Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 1

Re/Os cosmochronometer: measurements of relevant Os

cross sectionsMarita Mosconi1, Alberto Mengoni2, Michael Heil1, Ralf Plag1, Franz Käppeler1,

Kaori Fuji5, Gaelle Aerts3, Rita Terlizzi4 and the n_TOF collaboration

•186Os, 187Os and 188Os (n,) cross sections at the n_TOF facility operating at CERN

1 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,Italy

• Inelastic scattering cross section of 187Os at 3.7 MV FZK Van de Graaf

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 2

s-process local approximation: ANA= A-1NA-1

processsOs

)Os(n,

)Os(n,solarOs

radOs 186

187

186

187187 Nσ

σNN

At stellar temperatures ,187Os low-lying excited states are populated: theoretical calculations are needed to evaluate the stellar capture rate

The Re/Os cosmochronometer

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 3

187Os stellar rate

187Os

3/2-

3/2-

5/2-

7/2-

1/2-

9.75 keV

74.3 keV

75.0 keV

100.0 keV

Thermal population at 30 keV:P(gs) = 33%P(1st) = 47%P(all others) = 20%

Theoretical calculations can use as input experimental results:

the measurement of the inelastic scattering cross section for the first excited level allows to define the competition by the scattering channels under stellar conditions

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 4

Clock determination

Nuclear quantities

• temperature dependence of t1/2(187Re) (Bosch et al., Phys.Rev.Lett 77 (1996))

• 186186Os, Os, 187187Os(n,Os(n,) cross sections from 0.1 ) cross sections from 0.1 keV to 500 keVkeV to 500 keV

• evaluation of stellar cross sections including the effect of excited states (inelastic scattering cross section on inelastic scattering cross section on 187187OsOs )

Astrophysics• astration

• galactic history of the 187Re enrichment

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 5

Measurements of the (n) cross section at n_TOF

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 6

Experimental setup at n_TOF (2003)

Neutron flux monitor: Silicon detectors viewing a thin 6LiF foil

Pulse height weighting technique

Correction of the -response by weighting function to make the detector efficiency proportional to -ray energy

-ray detection: C6D6 scintillatorsSpallation neutron source: 20GeV protons on a lead target providing a wide range of neutron energies

C6D6

C6D6

Neutron beam

Sample changer

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 7

Samples measured and yield

• 186Os (2 g, 79 %) • 187Os (2 g, 70 %) • 188Os (2 g, 95 %)

• Al can environmental background

• natPb (2 g) in-beam gamma background

• natC (0.5 g) neutron scattering

background

• 197Au (1.2g) flux normalization (using Ratynski and Macklin cross

section measurements

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 8

Maxwellian averaged cross sections at 30 keV

n_TOF (mb)

Winters (mb)

Browne (mb)

186Os 409±20 418±16 438±30 187Os 931±47 874±28 919±28 188Os 298±15 401±15 395±24

Preliminary results

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 9

Scattering experiment at FZK Van de Graaff

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 10

• Time-of-Flight technique• neutron beam with a narrow energy distribution (FWHM≤10 keV)

Experimental method

p

thp

np

LiBe

E

EE

MM

MMarcsin||

77

• identification of the inelastic scattering component by energy separation• measure the cross section as closer as possible to the energies relevant for s-process

FWHM≤10 Ep-Eth≤0.3 keV

θ=0°

θ=30°θ=45°

θ=60°

θ=15°

θ=25°

1. narrow neutron distributions for En≥ 60 keV

2. Gaussian neutron energy distribution centered at 30 keV

• use of 7Li(p,n)7Be reaction:

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 11

Experimental setup and DAQ

PMTNeutron monitor

6Li-glass

Samples measured

• 187Os

• 188Os pure elastic component

• Empty canning background

• improved neutron detection efficiency by use of KG2 (NE912)• background subtraction by pulse shape analysis

Main drawback of 7Li(p,n)7Be at threshold: small yield

Beam unstable due to proton source instabilities:

computer monitoring of the neutron energy distribution

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 12

Data

Two week of data taking with proton current 3-6 μA

• Runs with similar FWHM extracted

• Parameter of the scattering component extracted

• Fit of the 187Os scattering distribution thought as made by two separated components

Data analysis

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 13

Summary

186,7,8Os(n) cross sections:

• background subtraction concluded

• resonances analysis in progress

• isotopic correction to be improved using the results of the previous item

187Os(n,n’) cross sections:

• data taken for neutron distribution centered at 30 keV and with FWHM from 7 to 10 keV

• analysis in progress

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 14

The End

Santa Tecla, 2-9 October 2005 Marita Mosconi,FZK 15

Derivation of the cross sections in the unresolved region

1. -response corrected by weighting functions

2. background subtraction

3. neutron flux determined by Au count rate (using Ratynski and Macklin cross section measurements)

4. isotopic correction

188Os with filtersEstimated level of background

• neutron scattering background evaluated using the C measurement (environmental background corrected) rescaled for the ratio of the neutron scattering cross sections of carbon and of the sample analyzed• shape of the -background determined using Pb measurement after subtraction of it own neutron scattering background and environmental background• level of the -background determined by GEANT3.21 simulations

Au 4.9 eV resonance SAMMY fit

Background 0.000

Normalisation 0.992