Compton Scattering at the High Intensity

g

-ray Source

Henry R. Weller Duke University and Triangle Universities Nuclear Laboratory HI g S PROGRAM Chiral Dynamics 2012

HI g S

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 (2x10 8 on target at 15 MeV as of June 2009)

Chiral Dynamics 2012

HIg S 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 •Optical Klystron •FEL •Mirror •RF Cavity •Booster Injector •LINAC Chiral Dynamics 2012

HI g S

–A free-electron laser generated

g

-ray source

Chiral Dynamics 2012

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 ISGQR GDR 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) Chiral Dynamics 2012

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

HIg

S beam along with the realization that the E1-E2 interference term flips sign when going from a forward to a backward angle.

Chiral Dynamics 2012

•The HINDA Array (

HIg

S

N

aI

D

etector

A

rray)

•NSF/MRI funded project—a high resolution high acceptance gamma-ray spectrometer consisting of eight 10”x12” NaI detectors in 3” thick segmented NaI shields.

•The Compton@HI

g

S Collaboration

Chiral Dynamics 2012

One of eight detectors which form the HINDA ARRAY

Paraffin n Shield 10”x10” NaI Core Detector 8 3” thick Optically Isolated NaI Shield Segments Pb Collimator Chiral Dynamics 2012

HINDA Setup

209 Bi Scattering Target 2” Diameter x 1/8” thick 9*10 21 nuclei/cm 2 6 Detectors 3 @ q= 60(55) (Left, Right,Down) 3@ q =120(125) (Left, Right, Down) D W=55 msr 12mm collimated HIg S beam 3 x 10 7 g ’s/sec D E/E=2.5 % Eg = 15-26 MeV Chiral Dynamics 2012

Backward angle detectors were

background free, but forward angle detectors required background subtraction.

Chiral Dynamics 2012

Scattering Theory

Assumptions: (GDR Dominates) Modified Thomson Amp included in E2 strength due to IVGQR Chiral Dynamics 2012

E=23+/-0.13 MeV RESULTS FOR 209 Bi

G

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

Chiral Dynamics 2012

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

Chiral Dynamics 2012

Preliminary parameters for the IVGQR in

89

Y

E

res

G

= = 28.0 +/- 0.4 MeV

11

+/- 0.9 MeV

IVQ-EWSR

= 0.93 +/- 0.11

Chiral Dynamics 2012

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

Chiral Dynamics 2012

Proposed experiments

IVGQR survey of 4 additional targets

Planning to study additional targets of (Raman) contributions.

51 V, 120 Sn, 142 Nd, and 152 required to be spherical nuclei in order to minimize inelastic Sm in the future. These were chosen to cover a range of A values, and are E g ~ 14 –40 MeV.

I ~ 10 7 g /s with D E ~ 2-3 %. 100% Linearly polarized.

Complete program in 2013-14.

Chiral Dynamics 2012

The Compton@HI

g

S 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.

Chiral Dynamics 2012

The electric and magnetic polarizabilities of the proton

The values of the polarizabilities of the proton given by the PDG are: a

p = 12.0 +/- 0.6 x 10 -4 fm 3

b

p = 1.9 +/- 0.5 x 10 -4 fm 3

Lensky and Pascalutsa (B c PT) with pion, Dirac nucleons and D find (EPJ C65, 195 (2010)): a

p = 10.8 +/- 0.7 x 10 -4 fm 3

b

p = 4.0 +/- 0.7 x 10 -4 fm 3

dof While a recent re-analysis of (selected) proton Compton scattering data using Chiral Effective Field theory gives: a

p = 10.7 +/- 0.3

stat +/- 0.2

Bald +/- 0.8

theory x 10 -4 fm 3

b

p = 3.1 +/- 0.3

stat +/- 0.2

Bald +/- 0.8

theory x 10 -4 fm 3

(Griesshammer et al., RPNP (2012); see also Beane et al., Phys. Letts.B567, 200 (2003)) Chiral Dynamics 2012

Comments

Measurements of Compton scattering from the proton using linearly polarized beams at 90 o 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 90 0 detector parallel to the plane of polarization wrt. the

Born (point)

value.

Chiral Dynamics 2012

Linearly polarized

g

s allow for independent measurements of the electric (

a

) and the magnetic (

b

) polarizabilities of the proton.

Chiral Dynamics 2012

The HI

g

S Experiment

PAC approved for 2013 Will use

100% linearly polarized beam (2 x 10 7

g

/s) at 80 MeV

.

Plastic scintillating target

, shown to be able to separate scattering from protons from that from 12 C, etc. 5 x 10 22 protons/cm 2.

Eight HINDA detectors at 90 o

(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 fm 3 )

Chiral Dynamics 2012

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.

Chiral Dynamics 2012

Compton scattering from 6 Li at 60 MeV No previous measurements Chiral Dynamics 2012

Easy to use 5.8 g/cm 2 target and ~10 times the cross section of the deuteron lead to clean spectra in a 50 hr experiment Chiral Dynamics 2012

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 6 Li, and their recent success in treating Compton scattering on the deuteron is encouraging. Chiral Dynamics 2012

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 E 2 . Set a=10.9 and b =3.6, fit data by varying Giant Resonance strengths, then vary a and b.

Chiral Dynamics 2012

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.

Chiral Dynamics 2012

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.

Chiral Dynamics 2012

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 HI g S.

• Polarized protons will be provided by our frozen-spin target. Chiral Dynamics 2012

The spin-polarizabilities of the nucleon

• At O( w 3 ) four new nucleon structure terms that involve nucleon spin-flip operators enter the RCS expansion. H ( 3 ), eff spin = 1 2 4   g E 1 E 1   E  E   g M 1 M 1   B  B  2 g M 1 E 2 E ij  j H j  2 g E 1 M 2 H ij  j E j • 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 spin polarizability.

Chiral Dynamics 2012

y E  p  J Circularly polarized photons moving in the z-direction incident on a proton initially polarized in the x-direction Chiral Dynamics 2012 x

Experiments

The GDH experiments at Mainz and ELSA used the Gell-Mann, Goldberger, and Thirring sum rule to evaluate the forward S.-P. g 0 g 0 = g E 1 E 1 g E 1 M 2 g M 1 M 1 g M 1 E 2 g 0 = 4 1  2 m     1 2 w 3  3 2 d w g 0 = ( 1 .

00  0 .

08  0 .

10 )  10 4 fm 4 Backward spin polarizability from dispersive analysis of backward angle Compton scattering g  = g E 1 E 1 g E 1 M 2  g M 1 M 1  g M 1 E 2 g  = ( 38 .

7  1 .

8 )  10 4 fm 4 Chiral Dynamics 2012

•

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

g

E1E1 by measuring

S

2x using a transverse polarized target and 8 HINDA detectors near 90 degrees ---4 left and 4 right.

Simulations indicate little sensitivity to

g

M1M1 , and we can use

g

o and

g 

to fix the other two.

Chiral Dynamics 2012

HIg

S Frozen Spin Polarized Target (HIFROST)

•

Butanol

•

Polarization ~ 80 %

•

Polarizing Field ~ 2.5 T

•

Holding Field ~ 0.6 T

Chiral Dynamics 2012

Low temp APD •Light capture with wavelength shifting fibers •Quartz chamber •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%

Chiral Dynamics 2012

Four HINDA detectors will be located clustered around 90 o in the horizontal plane on both the right and left sides of the beam.

Chiral Dynamics 2012

Count rate calculations and projections

Target thickness: 2.6 x 10 23 Beam intensity: 5 x 10 6 p/cm 2 ; 80% polarization 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.

Chiral Dynamics 2012

Anticipated result for g E1E1 is -1.4 +/- 0.4 x 10 -4 fm 4 (theory value from PRL 85 (2000)14).

Chiral Dynamics 2012

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 HIg S with a precision of ~ 0.4 x 10 -4 fm 4 , which is sufficient to test and differentiate between theoretical models. Full Lattice QCD calculations are imminent. Chiral Dynamics 2012

ACKNOWLEDGEMENTS

Thanks to my colleagues at TUNL/HI

g

S 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)

Chiral Dynamics 2012

Chiral Dynamics 2012