

• correction for multiple scattering and self shielding (SESH)

• subtraction of the (n,n’) events at high energy

• isotopic correction * background subtraction and SESH correction also to determine Au count rate

4 December 2006 n_TOF04: Os cross sections 5

background evaluation by sample changer technique

1. ambient background subtracted to the count rate of all samples (Pb, C included)

2. neutron scattering background by C count rate (fitted) rescaled according to the elastic scattering cross section of the sample we look for the background

3. shape of the in-beam -rays scattering background by Pb count rate minus its neutron scattering background

4. level of the -background determined by GEANT3.21 simulations

Crosschecks:1. Comparison with filters (next slides)

2. Comparison with SAMMY

4 December 2006 n_TOF04: Os cross sections 6

4 December 2006 n_TOF04: Os cross sections 7

4 December 2006 n_TOF04: Os cross sections 8

4 December 2006 n_TOF04: Os cross sections 9

Comparison with previous results

n_TOF in agreement with theoretical predictions

deviations from previous sets of data

cross sections by coupling continuum and resonance results

4 December 2006 n_TOF04: Os cross sections 10

Systematic errors

Error due to PHWT application:

comparison of the count rate obtained by different weighting functions

186Os 0.2%

187Os 0.1%

188Os 0.4%

Error from background determination: (previous slides on background)

186Os 2%

187Os 1%

188Os 3%

Error on flux determination:

difference between Ratysnki and the first Au saturated resonance

3 %

Error due to isotopic correction:

uncertainties on unmeasured isotopes

186Os 1.4%

187Os 3.0%

188Os 1.4%

4 December 2006 n_TOF04: Os cross sections 11

MACSs 186Os

Browne

Winters

Bao

n_TOF

MACS at 30 keV

4 December 2006 n_TOF04: Os cross sections 12

MACSs 187Os

Browne

Winters

Bao

n_TOF

Bokhovko

MACS at 30 keV

4 December 2006 n_TOF04: Os cross sections 13

MACSs 188Os

Browne

Winters

Bao

n_TOF

MACS at 30 keV

4 December 2006 n_TOF04: Os cross sections 14

cross sections down to thermal energy

188Os

cross section

count rate

resonances from contaminants (187Os, 189Os)

th=4.7 b

contribution from resonances 55.5 %

186Oscontribution from resonances 4.9 %

187Oscontribution from resonances 2.5 %

4 December 2006 n_TOF04: Os cross sections 15

The End

4 December 2006 n_TOF04: Os cross sections 16

187Os(n,n’) measurement at FZK

PMTNeutron monitor

6Li-glass

Monoenergetic neutron beam at 30 keV (FWHM≤10 keV) from 7Li(p,n)7Be reaction at threshold

Samples measured:

1. 187Os

2. 188Os (pure elastic component)

3. Empty can (background )

Fit of the 187Os scattering distribution assuming two separate components

ToF technique

4 December 2006 n_TOF04: Os cross sections 17

Comparisons of the yield of the 2 samples

1. TOF translated into energy

2. Fit of 188Os scattering count rate

3. Inelastic scattering defined by shifting in energy the elastic

4. Normalization of the 2 curves free parameters of the fit

DET4

Shape of the elastic scattering component

DET4

DET4

4 December 2006 n_TOF04: Os cross sections 18

geometry and presence of moderators strongly affecting the TOF spectra

inelastic spectrum

elastic spectrum

fit as addition of the two

experimental data

DET1

90o left

elastic spectrum

inelastic spectrum

fit as addition of the two

NU

MB

ER

OF

EV

EN

TS

200

400

600

800

1000

DET4

90o right

experimental data

GEANT simulation of detector response

4 December 2006 n_TOF04: Os cross sections 19

background subtraction

huge signature from the moderator material around the sample

This will define the background from contaminant for the three detectors

DET2

30o left

elastic spectrum

inelastic spectrum

fit as addition of the three

background by contaminant

experimental data

4 December 2006 n_TOF04: Os cross sections 20

Which challenges?Data needed: 186,7Os(n,) cross sections

187Os(n,n’) cross section

•186Os, 187Os and 188Os (n,) cross sections at the n_TOF facility operating at CERN• inelastic scattering cross section of 187Os at 3.7 MV FZK Van de Graaff

Range of (n,) cross sections down to lower energies

reliable MACS at low kT

186Os(n,) cross section determination

(n,)

HF calculation

Improved accuracy for 187Os(n,n’) cross section