Possibility of antiproton polarization by Spin transfer 1 Antiproton Polarization by Spin- exchange...

Possibility of antiproton polarization by Spin transfer

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Antiproton Polarization by Spin-exchange from Positrons?

PST2007 Brookhaven

September,10,2007

Kurt Aulenbacher

Institut für Kernphysik der Uni Mainz


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Outline

1. Pol-Antiprotons (pbar): Why?

2. Pol-pbar: Status.

3. Compact polarized positron sources


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Pol pbar:Why?

For the FAIR facility pbar-polarization could play the same role as pol.electrons did for SLAC, JLAB, MAMI.

….but so far no efficient way of polarizing pbar.

Antiproton-storage ring


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Ultrashort History of pbar-polarization schemes

1. 1992: FILTEX-experiment:Spin filtering (Spin dependent nuclear forces) in p (\vec p)-scattering demonstrated (polarized gas target in p-storage ring at 23 MeV). Filter cross section smaller than expected.

2. 1994: Horowitz and Mayer calculate large spin dependent cross section (Spin transfer coefficient‘ (STC)) in p(\vec e) scattering (1barn cross section at 5MeV)

3 2007: Distorted wave calculation of STC with realistic Coulomb wave functions by Arenhövel for very low relative velocities of lepton and pbar. No major increase of effect for electron/pbar but huge increase of STC for positron/electron scattering


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Small velocities

Advantage w.r.t. internal target: i) Increased beam life time,ii) directly applicable at all interesting pbar energies.iii) predictions may be tested conveniently in e-/proton interaction BUT:Even with high brightness source (30 A/cm2 over 1m at 4mm2 beam area), The cross section enhancement must make up for factor ~105 with respect to storage cell experiment (1barn cross section)


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A ‚huge‘ effect

• Non-screened interaction!• slow positrons are attracted to pbar• Interference between Coulomb- and Hyperfineamplitude creates large STC.

Reasons to worry:

1) Depolarization cross section at EH~1eV < 1010 barn (i.e. COSY exp.)2) Mayer (Aug. 2007) points out that STC may not necessarily lead to polarization of the pbar beam.


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STC is ‚not really‘ polarizing

Important is the Spin exchange: +- -+i.e. by Hyperfine interaction in Semiconductors.


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Preliminary Conclusion

1. Theoretical calculation of Spin exchange is underway.

2. Experiment at COSY Julich in Fall 2007probably has sensitivity to detect ~106 barns spin exchange c.s.

3. The smaller the cross section the more complicated the positronsource will have to be. What are the options?


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Compact polarized e+ sources

Generation Mechanism,i.e. radioactivity,Pair production

Low brightness: High average energy,Large spread in energy, Position, and momentum

Increase brightness by dissipative process

Decelleration:Dissipation by multiple scattering

Acceleration: Dissipation by syncrotron radiation


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Principle of moderation

•Absorber necessary in order to shift high intensity/high polarization part of -Spektrum to ~10keV.

•Absorber also suppresses positrons which large emission angles!

•~10keV Positrons are stopped close to moderator surface and escape because of negative positron work function.

•Escaping Positrons have thermal energy spread (!!)

J. van House et al. Phys. Rev. A 29,1 96 (1984)5*105/s at P=0.48 from Na-22. Efficiency: 2.5*10-4


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Possible d.c.-source parameters

Online C-11 production with Commercial superconducting Linac: 1.2 mA, 20MeV 1013 Bq source activity Expected Efficiency increase: factor 2compared to 1980-s experiment

Current: 5*109 e+/sPolarization: 0.7Energy width: <0.1eVNormalized beam emittance 1 mm mrad


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e+-Storage ring option

1. Profit from enhanced Beam current due to revolution frequency.

2. Conventional AG-storage ring offers beam lifetime of several seconds @1MeV

3. longitudinal spin stabilization with -rotating solenoid seems feasible.

4. Space charge limit 10mA.5. Aim at >3*105 stored particles (1

Mikroamp i.e 3 orders of mag. w.r.t. d.c. source)

6. Task: Produce 3*105 pol e+ in acceptance of 5keV*ns.

Antip ro to n b e a m in sto ra g e ring

Sp in tra nsfe r re g io n

C irc um fe re nc e ~ 14m (f = 21 M hz)re v

Kic ke r

Po la rize d e so urc e+So le no id fo r lo ng .

Sp in sta b iliza tio n


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General Idea:

eeSame as for the ILC sources….. ??


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T. Omori et al: PRL 96,114801 (2006)

Achieves about 103 polarized positrons with P=0.73 inside required acceptance.


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Compact Circularely polarized gamma source

1.) Conventional: Polarized Bremsstrahlung from polarized electrons (subsequent Brightness enhancement by moderation)2.) Unconventional: Laser accelerator(*) with small spread in all spacial dimensions +Compton backscattering from part of the drive laser pulse (tabletop e/Photon-collider(**).(Due to small initial pulse length no moderation needed)

*W.P. Leemans et al: Nature physics, 2, 696 (2006). **H Schwoerer et al. PhysRev.Lett., 96, 014802 (2006)


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Pulsed source with present day technology

• Generate circularely polarized gammas by Bremsstrahlung from polarized electrons

• If compared to the KEK-Ansatz we profit from the relatively long pulse length required.

• Will allow for compact (~15 m long) set up• only well established components


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A:Source: Pol. e pulse200keV,4sIpeak=5A

B:High charge r.f.-LINAC, 23MeV, Ipeak=1.7ANe-=4.3*1013

C;Conversion target1mm Tungsten(or liquid lead)

D:Positron energy (E) + Angular ()-selectionE=14.25MeVE=+-350keV,= 2deg.Conversion efficiency: 1=e+/e-=9*10-7

P=0.76

A B C D

E:Beam collimation

E


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F:Positron decelaration(r.f.-LINAC)14.25 to 1.75 MeV

Terminal with absorber/moderator (H) (at +1MV)and Buncher (I) (+ 0.99MV),reacceleration to ground potential (J)

Bunched e+ to storage ring E=1MeV, 50nsIpeak=2A

F G

G: Electrostaticdecelerationto Ekin= 1 MeV at terminal

H I J

{E

Moderator efficiency:2=1.4%.


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Expected parameters for pulsed source (conventional version)

• Bunch charge 6*105 (from 1.2*1014 pol. electrons)

• Polarization 0.76

• normalized emittance 9 mm mrad

• longitudinal phase space 4 keV*ns

• Current in storage ring 2 A.

• repetition rate 10 Hertz no lifetime problem for polarized electron source.


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Can it be improved even further?




Kic ke r



1. Increase storage time (and space charge limit)by longitudinal (toroidal)Magnetfield (Stellatron)2.) The Stellatron (aka LEPTA)is intended for electron cooling of stored positron beams! with ~mA current. 3.) But: Not investigated: i) Is it possible to stack?ii) Cooling time??iii) Lifetime???iv) positron spin dynamics


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Which device for which cross section?

-12 -10 -8 -6 -4 -2 0

4

6

8

10

12

14

16Current for

p~1hour

Limit for stable operation of Antiproton storage ring

Toroidal storage ring (Stellatron) with electron cooling and stacking (????)

Strorage ring with pulsed injection(polarized electrons)

d.c.-source with online isotope production

Michigan sourcelo

g(f

lip c

ross

se

ctio

n)

[ba

rn]

log (Positron current) [A]


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Summary

• e+/pbar spin exchange cross sections could provide a (pbar)-loss free polarization mechanism

• Theoretical and experimental investigations for the cross section are underway and will probably lead to conclusive results this year

• Compact polarized positron sources with present day technolgy would yield 1 hour of polarisation time if the cross section is ~1010barn, (106 Barn with advanced technolgy??)


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Laser-Plasma-accelerator*

*W.P. Leemans et al: Nature physics, 2, 696 (2006)

3T-Laser:1.5J,40fs, 40TWdfok=50ma0=4.8reprate:1Hz


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e/-collider

*H Schwoerer et al. PhysRev.Lett., 96, 014802 (2006)

using part of the drive beamfor compton backscattering!

sub-picosecond duration circularely polarized gamma beam may be possible,thus increasing long. brightness by two orders of magnitude


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Laser-Plasma-accelerator*

*W.P. Leemans et al: Nature physics, 2, 696 (2006)

Electronen-Puls:1GeV,30pC, E/E=0.05Peak current 10kA! E*T]~0.1MeV*ps [x*]~10nm*rad


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• Pocket accelerators may provide efficient

source for polarized gamma radiation • at small size and investment/running cost

(compared to 1GeV high charge storage ring)• Potential for far higher luminosity could result in

2 orders of magnitude increase of bunch charge. (3*10^5 /pulse in storage ring acceptance)

• but: technology not (yet) well established.


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Once established, an e+ source operates like an e- source…..

Figure from van House et al. (1984)Beam energy: 500eV


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Intensity Limit of d.c. source?

Commercial PET-Isotope production:Proton cyclotron 20 MeV, 14N(p,)11C:Yield: 0.8*1010Bq/A.

Isotope Emax

[MeV]T1/2 Amax

[Bq/mg]REM

22-Na 0.5 2.6 a 2*1011 Van House: 2*1010

64-Cu 0.6 12.7h 1.4*1014 Highest activity:1015

18-F 0.6 109.7m 3.3*1015 PET

11-C 1.0 20.38m 3.1*1016 PET

15-0 1.7 2.03m 2.2*1017


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P2I-optimization

‚Optimum‘ positron source: Carbon-11 (E0=1MeV).Higher Polarization, far higher activity

also from van Houseet al.


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Expected d.c.-source parameters

Online C-11 production with Commercial superconducting Linac*: 1.2 mA, 20MeV 1013 Bq source activity Expected Efficiency increase: factor 2compared to 1980-s experiment

Current: 5*109 e+/sPolarization: 0.7Energy width: <0.1eVNormalized beam emittance 1 mm mrad


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e+-Storage ring option

1. Profit from enhanced Beam current due to revolution frequency.

2. Conventional AG-storage ring offers beam lifetime of several seconds @1MeV

3. longitudinal spin stabilization with -rotating solenoid seems feasible.

4. Space charge limit 10mA.5. Aim at >3*105 stored particles (1

Mikroamp i.e 3 orders of mag. w.r.t. d.c. source)

6. Task: Produce 3*105 pol e+ in acceptance of 5keV*ns.




Kic ke r




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General Idea:

ee

As for the ILC sources…..


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P2I-optimization

‚Optimum‘ positron source: Carbon-11 (E0=1MeV).Higher Polarization, far higher activity

also from van Houseet al.

Possibility of antiproton polarization by Spin transfer 1 Antiproton Polarization by Spin- exchange...

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