Neutron capture cross section measurements for nuclear astrophysics at n_TOF Michael Heil on behalf of the n_TOF collaboration Outline   The CERN n_TOF Facility Motivation Basic parameters Experimental Setup   Experimental campaigns in Neutron capture measurements for astrophysics   Perspectives Michael Heil NIC IX, June 2006, Geneva

The n_TOF collaboration Michael Heil NIC IX, June 2006, Geneva 41 Research Groups 120 researchers

Neutron physics at n_TOF Nuclear Data for ADS n_TOF idea (C. Rubbia et al., 1998): Measurements of neutron cross sections relevant for Nuclear Waste Transmutation and related Nuclear Technologies n_TOF-ND-ADS: EC FP5 project running from November 2000 to December 2004 coordinated by CERN spokespersons: Noulis Pavlopoulos, Alberto Mengoni Basic nuclear Physics with neutrons Nuclear Astrophysics Neutron capture cross section measurements relevant for Nuclear Astrophysics Michael Heil NIC IX, June 2006, Geneva

The n_TOF facility at CERN

Basic parameters of n_TOF proton beam momentum20 GeV/c intensity (dedicated mode)7 x protons/pulse repetition frequency1 pulse/2.4s pulse width6 ns (rms) n/p300 lead target dimensions80x80x60 cm 3 cooling & moderation materialH2OH2O moderator thickness in the exit face5 cm Michael Heil NIC IX, June 2006, Geneva

Characteristics of the neutron beam neutron spectrum thermal – GeV neutron flux keV 7·10 4 n/pulse flight path187.5 m Ø 2 nd collimator beam size 1.8 cm < 4 cm  30 keV 1·10 -3

40 BaF2 crystals High detection efficiency ≈100% Good energy resolution so far, only used for (n,  ) measurements of actinides Experimental setup and detectors C6D6C6D6 C6D6C6D6 Neutron beam Sample changer -ray detection via C 6 D 6 scintillators Neutron flux monitor 4  BaF 2 detector array Pulse height weighting technique: Correction of the  -response by weighting function to make the detector efficiency proportional to  -ray energy Silicon detectors viewing a thin 6 LiF foilSince 2004 BaF 2 detector is operational Michael Heil NIC IX, June 2006, Geneva Sample changer and beam pipe made out of carbon fibre sample

Astrophysics at n_TOF Michael Heil NIC IX, June 2006, Geneva n_TOF featuresUse in astrophysics broad neutron energy range neutron capture cross sections for s-process studies (0.1 – 500 keV) high instantaneous flux small capture cross sections small sample quantities (isotopically enriched samples) radioactive samples (low intrinsic background) excellent energy resolutionresonance dominated cross sections low neutron sensitivity low backgrounds accurate cross section measurements even for large  el /  capture

ReactionMotivation 24,25,26 Mg(n,  ) Abundance anomalies in grains, Strength of neutron source 22 Ne( ,n) 25 Mg 90,91,92,93,94,96 Zr(n,  ) Neighborhood of s-process branching at A=95, Sensitivity to neutron flux 139 La(n,  ) Bottleneck at N=82 and s-process indicator for spectroscopic observations 151 Sm(n,  ) s-process branch point 186,187,188 Os(n,  ) Nuclear Cosmochronology (Re/Os clock) 204,206,207,208 Pb(n,  ) 209 Bi(n,  ) Termination of the s-process path see talk by Marita Mosconi today at 9 am see poster by Giuseppe Tagliente Neutron capture measurements Mg Zr La Sm Os Pb Bi see talk by C. Domingo Pardo today at 10:15 am see poster by Stefano Marrone Michael Heil NIC IX, June 2006, Geneva

90,91,92,93,94,96 Zr(n,  ) measurements Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva 90 Zr is neutron magic (N=50) small x-section (resonance dominated) bottleneck in the s-process flow Zr abundances are sensitive to neutron flux in AGB models s-process branching at 95 Zr

90,91,92,93,94,96 Zr(n,  ) measurements Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva Samples Isotopically enriched samples diameter 2.2 cm mass: 1.3 – 3.4 g Al can

90,91,92,93,94,96 Zr(n,  ) measurements Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva The extracted resonance parameters were compared with a previous measurement (Boldeman et al., 1976) L Marques, et al. - The n_TOF Collaboration

90,91,92,93,94,96 Zr(n,  ) measurements Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva ≈ 20% lower than previous data C Moreau, et al. - The n_TOF Collaboration ND2004 Conference, Santa Fe, NM – Sept Zr 96 Zr

90,91,92,93,94,96 Zr(n,  ) measurements Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva Preliminary Maxwellian averaged cross kT=30 keV n_TOFBao et al. 90 Zr20 ± 121 ± 2 91 Zr58 ± 360 ± 8 92 Zr30 ± 233 ± 4 94 Zr36 ± 226 ± 1 96 Zr7.5 ± ± 0.5 Analysis 93 Zr in progress!

139 La(n,  ) measurement Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva 139 La Neutron magic N=82 small x-section (resonance dominated) bottleneck in the s-process flow 139 La is mono-isotopic and easier to detect in stellar spectroscopy than Ba On the basis of accurate neutron capture cross sections 139 La can be used as s-process indicator (complement to Eu as r-process indicator)

139 La(n,  ) measurement Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva Remarkable energy resolution and background conditions have allowed to determine the resonance parameters up to 9 keV In the past, the best experimental data did not exceed 2.7 keV. R. Terlizzi et al. (INFN) Up to 2.7 KeV: n_TOF < 10% of Nakajima + Musgrove < 20% of databases MACS-30 in agreement with the FZK activation measurement(s)

151 Sm(n,  ) measurement Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva Sm Eu Gd 150 Sm 151 Sm 93 a 152 Sm 153 Sm 154 Sm 151 Eu 152 Eu 153 Eu 154 Eu 155 Eu 156 Eu 152 Gd 153 Gd 154 Gd 155 Gd 156 Gd 157 Gd First s-process branch point which was directly measured via TOF Sample: 206 mg 151 Sm 2 O 3 (Oak Ridge) 90 % enrichment t 1/2 = 93a, activity: 156 GBq encapsulated in Ti-can Branchings can be used to determine neutron density temperature mass density convection time scales in the interior of stars

151 Sm(n,  ) measurement Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva

151 Sm(n,  ) measurement Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva U Abbondanno et al. Phys. Rev. Lett. 93 (2004), MACS-30 = 3100 ± 160 mb = 1.48 ± 0.04 eV, S 0 = (3.87 ± 0.20)×10 -4 n_TOF S. Marrone et al. Phys. Rev. C 73 (2006) 03604

24,25,26 Mg(n,  ) measurements Motivation: Neutron poison for s process Strength of 22 Ne( ,n) 25 Mg neutron source Magnesium anomalies in presolar grains 26 Mg excess from in situ decay of radioactive 26 Al Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva

Mg Zr La Sm Os Pb Bi Michael Heil NIC IX, June 2006, Geneva Fitting of resonance parameter in progress! 24,25,26 Mg(n,  ) measurements first known Mg resonance at 24 keV Impurities In, Sb

Outlook - Plan for measurements in Phase-2 Michael Heil NIC IX, June 2006, Geneva Capture measurements Mo, Ru, Pd stable isotopes Fe, Ni, Zn, and Se (stable isotopes) 63 Ni, 79 Se A≈150 (isotopes varii) 147 Sm(n,  ), 67 Zn(n,  ), 99 Ru(n,  ), 58 Ni(n,p) r-process residuals calculation isotopic patterns in SiC grains s-process nucleosynthesis in massive stars accurate nuclear data needs for structural materials s-process branching points long-lived fission products p-process studies n_TOF will continue: Letter of Intent signed by 24 research labs of the n_TOF Collaboration + 4 newcomers (January 2005)

n_TOF target New Experimental Area (EAR-2) EAR-1 (at 185 m) ~ 20 m Flight-path length : ~20 m at 90° with respect to p-beam   expected neutron flux enhancement: ~ 100   drastic reduction of the t 0 flash   duty factor improved by factor of 10 Michael Heil NIC IX, June 2006, Geneva Second n_TOF beam line & EAR-2

Use of D 2 O moderator Michael Heil NIC IX, June 2006, Geneva Photon time distribution (E>1MeV)

EAR-2: Optimized sensitivity Michael Heil NIC IX, June 2006, Geneva Improvements (ex: 151 Sm case) sample mass / 3 s/bkgd=1 use BaF 2 TAC  x 10 use D 2 O  30 x 5 use 20 m flight path  30 x 100 consequences for sample mass 50 mg 5 mg 1 mg 10  g boosts sensitivity by a factor of 5000 ! (a factor of 100 ONLY from higher flux) problems of sample production and safety issues relaxed