Nuclear Astrophysics Research at HIγS

Transcription

1 Nuclear Astrophysics Research at HIγS Selected Examples of Work in Progress Werner Tornow Duke University & TUNL

2 Outline A. What is HIγS? B. Few-Body Physics (see T. Shima) C. Nuclear Astrophysics 4 9 a) He( αn, γ ) Be b) c) 4 12 He( 2α, γ ) C( α, γ ) O C d) Mg Mg ( γ, γ ) (γ, ) and 180m Ta( γ, γ ') 180 Ta e) 180m Ta n 179 Ta D. Applications Nuclear Resonance Fluorescence Studies on U, U,( Pu) (see C.T. Angell, Np ) 235

3 High-Intensity Gamma-ray Source (HIγS) at Triangle Universities Nuclear Laboratory (TUNL) at Duke University

4 Quasi monoenergetic γ-ray flux on target: > s Tunable from 1 to 90 MeV 100% linear or circular polarization Energy resolution determined by collimation; no need for tagging

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6 B. Few-Body Physics 3 He( γ, pd) and 4 He( γ, pt) [ 4 He( γ, 3 He) n]

7 3 He / Xe Gas Scintillator ~50 atm He-3 PMT

8 3 He(γ,p) 2 H Electrons from Compton scattering Two-body breakup Three-body breakup

9 3 He(γ,p) 2 H Recently, additional data obtained below 9 MeV and up to 16 MeV in 0.5 MeV steps

10 3 He(γ,p) 2 H

11 Electromagnetic interactions on 4N systems

12 Trento Group: G. Orlandini, W. Leidemann, S. Quaglioni, N. Barnea, V.D. Efros, Lorentz integral transform method, final-state interaction and Coulomb included, MTI-III Lisbon Group: A. Fonseca, A. Deltuva, work in progress with CD-Bonn

13 4 He( γ, pt) # of events Pulse Height

14 Very Trento

15 9 Be n ( γ, ) 2α Total Cross-Section Measurements The so-called rapid neutron capture process (r-process) produces about half of the abundance of nuclei heavier than iron. The actual site of the r-process is still under investigation. Models which assume that type II supernovae are the site of the r-process are 9 very sensitive to the astrophysical rate of formation of via the reaction 9 α + α + n Be + γ 8 Be This reaction can be studied only by investigating the inverse reaction Be 16 ( is unstable: s ) Neutrons are detected by a cylindrical detector composed 3 of 18 He proportional counters embedded in a polyethylene 9 moderator. The Be target is placed at the center of the cylinder. Data were taken between 1.5 and 5.2 MeV gammaray energy with 1% energy spread.

16 γ 9 + Be α + α + n Arnold et al. Utsunomiya et al. Conclusion: Less neutrons are available for r-process than previously thought

17 8 Be( α, γ ) 12 C 12 C is formed during stellar helium burning in the so-called triple-alpha process, which is governed by + the 0 state at an excitation energy in 12 C of MeV (2 nd 12 excited state in C ) 8 Be is unstable: decays into 2 α-particles, life time At higher temperatures (T> 1 GK) higher lying states may contribute Some theories predicts a broad 2 state at 9.11 MeV in C, + which is a member of the rotational band built on the 0 12 (Hoyle) state. It would increase the production of C by a factor of 15. Search performed at HIγS by M. Gai et al. using an Optical Time Projection Chamber (O-TPC). 16 s

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19 Predicted 9.11 MeV resonance known10.84 MeV resonance Conclusion: Hoyle state is a spherical alpha-particle condensate

20 12 C ( α, γ ) 16 O Thermonuclear burning in stars is responsible for the synthesis of most of the elements heavier than helium in the universe. 12 For example, C is formed by the triple α reaction, which is activated at T = K At slightly higher temperature the 12 C( α, γ ) 16 O reactions becomes dominant This nuclear reaction is responsible for defining the ratio of carbon to oxygen in the Universe (C and O are two of the most important constituents in organic matter and life). The abundance of most of the chemical elements is affected by the 12 C( α, γ ) 16 O reaction The C/O value has also extreme consequences on the structure and evolution of subsequent stellar burning stages and explosive scenarios: supernova (SN) explosion versus neutron star 12 C( α, γ ) 16 O The reaction has also cosmological implications: SN have been used as standard candles to determine distance of galaxies and rate of expansion of the Universe. Standard candles could be modeled if the C/O ratio in the progenitor star is known.

21 Level scheme of 16 O Helium burning energy window (red) is only 300 kev above threshold and is dominated by the tail of sub-threshold resonances at 6.92 and 7.12 MeV Coulomb barrier at such low energies reduces the reaction cross section to values as low as barn, making it impossible to measure with current technology

22 12 C ( α, γ ) 16 O Astrophysical S-factor: σ 2πη S ( E) = E ( E) e, η= Sommerfeld parameter Holy Grail of Nuclear Astrophysics Electric dipole component (E1) of S-factor Interference between E1 and E2 (electric quadrupole) components Extrapolation guided by R-matrix formalism of nuclear reactions

23 New experimental approach: Time reversed reaction 16 O ( γ, α ) 12 C Optical Time Projection Chamber (O-TPC) using oxygen containing gas Goal: Measure differential and total cross section M. Gai et al.

24 12 C ( α, γ ) 16 O Astrophysical S-factor: σ 2πη S ( E) = E ( E) e, η= Sommerfeld parameter Electric dipole component (E1) of S-factor E 5MeV = 9. γ Interference between E1 and E2 (electric quadrupole) components Extrapolation guided by R-matrix formalism of nuclear reactions

25 Energy resolution of 1% is required 18 O natural abundance: 0.2% Disadvantage of this method: low target density Won t be able to go much below 2.0 MeV in center-of mass system, before HIγS provides another factor of 10 more beam with 1% energy spread, or highly depleted gases become available

26 Parallel Effort by Rehm et al. (Argonne Group) using superheated water Superheated liquid forms bubbles when exposed to radiation: Bubble Chamber It takes only a small disturbance to induce vaporization in superheated state (4) Vaporization can be suppressed if the pressure in the liquid is promptly increased

27 10 ms between pictures

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29 Advantage: high target density, insensitive to gamma-rays Disadvantage: No energy resolution at all, sensitive to neutrons Water must be depleted of 2 H, 17 O, 18 O to a large degree

30 Count rate estimates based on incident gamma-ray flux of s Assumption: water is depleted by a factor of 1000 in and a factor of in 2 H 17 O, 18 O Natural abundance: 0.04% oxygen % oxygen % deuterium The Argonne group has acquired 5 liters of depleted water of sufficient depletion levels which should allow them to take data down to E = 8. 2MeV γ

31 12 C ( α, γ ) 16 O Electric dipole component (E1) of S-factor

32 26 Mg( γ, γ ') Mg Measurement of Excited States in 26 Mg The 22 Ne(α,n) 25 Mg reaction is one of the dominant neutron sources for the slow neutron capture process, also called s-process, which produces many of the heavier nuclei 14 N(α,γ) 18 F(β + ν) 18 O(α,γ) 22 Ne 22 Ne(α,n) 25 Mg or 14 C(α,γ) 18 O(α,γ) 22 Ne 22 Ne(α,γ) 26 Mg competing reaction A major factor in the reliability of nucleosynthesis calculations is the uncertainty in the reaction rates of these competing reactions. This uncertainty must be reduced at temperatures relevant to stellar s-process environments. Purpose: to identify states in 26 Mg, measure their energy, spin and parity 26

33 Nuclear Resonance Fluorescence (NRF)Technique S n Experimental observables in NRF Excitation energy E x 1 - Spin and parity J, π Decay width Γ 0 Branching ratio Γ i /Γ Mixing ratios δ A X N g.s. In a completely model independent way! HIGS Advantages σ el = f(e γ ) (from primary g.s. transitions) σ inel = f(e γ ) (from secondary transitions) σ tot = σ el + σ inel = σ abs A.P. Tonchev, NIM B (2005) G. Rusev, PRC 79, (2009)

34 Parity Measurements with a Linearly Polarized Photon Beam Azimuthal distribution E1 Dipole M E (θ,φ) = (135 0, 90 0 ) (θ,φ) = (90 0, 90 0 ) x E2 E z y x Quadruple E z M y W (9, 0 ) W0 (9, 9 ) = = π = W (9, 0 ) + W0 (9, 9 ) 0c 0 1 f o J J π π = 1 = 1 +, 2 +, 2 - y Experimental Asymmetry of 0.96 x E z x E z A. Tonchev, NIM B 241 (2005) 51474

35 The nuclear burning energy range at typical temperatures of T=300M K is near 600 kev. Due to the Coulomb barrier the cross section is so low that direct measurements are very difficult. The lowest directly measured resonance is at 830 kev, over 200 kev above the Gamow peak. 26 Mg Ground state transitions Γ 0 10 g (99.64%) Ne + α Secondary transitions Branching transitions Γ f HIGS detection sensitivities: resonance states with Γ tot 1meV R. Longland et al. PRC (2009)

36 Preliminary results of 26 Mg(γ,γ ) reaction study E x (MeV) J π i (ћ) 1808 (2+) 2938 (2+) 3588 (0+) E i (J π f) 4333 (2+) 4972 (0+) 5292 (2+) 7100 (2+) (9) 1 - * (9) 1 + * * * * * (2) * 1 - * (2) * 1 - * * * * known known (0 (4 + +,1,5,2,6 + +,3,7,4 ) + ) (3) 1 + * * * previously 1 * Known α width, but uncertain quantum number. The new spin-parity assignments for the and MeV states are expected to reduce the uncertainties in the Ne( α, γ ) Mg rate. A significant + finding is that the state at kev was found to have unnatural parity 1, contrary to previous findings. As a result this state cannot contribute to the 22 Ne +α reaction rates, and thus the neutron production rate is reduced compared to previous work.

37 180m Ta Z N 1. The rarest stable isotope in the solar system: Abundance = %. The only naturally occurring isomer in the universe m Ta is bypassed by the slow (s) neutroncapture process m Ta is shielded from the β - and β + decay paths following the rapid neutron-capture process A.P. Tonchev and F. Harmon, App. Rad. and Isotopes, 52 (2000) 873 A.P. Tonchev et al., Hyperfine Interactions, 107 (1997) 167

38 Production and Decay Schemes of 180m Ta 179 Ta(n,γ) 181 Ta(ν,n) Production balance 179 Hf(n,γ) 181 Ta(γ,n)? 180 Lu 180m Ta(γ,n) 180m Ta(γ, γ )? Difficulty of its production Can the gamma process account for the abundance deficiency? 180 Hf 180 Ta 180 W What is the destruction rate? 180m Ta(γ,n) and 180m Ta(γ,γ ) M. Arnould et al., Phys. Rep. 384 (2003) 1 P. Mohr et al., PRC (2007)

39 Motivation S n ( 181 Ta) = 7.6 MeV Photodisintegration cross section of 180m Ta near astrophysical important energies n J π = 8 +, 9 +, 10 + Photonuclear reaction on high-spin isomeric target γ S n ( 180 Ta) = 6.6 MeV No experimental data for 180m Ta(γ,n) γ E γ Experimental technique direct neutron counting activation measurement 179 Ta kev T 1/2 >10 15 y kev T 1/2 =8.1 h 180 Ta EC β -

40 Sp ( 180 Ta) Sn ( 180 Ta) Sn ( 181 Ta) E γ (MeV) 180m Ta(γ,γ ) 180m Ta(γ,n) 181 Ta(γ,n) Region of interest: 1 MeV around the neutron separation energy Direct neutron counting Reactions to be measured: m Ta(γ,γ ) by activation m Ta(γ,n) by He-3 neutron counter S. Goko et al., PRL 96, (2006)

41 Detector: He-3 neutron proportional counter (detector efficiency about 50% at 0.5 MeV) Target: 10 g natural tantalum = 4.0 x nuclei of 180 Ta Photon flux: 1x10 7 γ s -1 Cross section: 10 mb (from GNASH simulation) Photon beam: Circular Beam energies: MeV Neutron count rate = 723 s -1 Total beam time: 50 h X-rays Counts kev kev i.s. g.s. 180 Ta 93.4 kev kev 180 W 180 Hf C. Arnold (UNC), PhD dissertation Gamma Energy (kev)

42 X-rays from EC decay of 180m depopulation of the Ta 180 Ta ground state, indicating the isomer

43 Summary I hope I have convinced you that high-intensity and monoenergetic photon beams can contribute significantly to progress in nuclear astrophysics research. In most cases it is the time reversed reaction that does the trick. We are just at the beginning of exploiting the potential of photon beams in nuclear astrophysics. Stay tuned!