Compton Scattering at the High Intensity g-ray Source
Henry R. Weller
Duke University and Triangle Universities Nuclear Laboratory
HIgS PROGRAM
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HIgS
Nearly Mono-energetic g-rays from 2 to 160 MeV
Up to 100 MeV now
Up to ~160 MeV by 2015
•
•
~100% Linearly and Circularly Polarized g-rays
High Beam Intensities
(2x108 on target at 15 MeV as of June 2009)
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HIgS g-ray beam generation
•Circularly and linearly polarized, nearly monoenergetic g-rays from 2 to 100 MeV
•Utilizes Compton backscattering of FEL light to generate g-rays
•RF Cavity
•Optical Klystron
•FEL
•Booster Injector
•Mirror
•LINAC
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HIgS –A free-electron laser generated
g-ray source
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Measuring Giant Resonances
Giant resonances arise from the collective motion of
the nucleons within the nucleus
 Isovector Giant Dipole (GDR) -- well known
 Isoscalar Giant Quadrupole (IVGQR) -- well known
 Isovector Giant Quadrupole (IVGQR) -- poorly known
GDR
ISGQR
IVGQR
Precision measurements of the IVGQR would be useful for:
– Establishing the A-dependence of the parameters of the IVGQR
– Determining the density dependence of the nuclear symmetry
energy
Needed to predict the behavior of matter under extreme conditions
(e.g. within a neutron star)
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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
HIgS 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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•The HINDA Array
(HIgS NaI Detector Array)
•NSF/MRI funded project—a high resolutionhigh acceptance gamma-ray spectrometer
consisting of eight 10”x12” NaI detectors in
3” thick segmented NaI shields.
•The Compton@HIgS Collaboration
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One of eight detectors which form the HINDA ARRAY
10”x10” NaI
Core Detector
Paraffin n Shield
8 3” thick Optically
Isolated NaI Shield
Segments
Pb Collimator
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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 107 g’s/sec
DE/E=2.5 %
Eg =15-26 MeV
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Backward angle detectors were background free, but
forward angle detectors required background subtraction.
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Scattering Theory
Assumptions: (GDR Dominates)
Modified Thomson Amp included in
E2 strength due to IVGQR
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RESULTS FOR 209Bi
E=23+/-0.13 MeV
G=3.9 +/- 0.7 MeV
SR=0.56 +/- 0.04 IVQ-EWSRs
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A second target—89Y—was studied in February, 2012.
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Preliminary parameters for the IVGQR in 89Y
Eres =
G
=
IVQ-EWSR =
28.0 +/- 0.4 MeV
11 +/- 0.9 MeV
0.93 +/- 0.11
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The A-dependence of the IVGQR parameters is beginning
to take shape.
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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.
Eg ~ 14–40 MeV.
I ~ 107 g/s with DE ~ 2-3 %. 100% Linearly polarized.
Complete program in 2013-14.
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The Compton@HIgS Program
PAC approved experiments include:
1.
Use the 100% linearly polarized beam at ~80 MeV to
perform a model/sum-rule independent measurement
of the proton’s electric polarizability.
2.
Use a scintillating polarized proton target and
determine the proton spin-polarizabilities with circularly
polarized beam at 100 – 140 MeV.
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The electric and magnetic polarizabilities of the proton
The values of the polarizabilities of the proton given by the PDG are:
ap = 12.0 +/- 0.6 x 10-4 fm3
bp = 1.9 +/- 0.5 x 10-4 fm3
Lensky and Pascalutsa (BcPT) with pion, Dirac nucleons and D dof
find (EPJ C65, 195 (2010)):
ap = 10.8 +/- 0.7 x 10-4 fm3
bp = 4.0 +/- 0.7 x 10-4 fm3
While a recent re-analysis of (selected) proton Compton scattering data
using Chiral Effective Field theory gives:
ap = 10.7 +/- 0.3stat +/- 0.2Bald +/- 0.8theory x 10-4 fm3
bp = 3.1 +/- 0.3stat +/- 0.2Bald +/- 0.8theory x 10-4 fm3
(Griesshammer et al., RPNP (2012); see also Beane et al., Phys.
Letts.B567, 200 (2003))
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Comments
Measurements of Compton scattering from the proton using linearly
polarized beams at 90o with the detectors perpendicular to the plane
of polarization of the beam will provide a direct determination of a,
independent of b.
If contributions of order (w/M)4 are significant, they will show up as a
deviation in the measured value of the cross section in a 900
detector parallel to the plane of polarization wrt. the Born (point)
value.
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Linearly polarized gs allow for independent measurements of the electric
(a) and the magnetic (b) polarizabilities of the proton.
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The HIgS Experiment
PAC approved for 2013
Will use 100% linearly polarized beam (2 x 107 g/s) at 80 MeV.
Plastic scintillating target, shown to be able to separate scattering
from protons from that from 12C, etc. 5 x 1022 protons/cm2.
Eight HINDA detectors at 90o (l,r,u,d), using two independent setups.
Measure both parallel (x) and perpendicular (y) cross sections. Also,
determine asymmetries A = (y-x)/(y+x).
A 300 hour run is expected to give A to 0.25%.
The electric polarizability will be determined to within 5%
uncertainty
(eg 10.8 +/- 0.5 x 10-4 fm3)
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While the proton polarizabilities are “well-known”, the neutron values
come from Compton scattering from the deuteron which determines the
average of the n and p polarizabiities. After almost 20 years of effort,
the data are still in need of improvement.
We decided to begin with Compton on 6Li.
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Compton scattering from 6Li at 60 MeV
No previous measurements
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Easy to use 5.8 g/cm2 target and ~10 times the cross section of the
deuteron lead to clean spectra in a 50 hr experiment
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Theoretical treatment
A full theoretical treatment is required in order to extract
absolute values of the polarizabilities from these data.
At present, work is underway at Trento (W. Liedemann et
al.) using the Lorentz Integral Transform method.
They have previously calculated the total absorption
cross section for 6Li, and their recent success in treating
Compton scattering on the deuteron is encouraging.
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Meanwhile, we have performed a phenomenological model
calculation to examine the sensitivity of the data to the
isoscalar polarizabilities.
Main ingredients:
Modified Thomson scattering—
E1 and E2 Giant Resonances--Quasi-deuteron contribution—
Polarizabilities enter the amplitude at order E2. Set a=10.9 and b=3.6, fit data
by varying Giant Resonance strengths, then vary a and b.
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Angular distribution at 60 MeV along with calculations to demonstrate
the sensitivity to the isoscalar polarizabilities. The results indicate that
the statistical uncertainties in a and b are +/- 0.7, better than the 0.9
from 20 years of deuterium data.
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Hoped for results! Will run at 80 MeV early year. Increased sensitivity
(a factor of 2-to-3) will reduce the uncertainty in a and b to around 0.3.
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Spin polarizabilities.
• They tell us about the response of the spin of the
nucleon to the polarization of the photons. The stiffness
of the spin can be thought of as arising from the
nucleon’s spin interacting with the pion cloud.
•
Measuring these requires circularly polarized beams
and polarized targets – ideally suited to HIgS.
• Polarized protons will be provided by our frozen-spin
target.
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The spin-polarizabilities of the nucleon
• At O(w3) four new nucleon structure terms that
involve nucleon spin-flip operators enter the RCS
expansion.
(3), spin
Heff
  
  
1 
= - 4 g E1E1  E  E  g M1M1  B  B - 2g M1E2Eij jHj  2g E1M2Hij jEj 
2 

• A rotating electric field will induce a precession of
the proton spin around the direction of the polarized
photon, with a rate proportional to the spinpolarizability.
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y

E
p
Circularly polarized photons
moving in the z-direction
incident on a proton initially
polarized in the x-direction
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
J
x
Experiments
The GDH experiments at Mainz and ELSA used the Gell-Mann,
Goldberger, and Thirring sum rule to evaluate the forward S.-P. g0
g 0 = -gE1E1 - gE1M2 - g M1M1 - g M1E2
1
g0 =
42


m
1 2 - 3 2
3
w
dw
g 0 = ( -1.00  0.08  0.10)  10 -4 fm 4
Backward spin polarizability from dispersive analysis of backward
angle Compton scattering
g  = -gE1E1 - gE1M2  g M1M1  g M1E2
g  = ( -38.7  1.8)  10 -4 fm 4
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•Proton spin-polarizabilities will be measured using a
scintillating frozen-spin polarized target
Rory Miskimen et al., U. Mass.
Simulations have been performed. A working prototype is
under construction.
The initial experiment will run near 120 MeV.
The first experiment will determine gE1E1 by measuring S2x
using a transverse polarized target and 8 HINDA detectors
near 90 degrees ---4 left and 4 right.
Simulations indicate little sensitivity to gM1M1, and we can
use go and g to fix the other two.
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HIgS Frozen Spin Polarized Target (HIFROST)
•Butanol
•Polarization ~ 80 %
•Polarizing Field ~ 2.5 T
•Holding Field ~ 0.6 T
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•Light capture with wavelength shifting fibers
•Quartz chamber
Low temp APD
•Polarized scintillating disks 5 mm thick,
BC-490 doped with Tempo
•BCF-92 blue to green wavelength
shifting fiber, 1 mm square, double
clad, wrapped around clear shell
•Overall light transport efficiency ≈ 2%
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Four HINDA detectors will be located clustered around 90o in the
horizontal plane on both the right and left sides of the beam.
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Count rate calculations and projections
Target thickness: 2.6 x 1023 p/cm2 ; 80% polarization
Beam intensity: 5 x 106 g/s @ 100 MeV
The HINDA array with dets. 75 cm from the target
Running time: 800 hrs. transverse target polarization, 400
hrs. with +1 photon helicity, 400 hrs. with -1.
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Anticipated result for gE1E1 is -1.4 +/- 0.4 x 10-4 fm4
(theory value from PRL 85 (2000)14).
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Summary:
• Measuring the spin-polarizabilities of the nucleon is an
important next step. These are fundamental structure
constants of the nucleons.
•
Can measure the polarizabilities at HIgS with a precision
of ~ 0.4 x 10-4 fm4 , which is sufficient to test and
differentiate between theoretical models. Full Lattice
QCD calculations are imminent.
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ACKNOWLEDGEMENTS
Thanks to my colleagues at TUNL/HIgS for their
invaluable contributions to these experiments.
Special thanks to -Mohammad Ahmed (NCCU)
Seth Henshaw (Duke)
Luke Myers (Duke)
Jerry Feldman (GWU)
Mark Sikora (GWU)
Rory Miskimen (UMass)
Don Crabb (UVa)
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