Recent neutron capture measurements at FZK

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1 René Reifarth Recent neutron capture measurements at FZK Outline: Overview of s-process nucleosynthesis Nuclear data needs for the s-process where do we stand? Recent (n,γ) cross section measurements at FZK The planned neutron source at University Frankfurt: FRANZ Summary Workshop on Statistical Nuclear Physics and Applications in Astrophysics and Technology July , Ohio University, Athens Ohio

2 Nucleosynthesis of the elements s-process p-process Proton number rp-process r-process Neutron number Fusion up to iron Heavy elements (A>56) are produced by the s-process (~50%) and the r-process (~50%)

3 proton number Ni Co Fe (n,γ) (β ) (β + ) Zn Cu Beyond Iron mainly neutron induced p-,rp-process p-only Ge Ga Se As Kr Br neutron number Sr Rb r-only r-process

4 s-process nucleosynthesis Two components were identified and connected to stellar sites: Main s-process 90<A<210 Weak s-process A<90 TP-AGB stars 1-3 M massive stars > 8 M shell H-burning He-flash K K kt=8 kev kt=25 kev cm cm C(α,n) 22 Ne(α,n) core He-burning shell C-burning K ~ K kt=25 kev kt=90 kev 10 6 cm cm Ne(α,n)

5 The main s-process Success of the main s-process in TP-AGB stars of 1-3M r-residuals method: N r = N solar N s Arlandini et al. ApJ 525 (1999) 886

6 Nuclear data needs for the main s-process Success of the main s-process is based on reliable neutron capture cross sections (uncertainty < 5%) but also: stellar enhancement factors (SEF) and stellar β-decay rates are important Stellar neutron capture rate σv = 8 π μ 1 E σ(e) E exp de 3/2 kt ( kt ) 0 Neutrons Energy HkeV L We need to measure (n,γ) cross sections between 0.1 and 500 kev.

7 Michael Heil Nuclear Physics for Astrophysics, Dresden, March 2007 Nuclear data needs for the weak s-process 62 Ni(n,γ) 63 Ni Problems: small cross sections resonance dominated contributions from direct capture propagation effects Nassar et al. Phys. Rev. Lett. 94, (2005) previous:12.5 mb new: 28.4 mb

8 Branchings in the s-process path Classical analysis: f β = λ β λ λ n β + λ n = n n = ( σ N) Z+ 1 ( σ N) Z+ 1,A+ 1 σv A Z+1 (n,γ) (n,γ) A+1 Z+1 (β - ) (β - ) (n,γ) A-1 A A+1 Branchings can be used to determine neutron density temperature Every branch point is a sensitive mass density test of the stellar model! convection time scales in the interior of stars Experimental challenge: Measure (n,γ) of unstable isotopes

9 Connection between stellar model and experiment Modern s-process models (AGB stars) Classical s-process new n-facility (FRANZ) LANL

10 Activation technique at kt=25 kev Neutron production via 7 Li(p,n) reaction at a proton energy of 1991 kev. Induced activity can be measured after irradiation with HPGe detectors. Only possible when product nucleus is radioactive High sensitivity -> small sample masses or small cross sections Use of natural samples possible, no enriched sample necessary Direct capture component included

11 Results - neutron capture cross sections Isotope Bao et kt=30kev kt=30 kev in mbarn in mbarn 58 Fe 12.1 ± ± Fe 5.3 ± 2 (Rauscher 2000) 59 Co 38 ± 4 64 Ni 8.7 ± Cu 94 ± Cu 41 ± 5 79 Br 627 ± Br 313 ± Rb 15.5 ± ± ± ± ± ± ± ± ± 2.0

12 Motivation 60 Fe in the universe Detection of γ-ray lines from interstellar 60 Fe with SPI (INTEGRAL) Deep-Sea Manganese Crust E γ = 1173 and 1333 kev 60 Fe/ 26 Al = 0.11 ± 0.03 ongoing production in massive stars and distribution by subsequent supernovae tests stellar model and SN rate Harris et al, A&A 433 (2005) L49

13 Motivation 60 Fe on earth can be found in deep sea manganese crusts Gives hints about a nearby supernova 2.8 Ma ago Knie et al, PRL 93 (2004)

14 Production of 60 Fe Weak s-process component in massive stars During He-core and C-shell burning 60 Fe(n,γ) cross section needed no experimental data available yet (estimates: 1-10 mb)

15 Sample atoms 60 Fe (0.78 µg) (t 1/2 = 1.5(3) Ma) Retrieved from proton-irradiated copper beam stop (PSI) carrier: nat Fe, C active impurities: 55 Fe (t 1/2 = 2.7 y) 60 Co (ingrowth) 6 mm diameter activation only (presently) feasible method

16 γ-detection 70 mm 2 Ge-Clovers, face to face 1115 kev: sample single crystal: ε tot = 11 % ε peak = 1.1 % addback: ε peak = 15 %

17 61 Fe decay 6 min % 61 Fe % γ-rays used for 61 Fe detection: 1205 kev (single) 298 & 1027 kev (coincidence) 61 Co

18 Single spectra 1205 kev (single)

19 Coincidences: 298 & 1027 kev - almost no background - significantly reduced counts 61 Fe Sample background only 1333 kev ( 60 Co)

20 Neutron poisons Neutron capture on light elements. Most important neutron poisons are: 16 O(n,γ), 12 C(n,γ), 23 Na(n,γ), 25 Mg(n,γ) Measurement of 23 Na(n,γ) 24 Na with activation method Important during C-burning: 12 C( 12 C,p) 23 Na 16 O(n,γ) 17 O * 0.9 kt (kev) Bao et al. (mbarn) This work (mbarn) ± ± % lower cross section measured!

21 The Frankfurt neutron source at the Stern-Gerlach- Zentrum (FRANZ) Neutron beam for activation neutron flux: s -1 Design by Prof. Ratzinger, Prof. Schempp, O. Meusel and P. C. Chau 2 ma proton beam 250 khz < 1ns pulse width neutron flux: s -1 cm -2

22 Michael Heil Nuclear Physics for Astrophysics, Dresden, March 2007 Comparison with other neutron sources The Frankfurt neutron source will provide the highest neutron flux in the astrophysically relevant kev region (1 500 kev) worldwide. Facility Neutron flux at sample position* [cm -2 s -1 ] Repetition rate [Hz] Flight path [m] Pulse width [ns] Neutron energy range [kev] Frankfurt < FZ Karlsruhe DANCE at Los Alamos th n_tof at CERN th GELINA at Geel th ORELA at Oak Ridge th *Integrated flux between 1 kev and 100 kev

23 Experimental program at FRANZ The Frankfurt neutron source will provide the highest neutron flux in the astrophysically relevant kev region (1 500 kev) worldwide. Factor of 1000 higher than at FZK!!! Neutron capture measurements of small cross sections: Big Bang nucleosynthesis: 1 H(n,γ) Neutron poisons for the s-process: 12 C(n,γ), 16 O(n,γ), 22 Ne(n,γ). ToF measurements of medium mass nuclei for the weak s-process. Neutron capture measurements with small sample masses: Radio-isotopes for γ-ray astronomy 59 Fe(n,γ) and 60 Fe(n,γ) Branch point nuclei, e.g. 85 Kr(n,γ), 95 Zr(n,γ), 147 Pm(n,γ), 154 Eu(n,γ), 155 Eu(n,γ), 153 Gd(n,γ), 185 W(n,γ)

24 Double neutron capture X-1 X X+1 t 1/2 ~ d produce the sample on the fly n/s/cm 25 kev ~ n/cm 3

25 59 Fe(n,γ) at FRANZ (t 1/2 =45 d) activate 58 Fe, wait for 2 nd neutron capture measure 60 Fe/ 58 Fe ratio via AMS

26 Summary We have a good understanding of the main s-process. To do: - measure neutron capture cross section of branch points nuclei if possible - improve theoretical predictions Weak s-process is less understood. To do: Measure neutron capture cross sections of light and medium mass nuclei with sufficient accuracy. Neutron capture cross sections of neutron poisons are also important for both, the main and the weak s-process. Cross section measurements of neutron producing reactions, e.g. 13 C(α,n) and 22 Ne(α,n) are also important. New facilities like FRANZ will open a completely new area of cross section measurements, not only for astrophysics.

27 Thanks to M.Heil, F. Käppeler E. Uberseder, I. Dillmann R. Plag, F. Voss, S. Walter C. Domingo Pardo R. Gallino M. Pignatari U. Ratzinger O. Meusel E. Schempp L.P. Chau D. Schumann J. Görres, M. Wiescher U. of Notre Dame