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Nuclear Structure Studies at HI g S

Nuclear Structure Studies at HI g S. Henry R. Weller The HI g S Nuclear Physics Program. Nuclear Structure Publications from HIg S Research Parity Measurements of Nuclear Levels Using a Free-Electron-Laser Generated g -Ray Beam, Phys. Rev. Lett. 88, 012502 (2002).

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Nuclear Structure Studies at HI g S

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  1. Nuclear Structure Studies at HIgS Henry R. Weller The HIgS Nuclear Physics Program HIGS2 Workshop June 3-4, 2013

  2. Nuclear Structure Publications from HIgS Research Parity Measurements of Nuclear Levels Using a Free-Electron-Laser Generated g-Ray Beam, Phys. Rev. Lett. 88, 012502 (2002). First evidence for spin-flip M1 strength in 40Ar, T.C. Li et al., Phys. Rev. C 73, 054306 (2006). Multipole mixing ratios of transitions in 11B, G. Rusev et al., Phys. Rev. C 79, 047601 (2009). Photoexcitation of astrophysically important states in 26Mg, R. Longland et al., Phys. Rev. C 80, 055803 (2009). Photoexcitation of astrophysically important states in 26Mg. II. Ground-state-transition partial widths, R. J. deBoer et al., Phys. Rev. C 82, 025802 (2010). Spectral Structure of the Pygmy Dipole Resonance, A.P. Tonchev et al., Phys. Rev. Lett.104, 072501 (2010). Measurement of the 241Am(g,n)240Am reaction in the giant dipole resonance region, A. P. Tonchev et al., Phys. Rev. C 82, 054620 (2010). Investigation of low-lying electric dipole strength in the semimagic nucleus 44Ca, J. Isaak et al., Phys. Rev. C 83, 034304 (2011). Discrete deexcitations in 235U below 3 MeV from nuclear resonance fluorescence, E. Kwan et al., Phys. Rev. C 83, 041601(R) (2011) New Method for Precise Determination of the Isovector Giant Quadrupole Resonances in Nuclei, S.S. Henshaw et al., Phys. Rev. Lett. 107, 222501 (2011). HIGS2 Workshop June 3-4, 2013

  3. Dipole response of 238U to polarized photons below the neutron separation energy, S. L. Hammond et al., Phys. Rev. C 85, 044302 (2012). Electromagnetic dipole strength of 136Ba below neutron separation energy, R. Massarczyk et al., Phys. Rev. C 86, 014319 (2012). Fine Structure of the Giant M1 Resonance in 90Zr, G. Rusev et al., Phys. Rev. Lett.110, 022503 (2013). Pygmy dipole strength in 86Kr and systematics of N = 50 isotones, R. Schwengner et al., Phys. Rev. C 87, 024306 (2013). Unambiguous Identification of the Second 2+ State in 12C and the Structure of the Hoyle State, W. R. Zimmerman et al., Phys. Rev. Lett.110, 152502 (2013). Exploring the multihumped fission barrier of 238U via sub-barrier photofission, L. Csige et al., Phys. Rev. C 87, 044321 (2013). Decay Pattern of the Pygmy Dipole Resonance in 60Ni, M. Scheck et al., Phys. Rev. C 87, 051304(R) (2013). The high-efficiency g-ray spectroscopy setup g3 at HIgS, B. Loeher et al., accepted for publication in NIM-A. HIGS2 Workshop June 3-4, 2013

  4. HIGS2 Workshop June 3-4, 2013

  5. Demonstration of the power of doing Nuclear Resonance Fluorescence at HIgS with ~100% linearly polarized g-rays HIGS2 Workshop June 3-4, 2013

  6. A lot of attention has been given to studying the Pygmy Dipole Resonance…a collective excitation below neutron separation energy which is viewed as an oscillation of the neutron skin against a T=0 isospin symmetric core. HIGS2 Workshop June 3-4, 2013

  7. Why is the PDR of Astrophysical Interest?Piekarewicz (PRC 73, 044325 (2006) • Systematics of the PDR may be used to constrain the density dependence of the symmetry energy, a property which has a strong impact on neutron-star properties such as composition, radius and cooling mechanisms. • The existence of low-energy dipole strength in neutron-rich nuclei significantly enhances the cross section for radiative capture of low-energy (~10 MeV) neutrons. Process is fundamental to creation of heavy elements via the rapid neutron capture process. • The PDR may aid the supernovae explosion mechanism. Neutrinos (99% of E) interact strongly with neutrons (large weak vector charge) and can therefore couple to the neutron rich skin of the PDR allowing for a significant energy transfer to the nuclear medium. This could revive the supernovae shock. HIGS2 Workshop June 3-4, 2013

  8. HIGS2 Workshop June 3-4, 2013

  9. Identified 87 new dipole states below n-separation energy. Measured elastic and total absorption cross sections. The PDR is seen as excess strength above the extrapolated GDR strength. HIGS2 Workshop June 3-4, 2013

  10. Parity assignments to all of the observed states proved gave a definite proof that the PDR is an electric dipole phenomenon. HIGS2 Workshop June 3-4, 2013

  11. HIGS2 Workshop June 3-4, 2013

  12. The striking difference between the (a,a’g) and the (g,g’) data sets is reproduced by the quasi-particle phonon model (QPM).The low lying strength corresponds to the more isoscalar neutron-skin oscillation, while the higher lying states belong to a transitional region on the tail of the GDR. HIGS2 Workshop June 3-4, 2013

  13. The quasi-monochromatic beam at HIgS was exploited to study the decayof the PDR. HIGS2 Workshop June 3-4, 2013

  14. The depopulaton of low-lying levels yield information on the summed feeding from spin-1 states in the energy range covered by the beam. HIGS2 Workshop June 3-4, 2013

  15. The contribution of 1p1h components of the wave functions for PDR states is large, giving a large branching ratio to the ground state. Higher energy states associated with the GDR are, on the other hand, expected to exhibit a statistical decay via cascades since they have very small 1p1h components and the density of nearby intermediate states is very high. HIGS2 Workshop June 3-4, 2013

  16. Using HIgS to observe the fine structure of the Giant M1 Resonance HIGS2 Workshop June 3-4, 2013

  17. The Giant M1 Resonance in 90Zr observed via inelastic proton scatteringG. Crawley et al., Phys. Lett. B 127, 322 (1983). HIGS2 Workshop June 3-4, 2013

  18. Fine Structure of the Giant M1 Resonance in 90ZrPRL 110, 022503 (2013)Over 40 1+ states revealed the fine structure of the Giant M1 Resonance in 90Zr centered at 9 MeV for the first time. Three-phonon QPM calculations confirmed the importance of multi-phonon states in describing the observed fragmentation. Excellent agreement between the total B(M1) (4.6 mN2) and centroid energy (9 MeV) were found. HIGS2 Workshop June 3-4, 2013

  19. HIGS2 Workshop June 3-4, 2013

  20. Discovery: 1+ (M1) state was found at 9.757 MeV having B(M1)up= 0.148(59) mN2 The first observation of a 1+ state in 40Ar. Could be important for the Low Baseline Neutrino Experiment which will use 40Ar. Low energy supernovae neutrinos (<50 MeV) could be detected via (n,n’) neutral current excitation of the M1 state(s), and detecting the subsequent 9.757 MeV g-ray. (Anna Hayes) Need another M1 state to determine the shape of the n-spectrum and thus the temperature of the supernovae. NEW (preliminary) RESULT: A second 1+ state at 4.473 MeV having B(M1)up= 0.2 mN2. Being considered by the Neutrino Experimental Group at LANL. HIGS2 Workshop June 3-4, 2013

  21. The g3 CampaignSetup installed at HIgS in summer of 2012.Four 60% HPGe, four 3”x 3” LaBr3 and four 1.5” x 1.5” LaBr3 HIGS2 Workshop June 3-4, 2013

  22. The g3 Campaign The combination of NRF with g-g coincidence spectroscopy is the ideal method for investigating the decay pattern of the Scissors M1 mode and the Pygmy Dipole Resonance. Recent experiments have shown that a major part of the dipole-excited states decay through cascades instead of direct transitions to the ground state. But the detailed decay pattern is unknown. This should reveal new information on the detailed structure of the M1 and E1 excited states. This is the main intention of the experimental campaign using the new g3installation. This work has begun at HIgS but is clearly limited by the beam intensities and the energy resolution of the beams. HIGS2 Workshop June 3-4, 2013

  23. Example of coincidence spectra HIGS2 Workshop June 3-4, 2013

  24. Discovery of the second 2+ State in 12CW.R. Zimmerman et al., PRL 110, 152502 (2013) HIGS2 Workshop June 3-4, 2013

  25. Track Images from the Optical Time Projection Chamber HIGS2 Workshop June 3-4, 2013

  26. PM tube signals provide the time projection response functions HIGS2 Workshop June 3-4, 2013

  27. Fitting these differentiates 12C from 16O events HIGS2 Workshop June 3-4, 2013

  28. HIGS2 Workshop June 3-4, 2013

  29. The (updated) E2 cross section data HIGS2 Workshop June 3-4, 2013

  30. The experimentally determined relative phase compared to the prediction of a two-resonance model HIGS2 Workshop June 3-4, 2013

  31. Results HIGS2 Workshop June 3-4, 2013

  32. Study of the Isovector Giant Quadrupole Resonance in Nuclei(Dissertation of Seth Henshaw, PhD, 2010)Phys. Rev. Lett. 107, 222501 (2011) • Linearly polarized Compton scattering • Exploits the 100% polarization of the HIgS beam along with the realization that the E1-E2 interference term flips sign when going from a forward to a backward angle. HIGS2 Workshop June 3-4, 2013

  33. Scattering Theory • Assumptions: (GDR Dominates) • Modified Thomson Amp included in • E2 strength due to IVGQR HIGS2 Workshop June 3-4, 2013

  34. HINDA Setup 209Bi Scattering Target • 2” Diameter x 1/8” thick • 9*1021 nuclei/cm2 6 Detectors 3@ q=60(55) (Left, Right,Down) 3@ q=120(125) (Left, Right, Down) • D W=55 msr 12mm collimated HIgS beam • 3 x 107g’s/sec • DE/E=2.5 % • Eg =15-26 MeV HIGS2 Workshop June 3-4, 2013

  35. RESULTS FOR 209BiE=23+/-0.13 MeVG=3.9 +/- 0.7 MeV SR=0.56 +/- 0.04 IVQ-EWSRs HIGS2 Workshop June 3-4, 2013

  36. A second target—89Y—was studied in February, 2012. HIGS2 Workshop June 3-4, 2013

  37. Preliminary parameters for the IVGQR in 89Y Eres = 28.0 +/- 0.4 MeV G= 11 +/- 0.9 MeV IVQ-EWSR = 0.93 +/- 0.11 HIGS2 Workshop June 3-4, 2013

  38. The A-dependence of the IVGQR parameters is beginning to take shape. HIGS2 Workshop June 3-4, 2013

  39. Proposed experiments IVGQR survey of 4 additional targets Planning to study additional targets of 51V, 120Sn, 142Nd, and 152Sm in the future. These were chosen to cover a range of A values, and are required to be spherical nuclei in order to minimize inelastic (Raman) contributions. Eg ~ 14–40 MeV. I ~ 107 g/s with DE ~ 2-3 %. 100% Linearly polarized. Complete program in 2013-14. HIGS2 Workshop June 3-4, 2013

  40. Compton scattering at very low energies to determine the polarizabilities of6Li (running right now) and 4He (to be proposed on Wednesday) Polarizabilities of light nuclei Polarizabilities have been measured for d and 3He, but only the sum rule result exists for 4He. These are fundamental constants of these nuclei. Predictions based on modern two and three nucleon potential models exist and need to be tested. These quantities are also important for high precision tests of quantum electrodynamics and for accurate determination of the nuclear charge radius from spectroscopic measurements in helium atoms where they amount to 28(3) kHz for the 1S-2S transition in 4He+, for example. HIGS2 Workshop June 3-4, 2013

  41. Can obtain a value of a + b using the energy-weighted sum rule and total absorption cross section data. Large discrepancies in the data lead toa factor of ~2 uncertainty in a + b for 4He. Must assume b is negligible to obtain a. HIGS2 Workshop June 3-4, 2013

  42. Presently running an experiment using Compton scattering to determine the polarizabilities of 6Li. HIGS2 Workshop June 3-4, 2013

  43. Polarizability of 4He HIGS2 Workshop June 3-4, 2013

  44. The HIgS cryo-target assembly will provide liquid H, D, and He targets.Scheduled to be available for use by September 2013. HIGS2 Workshop June 3-4, 2013

  45. HIGS2 Workshop June 3-4, 2013

  46. Projected results for running at 3 energies (3, 9 and 15 MeV) for a total of 365 hours. HIGS2 Workshop June 3-4, 2013

  47. HIGS2 Workshop June 3-4, 2013

  48. An array of parallel plate avalanche counters consisting of 23 electrolytically deposited 238UO2 (2 mg/cm2) detecting both fission fragments in coincidence. HIGS2 Workshop June 3-4, 2013

  49. The double-humped potential model could not fit the new data in a consistent manner.The triple-humped barrier parameters were adjusted to best describe the photofission data and the (g,n) data using the EMPIRE-3.1 code. The hyperdeformed third potential minimum, when adjusted to reproduce the 5.5 MeV resonance, predicts an additional resonance at 4.55 MeV. HIGS2 Workshop June 3-4, 2013

  50. The triple-humped fission barrier of 238U as determined from the present study. HIGS2 Workshop June 3-4, 2013

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