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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 HIS 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.
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Pb IVGQR Studies208
perpendicular to polarization plane
parallel to polarization plane
Unpolarized
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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 S beam.
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Scattering TheoryAssumptions: (GDR Dominates)
Modified Thomson Amp included in CE1
E2 strength due to IVGQR
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Scattering TheoryAssumptions: (GDR Dominates)
Modified Thomson Amp included in E2 strength due to IVGQR
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Polarization Ratio
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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
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Analysis Fit 12C NRF spectra with GEANT4 simulation to determine Response Function for monoenergetic s
Fit Data with Lineshape + Background Subtract Background
Sum Resulting Data
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Results
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ResultsSR=0.6 +/- 0.04 IVQ-EWSRsE=23+/-0.13 MeV =3.9 +/- 0.7 MeV
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Results
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Results
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Results
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Results
Energy Width Strength Probe Year Reference
22.5 9 1.0 1988 [Sch88]
1992 [Dal92]
2010 Current Work
(MeV) (MeV) (IVQ-EWSRs)208Pb
20.2 0.5 5.5 0.5 1.4 0.3 208Pb
23.0 0.13 3.9 1.4 0.6 0.04 09Bi
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Proposal
Perform similar measurements on 8 targets between A=60 and A=240 at HIS.
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.
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A New Method for Identifying Special Nuclear Materials Based Upon Polarized
(,n) AsymmetriesA TUNL/HIS 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—TUNLB. Davis and D. Markoff—NCCUG. Feldman—GWUL. Myers—UIUCM. S. Johnson--LLNL
A New Method for Identifying Special Nuclear Materials Based Upon Polarized
(,n) AsymmetriesA TUNL/HIS 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—TUNLB. Davis and D. Markoff—NCCUG. Feldman—GWUL. Myers—UIUCM. S. Johnson--LLNL
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Introduction
• Premise: Linearly polarized rays having energies 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 (,n) threshold
–
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Formalism
• where a2=A2/A0 ,P2 is the second Legendre polynomial
• Using Satchler ‘s expressions for linearly polarized rays (Proc. Phys. Soc., 68A:1041, 1955), when both detectors are at 90 degrees:
•
• Ipar/Iperp depends only on a2
•For unpolarized -ray beams, the angular distribution of the outgoing neutrons assuming pure electric dipole absorption can be written as:
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Overview of a2
From Baker and McNeill, Can. J. Phys., 39:1158, 1961
a2 varies from -0.1 to -0.7 for Z between 23 (Vanadium) and 92 (Uranium)
Leads to a range of Ipar/Iperp from 1.0 to 8.0
Ipar/Iperp has not been measured before this project began.
•These are the targets that were used in our intial
measurements.
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Sensitivity when using 2-detectors
Ipar/Iperp
-a2
• Linearly polarized beam increases sensitivity over unpolarized measurement
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Experiment Setup—Four detectors left, right, up and down at 90o.
BC-501ALiquid scintillators
Target at =45˚,=45˚to make the out-going pathmaterial length similar for all =90˚ detectors
-ray beam directioninto the screen
Using 1” collimatorApproximate flux: 1x107 /s
1 meter flightpath
Ipar Ipar
Iperp
Iperp
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238U target: 15.5 MeV Linear pol.
Peaking seen in-plane only
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Average from 5 MeV to max En
238U target: Linear pol.238U target: 15.5 MeV Linear pol.
Ipar/Iperp
Peaking at 2.5 near max En
1 at lowerenergies
Uncertaintiesare from statistics and a detector efficiency correction
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Flight path is one meter. Up, down, left and right detectors at 55, 90 and 125 degrees.
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Preliminary results from the Feb. 22-28, 2010 run for 238U
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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.
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Neutron production below (,n) threshold
Running at a -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 promising tool for interrogation and is receiving further study.
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238U target: 6.2 MeV Circular pol.
Same
neutron
yields both
in- and out-
of-plane, as
expected
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238U target: 6.2 MeV Linear Pol.
Neutron yield
enhancement
is observed
in both in-
plane
detectors
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Understanding the Ratio for 238U
• First take the measured angular distribution of fission fragments as a function of E for 238U from Rabotnov [Yad. Fiz. 11, 508 (1970)]
• Using the formalism for linearly polarized rays from Ratzek [Z. Phys. A 308, 63 (1982)] the angular distribution of fission fragments can be written as:
• where is the CM polar angle and is the CM azimuthal angle of the emitted fragment measured with respect to the plane of polarization; Pis the linear polarization of the -ray beam
)sin4sin(2cos
2sinsin),(42
22
cdP
cbaW
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Angular Distribution of Fragments
E (MeV) a b c d
5.65 0.034 0.966 0.040 1.380
5.95 0.078 0.922 0.039 1.079
6.40 0.127 0.873 0.034 1.032
•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
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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
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Fission fragment mass distribution
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
235U fission fragment masses and relative yields from NNDC
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Simulation Results for 238UData and Simulation at 90 degrees
•Both trends as a function of incident -ray energy and outgoing neutron energy are recreated by the simulation
•Simulation tends to under-predict at low and over-predict at higher -ray energies
•Rabotnov data taken using a brem. beam
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ConclusionIpar/Iperp has been measured for 238U with E=5.7 to 6.5 MeV
The results show ratios which deviate significantly from 1.0 and change as a function of -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
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Summary
We have begun to create a catalogue (graphical and tabular) of polarization asymmetries both for incident -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.
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