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

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HIGS2 Workshop June 3-4, 2013 Nuclear Structure Publications from S Research Parity Measurements of Nuclear Levels Using a Free-Electron-Laser Generated -Ray Beam, Phys. Rev. Lett. 88, 012502 (2002). First evidence for spin-flip M1 strength in 40 Ar, T.C. Li et al., Phys. Rev. C 73, 054306 (2006). Multipole mixing ratios of transitions in 11 B, 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 26 Mg. 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 241 Am(g,n) 240 Am 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 44 Ca, J. Isaak et al., Phys. Rev. C 83, 034304 (2011). Discrete deexcitations in 235 U 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).

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

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HIGS2 Workshop June 3-4, 2013

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Demonstration of the power of doing Nuclear Resonance Fluorescence at HI S with ~100% linearly polarized -rays

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HIGS2 Workshop June 3-4, 2013 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.

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HIGS2 Workshop June 3-4, 2013 Why is the PDR of Astrophysical Interest? Piekarewicz (PRC 73, 044325 (2006) 1.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. 2.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. 3.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.

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HIGS2 Workshop June 3-4, 2013

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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.

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

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HIGS2 Workshop June 3-4, 2013

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The striking difference between the ( ’ and the ( ’) 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.

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HIGS2 Workshop June 3-4, 2013 The quasi-monochromatic beam at HI S was exploited to study the decay of the PDR.

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

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HIGS2 Workshop June 3-4, 2013 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.

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HIGS2 Workshop June 3-4, 2013 Using HI S to observe the fine structure of the Giant M1 Resonance

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

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HIGS2 Workshop June 3-4, 2013 Fine Structure of the Giant M1 Resonance in 90 Zr PRL 110, 022503 (2013) Over 40 1 + states revealed the fine structure of the Giant M1 Resonance in 90 Zr 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 N 2 ) and centroid energy (9 MeV) were found.

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HIGS2 Workshop June 3-4, 2013

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Discovery: 1+ (M1) state was found at 9.757 MeV having B(M1) up = 0.148(59) N 2 The first observation of a 1 + state in 40 Ar. Could be important for the Low Baseline Neutrino Experiment which will use 40 Ar. Low energy supernovae neutrinos (<50 MeV) could be detected via ( ’) neutral current excitation of the M1 state(s), and detecting the subsequent 9.757 MeV -ray. (Anna Hayes) Need another M1 state to determine the shape of the -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 N 2. Being considered by the Neutrino Experimental Group at LANL.

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HIGS2 Workshop June 3-4, 2013 The 3 Campaign Setup installed at HI S in summer of 2012. Four 60% HPGe, four 3”x 3” LaBr 3 and four 1.5” x 1.5” LaBr 3

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HIGS2 Workshop June 3-4, 2013 The 3 Campaign The combination of NRF with 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 installation. This work has begun at HI S but is clearly limited by the beam intensities and the energy resolution of the beams.

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

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

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

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

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

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HIGS2 Workshop June 3-4, 2013

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The (updated) E2 cross section data

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

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HIGS2 Workshop June 3-4, 2013 Results

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HIGS2 Workshop June 3-4, 2013 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 S beam along with the realization that the E1-E2 interference term flips sign when going from a forward to a backward angle.

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

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HIGS2 Workshop June 3-4, 2013 HINDA Setup 209 Bi Scattering Target 2” Diameter x 1/8” thick 9*10 21 nuclei/cm 2 12mm collimated S beam 3 x 10 7 ’s/sec E/E=2.5 % = MeV 6 Detectors 3 @ 60(55) (Left, Right,Down) 3@ =120(125) (Left, Right, Down) msr

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

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

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HIGS2 Workshop June 3-4, 2013 Preliminary parameters for the IVGQR in 89 Y E res =28.0 +/- 0.4 MeV = +/- 0.9 MeV IVQ-EWSR = 0.93 +/- 0.11

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

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HIGS2 Workshop June 3-4, 2013 Proposed experiments IVGQR survey of 4 additional targets Planning to study additional targets of 51 V, 120 Sn, 142 Nd, and 152 Sm 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. E ~ 14–40 MeV. I ~ 10 7 /s with E ~ 2-3 %. 100% Linearly polarized. Complete program in 2013-14.

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HIGS2 Workshop June 3-4, 2013 Compton scattering at very low energies to determine the polarizabilities of 6 Li (running right now) and 4 He (to be proposed on Wednesday) Polarizabilities of light nuclei Polarizabilities have been measured for d and 3 He, but only the sum rule result exists for 4 He. 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 4 He +, for example.

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

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

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

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

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HIGS2 Workshop June 3-4, 2013

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Projected results for running at 3 energies (3, 9 and 15 MeV) for a total of 365 hours.

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HIGS2 Workshop June 3-4, 2013

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An array of parallel plate avalanche counters consisting of 23 electrolytically deposited 238 UO 2 (2 mg/cm 2 ) detecting both fission fragments in coincidence.

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HIGS2 Workshop June 3-4, 2013 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 ( ,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.

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

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HIGS2 Workshop June 3-4, 2013 51

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HIGS2 Workshop June 3-4, 2013

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HIGS2 --- An increase of 2 to 3 orders of magnitude in beam intensity, better energy resolution and rapid spin reversal.

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HIGS2 Workshop June 3-4, 2013 Opportunities created by HI S2 Assume: E – 12 MeV 0.5% energy spread Beam on target ~ 8 x 10 10 /s

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HIGS2 Workshop June 3-4, 2013 The energy spread and the intensity have major impacts on nuclear structure experiments 1.Increased intensities will allow for much greater speed in obtaining results in NRF studies, especially in coincidence experiments. This will have a great impact on studies of decay patterns of low lying collective modes such as the M1 Scissors Mode and the Pygmy Dipole Resonance. 2.Reducing the present energy spread of ~3% to a value of 0.5% (or less?) will have a major impact on NRF studies. 3.Will provide precision measurements of the polarizabilities of light nuclei via Compton Scattering.

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HIGS2 Workshop June 3-4, 2013 Impact on Photofission Studies An increase in the beam intensity from the present ~100 /eV s to 10 6 /eV s with a decrease in energy resolution to ~0.5% or better will make it possible to identify sub-barrier transmission resonances in the fission channel having integrated cross sections 10 times smaller than presently possible. ( eV b instead of 10 eV b). The narrow energy bandwidth will lead to a significant reduction in the background from non-resonant processes. This will allow for preferential population and identification of vibrational resonances in the photofission cross section. The present model for 238 U predicts a resonance at E = 4.55 MeV. This could be confirmed using the HI S2 beams.

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HIGS2 Workshop June 3-4, 2013 Present and future performance of HIGS

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HIGS2 Workshop June 3-4, 2013 HIGS2

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HIGS2 Workshop June 3-4, 2013

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