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Polarized Neutron Technique Network in NMI3 Larmor precession devices M. Janoschek, S. Klimko, H. Lauter, R. Gähler, ILL Grenoble L.P. Regnault, CEA Grenoble. MUPAD - 3D polarization analysis bunching of continuous polychromatic beams Larmor precession and reflectometry.

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slide1
Polarized Neutron Technique Network in NMI3

Larmor precession devices

M. Janoschek, S. Klimko, H. Lauter, R. Gähler, ILL Grenoble

L.P. Regnault, CEA Grenoble

  • MUPAD - 3D polarization analysis
  • bunching of continuous polychromatic beams
  • Larmor precession and reflectometry
slide2
A) Conventional magnetic scattering:

Intensities from nuclear Bragg peaks: Inuclear N*k Nk; NkFT{nuclear(r)};

Intensities from magnetic Bragg peaks: ImagneticM*k Mk; M k k s(k)  k ;

s(k)  FT{magnetic(r)};

magnetic structure factor

Iinter[N*k Mk+ M*k Nk] Pi;

Higher visibility of magnetic(r) by nucl.-magn. Interf.:

Iinter depends on direction of incom. Polarisation Pi to magn. interaction vector Mk

The component of Mkalong Pi is measured.

slide3
B) Spherical neutron polarimetry

Blume (1963) : calculation of polarisation Pf(Pi) for magnetic + nuclear scattering:

Pf= N*k NkPi - M*k  Mk Pi + M*k ( Pi  Mk ) + ( Pi  M*k )  M k +

N*k Mk+ M*k Nk - i [ N*k Mk- Nk M*k] Pi

Measuring direction and magnitude of Pf for few reflexes as function of of Pi

is avery efficient way to determine magnitude and direction of Mk .

This method requires B = 0 at the sample!

In practice, complete information on Mk is obtained from measuring

3 orthogonal components of Pffor 3 orthogonal components of Pi

slide4
Cryopad principle

top view

field free area

B  5 mG

two Meissner shields

nutator

ki

precession fields

nutator

secondary coil

primary coil

kf

slide7
Mupad, 3-coil version

zi

coupling-coil in

zi

i

yi

 i-coil

xi

zi

yi

 i

sample

the action of each coil is

shown downstream of it !

No spin turn on scattering is assumed in the drawing !

xi

i -coil

Field free area

2

zf

 f =  i - 2

yf

xf

 f

 f

f -coil

 f-coil

zi  zf

coupling-coil out

field direction

yf

zf

Neutron spin

Assume spin turns on scattering:

by  s in the scattering plane  set  f -coil to f =- i -  s+2

by  s with respect to z-axis  set  f-coil to  f = - i -  s

yf

xf

slide8
MUPAD: general setup

side view

top view

opening for

cryostat

mu-metal links

between cylinders

and coupling screens

outer mu-metal-screen

inner mu-metal

-screen;optional;

B  5 mG

ki

B  5 mG

-coil

mu-metal shield

around all coils

to guide return fields

in low-field region

 -coil;

DC-compensation of mean

vertical component of earth field;

likely superfluous

kf

coupling-coil;

slide11
Design of new coils, goal: outer field integral < 1/1000 of inner field integral

How to avoid field from current Iz?

Al frame anodized

Cu contacts for return field

Iret

Open area 40 x 40

Al foil for

return current

Al wire 1

Iz

Mu metal yoke

Mu metal screen

Outer dimensions: 118 x 118 x 60

demagnetisation coils for all mu-metal screens!

slide16
Mupad for 2D- analysis

top view

He3 spin filter

in -coil

 -coil

in -coil

-coil

ki

cryostat

- -coil;

return field

for -coil

coupling-coil

kf

kf

Mu metal for return field

slide18
MIEZE - Principle

L1

L2

Coherent

frequency splitting

Coherent reversal

of frequency splitting

d

0 + (s - e)

e

s

total energy

k + e/v;

0 + e

+ s

+ e

plane of detection

E0

i(kx - 0t)

e

0 =

- e

k0 - e/v;

0 - e

0 - (s - e)

+i(keL1 - ksL2)

-i(s - e)t

 ke = e/v;

e

e

0

-i(keL1 - ksL2)

+i(s - e)t

 ks = (s -e)/v;

e

e

 =

For eL1 = (s -e)L2 ,  gets independent from v.  beats in time with d= (s -e)

detector

detector

slide19
MIEZE (without bunching!) using transmitted and reflected beam after analyser

L1

L2

Ce

Cs

B-field

 spin down

A

B = 0

P

d

fast

detectors

d

e

analyzer

s

 spin up

B

coupling coil

Idet

IB

IA

1

IA = 1/2[cos d·td +1],

IB = 1/2[cos (d·td +)+1],

with IB + IA = 1;

td: time of arrival at A

d = 2(s - e).

time

T

slide20
II) MIEZE setup with bunching using the full beam

replace analyzing mirror by RF-coil (resonance frequency F=2B;  -condition )

E   T/2 in arrival time for spin up and spin down at the detector.

E = E·T/t  v3 ·T(like in spin echo); T is fairly constant for a velocity band of 5-10%.

+

+

-

L1

L2

fast

detector

Ce

Cs

B-field

P

B = 0

d

s

e

RF-field; F

Signal function after bunching

buncher: sequence of high-field-RF-flippers

Idet

1

EF = 8B

EF = -8B

T

time

slide21
Estimate of the necessary energy chances ffor bunching:

Intensity

Signal function after bunching

Necessary condition for overlap of both signals:

 = f L2 / (mv3) with /4 = D

1

f : change of energy by the bunching flipper

For L=10m; =10Å; D = 2 1 MHz:

f = 0.8 10-8 eV;

f = 2 B;  = 6.8 10-8 eV/T

T

time

I = cos2Dt

I = sin2Dt

I = cos2(Dt - /4) = sin2(Dt + /4)

slide22
Quantum-mechanical view of bunching; there is no classical view of it!

The variation of k = k1 - k2 due to f is not cancelled

by the MIEZE condition and leads to an extra phase

difference d between both waves at the detector.

2+f

1+f

f

2

1

-f

This determines f as before.

2-f

1-f

State behind the bunching flipper: [ = (s - e)t]

 sin (+d)

k1

k2

k

 cos (-d)

k1 - k2 depends on f !

slide23
Application of bunching
  • MIEZE at SANS instruments (no analyzer near sample)
  • SPAN-like NRSE spectrometer with very high resolution?
slide24
3) Larmor precession and reflectometry

using ZETA at IN3

A) Off-specular scattering from polymer-multilayer

B) Larmor pseudo-precession in reflection

slide25
Mono SEU1 sample* SEU2 Ana Det (CCD camera)

Lf

ki

 = 2.44 Å

Li Lfr Lfy

Lenghts Lf depend to first

order on scattering angle;

‘angular encoding’

Rf flippers arranged symmetrically w.r.t. ki

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