Challenges for the HI γs Compton Program

Transcription

1 Challenges for the HI γs Compton Program Mohammad W. 1 1 Duke University & Triangle Universities Nuclear Laboratory June 17, 2008

2 Compton Scattering at HI γs Remember When expanding Compton scattering amplitude in the energy of the photon, O(ω 0 ) charge, mass O(ω 1 ) anomalous magnetic moment (I GDH, γ 0 ) O(ω 2 ) nucleon response to E & M dipole field (α, β) O(ω 3 ) spin response (γ i ) Following quantities are proposed to be measured at HIγS α p, and β p ; α n, and β n ; γ p, and γ n ;

3 Experimental Status of α and β for proton and neutron α p 10 4 fm 3 Exp. Error Experiment 12.2 ±0.3(stat) 0.4(syst)±0.3(mod) 1 2 % β p 10 4 fm 3 Experiment 1.8 ±0.4(stat)±0.4(syst)±0.4(mod) 22 % α n 10 4 fm 3 Exp. Error Experiment 12.5 ±1.8(stat) (syst) ± 1.1(model)2 14 % β n 10 4 fm 3 Experiment (stat) (syst) 1.1(model) 66 % 1 Olmos de Leon, EPJ, A10, 207(2001) and L vov NP, A674, 449 (2000) 2 Kossert, PRL, 88, (2002)

4 The Spin Polarizabilities There are 6 independent structure functions (A i (ω, θ)) in T-Matrix of real Compton scattering. These can be expressed as combinations of γs : γ 0 = γ 1 + γ 5 γ π = γ 1 γ 5 where; γ 5 = γ 2 2γ 4 γ M1M1 γ 4 γ E1M2 γ 3 γ E1E1 = γ 1 γ 3 γ M1E2 = γ 2 + γ 4

5 The Spin Polarizabilities γ p 3 n p 4 n Exp p Exp n γ ± 0.08 ± γ π ± ± 4 γ γ γ γ Table: Predictions for γ s by Judith McGovern and Gellas, Hemmert, Meißner. Experimental Data is from Mainz, J. Ahrens, PRL, 87, (2001). Units of γ are 10 4 fm 4 3 J. McGovern, CD GHM, PRL, 85, 14 (2000)

6 A program is in place now at HIγS to carry out a composite study of static EM-dipole polarizabilities and spin polarizabilities of neutron and proton Linear and circularly polarized γ-rays from upgraded HIγS; Polarized and unpolarized targets; Recently upgraded HIγS NaI Array (HINDA).

7 α p, and β p Method : 100 % Linearly Polarized Beam on Unpolarized Proton Target dσ dω θ=90 dσpt dω θ=90 = K α cos 2 θ( dσ dω dσpt dω ) (dσ dω dσ dω ) = K βcosθsin2 θ where K = 2( e2 Mc 2 )( ω ω )2 ωω pt

8 A θ lab =90 will give a pure α measurement, and an appropriate choice of angle (e.g., θ lab =160 ) will yield a pure β measurement. Rate Calculation and Measurement Prediction 300 hours with 10 7 γ/s, 80 mg/cm 2 target will yield 5 5 % errors on both α p and β p 5 Calculations by B. Norum

9 α n E, and βn M (50-80 MeV) Method: Un-Polarized Beam on Unpolarized Deuteron Target

10 Rate Calculation and Measurement Prediction 6 N γ = 10 7 γ E γ = MeV Target = Unpolarized scintillating ( cm 2 ) Detector = HINDA RATE 6.4 counts/hr MEASUREMENT Four beam energies ( 11 days) This gives a 5 % (stat) measurement of the neutron static EM-dipole polarizabilities 6 J. Feldman, HIGS Upgrade Review Talk to DOE

11 Summary of Predicted Measurements at HIγS Error (stat) Error (stat) Energy/Flux/Target Time (hr) Current HIγS MeV / Hz α p 2 % 5 % 100 / 10 7 /Unpol 300 β p 22 % 5 % 100 / 10 7 /Unpol α n 14 % 5 % 60 / 10 7 /Unpol 250 β n 66 % 5 % 60 / 10 7 /Unpol

12 γ p (110 MeV) Method: Circular Polarized Beam on Transversed and Longitudinal Polarized Scintillating Proton Target nanobarns per steradian 100 MeV CM energy Longitudinal target polarization p p longitudinal polarized target, 110 MeV +z +z no spin pol -z +z no spin pol. -z -z no spin pol -z no spin pol. Projected errors for 100 hours running Sensitivity to s nanobarns per steradian 100 MeV CM energy transverse polarized target, 110 MeV Transverse target polarization p p +x +x +x no spin pol. +x no spin pol -x -x -x no spin pol. -x no spin pol Projected errors for 100 hours running Sensitivity to s angle (degrees) angle (degrees)

13 Rate Calculation and Measurement Prediction 7 N γ = 10 7 γ E γ = 110 MeV / Circular Polarization Target = Polarized ( and ) scintillating 5 cm long; Detector = HINDA MEASUREMENT 800 hour ( 400 in each spin state): Proton HIγS Measurement γ γ γ γ Rory Miskimen, HIGS Upgrade Review Talk to DOE

14 γ n (120 MeV, 2009) Method: Circular Polarized Beam on Polarized 3 He Target 8 New theoretical calculations 9 on Extraction of spin polarizabilities 3 He( γ, γ ) High pressure spin-polarized 3 He target. 8 PI: H. Gao 9 CNP, PRL, 98, (2007)

15 Rate Calculation and Measurement Prediction N γ = γ E γ = 120 MeV / Circular Polarization Target = 3 He Polarized (, and ); Detector = HINDA MEASUREMENT 2000 hour: Neutron HIγS Measurement γ γ γ γ

16 Okay, this all sounds good! Can we really do all we say we can? What are the improvements since the 16 O Compton scattering experiment? What about detectors? Do we have the right targets? What is the truth about the (scintillating) and polarized target? Do we have the needed flux (what s the proof?), can we measure nb cross sections?, how well can we measure flux? Do we have right beam polarizations? What about the beam energies, can we really do 100 MeV? Mirrors?

17 Since 16 O... We can store 100 ma of beam current (I) instead of 20 ma. The γ-ray flux I 3 2 orders of magnitude increase. No beam lifetime issues. Electrons are now top loaded and flux maintained at all energies (higher the energy, better it works) Added the DOK-1 wigglers for linear and circular polarized beam A total dω coverage from 200 msr to 800 msr (double the number of detectors at a shorter distance) Installed an attenuator system for flux determination New, faster electronics ( 10 % increase of livetime)

18 The detector status The collaboration was awarded an NSF (MRI) (via JMU) grant to purchase active NaI shields for 8 core detectors DOE funded upgrade of 8 NaI core detectors and electronics to make the HINDA system All, except two, core detectors are now ready. All shields will be delivered by fall Testing of first HINDA elements have been performed

19 One Detector of the HINDA Array

20 HINDA Detector Test 4.4 MeV CORE + Shield as One Detector CORE without Shield CORE AND Shield Anti-Coincidence CORE AND Shield Coincidence CORE AND Shield 511-Coincidence 6.8 MeV SEP of 4.4 SEP of E (KeV)

21 The HINDA Array: 8 NaI Core Detectors with Segmented Shields

22 Polarized Targets 3 He polarized target exists and was recently used at HIγS during the GDH run; Frozen spin polarized target (HIFROST) is in its final stages of testing at U. Va. Delivery to HIγS and installation is expected in Fall of All critical components have arrived at HIγS; Currently, HIFROST is only polarized and non-scintillating; Active R&D is in works for polarized target; Active R&D is in works for scintillating polarized target; Target room is being prepared for the installation of the target.

23 Saddle Coil for Polarized HIFROST

24 Scintillating HIFROST

25 The γ-ray beam We can produce γ-ray fluxes in excess of 10 7 γ/s for the entire Compton program; two experiments ( 16 O(γ, α), and 3 He GDH) took beams and obtained count rates based upon excess of 10 7 γ/s. Beams upto 60 MeV can be produced right now; Beams between 60 and 100 MeV will be available after summer of 2009; Linear polarization at 100 MeV has not been attempted yet. Issues relate to mirror demage. There are atleast two solutions which have been expimentally tested; Beams above 100 MeV can be realized in three to five years

26 The γ-ray beam

27 The Proposed Timeline to Execute the Compton HIγS Program An in-beam test sometime this fall (maybe 12 C(γ, γ) 12 C); Compton Scattering on Unpolarized Deuterons (α n, β n ); Compton Scattering on Unpolarized Protons (α p, β p ); Compton Scattering on Polarized Protons (γ p ); and Compton Scattering on Polarized 3 He (γ n ).