Møller Polarimetry on Atomic Hydrogen

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1 E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 1 Møller Polarimetry on Atomic Hydrogen E.Chudakov 1 1 JLab Meeting at UVA

2 Outline E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 2 1 Møller Polarimetry 2 Møller with Atomic Hydrogen Target 3 Path Forward

3 Outline E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 2 1 Møller Polarimetry 2 Møller with Atomic Hydrogen Target 3 Path Forward

4 Outline E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 2 1 Møller Polarimetry 2 Møller with Atomic Hydrogen Target 3 Path Forward

5 Electron Polarimetry for PV at JLab: Features E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 3 Energy range E beam = GeV Current range E beam = µa Beam pulse σ t = 0.5 ps, repetition rate σ R 100 µm Statistical error for a period of a possible polarization change ( 1 h) Systematic error Does polarimetry use the same beam (energy, current, location) as the experiment? Continuous or intermittent (invasive?)

6 E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 4 σ σ Compton Polarimetry σ +σ = A P b P t Møller Polarimetry e + (hν) σ e + γ QED. e + e e + e QED. A(E) = 7 9 σ lab 180 mb ster Rad. corrections to Born < 0.1% Detecting: γ (0 ), e E < E Strong da - good σe dk γ/e γ needed A ke at E < 20 GeV T 1/(σ A 2 ) 1/k 2 1/E 2 P laser 100% Non-invasive measurement Syst. error 3 50 GeV: % Rad. corrections to Born < 0.3% Detecting the e at θ CM 90 da dθ CM good systematics Beam energy independent Coincidence - no background Ferromagnetic target P T 8% I B < 3 µa (heating) Levchuk effect Low P T dead time Syst. error σ(p T ) 2% (0.5%?)

7 Hall A Møller Polarimeter with Saturated Iron Target E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 5 Y cm X cm Target Collimator Coils Quad 1 Quad 2 Quad 3 Dipole non-scattered beam Detector (a) Z cm (b) Z cm Upgraded to saturated foils (as in Hall C). B Minimal Levchuk σ stat = 1% in 2 3 min B Z 4 T field Foil at 0 to field Foils 1 10µm Beam <5µA Syst. error 1%

8 E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 6 Possible Breakthrough in Accuracy Møller polarimetry with 100% polarized atomic hydrogen gas, stored in a ultra-cold magnetic trap. E.Chudakov and V.Luppov IEEE Trans. on Nucl. Sc., 51, 1533 (2004) Advantages: 100% electron polarization very small error on polarization sufficient rates no dead time false asymmetries reduced 0.1 Hydrogen gas target no Levchuk effect low single arm BG from rad. Mott ( 0.1 of the BG from Fe) high beam currents allowed: continuous measurement Operation: density: atoms/cm 2 Stat. error at 50 µa: 1% in 10 min

9 E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 7 Møller Systematic Errors Proposed: 100%-polarized atomic hydrogen target ( atoms/cm 2 ). Variable Hall C Hall A Present Upgrade Proposed Target polarization 0.25% 2.00% 0.63% 0.01% Target angle 0.00% 0.50% 0.00% 0.00% Analyzing power 0.24% 0.30% 0.30% 0.10% Levchuk effect 0.30% 0.20% 0.50% 0.00% Target temperature 0.05% 0.00% 0.02% 0.00% Dead time % 0.30% 0.10% Background % 0.30% 0.10% Others 0.10% 0.30% 0.30% 0.30%? Total 0.47% 2.10% 1.00% 0.35% (Hall A different targets) 0.3 (Hall A saturation effects)

10 E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 8 Hydrogen Atom in Magnetic Field H 1 : µ µ e ; H 2 : opposite electron spins Consider H 1 in B = 7 T at T = 300 mk At thermodynamical equilibrium: n + /n = exp( 2µB/kT ) Complication from hyperfine splitting: Low energy b = a = cos θ sin θ High energy d = c = cos θ+ sin θ where tan 2θ 0.05/B(T ), at 7 T sin θ Mixture 53% of a > and 47% of b >: P e 1 δ, δ 10 5, P p 0.06 (recombination 80%)

11 Storage Cell E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 9 H beam 0.3K 30K 40 cm Solenoid 8T Storage Cell First: 1980 (I.Silvera,J.Walraven) p jet (Michigan) Never put in high power beam 4 cm ( µ H B) force in the field gradient pulls a, b into the strong field repels c, d out of the field H+H H 2 recombination (+4.5 ev) high rate at low T parallel electron spins: suppressed gas: 2-body kinematic suppression gas: 3-body density suppression surface: strong unless coated 50 nm of superfluid 4 He Density cm 3. Gas lifetime > 1 h.

12 Dynamic Equilibrium and Proton Polarization E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 10 Proton polarization builds up, because of recombination of states with opposite electron spins: a = α+ β and b = As a result, a dies out and only b = is left! P 0.8

13 Contaminations and Depolarization of the Target Gas E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 11 Ideally, the trapped gas polarization is nearly 100% ( 10 5 contamination). Good understanding of the gas properties (without beam). Contamination and Depolarization Gas Properties No Beam Atom velocity 80 m/s Hydrogen molecules 10 5 Atomic collisions s 1 Upper states c and d < 10 5 Mean free path λ 0.6 mm Excited states < 10 5 Wall collision time t R 2 ms Escape (10cm drift) t es 1.4 s CEBAF Beam Bunch length σ=0.5 ps Repetition rate 497 MHz Beam spot diameter 0.2 mm Based on σ HH = cm 2 needs verification Helium and residual gas <0.1% - measurable with the beam 100 µa Beam Depolarization by beam RF < Ion, electron contamination < 10 5 Excited states < 10 5 Ionization heating < Expected depolarization <

14 Contaminations and Depolarization of the Target Gas E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen µa CEBAF beam: Gas Ionization Beam RF influence 10 5 s 1 of all atoms a d and b c 200 GHz 20% s 1 in the beam area RF spectrum: flat at <300 GHz Problems: 10 2 No transverse diffusion 10 Recombination suppressed 1 Contamination 40% in beam 1/N dn/dν (GHz -1 ) Transition frequency ν (GHz) 10 4 s 1 conversions (all atoms) 6% s 1 conversions (beam area) Diffusion: contamination in the beam area Solenoid tune to avoid resonances Solution: electric field 1 V/cm Drift v = E B/B 2 12 m/s Cleaning time 20 µs Contamination < 10 5 Ions, electrons: same direction Beam E r (160µm) 0.2 V/cm E B beam V drift

15 Main Components E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 13 Target Dilution refrigerator for 0.3 K Solenoid 8 T Hydrogen dissociator Storage cell, superfluid He supply, film burners Interface to the beam pipe, pumping, super-insulation Optimized Møller spectrometer: No Levchuk effect - Møller θ E correlation can be used Large acceptance (the target is thin) Vertex detector (easier for low energies) Event-like data at < 1 khz (not simply the counting rates) vertex reconstruction (measurement of the residual gas); Kinematics on the event basis - reduction of the error on the analyzing power

16 Proof of Principle H-H atomic cross section calculations σ HH cm 2 we used σ HH = cm 2 acceptable cleaning time Larger cross section longer time (worse) Needs consultations with atomic physicists The calculations have been done for the CEBAF beam of 100 µa, 1 GeV. Should be adapted/verified for the given beam parameters: Beam pulse length σ t (the RF effect) longer better Beam spot size σ R larger better Rep. rate - to tune the magnetic field off the RF resonances Beam energy: Møller spectrometer and the optics in the 8 T field ionization losses Technical R&D and challenges: Electrodes in the storage cell with the superfluid He layer - field, thermal conductivity, extra recombination etc. Residual gas (may depend on the recombination rate): vertex detector or special optics to select different ranges of Z. Spectrometer engineering and design including the super-insulation for 0.3 K E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 14

17 E.Chudakov June 21, 2011 Møller Polarimetry on Atomic Hydrogen 15 Cost Evaluation (very crude!) It is assumed that there is a 4 He 4 K cryo plant Møller spectrometer is not included Item Initial R&D Construction Labor Cost Labor Cost FTE Year $k FTE Year $k Scientist Engineer Designer Technician R&D materials, instr Dilution refrigerator He Pumps He 20 4 He film system? 20 Dissociator 20 Solenoid 150 Misc. pumps, valves 50 Instrumentation 70 Assembly 150 Total material Total

Møller Polarimetry for PV Experiments at 12 GeV

Outline E.Chudakov Jan 15, 2010, MOLLER Review Møller Polarimetry 1 Møller Polarimetry for PV Experiments at 12 GeV E.Chudakov 1 1 JLab MOLLER Review Outline E.Chudakov Jan 15, 2010, MOLLER Review Møller