Mott polarimeter at MAMI - fe.infn.it file07.09.09 V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany 3 MAMI polarimeter technique • Mott scattering: electron at gold nuclei (Z=79),

07.09.09 V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany 1

Mott polarimeter at MAMI


Contains

• Introduction

• Design of Mott polarimeter

• Measurements• Results

• Conclusion


MAMI polarimeter technique

• Mott scattering: electron at gold nuclei (Z=79), due to spin-orbit coupling of electron spin appears asymmetry in backscattering,

• Used in MAMI 14 MeV and JLAB 2-8 MeV • Main parameters: Asymmetry at foil zero thickness -A0, beam

Polarization - P, Sherman Function - S are related by: A0=P x S

• To measure beam polarization need to know both Sherman function and asymmetry

• Sherman function needs theoretical calculation• Asymmetry needs experimental measurements:

• A=(Nl-Nr)/(Nl+Nr)


Analyzing Power - Sherman Function

• For spherical symmetric potentials, the Sherman function can be calculated exactly by the Mott phase shift formula

• Low energy region (< 1 MeV) must take into account electron cloud (theory) and strong multiple scattering increases experimental problems.

• High energy region (>5 MeV) must take into account nuclear size effect. (P. Uginčius,et. al. 1970.) Considerable deviations from point nucleus values appear above 5 MeV.

• For energies from 1 MeV to 3.5 MeV scattering in Coulomb field with negligible nuclear size effect and electron cloud is possible. This region has preference.


Nuclear size effect

-0.60

-0.55

-0.50

-0.45

-0.40

-0.35

-0.30

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

130 135 140 145 150 155 160 165 170 175 180

angle, [degrees]

SF

,[a

.u.]

Pb 10 MeV exten

Pb 10 MeV point

Pb 5 MeV exten

Pb 5 MeV point

Pb 1 MeV point

Pb 1 MeV exten

Au 3,5 MeV exten

Au 3.5 MeV point

Au Coulomb

For Pb from P. Uginčius, H. Überall and G.H. Rawitscher. 1970


Polarised electron source

Polarised beam produced by photo cathodes based on super lattice A3B5 cathodes from Sankt Petersburg State Technical University, Russia and SVTA, USA

Activation by Cs:O layers. by illuminating by circular polarised laser light wave length 780-830 nm.

Quantum efficiency of photo cathodes about 1%

Beam polarisation more then 80 %

Manipulation of spin direction using Wien filter


Mott layout

• Mott polarimeter located in the 3.5 MeV region of the MAMI accelerator • Beam energy: variable from 1.0 to 3.6 MeV • Beam current: 5 nA to 45 µA• Scattered electron energy selection by magnetic spectrometers• Empty Target / Target signal ratio > 1000:1 typically • Targets: 0.1 to 15 µm Au foil (Z=79)• Fixed scattering angle 164 degree, yields Sherman function S>0.45 for all

energies


Mott design

Au target

ScintillatorCollimator

Ø 4 mm

Primary beamBending magnets

Photo multiplier

Spin direction


Mott photo


Asymmetry measurements

0,1 1 100,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

0,40

0,45

0,096 0,098 0,100 0,102 0,104

0,36

0,38

0,40

0,42

1.0 MeV 2.0 MeV 2.5 MeV 3.0 MeV 3.5 MeV

Asy

mm

etr

y, [1

= 1

00

%]

Foil thickness, [mkm]


Measured asymmetry in a range from 0.005 to 45 μA

0.001 0.01 0.1 1 10 10012.8

12.9

13.0

31.8

31.9

32.0

32.1

36.5

36.6

36.7

36.8

Beam current, mkA

foil thickness 1 mkm

foil thickness 100 nm

foil thickness 15 mkm

Relative variation ±0.5% for each foil.

Required measurement times between 2 min to 2 hours per point


Extrapolation procedure

Fit procedure and extrapolation to foil zero thickness should have an the property: A(∞)=const and ≠ 0 because of the energy resolution of the detector system.

Exponential extrapolation proposed by Gay, Khakoo, RSI 1992, based on phenomenological model:

A(t)=Ac + Av exp(-t/d)Where t- foil thickness, Ac , Av , d - fit parameters, evidently A(0)= Ac +Av

Wegener 1958 proposed detector count rate as:

Nl,r ~(1 ± P Ao) t + α t2

proposed rational or polynomial fit:

A(t)=Ao /(1+a t)But double scattering term can (should) depend on beam polarisation:

α -> (α l , α r)

Nl,r ~(1 ± P Ao) t + α l,r t2

following fit function: A(t)=Ac + Av /(1+a t)


Fit example at 2.0 MeV

0,1 1 100,0

0,1

0,2

0,3

0,4

Asymmetry 2.0 MeV Exponential Fit 95% Confidence Limit Rational Fit 95% Confidence Limit

Me

asu

red

asy

yme

try,

[1=

10

0%]

Foil thickness, [mkm]

Both fits predict A(t=0) with accuracy better than 1.5%


Measured beam polarisation vs beam energy

1,0 1,5 2,0 2,5 3,0 3,5 4,00,79

0,80

0,81

0,82

0,83

0,84

ExpFit PolFit

Bea

m P

olar

isat

ion,

[1=

100%

]

Beam Energy, [MeV]


Error Sources• The main source is presently the quality of the fit (± 1.5%), which can be

further improved by better thickness measurement or more targets.

• Statistical error by each asymmetry measurement is smaller than ∆A/A= ± 0.2 % negligible.

• Sherman Function. Nucleus extended or point. At energy from 1.0 to 3.5 MeV at gold the effect smaller than ∆S/S=±0.5 %.

• Choice of approximation function – no significant difference for two reasonable fit functions.

• Target induced Background? A(0) reduction due to background would have to be the same for all energies.


Conclusion•A Mott electron polarimeter has been developed for MAMI

•Background discrimination by magnetic spectrometers allows for a good signal to noise ratio

•High beam energy allows to measure asymmetries at ‘robust’ target thicknesses from 0.10 to 15 µm

• Accuracy of extrapolation better than 1.5% in an energy range between 1 and 3.5 MeV

• Asymmetry measurements are independent of the primary beam current in a range from 0.005 to 45 microamperes at a level of 0.5% relative variation.


The End

Documents

Mott polarimeter at MAMI - fe.infn.it file07.09.09 V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany 3 MAMI polarimeter technique • Mott scattering: electron at gold nuclei (Z=79),