Nuclear astrophysics Act II - Stellar obituary A survey in 3 acts
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Nuclear astrophysics A survey in 3 acts log (abundance) Jeff Blackmon, Physics Division, ORNL Where did this come from? Act II - Stellar obituary 4. Stellar evolution s process 5. Supernovae r process Mass Stellar Classification Aldebaran Betelgeuse Alnitak Rigel Sirius Arneb Stellar evolution Brighter Globular cluster Most stars formed at about the same time AGB He burning Asymptotic Giant Branch Star Giant branch H-shell burning Convective envelope Core H2 exhausts H burn Cooler He burn CO core He burning & the “Hoyle” state t1/2(8Be)=9.7x10-17 s N 8 B N 5 1010 7.367 8Be+ 0+ resonance near the Gamow energy was predicted by Hoyle 0+ 7.654 e+e- 8Be 4.439 2+ Phys Rev 92 (1953) 1095. Numerous complementary techniques 12C(p,p’)12C* , 3, e+e13C(3He,)12C* Largest uncertainty ee~12% Experiments now at West. Mich. U. 0+ 12C 12C(,)16O - the “holy grail” ? The 12C(,)16O reaction rate fixes the ratio of 12C/16O in the core Ecm The 12C/16O ratio substantially affects the subsequent evolution of the star: Size of Fe core Supernova? 300 keV 16O Influence of subthreshold states substantial uncertainties in extrapolation New Stuttgart measurements: Kunz et al., PRL (2001) improvement? = 0.1 fb 12C(,)16O - via 16N decay Azuma et al. PRC 50 (1994) Ecm 16N 12C 16O New WNSL Measurement France et al., PRC 75 (2007) 065802. Approach @ ANL (Tang et al.) 12C(,)16O via ANC A nucleon or “cluster” of nucleons (no internal degrees of freedom) is transferred from one nucleus to another. The core nuclei are unperturbed. exp=S1S2DWBA W (r) C r 12C(,)16O SubCoulomb transfer to subthreshold states via ANC Brune et al. PRL 83 (1999) 6.92 (2+) 7.12 (1-) 12C 16O DWBA C2(2+)=(1.30.2) x 1010 fm-1 C2(1-)=(4.30.8) x 1028 fm-1 w/ 16N decay 16 SE 2 (300keV) 4223 keV b SE1(300keV) 10117keV b Neutron sources in AGB Stars Stars are thermally unstable: mixing, convection, mass loss 12C(p,)13N(n)13C(,n)16O 22Ne(n)25Mg H envelope radius convective envelope driven off Flash mixing 13C(,n) 13C(,n) Convective pocket He intershell CO core (white dwarf) CO core time Synthesis of heavy elements • s process ~ 80% of isotopes (n,) rates needed Branch points crucial • r process ~ 70% of isotopes Far from stability See supernovae • p process ~ 10% of isotopes Very low abundance n, ~ 1 v ~ constant v Secondary process (s-wave) Neglected here Recipe for untangling r & s abundances Calculate s process yields and fit to s only isotopes log (abundance) Subtract s abundances from solar system to get r abundances Mass Stardust in a haystack Nittler, Earth Planetary Sci Lett (2003) Tiny grains isolated from meteorites Unusual grains identified with SIMS Nguyen & Zinner, Science 303 (2004) 1496. ( Nd/ Relative abundances for isotopes of a given element from a single AGB star 144Nd)/(solar) (XNd/ 144 i Some grains have preserved isotopic composition from solar environment Nd)/s olar Nd Isotope Ratios in SiC Grains Meteorite data Stellar model before ORELA data Stellar model with new ORELA data 2.0 1.5 Solar Nd 1.0 0.5 Guber et al. 0.0 142 143 144 145 146 Mass Number 147 148 (n,) cross sections for the s process Good data on most stable isotopes Spallation n sources TOF techniques Good energy resolution Often high level densities ORELA Maxwellians at kT = 8 and 30 keV 8 keV 30 keV 1.0 Resonance Areas Influence of low-energy levels on <v> at low temp Effect of thermal excitations in stellar environment Branch point isotopes Relative Height Major outstanding issues 0.8 0.6 0.4 0.2 0.0 0 10 20 En (keV) 30 The new frontier Source flight path (m) resolution (ns/m) power (kW) flux (n/s/cm2) FOM (n/s/cm2) ORELA Lujan n TOF 40 0.2 8 2x104 5x105 20 6.2 64 5x106 6x109 180 0.05 45 3x105 5x108 SNS 20 18 2000 2x108 9x1010 Experiments now possible with samples of only ~ 1016 atoms/cm2. Important s process branch points High efficiency detector arrays High segmentation to handle rate from radioactive sources DANCE status feasible Synthesis of heavy elements • s process ~ 80% of isotopes (n,) rates needed Branch points crucial • r process ~ 70% of isotopes Far from stability See supernovae • p process ~ 10% of isotopes Very low abundance n, ~ 1 v ~ constant v Secondary process Neglected here The r process site Galactic chemical evolution arguments favor supernovae as the dominant source for elements early in the history of the Galaxy an r process Argast et al., A&A 416 (2004) 997. Creation of elements in the early Galaxy Now many observations of unmixed supernova nucleosynthesis in the Galactic halo Cowan & Sneden, Nature 440 (2006) 1151. CS22892-052 Fe/H = (8x10-4) solar = very old r/Fe = 50 solar Only 2 known in 2000 Now extensive surveys Z>55 pattern matches solar e.g. see Frebel et al., ApJ 652 (2006) 1585 SEGUE (Sloan DSS) Spectra of >2x105 selected halo stars Expect ~ 1% with Fe/H < 0.001solar ~36 known r process stars 11 with r/Fe > 10 solar Distribution Fe/H puzzling Lowest Fe/H stars intriguing Frebel et al., Nature 434 (2005) 871. CS22892 Fe/H < 10-5solar (C&S, Nature 440) Z<50 abundances vary Anatomy of a supernovae Stars > 10 solar masses Higher gravity Faster burning stages Less mass loss C burning O burning Si burning In rapid succession • Fermi degeneracy initially supports core • Shell Si burning increases core size of • Electron capture on nuclei in core begins to reduce pressure support • Core undergoes runaway collapse • Reaches supernuclear densities & shock rebounds -- EOS important • Mechanism involves interplay of hydrodynamics and nuclear physics • Spherical models fail to explode • Multidimensional effects are critical Standing Accretion Shock Instability History of SN1987a QuickTime™ and a Video decompressor are needed to see this picture. Nucleosynthesis sites in supernovae Fe group nuclei produced from nuclear statistical equilibrium Environment above neutron star is likely site for the r process Influence of weak interaction Effect of e-capture rates on formation of the shock Electron capture rates affect the formation of the shock wave. Neutrino interactions play a role in driving the explosion. Neutrino induced reactions alter nucleosynthesis. Weak rates in this mass region are not well understood: GT strength distributions first-forbidden contribution Fröhlich et al., PRL 96 (2006) Abundaces relative to solar with n reactions without n reaction Cole et al., PRC 74 (2006) 034333. Charge exchange reactions with fast beams at the NSCL Charge exchange reactions such as (t,3He) are sensitive probes for GT strength at 100 – 200 MeV/u Needed for • core collapse supernova models • type Ia supernova models • neutron star crust processes Special case or systematic issue? Need systematic measurements for entire relevant range (especially beyond fp shell where nuclear models become much simpler) can help decide which theoretical model to use and can help to improve theoretical models for supernova usage Need to develop technique for inverse kinematics and radioactive beams nSNS SNS neutrino spectrum nm 0.04 0.035 Neutrino Flux A proposal has been submitted to DOE to construct a facility for neutrino reaction measurements at the Spallation Neutron Source. http://www.phy.ornl.gov/nusns 0.03 nm 0.025 ne 0.02 0.015 0.01 0.005 0 GeV protons BL18 ARCS 0 5 10 15 20 25 Segmented Accumulator Detector Likely initial program ne+OF+e- (450 events/yr) ne+FeCo+e- (1100 events/yr) ne+AlSi+e- (1100 events/yr) ne+Pb Bi+e- (4900 events/yr) 35 40 45 50 Proton beam (RTBT) Homogeneous Det. Hg target 30 Energy, MeV Cartoon r process Y(A 1) 1 2 2 Sn /(kT ) n n e Y(A) 2 mu kT Large Sn Small Sn (n,) >> (,n) >> t1/2 (,n) >> (n,) >> t1/2 Free parameters nn, kT, t Instantaneous freezeout & decay to stability Only masses, t1/2, and Pn needed Calculated r process QuickTime™ and a None decompressor are needed to see this picture. Results of r process calculations Many different n densities needed Reasonable fits to A=130,190 peaks Not so nice reproduction of intermediate nuclei Fission? (Qian & Wasserburg) Evidence for quenching of the shell gaps? (Kratz et al.) Astrophysical environment? Freezeout effects? Masses? P. Hosmer et al. PRL 94 (2005) 112501. NSCL fast beam r-process campaign: the half-life of 78Ni 3He t1/2(78Ni): 110 +100-60 ms + n -> t + p Effect of new t1/2 on r process abundances neutron Si stack ~ 100 MeV/u NERO Particle identification in rare isotope beam Model Calculation: Half-Lives from Moeller, et al. 97 Abundance (A.U.) r-process beam Observed Solar Abundances 1.E+02 Same but with present 78Ni Result 1.E+01 1.E+00 1.E-01 1.E-02 70 120 170 Mass (A) 78Ni Half-life of 78Ni measured with 11 events. Shorter 78Ni half-life leads to greater production of A=190 peak The properties of neutron-rich nuclei are crucial for understanding the site(s) of the r process and the chemical history of the Galaxy 220 Mass measurements Large number of isotopes circulate and are measured in ring Matos, Ph.D. Univ. Giessen Yu. Litvinov et al., NPA756 (2005) 3. 2 modes: Schottky - slow, more precise isochronous - fast, less precise Measurements now crossing into regime of light r process The Chart of the Nuclides http://www.nndc.bnl.gov/chart/ The Chart of the Nuclides http://www.nndc.bnl.gov/chart/ = half-life measurements since 2000 (6th ed.) (neutron-rich nuclei only) The Chart of the Nuclides http://www.nndc.bnl.gov/chart/ r process = half-life measurements since 2000 (6th ed.) (neutron-rich nuclei only) Only a few measurements in r process path Structure n-rich nuclei and the r process Masses, half-lives and Pn are crucial direct impact on r process abundances. Must rely on theory. Dillman et al., PRL 91 (2003) 162503. Properties like level energies and B(E2) values provide some direct benchmarks. Radford et al., PRL 88 (2002) 222501. Varner et al., EPJ 25 (2005) 391. @ HRIBF Jones et al. Understanding the structure of neutron-rich nuclei is crucial to improving extrapolations to more neutron-rich (unmeasured nuclei). 2f7/2 2f5/2 HRIBF 3p1/2? 3p3/2 132Sn(d,p)133Sn EP (channels) Ex The HRIBF CARIBU CPT measurements of very neutron-rich nuclei Intense beams and high energy will allow unique structure studies, e.g. (p,t) Intense 252Cf fission source under construction at ATLAS Gas stopping technology Neutron-rich RIBs will push the boundaries of our knowledge Different region on nuclei complementary to HRIBF Atomic number (Z) Next-generation RIB Facilities RIBF (RIKEN), FAIR (GSI), SPIRAL-II (GANIL), RIA (USA) Neutron number (Z) Ground state properties of nearly all r process nuclei up to the A=190 peak can be measured Atomic number (Z) Nuclear structure studies far from stability will greatly improve our ability to extrapolate to the unknown Understanding observations of the oldest stars and the origin of the heavy elements in our Galaxy Neutron number (Z)
