Nuclear Structure Studies at HI g S

HIGS2 Workshop June 3-4, 2013

Nuclear Structure Studies at HIS

Henry R. WellerThe HIS Nuclear Physics Program


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 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).


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 -ray spectroscopy setup 3 at S, B. Loeher et al., accepted for publication in NIM-A.



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


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.


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.



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.


Parity assignments to all of the observed states proved gave a definite proof that the PDR is an electric dipole phenomenon.



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.


The quasi-monochromatic beam at HIS was exploited to study the decayof the PDR.


The depopulaton of low-lying levels yield information on the summed feeding from spin-1 states in the energy range covered by the beam.


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.


Using HIS to observe the fine structure of the Giant M1 Resonance


The Giant M1 Resonance in 90Zr observed via inelastic proton scatteringG. Crawley et al., Phys. Lett. B 127, 322 (1983).


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 N2) and centroid energy (9 MeV)

were found.



Discovery: 1+ (M1) state was found at 9.757 MeV having B(M1)up= 0.148(59) N2

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 (’) 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 N2.

Being considered by the Neutrino Experimental Group at LANL.


The 3 CampaignSetup installed at HIS in summer of 2012.

Four 60% HPGe, four 3”x 3” LaBr3 and four 1.5” x 1.5” LaBr3


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 HIS but is clearly limited by the beam intensities and the energy resolution of the beams.


Example of coincidence spectra


Discovery of the second 2+ State in 12CW.R. Zimmerman et al., PRL 110, 152502 (2013)


Track Images from the Optical Time Projection Chamber


PM tube signals provide the time projection response functions


Fitting these differentiates 12C from 16O events



The (updated) E2 cross section data


The experimentally determined relative phase compared to the prediction of a two-resonance model


Results


•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.


•Scattering Theory•Assumptions: (GDR Dominates)

Modified Thomson Amp included in E2 strength due to IVGQR


HINDA Setup

209Bi Scattering Target 2” Diameter x 1/8” thick 9*1021 nuclei/cm2

12mm collimated S beam 3 x 107 ’s/sec E/E=2.5 % = MeV

6 Detectors 3@ 60(55) (Left,

Right,Down)3@ =120(125) (Left,

Right, Down) msr


•RESULTS FOR 209Bi

E=23+/-0.13 MeV =3.9 +/- 0.7 MeV SR=0.56 +/- 0.04 IVQ-EWSRs


A second target—89Y—was studied in February, 2012.


Preliminary parameters for the IVGQR in 89Y

Eres = 28.0 +/- 0.4 MeV

=+/- 0.9 MeV IVQ-EWSR = 0.93 +/- 0.11


The A-dependence of the IVGQR parameters is beginning to take shape.


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.

E ~ 14–40 MeV.I ~ 107 /s with E ~ 2-3 %. 100% Linearly polarized.Complete program in 2013-14.


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.


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 4He. Must assume is negligible to obtain .


Presently running an experiment using Compton scattering to determine the polarizabilities of 6Li.


Polarizability of 4He


The HIS cryo-target assembly will provide liquid H, D, and He targets.Scheduled to be available for use by September 2013.



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



An array of parallel plate avalanche counters consisting of 23 electrolytically deposited 238UO2 (2 mg/cm2) detecting both fission fragments in coincidence.


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.


The triple-humped fission barrier of 238U as determined from the present study.




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


Opportunities created by HIS2

Assume: E– 12 MeV

0.5% energy spread

Beam on target ~ 8 x 1010 /s


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.


Impact on Photofission Studies

• An increase in the beam intensity from the present ~100 /eV s to 106 /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 238U predicts a resonance at E = 4.55 MeV. This could be confirmed using the HIS2 beams.


Present and future performance of HIGS


HIGS2


Documents

Nuclear Structure Studies at HI g S