The origin of heavy elements in the solar system

Transcription

1 The origin of heavy elements in the solar system (Pagel, Fig 6.8) each process contribution is a mix of many events! 1

2 Heavy elements in Metal Poor Halo Stars recall: [X/Y]=log(X/Y)-log(X/Y) solar CS red (K) giant located in halo distance: 4.7 kpc mass ~0.8 M_sol [Fe/H]= 3.0 [Dy/Fe]= +1.7 old stars - formed before Galaxy was mixed they preserve local pollution from individual nucleosynthesis events 2

3 A single (or a few) r-process event(s) abundance log(x/h) CS (Sneden et al. 2003) solar r Cosmo Chronometer element number other, second r-process to fill this up? (weak r-process) main r-process matches exactly solar r-pattern conclusions? NEW: CS with U (Cayrel et al. 2001) Age: Gyr (Schatz et al ApJ 579, 626) 3

4 Overview heavy element nucleosynthesis process conditions timescale site s-process (n-capture,...) T~ 0.1 GK τ n ~ yr, n n ~ /cm yr and yrs Massive stars (weak) Low mass AGB stars (main) r-process (n-capture,...) T~1-2 GK τ n ~ µs, n n ~10 24 /cm 3 < 1s Type II Supernovae? Neutron Star Mergers? p-process ((γ,n),...) T~2-3 GK ~1s Type II Supernovae 4

5 The r-process Temperature: ~1-2 GK Density: 300 g/cm3 (~60% neutrons!) neutron capture timescale: ~ 0.2 µs Rapid neutron capture β-decay Proton number Seed (γ,n) photodisintegration Neutron number Equilibrium favors waiting point 5

6 show movie 6

7 Waiting point approximation Definition: ASSUME (n,γ)-(γ,n) equilibrium within isotopic chain How good is the approximation? This is a valid assumption during most of the r-process BUT: freezeout is neglected Freiburghaus et al. ApJ 516 (2999) 381 showed agreement with dynamical models Consequences During (n,γ)-(γ,n) equilibrium abundances within an isotopic chain are given by: 2 Y ( Z, A + 1) G( Z, A + 1) A + 1 2πh = nn Y ( Z, A) 2G( Z, A) A mukt 3/ 2 exp( S n / kt ) time independent can treat whole chain as a single nucleus in network only slow beta decays need to be calculated dynamically neutron capture rate independent (therefore: during most of the r-process n-capture rates do not matter!) 7

8 Endpoint of the r-process r-process ended by n-induced fission or spontaneous fission (different paths for different conditions) (Goriely & Clerbaux A&A 348 (1999), 798 n-induced fission β-delayed fission spontaneous fission (Z,A) n-capture (DC) fission (Z,A) β fission (Z,A+1) fission (Z,A+1) (Z+1,A) fission barrier 8

9 Consequences of fission Fission produces A~A end /2 ~ 125 nuclei modification of abundances around A=130 peak fission products can serve as seed for the r-process - are processed again into A~250 region via r-process - fission again fission cycling! Note: the exact endpoint of the r-process and the degree and impact of fission are unknown because: Site conditions not known is n/seed ratio large enough to reach fission? (or even large enough for fission cycling?) Fission barriers highly uncertain Fission fragment distributions not reliably calculated so far (for fission from excited states!) 9

10 Role of beta delayed neutron emission Neutron rich nuclei can emit one or more neutrons during β-decay if S n <Q β (the more neutron rich, the lower S n and the higher Q β ) (Z,A) β n γ S n (Z+1,A-1) (Z+1,A) If some fraction of decay goes above S n in daughter nucleus then some fraction P n of the decays will emit a neutron (in addition to e - and ν) (generally, neutron emission competes favorably with γ-decay - strong interaction!) 10

11 Effects: during r-process: none as neutrons get recaptured quickly during freezeout : modification of final abundance late time neutron production (those get recaptured) Calculated r-process production of elements (Kratz et al. ApJ 403 (1993) 216): before β-decay after β-decay smoothing effect from β-delayed n emission! 11

12 Cs (55) Xe (54) I (53) Te (52) Sb (51) Sn (50) In (49) Cd (48) Ag (47) Cs131 Cs132 Cs133 Cs134 Cs135 Cs136 Cs137 Cs138 Cs139 Cs140 Cs141 Cs142 Cs143 Cs144 Cs145 Cs146 Cs147 Te128 Te129 Sb127 Sb128 Sb129 Sb130 Sb131 Sb132 Sb133 Sb134 Sb135 Sb136 Sb137 Sb138 Sb139 Sb140 Sb141 Sb142 Sb143 Sn126 Sn127 Sn128 Sn129 Sn130 Sn131 Sn132 Sn133 Sn134 Sn135 Sn136 Sn137 Sn138 Sn139 Sn140 Sn141 Sn142 In Cd124 Cd125 Cd126 Cd127 Cd128 Cd129 Cd130 Cd131 Cd132 Cd133 Cd134 Cd135 Cd Ag123 Ag124 Ag125 Ag126 Ag127 Ag128 Ag129 Ag130 Ag131 Ag132 Ag I129 I130 In I131 Te130 Te131 In I132 In I133 Te132 In Xe130 Xe131 Xe132 Xe133 Xe134 Xe135 Xe136 Xe137 Xe138 Xe139 Xe140 Xe141 Xe142 Xe143 Xe144 Xe145 Xe146 P n =0% I134 Te In I135 Te In I Te In I Te P n =99.9% abundance (per 10 6 Si) In I Te I Te I I I142 I143 I144 I145 Te139 Te140 Te141 Te142 Te143 Te144 In134 In135 In136 In137 In138 In139 In140 In141 Cd138 Cd139 Cd140 Ag135 Ag136 Ag137 Ag138 Ag Example: impact of P n for 137 Sb r-process waiting point Pn=0% solar Pn=99% A=136 ( 99%) A=137 ( 0%) r-process waiting point mass number A 12

13 Summary: Nuclear physics in the r-process Quantity S n T 1/2 P n fission (branchings and products) G N A <σv> neutron separation energy β-decay half-lives β-delayed n-emission branchings partition functions neutron capture rates Effect path abundance pattern timescale final abundance pattern endpoint abundance pattern? degree of fission cycling path (very weakly) final abundance pattern during freezeout? conditions for waiting point approximation 13

14 RIA Reach r-process path 82 Known New MSU/NSCL Reach r-process abundance distribution First NSCL experiment The r-process path (Reach for half-life)

15 National Superconducting Cyclotron Laboratory at Michigan State University New Coupled Cyclotron Facility experiments since mid 2001 Ion Ion Source: Source: Kr Kr beam beam Kr Kr beam beam MeV/u MeV/u Kr Kr hits hits Be Be target target and and fragments fragments Tracking (=Momentum) TOF start Separated beam of r-process nuclei TOF stop de detector Implant beam in detector and observe decay Fast beam fragmentation facility allows event by event particle identification 15

16 Installation of D4 steel, Jul/

17 First r-process experiments at new NSCL CCF facility (June 02) Measure: β-decay half-lives Branchings for for β-delayed n-emission Detect: Particle type type (TOF, de, de, p) p) Implantation time time and and location β-emission time time and and location n neutron-β coincidences New NSCL Neutron detector NERO neutron 3 He + n -> t + p Fast Fragment Beam (fragment. 140 MeV/u 86 Kr) Si Stack 17

18 NSCL BCS Beta Counting System 4 cm x 4 cm active area 1 mm thick 40-strip pitch in x and y dimensions ->1600 pixels Si BCS β Si Si 18

19 NERO Neutron Emission Ratio Observer 3 He Proportional Counters BF 3 Proportional Counters Specifications: 60 counters total (16 3 He, 44 BF 3 ) 60 cm x 60 cm x 80 cm polyethylene block Extensive exterior shielding 43% total neutron efficiency (MCNP) Polyethylene Moderator Boron Carbide Shielding 19

20 June 2002 Data preliminary results 77 Ni 74 Co r-process path Mainz: K.-L. Kratz, B. B. Pfeiffer PNNL: P. P. Reeder Maryland/ANL: W.B. Walters, A. A. Woehr Notre Dame: J. J. Goerres, M. M. Wiescher NSCL: P. P. Hosmer, R. R. Clement, A. A. Estrade, P.F. P.F. Mantica, F. F. Montes, C. C. Morton, M. M. Ouellette, P. P. Santi, A. A. Stolz Stolz Energy loss 71 Fe Gated β-decay time curve (implant-decay time differences) Time of flight Fast RIBs: cocktail beams no inflight decay losses measure with low rates (>1/day) 20

21 Neutron Data Nuclei with decay detected With neutron in addition N n E (arb units) Ni 73 Co E (arb units) Ni 73 Co TOF (arb units) TOF (arb units) P n = Nn N ε β n neutron detection efficiency (neutrons seen/neutrons emitted) 21