Compton Scattering at HIGS
with Polarized Photons
Compton@HIGS Collaboration
 George Washington University
 Jerry Feldman
 Mark Sikora
 Duke University/TUNL
 Luke Myers
 Henry Weller
 Mohammad Ahmed
 Jonathan Mueller
 Seth Henshaw
 University of Kentucky
 Mike Kovash
Outline
 What (and where) is HIGS?
 What have we done so far at HIGS?
 polarized Compton scattering study of IVGQR
 elastic Compton scattering on 6Li
 high energy (60-86 MeV) and low energy (3-5 MeV)
 What are we planning to do at HIGS?
 elastic Compton scattering on deuterium
 neutron polarizability
 polarized Compton scattering on proton
 proton electric polarizability
 double-polarized Compton scattering on proton
 proton spin polarizability
Background
Information
on HIGS
United States
North Carolina
Duke University
TUNL
HIGS
TUNL
Triangle Universities
Nuclear Laboratory
Duke Free-Electron Laser Lab
Storage Ring and Booster
Circularly and linearly polarized g rays, nearly monoenergetic (Eg = 2–90 MeV)
Utilizes Compton backscattering to generate g rays
RF Cavity
Optical Klystron
FEL
Booster Injector
Mirror
LINAC
HIGS Photon Beam
to target room
HIGS Photon Beam
 monoenergetic photons up to ~90 MeV
 energy will reach ~160 MeV by 2015
 100% linear or circular polarization
 high photon beam intensity
 ~107 Hz at 20-60 MeV
 ~108 Hz below 15 MeV
 low beam-related background

no bremsstrahlung typical of tagged photons
Polarized Compton Scattering
for IVGQR Systematics
Giant Resonances
 collective nuclear excitations
DT = 0
DT = 1
 GDR and ISGQR well known
 IVGQR poorly known
 photon as isovector probe
L=1
 use pol. photons for IVGQR
 map systematics vs. A
 nuclear symmetry energy
 neutron star eqn. of state
L=2
• ratio of H/V scattered photons is sensitive to E1/E2 interference
• sign difference in interference term at forward/backward angles
Photon Asymmetry in IVGQR
pure E1
E1/E2 interference
HINDA Array
o
55
HIGS NaI Detector Array
55o
125o
125o
Results for
209
Bi
209
Bi
Results for
89
Y
 extend measurements to
 measured
 lease
142
124
89
Y
Sn last month!
Nd target from ORNL for $15k
 other targets include A ~ 56, 180, 238
89
Y
E0 = 27.7  0.2 MeV
 = 8.23  1.88 MeV
S = 110%  18% EWSR
preliminary
Results for
124
Sn
IVGQR Systematics
89
Y
124
Sn
Pitthan 1980
209
Bi
Compton Scattering
6
on Li
World Data Set
D(g,g)D
 Lucas – Illinois (1994)
Eg = 49, 69 MeV
 Hornidge – SAL (2000)
Eg = 85-105 MeV
 Lundin – Lund (2003)
Eg = 55, 66 MeV
 Myers and Shonyozov
(coming 2013)
Illinois, GW, UK, Lund
Eg = 58-115 MeV
EFT Fits to Deuteron Data
Lucas
Lucas, Lundin
Lundin
Hornidge
Griesshammer 2012
Summary of Neutron Results
 Neutron scattering
 Schmiedmayer (91)
an = 12.6  1.5(stat)  2.0(syst)
 Quasi-free Compton scattering
an = 12.5  1.8(stat)

bn = 2.7
+1.1
–0.6 (syst)
+0.6
 1.1(model)

 Kossert (03)
1.8(stat) –1.1 (syst) 1.1(model)
 Elastic Compton scattering
 data from Lucas (94), Hornidge (00), Lundin (03)

bn = 4.1
1.8 (stat)  0.4 (Baldin)  (0.8 (theory)
Griesshammer 12
bn = 3.6

an = 11.1  1.8 (stat)  0.4 (Baldin)  0.8 (theory)
an = 11.6  1.5 (stat)  0.6 (Baldin)
1.5 (stat)  0.6 (Baldin)
Hildebrandt 05
Experiment on 6Li at HIGS
 experiment motivation
 exploit higher nuclear cross section to measure a and b
 cross section scales as Z2, so factor of 9x higher than 2H
 solid 6Li target is simple
 provided by Univ. of Saskatchewan
 no previous Compton data on 6Li exists
(except Pugh 1957)
 energies: Eg = 60, 86 MeV
 angles: qg = 40°-160° (Dq = 17°)
 target: solid 12.7 cm long 6Li cylinder (plus empty)
 detectors: eight 10”12” NaI’s (HINDA array)
 good photon energy resolution (DEg/Eg < 5%)
HINDA Array
HIGS NaI Detector Array
Experimental Setup
Sample Spectra
Full  Empty subtraction
Full and Empty Targets
6
Li(g,g)6Li
Eg = 60 MeV
Cross Section for
16
O(g,g)16O
6
6
Cross Section for Li(g,g) Li
L. Myers et al.
Phys. Rev. C86
(2012)
Eg = 60 MeV
sum rule: a+ b = 14.5
Eg = 80 MeV
7.4%
12.8%
Eg = 60 MeV
Eg = 100 MeV
(a, b) = (10.9, 3.6)
Da = 2
Db = 2

sum rule: a+ b = 14.5
20.9%
6
6
Cross Section for Li(g,g) Li
Eg = 86 MeV
preliminary
LIT Method for Compton Scattering
D(g,g)D
Lundin (Lund) – 55 MeV
Lucas (Illinois) – 49 MeV
Bampa 2011
Nuclear Polarizability
6
4
of Li (and He?)
Nuclear Polarizability
 nuclear polarizability affects energy levels of light atoms
 non-negligible corrections for high-precision tests of QED
 extraction of nuclear quantities from atomic spectroscopy
 nuclear charge radius from Lamb shift in muonic atoms
 usually determined from photoabsorption sum rule
Nuclear Polarizability of 6Li
aE = 0.163  0.064
bM = 0.018  0.012
q = 55 f = 90
q = 125 f = 90
6
Li(g,g)6Li
Eg = 3.0 MeV
q = 55 f = 0
q = 125 f = 0
q = 55 f = 90
q = 125 f = 90
6
Li(g,g)6Li
Eg = 4.2 MeV
q = 55 f = 0
q = 125 f = 0
Compton Scattering
on the
Proton and Deuteron
Compton Scattering on Deuterium
 unpolarized photon beam and unpolarized deuterium target
 first use of our new LD2 cryogenic target
 scattering angles 45o, 80o, 115o, 150o (Eg = 65, 100 MeV)
 requires 300 hrs (65 MeV) + 100 hrs (100 MeV)
 detectors: eight 10”12” NaI’s (HINDA array)
 arranged symmetrically left/right
Cryogenic Target
LH2/LD2/LHe
(3.5 K  24 K)
 paid by GWU and TUNL
 procured from vendors
 assembled at HIGS
 first run Oct. 2013?
HINDA Array
HIGS NaI Detector Array
55o
55o
125o
125o
Sum-Rule-Independent Measurement of ap
 linearly polarized photon beam (unpolarized target)
 scintillating active target (detect recoils in coincidence)
 measure scattered photons at 90o (Eg = 82 MeV)
 scattering cross section is independent of bp
 extraction of ap is independent of the Baldin sum rule
 extraction of ap is model-independent
 requires 300 hrs for 5% uncertainty in ap
 detectors: four 10”12” NaI’s (HINDA array)

located left, right, up, down
Sum-Rule-Independent Measurement of ap
(point)
(point)
Polarizability of the Proton
Scintillating Target
simulations: R. Miskimen
Nucleon Spin Polarizability
forward and backward spin polarizabilities

k
Polarization
Observables
yˆ

k
xˆ
zˆ
g
d
Circular polarization
 2x =
  
  
Circular polarization
 2z =
  
  
Linear polarization
3 =
RCP (+)
||   
||   
LCP ()
Spin Polarizabilities of the Proton
 measure 2x for first determination of proton gE1E1
 circularly polarized photon beam
 scintillating active transverse polarized target (P ~ 80%)
 scattering angles 65o, 90o, 115o (Eg = 100 MeV)
 requires 800 hrs for DgE1E1 = 1
 detectors: eight 10”12” NaI’s
 4 in plane, 4 out of plane
Circular polarization
 2x =
  
  
Spin
Polarizabilities
of the Proton
expand
simulations: R. Miskimen
Summary

Early measurements of Compton scattering at HIGS
 polarized A(g,g)A

6
Li(g,g)6Li
for A = 89-209 (IVGQR systematics)
at 60, 86 MeV
 polarized 6Li(g,g)6Li

(nucleon polarizability)
at 3.0-4.2 MeV (nuclear polarizability)
Next generation of experiments on light nuclei

D(g,g)D
at 65 and 100 MeV (neutron polarizability)
 polarized p(g,g)p
at 82 MeV
 polarized 4He(g,g)4He
(proton electric polarizability)
at 3-15 MeV (nuclear polarizability)
 double-polarized Compton scattering on proton/deuteron
 nucleon spin polarizability

HIGS can contribute high-quality polarized data!
 stay tuned for further developments in the future…
Extra slides
Phenomenological Formalism
RE,q  = R GR E,q   R QD E,q   R1SG E,q   R2SG E,q 
R GR E ,q  = f E1 ( E ) g E1 (q )  f E 2 ( E ) g E 2 (q ) 
NZ
r0 1   GR g E1 (q )
A
NZ


R QD E,q  =  f QD ( E ) 
r0 QD  F2 (q) g E1 (q )
A


2
2






E
E

 
 
SG
4 
R1 E,q  =  F1 (q)Zr0    Aa  g E1 (q )    Ab  g M 1 (q )  O E 
 c 



 c 



 
R2SG E ,q  =  F2 (q)
NZ
r0  GR   QD 
A
Cross-Section Ratios for Deuterium
DbN = 1
Level Scheme of 6Li
Nuclear Polarizability
Calculations by Trento Group
 photoabsorption on 6Li
• Lorentz Integral Transform method
• extend calculations to case of Compton scattering
Bacca 2002
NaI Detectors
Paraffin n shield
10"  10" NaI
core detector
3" thick optically isolated NaI
shield segments (8 in total)
Pb collimator
q = 55
q = 125
Eg = 3.0 MeV
q = 55
q = 125
Eg = 4.2 MeV
Nuclear
Polarizability
of 4He
aE = 0.061  0.007 (stat)
 0.020 (syst)
bM = 0.007  0.001 (stat)
 0.002 (syst)
Nuclear Polarizability of 4He
aE = 0.061  0.007 (stat)  0.020 (syst)
bM = 0.007  0.001 (stat)  0.002 (syst)
Light Output
Compton Scattering with scintillating target
Missing Energy (MeV)
deuteron
proton
simulations: R. Miskimen
Nucleon Spin Polarizability
 classical analogy: Faraday rotation of linearly polarized
light in a spin-polarized medium
 four spin polarizabilities: g1, …,g4
 forward spin polarizability: g0 = g1 – g2 – 2g4
 backward spin polarizability: gp = g1 + g2 + 2g4
 expt. asymmetries with circularly polarized photons
 x : target spin  photon helicity (in reaction plane)
 z : target spin parallel to photon helicity