TUNL_Retreat_2011

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.

TUNL_Retreat_2011

Pb IVGQR Studies208

perpendicular to polarization plane

parallel to polarization plane

Unpolarized

TUNL_Retreat_2011

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.

TUNL_Retreat_2011

Scattering TheoryAssumptions: (GDR Dominates)

Modified Thomson Amp included in CE1

E2 strength due to IVGQR

TUNL_Retreat_2011

Scattering TheoryAssumptions: (GDR Dominates)

Modified Thomson Amp included in E2 strength due to IVGQR

TUNL_Retreat_2011

Polarization Ratio

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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

TUNL_Retreat_2011

Results

TUNL_Retreat_2011

ResultsSR=0.6 +/- 0.04 IVQ-EWSRsE=23+/-0.13 MeV =3.9 +/- 0.7 MeV

TUNL_Retreat_2011

Results

TUNL_Retreat_2011

Results

TUNL_Retreat_2011

Results

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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.

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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

–

TUNL_Retreat_2011

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:

TUNL_Retreat_2011

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.

TUNL_Retreat_2011

Sensitivity when using 2-detectors

Ipar/Iperp

-a2

• Linearly polarized beam increases sensitivity over unpolarized measurement

TUNL_Retreat_2011

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

TUNL_Retreat_2011

238U target: 15.5 MeV Linear pol.

Peaking seen in-plane only

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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

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.

TUNL_Retreat_2011

238U target: 6.2 MeV Circular pol.

Same

neutron

yields both

in- and out-

of-plane, as

expected

TUNL_Retreat_2011

238U target: 6.2 MeV Linear Pol.

Neutron yield

enhancement

is observed

in both in-

plane

detectors

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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

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

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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

TUNL_Retreat_2011

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.

TUNL_Retreat_2011