Polarization for precision measurements. Torsten Soldner Institut Laue Langevin

Transcription

1 olarization for precision measurements Torsten Soldner Institut Laue Langevin

2 olarization Definition More precisely: spin polarization Degree to which the spin of particles is aligned with a given direction For spin ½ particles: roperty of an ensemble of particles, not of a single particle! olarization vector: i i

3 Asymmetry measurements Ap e W d d d Beam Detector d e Ap Beam Detector Beam Beam Detector d ) ( d ) ( e e p r p r Beam Detector d e Ap Beam Detector d p e A

4 Solution I Detector averages beam Detector average of Beam average of Beam average Detector average Beam average Detector average

5 Solution II olarization homogeneos Detector average of Beam average of Beam average Detector average Beam average Detector average

6 Content Techniques for polarization and spin flipping olarization analysis ushing

7 U U olarization Supermirrors opt B -7 ev -7 ev U -7 ev Match index of refraction B Increase critical angle (see optics lecture)

8 Example: m=4 FeSi super mirror ote expanded scale for Matching works better for CoTi, but activation worse icture from Swiss eutronics web page

9 SM bender Dependencies q q q q Wavelength dependence Angle dependence

10 SM bender angular dependence A osition [mrad] Single Analyzer Crossed Analyzer Old example, modern polarizers are much better

11 SM bender wavelength dependence A Single Analyzer Crossed Analyzer [A] Old example, modern polarizers are much better

12 3 He spin filter n+ 3 He t + p σ c, = 5333(7) barn σ c / σ c, =.(3) (often assumed that σ c = but known only on % level) T n T exp [He] l ( tanh O( ) He He ) O( ) e cosh O( n ) t) He ( He ()e t T He relaxation of hyperpolarized 3 He σ c >> σ c.733 O( ) pl opacity bar cm Å Angle-independent (cos(θ)) n up to % He, n drop in time dependent

13 3 He spin filter n tanh O( ) He He =.7, p=bar, l=cm

14 3 He spin filter T O( ) e cosh O( n ) He He =.7, p=bar, l=cm

15 Cells

16 Techniques MEO (metastability exchange optical pumping) ump 3 He to metastable state olarize nuclei Works at mbar pressure offline pumping, compression SEO (spin exchange optical pumping) ump electrons in alkali metal Transfer to 3 He by collision Works at bar pressure online pumping Strong performance loss in high neutron flux

17 Flipper I Current sheet on-adiabatic transition B B Typical parameters: Wire: D = mm Current: I = A Good for fast neutrons, bad for slow (adiabaticity) Difficult to adjust: OFF: adiabatic transport O: non-adiabatic transition, no other field components Requires material in the beam, scattering may be currentdependent

18 Flippers II Adiabatic fast passage B (x) ω=γb R ω B Frame rotating with ω:.5 ΔB=B(x)-B R B x Typical values: f=ω/π=5 khz, B R =7 G Flipper efficiency Radio Frequency Flipper <f> =.9993±. Current Sheet Fliper [A]

19 Flippers III Small-band Mezei flipper: on-adiabatic transition, adiabatic rotation Resonance spin flipper See Florian s lecture More complex configuration for arbitrary rotation angle For π/ flip (as needed for EDM or spin echo experiments) Works only for fixed wavelength (tunable to λ at pulsed beam)

20 olarization Analysis

21 A handy formalism I articles spin up articles spin down n B Unpolarized beam olarizer n B

22 A handy formalism II articles spin up articles spin down n B Unpolarized beam olarizer F F F F F Spin flipper ) ( ) ( F F n FB ) ( F F Spin flip efficiency ε = -F Spin flip inefficiency f = F- Flipper efficiency (modification of beam polarization by flipper)

23 A handy formalism III articles spin up articles spin down n B Unpolarized beam olarizer F F F F F Spin flipper d d D Detector Insensitive to polarization A A A Analyzer Detection efficiency n d (...)... B D Detected neutrons:

24 olarization analysis I One flipper B F A D Flipper off Flipper on DAB ( A) db DAFB ( A AF ) db AF A( F) Flipping ratio r A A( F) r r AF cannot be separated

25 olarization analysis II Two flippers B F F A D D A D A D A F D A F F F B B B B Requires a lot of statistics White Black Black White A F F A cannot be separated F and F can

26 olarization analysis with SM? F F F F A ( ) ( ) ( ) ( ) is property of device AD beam A (assuming A=) does not work, strictly speaking

27 olarization analysis III Analyzer flip B F A D D A( A) D A( A) B B D A( A) F B White Black Black A Works for 3 He analyzer We have 3 He flippers with ε ~ 3-6 This can be measured easily by performing many flips A cannot be separated F can n F tanh O( ) He

28 Example X-SM geometry -, -,995 -,99 -,985 olarization -,98 -,975 -,97 -,965 -,96 -,955 <> = (99.7 +/-.)% p 5.5 Bar p 4. Bar p. Bar p3.5 Bar p4. Bar p5.67 Bar Spectrum -, eutron wavelength (A) Used in ERKEO II, Mund et al, RL (3) 75

29 Analysis of inhomogeneous polarization Exploit features of 3 He: o angular dependence large area average possible Accurate scan of beam: Same areas for intensity and polarization scans Accurate average

30 ushing

31 Combining polarizers? olarisers not independent ) ( ), ( ) ( ) ( ), ( ) ( R R f R R f ) ( ) ( ) ( ) (

32 The X-SM geometry also known as crossed, but different meaning in optics olarizers independent ) ( ), ( ) ( ) ( ), ( ) ( y y y x x x R R f R R f y x y x y x y x ) ( ) ( ) ( ) (,

33 olarization analysis with X-SM? F F F F A x y x ( x) ( y ) ( x), ( ) y y is property of device AD beam A and independent, but numbers depend on beam (incoming divergence ) Cannot separate

34 But very powerful polarizer X Y Z n n B B B R adiabatic rotation.5 redictable olarisation... ( )(...and Transmission T T T ) Measured single olarizer rediction for Crossed Geometry [A]

35 Measured performance A.96 A But: osition [mrad] ( )( ) Single Analyzer Crossed Analyzer.8 (with old polarizers) Single Analyzer Crossed Analyzer [A] i =.95 expect =.999 Much more than measured!

36 . Limit of X-SM polarizer X-SM polarizer with X-SM analyzer:.99 A State of the art ILL SM olariser Magnetic housing field kg Fields in olarizer / Analyzer B = G, B A = G B = G, B A =35 G B =35 G, B A =35 G [A] Magnetic layers still not completely saturated

37 Accuracy of 3 He spin filters The opaque test bench >99.995% A>99.995% F>99.999% tanh O( ) He A tanh O( ) He Idea: Clean systematics by pure beam Motivation: Test ultimate performance of 3 He spin filters recision characterization of polarizing elements

38 Analysing ower of 3 He Be filter Chopper tanh O( ) He Accuracy better than -4 l = 4cm p He =.69 bar (and:. bar,.4 bar) C. Klauser, hd thesis (3)

39 AF Flipper Be filter Chopper f Detects only unflipped neutrons very sensitive F > 99.99% from 3 to 9 Å C. Klauser, hd thesis (3)

40 Depolarisation in super mirrors Selector Be filter Electromagnet.85T (Basic reflectometer) C. Klauser, hd thesis (3)

41 Depolarisation in super mirrors (Basic reflectometer) Depolarization can be reduced by higher magnetizing field lower m choice of material C. Klauser, hd thesis (3)

42 SuperADAM Reflectometer with polarization analysis

43 Origin of depolarisation in SMs Off-specular scattering: scattering by domains that are not aligned Can be reduced by increasing the magnetizing field

44 Selector Be filter Apply to X-SM Electromagnet.85T (Basic reflectometer) A = 99.97(3)% achieved with SMs only FeSi. CoTi Magnetising-Field st Mirror [Tesla] C. Klauser, hd thesis (3)

45 Summary Super mirrors Constant in time Wavelength dependent Angle dependent 98% routine 99.7% with X-SM 99.97% with pushed X-SM 3 He spin filters Changes in time Wavelength dependent egligible angle dependence > 99.99% possible AF flipper > 99.99% possible in λ range -4 accuracy achievable but requires careful design of polarizer and analysis