n_TOF04: 186 Os, 187 Os, 188 Os capture cross sections
description
Transcript of 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