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A new opportunity to study the IVGQR in nuclei

A new opportunity to study the IVGQR in nuclei. Studies in the 70s and 80s produced data on the energy, width and strength of this global feature of nuclei with large uncertainties.

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A new opportunity to study the IVGQR in nuclei

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  1. A new opportunity to study the IVGQR in nuclei Studies in the 70s and 80s produced data on the energy, width and strength of this global feature of nuclei with large uncertainties. Advances in nuclear structure computational techniques make a new high accuracy investigation both interesting and timely. Some of the most robust and unambiguous results came from studies using Compton scattering where the IVGQR was observed via its interference with the GDR. The 100% polarized beams at HIgS along with a new technique and a world class detector system will allow for an order of magnitude improvement in the determination of the properties of the IVGQR of nuclei. TUNL_Retreat_2011

  2. Pb(g,g) IVGQR Studies 208 perpendicular to polarization plane parallel to polarization plane Unpolarized TUNL_Retreat_2011

  3. Our Measurement Technique The [Dal92] experiment only had beam polarization ~25% and only measured at a backward angle. Will see that simultaneous forward and backward measurements lead to unambiguous IVGQR parameters. Our recent experiment (PhD for Seth Henshaw) exploits the 100% polarization of the HIgS beam. TUNL_Retreat_2011

  4. Scattering Theory Assumptions: (GDR Dominates) Modified Thomson Amp included in CE1 E2 strength due to IVGQR TUNL_Retreat_2011

  5. Scattering Theory Assumptions: (GDR Dominates) • Modified Thomson Amp included in • E2 strength due to IVGQR TUNL_Retreat_2011

  6. Polarization Ratio TUNL_Retreat_2011

  7. 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 TUNL_Retreat_2011

  8. Analysis Fit 12C NRF spectra with GEANT4 simulation to determine Response Function for monoenergetic gs Fit Data with Lineshape + Background Subtract Background Sum Resulting Data TUNL_Retreat_2011

  9. Results TUNL_Retreat_2011

  10. Results G=3.9 +/- 0.7 MeV SR=0.6 +/- 0.04 IVQ-EWSRs E=23+/-0.13 MeV TUNL_Retreat_2011

  11. Results TUNL_Retreat_2011

  12. Results TUNL_Retreat_2011

  13. Results TUNL_Retreat_2011

  14. Results TUNL_Retreat_2011

  15. Proposal Perform similar measurements on 8 targets between A=60 and A=240 at HIgS. Use the full 8-detector HINDA array. Data as good or better than obtained for 209Bi can be obtained in 40 - 100 hours per target (depending on Z). A ~500 hour program will produce accurate results (x10) for the energy, width and strength of the IVGQR in nuclei as a function of A. This will allow testing of model predictions of quantities such as the A-dependence of the energy, the splitting and/or fragmentation of the IVGQR, and the search for missing strength. Both collective models and no-core shell models can be applied and refined, and extended to exotic nuclei. TUNL_Retreat_2011

  16. A New Method for Identifying Special Nuclear Materials Based Upon Polarized (g,n) Asymmetries A New Method for Identifying Special Nuclear Materials Based Upon Polarized (g,n) Asymmetries A TUNL/HIgS Project funded by the NSF/DNDO through their Academic Research Initiative program H. R. Weller—PI M. Ahmed and Y. Wu -- Co PIs Collaborators: N. Brown, S.S. Henshaw, H. J. Karwowski, J. M. Mueller, S. Stave, B. A. Perdue, J. R. Tompkins—TUNL B. Davis and D. Markoff—NCCU G. Feldman—GWU L. Myers—UIUC M. S. Johnson--LLNL A TUNL/HIgS Project funded by the NSF/DNDO through their Academic Research Initiative program H. R. Weller—PI M. Ahmed and Y. Wu -- Co PIs Collaborators: N. Brown, S.S. Henshaw, H. J. Karwowski, J. M. Mueller, S. Stave, B. A. Perdue, J. R. Tompkins—TUNL B. Davis and D. Markoff—NCCU G. Feldman—GWU L. Myers—UIUC M. S. Johnson--LLNL TUNL_Retreat_2011

  17. Introduction • Premise: Linearly polarized g rays havingenergies between threshold and 20 MeV can be a useful tool for the interrogation of materials • Induce the emission of several MeV neutrons which can then be detected as a function of energy and emission angle relative to the plane of polarization • In fissionable nuclei, energetic neutrons are produced even at energies effectively below (g,n) threshold TUNL_Retreat_2011

  18. Formalism • For unpolarizedg-ray beams, the angular distribution of the outgoing neutrons assuming pure electric dipole absorption can be written as: • where a2=A2/A0 ,P2 is the second Legendre polynomial • Using Satchler ‘s expressions for linearly polarized g-rays (Proc. Phys. Soc., 68A:1041, 1955), when both detectors are at 90 degrees: • Ipar/Iperpdepends only on a2 TUNL_Retreat_2011

  19. Overview of a2 a2 varies from -0.1 to -0.7 for Z between 23 (Vanadium) and 92 (Uranium) Leads to a range of Ipar/Iperpfrom 1.0 to 8.0 Ipar/Iperphas not been measured before this project began. • These are the targets that were used in our intial measurements. From Baker and McNeill, Can. J. Phys., 39:1158, 1961 TUNL_Retreat_2011

  20. Sensitivity when using 2-detectors • Linearly polarized beam increases sensitivity over unpolarized measurement Ipar/Iperp -a2 TUNL_Retreat_2011

  21. Experiment Setup—Four detectors left, right, up and down at 90o. Iperp BC-501A Liquid scintillators g-ray beam direction into the screen 1 meter flightpath Ipar Ipar Target at q=45˚,f=45˚ to make the out-going path material length similar for all q=90˚ detectors Using 1” collimator Approximate flux: 1x107 g/s Iperp TUNL_Retreat_2011

  22. 238U target: 15.5 MeV Linear pol. Peaking seen in-plane only TUNL_Retreat_2011

  23. 238U target: 15.5 MeV Linear pol. 238U target: Linear pol. Ipar/Iperp Uncertaintiesare from statistics and a detector efficiency correction Average from 5 MeV to max En Peaking at 2.5 near max En 1 at lower energies TUNL_Retreat_2011

  24. Flight path is one meter. Up, down, left and right detectors at 55, 90 and 125 degrees. TUNL_Retreat_2011

  25. Preliminary results from the Feb. 22-28, 2010 run for 238U TUNL_Retreat_2011

  26. New data were obtained on Pb, 235U, and 238U; results at 15.5 MeV are shown here and compared to results on other targets at 90o. TUNL_Retreat_2011

  27. Neutron production below (g,n) threshold Running at a g-ray energy of ~6.0 MeV and looking at neutrons above 2 MeV only produces counts for fissionable nuclei, except for d, Li and 9Be. These can be identified by their unique spectra. This provides a very promisingtool for interrogation and is receiving further study. TUNL_Retreat_2011

  28. 238U target: 6.2 MeVCircular pol. Same neutron yields both in- and out-of-plane, as expected TUNL_Retreat_2011

  29. 238U target: 6.2 MeV Linear Pol. Neutron yield enhancement is observed in both in-plane detectors TUNL_Retreat_2011

  30. Understanding the Ratio for 238U • First take the measured angular distribution of fission fragments as a function of Eg for 238U from Rabotnov [Yad. Fiz. 11, 508 (1970)] • Using the formalism for linearly polarized g rays from Ratzek [Z. Phys. A 308, 63 (1982)] the angular distribution of fission fragments can be written as: • where q is the CM polar angle and f is the CM azimuthal angle of the emitted fragment measured with respect to the plane of polarization; Pg is the linear polarization of the g-ray beam TUNL_Retreat_2011

  31. Angular Distribution of Fragments • a, b, and c terms from Rabotnov • d term can be calculated using formalism given in Ratzek with the simplification that the low lying transition states can be represented by dipole plus the K=0 quadrupole terms [Huizenga and Vandenbosch, Nuclear Fission (1973)] • Dominated by dipole transition but with a small quadrupole contribution TUNL_Retreat_2011

  32. Neutron energy distribution • Then assume the neutrons are emitted isotropically in the center-of-mass frame of each fragment and have an energy distribution based upon an evaporation model from Fraser[Phys. Rev. 88, 536 (1952)] Neutron energy distribution TUNL_Retreat_2011

  33. Fission fragment mass distribution 235U fission fragment masses and relative yields from NNDC Distribution of fragment masses taken from neutron induced fission data for 235U All the neutrons emitted from the fragments are boosted back into the lab frame Ratios are then formed at 90 degrees using simulated detectors both in and out of the plane of polarization TUNL_Retreat_2011

  34. Simulation Results for 238U Data and Simulation at 90 degrees • Both trends as a function of incident g-ray energy and outgoing neutron energy are recreated by the simulation • Simulation tends to under-predict at low and over-predict at higher g-ray energies • Rabotnov data taken using a brem. beam TUNL_Retreat_2011

  35. Conclusion Ipar/Iperphas been measured for 238U with Eg=5.7 to 6.5 MeV The results show ratios which deviate significantly from 1.0 and change as a function of g-ray energy The results agree well with a new simulation based upon previously measured unpolarized angular distributions of fission fragments along with the assumption of dipole plus quadrupole excitations Higher statistics data have already been taken on 238U as well as 235U, 239Pu, and 232Th The analysis and interpretation are underway with results expected within the next year TUNL_Retreat_2011

  36. Summary We have begun to create a catalogue (graphical and tabular) of polarization asymmetries both for incident g-ray energies from 11 to 15.5 MeV and in the threshold region where photofission neutrons can be isolated. Targets to date include Ta, Cd, Sn, Pb, Bi, Fe, Cr, Cu, Be, 238U, 235U, 239Pu, 232Th. Next: 233U, 237Np, 241Am, B, N, Ni, Al, W, V, As, Rb, Sr, Ag, Ba, La, Ce, Hg. TUNL_Retreat_2011

  37. TUNL_Retreat_2011

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