R-Process Abundances: Calculations vs. Observations

Transcription

1 R-Process Abundances: Calculations vs. Observations Karl-Ludwig Kratz - Institut für Kernchemie, Univ. Mainz, Germany - HGF VISTARS, Germany - Department of Physics, Univ. of Notre Dame, USA

2 B²FH, the bible of nuclear astrophysics Historically, nuclear astrophysics has always been concerned with interpretation of the origin of the chemical elements from astrophysical and cosmochemical observations, description in terms of specific nucleosynthesis processes.

3 Fit of N r, from B²FH assumption (n,γ) (γ,n) equillibrium waiting-point concept static calculation astrophysical conditions explosive He-burning in SN-I T 9 1 n n cm -3 τ r s neutron source: 21 Ne(α,n) mainly nuclear physics: Q β - Weizsäcker mass formula + empirical corrections (shell, deformation, pairing) T 1/2-1 allowed transition to excited state, logft = 3.85

4 Observational instrumentation r-process observables today Solar system isotopic abundances, N r, meteoric and overall solar abundances ground- and satellite-based telescopes like Imaging Spectrograph (STIS) at Hubble or HIRES at Keck, and γ-ray satellites like INTEGRAL, or X-ray observatories CHANDRA and XMM-Newton. T 9 =1.35; n n = Pb,Bi r-process observables Ga Ge Sr Zr Ru Cd Mo Pd Sn Y Rh Nb Ag CS abundances scaled solar r-process scaled theoretical solar r-process Pt Ba Os Nd Dy Gd Ce Sm Er Yb Ir Hf La Pr Eu Ho Tb Lu Tm Elemental abundances in UMP halo stars Au Pb Th U δ [ ] ALLENDE INCLUSION EK Mass number Mass number isotopic composition Ca, Ti, Cr, Zr, Mo, Ru, Nd, Sm, Dy r-enhanced FUN anomalies in meteoritic samples

5 Abundance Clues and Constraints New observations of n-capture elements in lowmetallicity Galactic halo stars providing clues and constraints on: 1. Synthesis mechanisms for heavy elements early in the history of the Galaxy 2. Identities of earliest stellar generations, the progenitors of the halo stars 3. Suggestions on sites, particularly site or sites for the r-process 4. Galactic chemical evolution 5. Ages of the stars and the Galaxy From J. Cowan 2006

6 CS Abundances Cowan et al. (2005) Germanium Platinum (64 HST Orbits) 57 elements observed. More than any star except the Sun. Log (A) = Log 10 (N A /N H ) + 12 From J. Cowan 2006

7 Observational Summary of Total Abundances CS HD BD CS HD r-process rich stars Same abundance pattern at the upper end and? at the lower end. From J. Cowan 2006

8 Abundance Comparisons Among Four Halo Stars Note agreement at upper end Ivans et al. (2006)

9 Nuclear-Data Needs for the Classical r-process nuclear masses S n -values r-process path Q β, S n -values theoretical β-decay properties, n-capture rates β-decay properties T 1/2 r-process progenitor abundances, N r,prog β-decay freeze-out P n smoothing N r,prog N r,final (N r, ) n-capture rates nuclear structure development fission modes σ RC + σ DC smoothing N r,prog during freeze-out extrapolation into unknown regions SF, βdf, n- and ν-induced fission fission (re-) cycling ; r-chronometers

10 History and progress in measuring r-process nuclei Definition: r-process isotopes lying in the process path at freeze-out when r-process falls out of (n,γ)-(γ,n) equilibrium even-neutron isotopes waiting points important nuclear-physics property T 1/2 odd-neutron isotopes connecting the waiting points important nuclear-physics property S n σ n.γ In 1986 a new r-process astrophysics era started: at the ISOL facilities OSIRIS, TRISTAN and SC-ISOLDE T 1/2 of N=50 waiting-point isotope 80 Zn 50 (top of A 80 N r, peak) T 1/2 of N=82 waiting-point isotope 130 Cd 82 (top of A 130 N r, peak) In 2006, altogether more than 50 r-process nuclei have been measured (at least) via their T 1/2, which lie in the process path at freeze-out. These r-process isotopes range from 68 Fe to 139 Sb. The large majority of these exotic nuclei was identified at CERN/ISOLDE via the decay mode of β delayed neutron emission.

11 What we knew already in Shell-model (QRPA; Nilsson/BCS) prediction T 1/2 = 230 ms T 1/2 = (195 ± 35) ms Q β = 8.0 MeV IKMz 155R(1986) K.-L. Kratz et al (Z. Physik A325; 1986) Exp. at old SC-ISOLDE with plasma ion-source and βdn counting νg 7/2, πg 9/2 4.1 Problems: high background from -surface ionized 130 In, 130 Cs -molecular ions [ 40 Ca 90 Br] T 1/2 (GT) = 0.3 s Request: SELECTIVITY!

12 Request: Selectivity! Why? the Ag needle in the Cs haystack How? at an ISOL facility Fast UC x target Neutron converter Laser ion-source Hyperfine splitting Isobar separation Chemical separation Multi-coincidence setup >10 5 Ag Cd In Sn Sb Te I Xe Cs

13 Request: Selectivity! UC x target and neutron converter Proton-beam on neutron converter only fission, avoids p-rich isobars

14 Request: Selectivity! Laser ion-source (RILIS) Laser ON Laser OFF Comparison of Laser ON to Laser OFF spectra γ-singles spectrum Chemically selective, three-step laser ionisation of Ag into continuum Properties of the laser system: Efficiency 10% Selectivity Cd 1669 kev Laser ON Laser OFF 130 Cd 1732 kev 130 Sb 1749 kev Energy [kev]

15 The r-process waiting-point nucleus 130 Cd...obtain a physically consistent picture! T 1/2, Q β, E(1 + ), I β (1 + ), log ft S n Q β J π =1 + {νg 7/2, πg 9/2 } 2QP 4QP Various model predictions: P. Möller et al. T 1/2 (GT) = 1.08 s (FY/LN) log ft = 4.4 E(1 + ) = 2.87 MeV (Nils./BCS) T 1/2 (GT) = 233 ms log ft = 4.1 E(1 + ) = 1.19 MeV Engel et al. T 1/2 (GT) 250 ms Martinez-Pinedo et al. T 1/2 (GT) = 146 ms log ft = 3.8 E(1 + ) = 1.55 MeV A. Brown et al. T 1/2 (GT) = 180 ms old log ft = 4.2 E(1 + ) = 1.38 MeV T 1/2 (GT) = 233 ms new log ft = 4.2 E(1 + ) = 2.18 MeV

16 Full spectroscopy 130 Cd decay Surprises high [νg 7/2 πg 9/2 ] 1 + state weakening of the νg 7/2 - πg 9/2 residual interaction high Q β -value OXBASH (B.A. Brown, Oct. 2003) (new) (old) reduction of the TBME (1+) by 800 kev In 81

17 β-decay Properties T 1/2, P n gross β-strength properties from theoretical models, e.g. QRPA in comparison with experiments. Requests: (I) prediction / reproduction of correct experimental number (II) detailed nuclear-structure understanding full spectroscopy of key isotopes, like 80 Zn 50, 130 Cd 82. Half-lives Total Error = 3.73 QRPA (GT) P n -Values Total Error = 5.54 QRPA (GT) QRPA (GT+ff) QRPA (GT+ff) (P. Möller et al., PR C67, (2003)) Total Error = 3.08 Total Error = 3.52

18 Effects of T 1/2 on r-process Matter Flow Mass model: ETFSI-Q - all astro-parameters kept constant T 1/2 (GT + ff) r-process model: waiting-point approximation T 1/2 x 3 T 1/2 : 3 r-matter flow too slow r-matter flow too fast

19 Nuclear Masses Over the years, development of various types of mass models / formulas: Weizsäcker formula Local mass formulas (e.g. Garvey-Kelson; N π N ν ) Global approaches (e.g. Duflo-Zuker; KUTY) Macroscopic-microscopic models (e.g. FRDM, ETFSI) Microscopic models (e.g. RMF; HFB) Comparison to NUBASE (2001) // (2003) FRDM (1992) σ rms = // [MeV] ETF-Q (1996) σ rms = // [MeV] HFB-2 (2002) σ rms = [MeV] HFB-3 (2003) σ rms = [MeV] HFB-4 (2003) σ rms = [MeV] HFB-8 (2004) σ rms = [MeV] HFB-9 (2005) σ rms = [MeV] HFB-12 (2005) σ rms =? No improvement of σ rms J. Rikovska Stone, J. Phys. G: Nucl. Part. Phys. 31 (2005) Main deficiencies at N magic! D. Lunney et al., Rev. Mod. Phys. 75, No. 3 (2003)

20 The N=82 Shell Gap as a Function of Z (1) The N=82 shell closure dominates the matter flow of the main r-process (n n ). Definition shell gap : S 2n (82) S 2n (84) paired neutrons. Therefore request: experimental masses and reliable model predictions for the respective N=82 waiting-point nuclei 125 Tc to 131 In 130 N r, peak FRDM Exp FRDM Exp DZ Groote HFB-9 HFB-8 EFTSI-Q HFB-2

21 The N=82 Shell Gap as a Function of Z (2) Definition shell gap : S 2n (81) S 2n (83) unpaired neutrons. Large odd-even Z staggering for microscopic HFB models FRDM HFB-9 FRDM EFTSI-Q DZ Groote HFB-8 Exp Exp HFB-2

22 Effects of Nuclear Masses on N r, Fits T 1/2 +P n and all astro-parameters kept constant!

23 Snapshots: r-process paths for different neutron densities Sn In Cd Ag Pd Rh Ru Tc Mo Nb Zr Y Sr Rb Kr Br Se As Ge Ga Zn Cu Ni Co Fe Z N heaviest isotopes with measured T 1/2 Ba Cs Xe I Te Sb g 9/2 d 5/2 s 1/2 g 7/2 d 3/2 h 11/ g 9/2 p 1/2 p 3/2 f 5/2 f 7/2

24 n n =10 20 r-process paths for n n =10 20, and Sn In Cd Ag Pd Rh Ru Tc Mo Nb Zr Y Sr Rb Kr Br Se As Ge Ga Zn Cu Ni Co Fe Z N Ba Cs Xe I Te Sb n n =10 26 waiting-point isotopes at n n =10 26 freeze-out (T 1/2 exp. : 28 Ni, 29 Cu, 47 Ag 50 Sn) n n =10 23

25 CS Abundances Cowan et al. (2005) Good agreement with SS r-abundances beyond Z=50 Lighter elements appear underabundant relative to SS r-pattern. Y, Pd, Ag particularly low! Evidence for a second ( weak ) r-process? Log (A) = Log 10 (N A /N H ) + 12 From J. Cowan 2006

26 If There Are Two r-processes Where Is the Split? Two separate r-process sites based upon SS meteoritic data on Iodine and Hafnium (Wasserburg, Busso & Gallino); suggested split at Iodine Check by r-process calculations: Is Iodine made with Barium and Hafnium in the primary main r-process or is it made in the secondary weak r-process? Problem: no observations yet of any elements between Z = (only upper limits for Sn) From J. Cowan 2006

27 r-process elemental abundances Conditions for weak r-process main r-process Solar system Simmerer et al., 2004 CS Sneden et al., 2003 Calculations Pfeiffer & Kratz, 2005

28 I, Ba and Hf Production vs. N n The production of I seems to be coupled to the abundances of the heavier Ba-Pb abundances Solar value Sum of N n components K.-L. Kratz 2006

29 [I/Ba], [Ba/Hf], [I/Hf] vs. N n Again see that if Ba & Hf made at solar levels in main r-process make at least 90% I coupled results more element synthesis complications K.-L. Kratz 2006

30 Calculated r-process abundances as function of neutron density (K.-L. Kratz, B. Pfeiffer, 2005)

31 r-process Elemental Abundance Predictions Elemental abundances normalized at log N = 0 Note Hf seems to follow abundances of 3 rd r-process peak elements Kratz et al. (2006) main r-process

32 The age of the Universe holy grail of cosmology BPS CS in AQUARIUS [Fe/H]= -3.1 HD in CANES VENATICI [Fe/H]= -3.0 BPS CS in CETUS [Fe/H]= -2.9 We compare r-process model predictions with astronomical observations (Hubble, KECK) from UMP halo stars. We deduce - astrophysical conditions, and - criteria to determine actinide chronometers. age of UMP star -> Galaxy -> Universe 14 ± 2 Gyr Th/Eu too far in Z Th/U not conclusive without Pb, 3 rd peak U,Th/Hf U, Th/ 3 rd peak new chronometers Ga Ge Sr Zr Ru Cd Mo Pd Sn Y Rh Nb Ag CS abundances scaled solar r-process scaled theoretical solar r-process Pt Ba Os Nd Dy Gd Ce Sm Er Yb Ir Hf La Pr Eu Ho Tb Lu Tm Au Pb Th U

33 Conclusion Nuclear-physics data for r-process calculations still unsatisfactory! better global models for all nuclear shapes (spherical, prolate, oblate, triaxial, tetrahedral, ) and all nuclear types (even-even, even-odd, odd-even, odd-odd) more measurements masses! gross β-decay properties fission properties full spectroscopy of selected key waiting-point isotopes

34 THE END