Relativistic Spin-Polarized Positron Beam at MLL filePolarization of betas: Christoph Hugenschmidt 5 Polarized Positron Beam M. Maekawa et al.; NIM B 308 (2013) 9–14 68Ge production

Relativistic Spin-Polarized Positron Beam at MLL

Christoph Hugenschmidt

Technische Universität München

MLL2020 16 December 2015

Christoph Hugenschmidt 2

Outline

NEPOMUC & Limits

Polarized e+

e+ beam at MLL


Positron Sources & Beams

+ isotopes p n e+ e high e+ yield long t

lab beams

pair production e+ e– E > 2mec2

high Z

large scale facilities

Ee+ ~keV

DE < 2 eV

NEPOMUC: Pt as converter AND moderator f+

Pt = -1.95(5) eV

1 eV

C. H. et al. NIM B 198 (2002) 220

+ spin polarized !


+ Sources

Nuclide Half-life Emax [MeV] Em [MeV] v/c f [%] E[MeV] Appl. Prod.

22Na 2.6 y 0.546 0.20 0.70 90 1.275 t, S, Beam Zyc

64Cu 12.7 h 0.653 0.27 0.76 19 - S, Beam Reac

68Ge/68Ga 271 d 1.90 0.99 0.94 90 - t, S Zyc

Polarization of betas:


Polarized Positron Beam

M. Maekawa et al.; NIM B 308 (2013) 9–14

68Ge production using p beam Beam flux: 104 e+/s (max) Polarization: 0.47 (max)


Spin Currents at Surfaces

Zhang et al. PRL 114, 166602 (2015)

Change relative spin orientation of e+ & e- Polarization dependent annihilation rate Spin currents at surface

2D-ACAR Spectrometer at MLL/TUM

Ceeh et al. Rev. Sci. Instrum. 84, 043905 (2013)

ACAR

Angular Correlation of Annihilation Radiation

Electronic structure

Spin channels of DOS


Features of ACAR:

special conditions not required

T >> 0, “no“ B-field

e+ beam surface, interface, thin layers, bulk

+ source bulk spin-polarized ACAR

Spin Polarized ACAR

22Na , mean longitudinal polarization:

P = vm/c = 0.67

Measured polarization: P = 0.368(5)

Ceeh et al. Rev. Sci. Instrum. 84, 043905 (2013)


Motivation: Ferromagnetic Materials

FRM II News 8


Nickel

Magnetic FCC metal

One unpaired 3d-electron

"simple" test case for theory and experiment

Theory

6eV satellite peak arises when correlations are included (DMFT)

Electron states are relocated

Effect of correlation change the appearance of the Fermi surface

http://www.phys.u.edu

J Kolorenc, et al., arXiv:1202.6595v1

Prime Example: Ni


Electron-electron interaction strength in ferromagnetic Ni determined by spin-polarized positron annihilation

H. Ceeh, J. Weber, P. Böni, M. Leitner, D. Benea, L. Chioncel, D. Vollhardt, H. Ebert, J. Minar, C. H.; arXiv:1501.02584

Result

Best agreement between experiment and theory DMFT(U) for U = 2.0eV

Heusler Compounds: Cu2MnAl


Cu2MnAl:

• L21-structure

• Prototype of Heusler alloys

• Ferromagnetic with no

ferromagnetic element

Heusler Compounds: Cu2MnAl


Individual contribution of each Fermi sheet to magnetisation !

J. Weber et al. PRL 115 (2015) 206404

NEPOMUC

Cd Pt

fast e+

slow e+

n therm

n capture

emission

e+ e- conversion e+ moderation e+ emission

Neutron Source

FRM II

Positron Beam

NEPOMUC

Principle:

NEutron induced POsitron Source MUniCh


C. H. et al. Appl. Phys. A 74 (2002) 295; Nucl. Instr. Meth. A 593 (2008) 616


NEPOMUC NEutron induced POsitron Source MUniCh

e+

Intensity: > 109 e+/s

C. H. et al. New J. Phys. 14 (2012) 055027; J. Phys. Conf. Ser. 443 (2013) 012079


PAES

Remoderator

Open beam port

CDBS PLEPS

SR 11 Switch

SPM interface

Positron Beam Facility at NEPOMUC

C. H. et al. NIM A 593 (2008) 616 New J. Phys. 14 (2012) 055027

J. Phys. Conf. Ser. 443 (2013) 012079

What we Measure:

ideal crystal

crystal with defects

Doppler-Broadening Spectroscopy – DBS

Positron Lifetime Spectroscopy – PLS

Coincident Doppler-Broadening Spectroscopy – CDBS

Cr


Angular Correlation of Annihilation Radiation – ACAR


What are the Limitations at NEPOMUC ?

Maximum beam energy

NEPOMUC provides a dedicated low-energy beam of high brightness

Maximum beam intensity

NEPOMUC provides highest beam intensity – can we do better?

Spin polarization

Pair production without distinct spin polarization

z0(n,m,A, E): n,m,A material dependent parameters

Makhovian implantation profile:

Puska et al. Rev. Mod. Phys. 66 (1994) 841


From the Surface to the Bulk

e+ Mean implantation depth (Al):

20 eV surface

10 keV layers 600 nm

30 keV bulk 4 µm 22Na: 200 keV bulk 150 µm

MLL: 10 MeV bulk 19 mm


(n, ) Reaction: 113Cd

Full absorption in W 75% NOT used for PP

114Cd: - spectrum

Solution: – “Beam“

Predicted spectrum of MEGa-ray source at ~2.54 MeV

Positron Source Based on Inverse Compton Scattering

nsc ~ 2n0

~ 1000


e+

C. H. et al. Appl. Phys. B: 106 (2012) 241 see White Book ELI


Beam Based e+ Source at ELI-NP

Features at first stage:

beam diam.: few mm

E = 3.5 MeV

I = 2.4 x 1010 γ/s

Expected key values:

slow positron intensity 1.9 x 106 s-1

spin polarization 31%

First user facility offering a polarized positron beam

ELI-NP/TDR/RA2/G2P/3/Jan2015


New Positron Beam at MLL

Idea: Put + emitter & e+ moderator inside the HV terminal of the tandem and apply a couple of MV !



Layout of terminal with positron source section:



Some numbers for 22Na and W moderator:

A= 50mCi Ye+ = 0.9 emod = 5(1)x10-3

etrans = 3(1)x10-1

Ie+ > 106 s-1 moderated & polarized !

Maximum beam energy

MeV range

Maximum beam intensity

Lower than NEPOMUC

Spin polarization

Almost 100% longitudinal spin polarization


Experiments with Relativistic Spin Polarized Positrons

Bulk studies:

Spin 2D-ACAR

AMOC

Spin CDBS

Spin lifetime


Summary

Positron Beam at MLL

easy to realize

high-energy polarized positron beam

Spin Dependent Positron Beam Experiments

bulk electronic structure spin channels of DOS in correlated systems

spin dependent positron lifetime complementary to spin-ACAR

AMOC simultaneous measurement of momentum & e+ lifetime

Unique positron beam facility for polarized positrons

Thank you !

Documents

Relativistic Spin-Polarized Positron Beam at MLL filePolarization of betas: Christoph Hugenschmidt 5 Polarized Positron Beam M. Maekawa et al.; NIM B 308 (2013) 9–14 68Ge production