Compton Scattering at the High Intensity g -ray Source

1. Compton Scattering at the High Intensity g-ray Source Henry R. Weller Duke University and Triangle Universities Nuclear Laboratory HIgS PROGRAM Chiral Dynamics 2012

2. 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) Chiral Dynamics 2012

3. RF Cavity • Optical Klystron • FEL • Booster Injector • Mirror • LINAC HIgSg-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 Chiral Dynamics 2012

4. HIgS–A free-electron laser generated g-ray source Chiral Dynamics 2012

5. 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 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) ISGQR GDR IVGQR Chiral Dynamics 2012

6. 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. Chiral Dynamics 2012

7. The HINDA Array(HIgS NaI Detector Array) • 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@HIgS Collaboration Chiral Dynamics 2012

8. 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

9. 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 107g’s/sec • DE/E=2.5 % • Eg =15-26 MeV Chiral Dynamics 2012

10. Backward angle detectors were background free, but forward angle detectors required background subtraction. Chiral Dynamics 2012

11. Scattering Theory Assumptions: (GDR Dominates) • Modified Thomson Amp included in • E2 strength due to IVGQR Chiral Dynamics 2012

12. RESULTS FOR 209BiE=23+/-0.13 MeVG=3.9 +/- 0.7 MeV SR=0.56 +/- 0.04 IVQ-EWSRs Chiral Dynamics 2012

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

14. Preliminary parameters for the IVGQR in 89Y Eres = 28.0 +/- 0.4 MeV G= 11 +/- 0.9 MeV IVQ-EWSR = 0.93 +/- 0.11 Chiral Dynamics 2012

15. The A-dependence of the IVGQR parameters is beginning to take shape. Chiral Dynamics 2012

16. 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. Chiral Dynamics 2012

17. The Compton@HIgS ProgramPAC approved experiments include: • 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

18. 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)) Chiral Dynamics 2012

19. 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. Chiral Dynamics 2012

20. Linearly polarized gs allow for independent measurements of the electric (a) and the magnetic (b) polarizabilities of the proton. Chiral Dynamics 2012

21. The HIgS ExperimentPAC 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) Chiral Dynamics 2012

22. 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

23. Compton scattering from 6Li at 60 MeVNo previous measurements Chiral Dynamics 2012

24. 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 Chiral Dynamics 2012

25. 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. Chiral Dynamics 2012

26. 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. Chiral Dynamics 2012

27. 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

28. 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

29. 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. Chiral Dynamics 2012

30. The spin-polarizabilities of the nucleon At O(w3) four new nucleon structure terms that involve nucleon spin-flip operators enter the RCS expansion. • 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

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

32. Experiments The GDH experiments at Mainz and ELSA used the Gell-Mann, Goldberger, and Thirring sum rule to evaluate the forward S.-P. g0 Backward spin polarizability from dispersive analysis of backward angle Compton scattering Chiral Dynamics 2012

33. 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 gp to fix the other two. Chiral Dynamics 2012

34. HIgS Frozen Spin Polarized Target (HIFROST) Butanol Polarization ~ 80 % Polarizing Field ~ 2.5 T Holding Field ~ 0.6 T Chiral Dynamics 2012

35. Light capture with wavelength shifting fibers • Quartz chamber Low temp APD BCF-92 blue to green wavelength shifting fiber, 1 mm square, double clad, wrapped around clear shell Polarized scintillating disks 5 mm thick, BC-490 doped with Tempo Overall light transport efficiency ≈ 2% Chiral Dynamics 2012

36. Four HINDA detectors will be located clustered around 90o in the horizontal plane on both the right and left sides of the beam. Chiral Dynamics 2012

37. Count rate calculations and projections Target thickness: 2.6 x 1023 p/cm2 ; 80% polarization Beam intensity: 5 x 106g/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

38. Anticipated result for gE1E1 is -1.4 +/- 0.4 x 10-4 fm4 (theory value from PRL 85 (2000)14). Chiral Dynamics 2012

39. 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. Chiral Dynamics 2012

40. 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) Chiral Dynamics 2012

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