Applications of polarized neutrons

Applications of polarized Applications of polarized neutronsneutrons

V.R. SkoyV.R. SkoyFrank Laboratory of Neutron Physics, Joint Institute for Nuclear Frank Laboratory of Neutron Physics, Joint Institute for Nuclear

Research141980 Dubna, Moscow Region, RussiaResearch141980 Dubna, Moscow Region, Russia

Currently atCurrently at

Pohang Accelerator Laboratory, Pohang University of Science and Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang, 790 – 784, Korea Technology, Pohang, 790 – 784, Korea

Main fields of ApplicationMain fields of Application

• Nuclear structure parametersNuclear structure parametersAngular and polarization correlations in neutron scattering and capture reactions. Angular and polarization correlations in neutron scattering and capture reactions. Scattering lengths and nuclear pseudomagnetismScattering lengths and nuclear pseudomagnetism..  

• Tests of fundamental symmetriesTests of fundamental symmetriesSpace parity nonconservation in transmission, fission and capture reactions with Space parity nonconservation in transmission, fission and capture reactions with unpolarized nuclei. Time reversal invariance test in neutron reactions with polarized or unpolarized nuclei. Time reversal invariance test in neutron reactions with polarized or aligned nuclei. aligned nuclei.   

• Fundamental properties of neutronFundamental properties of neutronAnomalous neutron dipole moment.Anomalous neutron dipole moment.  

• Magnetic properties of matterMagnetic properties of matterInvestigations of domen structure of ferromagnetic. Dynamics of the phase transitionsInvestigations of domen structure of ferromagnetic. Dynamics of the phase transitions.

Definition of PolarizationDefinition of Polarization

NNNN

P

P = 0P = 0

P = 6/12 = 0.5P = 6/12 = 0.5

Polarized neutrons Polarized neutrons productionproduction

• Transmission through polarized targetsTransmission through polarized targetsPolarization via Scattering (n,pPolarization via Scattering (n,p) reaction) reactionPolarization via Capture Polarization via Capture 33HeHe(n,p) reaction(n,p) reaction

  • Reflection from magnetic single crystals or magnetic mirrors Reflection from magnetic single crystals or magnetic mirrors

(cold and thermal neutrons)(cold and thermal neutrons)

Polarization via ScatteringPolarization via Scattering

k

I

Proton TargetProton Target

For (n,pFor (n,p) scattering cross section) scattering cross section

below 100 keVbelow 100 keV

20

Dynamical Nuclear Dynamical Nuclear PolarizationPolarization

pe ww

pw

ew pe ww

RF - FieldRF - Field

For polarization via For polarization via scattering LMN single scattering LMN single crystal was initially crystal was initially used. used.

Now some sorts of Now some sorts of alcohol and alcohol and polyethylene are used. polyethylene are used.

Dynamical Nuclear Dynamical Nuclear Polarization method Polarization method which requirewhich requires:s:

This method can be used for This method can be used for polarization of other nuclei.polarization of other nuclei.

For example, La in LaAlOFor example, La in LaAlO33

T < 1 KT < 1 K

H > 1 TeslaH > 1 Tesla

RF - pumpingRF - pumping

Polarization via CapturePolarization via Capture

k

I

33He TargetHe Target

For For 33HeHe(n,p) capture cross section(n,p) capture cross section

and and

tot

ntot E/1

Optical polarization of the Optical polarization of the Noble GasesNoble Gases

The high nuclear polarization of odd isotopes of The high nuclear polarization of odd isotopes of any noble gasesany noble gases can be built can be built by means of the following by two – stage processby means of the following by two – stage process

1.1. The circularly polarized resonance laser light optically pumps an alkali The circularly polarized resonance laser light optically pumps an alkali – metal vapor (usually Rb), aligning the optical electron spins along the – metal vapor (usually Rb), aligning the optical electron spins along the quantization axis (quantization axis (direction of laser beamdirection of laser beam). ).

2.2. In the presence of noble gas nuclei with non – zero spins, the electron In the presence of noble gas nuclei with non – zero spins, the electron polarization of the alkali – metal atoms is transferred to the nuclear polarization of the alkali – metal atoms is transferred to the nuclear polarization of these nuclei via the polarization of these nuclei via the hyperfinehyperfine interactioninteraction during the during the collisions (the spin – exchange process).collisions (the spin – exchange process).

Optical Pumping of Rb Optical Pumping of Rb atomsatoms

But in presence of buffer gas…But in presence of buffer gas…

1.1. Helmholtz Coils (20 – 40 Gauss)Helmholtz Coils (20 – 40 Gauss)

2.2. RF – Coils (NMR)RF – Coils (NMR)

3.3. Glass Cell with Glass Cell with 33He (1 – 10 atm.)He (1 – 10 atm.)

4.4. Pick – Up Coil (NMR)Pick – Up Coil (NMR)

5.5. Laser Beam (795 nm, > 15 Watt)Laser Beam (795 nm, > 15 Watt)

6.6. PhotodiodePhotodiodeTypical Installation DesignTypical Installation Design

Relaxation (decay) of Relaxation (decay) of 33He polarizationHe polarization

The origin of The origin of 33He polarization relaxation are:He polarization relaxation are:1.1. The impurities of the paramagnetic atoms inside cell wall and bulk. Thus, to The impurities of the paramagnetic atoms inside cell wall and bulk. Thus, to

achieve of long decay time (> 10 hours), the cell and gases mast be very clean.achieve of long decay time (> 10 hours), the cell and gases mast be very clean.

2.2. Inhomogeneity Inhomogeneity of the external magnetic fields.of the external magnetic fields.

The last characteristics time can be expressed as:The last characteristics time can be expressed as:2

||

1

HH

DT ng

m

If one put a cell inside the cylindrical magnetic shield, thenIf one put a cell inside the cylindrical magnetic shield, then

mmsh

m TS

S

TT

1112

||

Here, SHere, S is transverse field attenuation factor, and S is transverse field attenuation factor, and S |||| is longitudinal one. is longitudinal one.

Usually: SUsually: S > S> S||||

Neutron Spin Filter Based On Optically Polarized Neutron Spin Filter Based On Optically Polarized 33He In a Near – Zero Magnetic FieldHe In a Near – Zero Magnetic Field

Frank Lab. of Neutron Physics, JINR 2001Frank Lab. of Neutron Physics, JINR 2001

ResultsResults::

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.30.00

0.05

0.10

0.15

0.20

0.25

0.30Neutron polarization with 3.4 atm. cell

pola

rizat

ion

neutron energy (eV)

with guide field witout guide field fit for P

He=0.30 ± 0.03

Reflection from magnetic single Reflection from magnetic single crystals or magnetic mirrorscrystals or magnetic mirrors

Polarization in outgoing neutron beam after reflection from magnetic Polarization in outgoing neutron beam after reflection from magnetic single single crystalcrystal arise due to interference between nuclear and magnetic scattering arise due to interference between nuclear and magnetic scattering amplitudes:amplitudes:

nmmnmnn fMffffMsf 21222

Polarization in outgoing neutron beam after reflection from Polarization in outgoing neutron beam after reflection from magnetic mirror magnetic mirror arise arise due to the difference of the refraction coefficients due to the difference of the refraction coefficients nn for two neutron spin states: for two neutron spin states:

Hnn 01

Thus, by adjustment of field Thus, by adjustment of field H,H, for one of spin states the mirror reflection can be for one of spin states the mirror reflection can be realizedrealized..

Both above methods may be effectively used for thermal and cold neutrons (E Both above methods may be effectively used for thermal and cold neutrons (E 0.025 eV) because the wavelength should be of order of a distance between the 0.025 eV) because the wavelength should be of order of a distance between the crystal planes or the thickness of mirror surface coating.crystal planes or the thickness of mirror surface coating.

Angular and polarization correlations Angular and polarization correlations in (n,in (n,) reactions) reactions

Differential cross section of neutron capture by unpolarized nuclei (17 terms):Differential cross section of neutron capture by unpolarized nuclei (17 terms):  

: unit vector along incident neutron momentum: unit vector along incident neutron momentum : unit vector along outgoing : unit vector along outgoing - ray momentum - ray momentum :: unit vector of neutron polarizationunit vector of neutron polarization

...,

43210

ksaksakksakkaad

ndnnnnnn

knk

ns

Terms:Terms:  

  Indexes Indexes ““s,ps,p”” refer to capture of neutron with orbital momenta 0 (s – wave refer to capture of neutron with orbital momenta 0 (s – wave)) and 1 and 1 ((p – wavep – wave)) respectively. Some of terms depend from space parity nonconservation respectively. Some of terms depend from space parity nonconservation

weak matrix element weak matrix element wwpp..

Simultaneous investigation of above correlations can give Simultaneous investigation of above correlations can give information about the parameters of the s – and p - wave information about the parameters of the s – and p - wave

resonances of nuclei and matrix elements of the weak interaction.resonances of nuclei and matrix elements of the weak interaction.

Pppnppssnssii wEEaa ;,,,;,,,

Nuclear PseudomagnetisNuclear PseudomagnetisLet neutron momentum coincides with nuclear polarization. Then, neutron Let neutron momentum coincides with nuclear polarization. Then, neutron forward scattering amplitudes for parallel (+) and antiparallel (-) directions of forward scattering amplitudes for parallel (+) and antiparallel (-) directions of neutron and nuclear spins would be:neutron and nuclear spins would be:

b

I

Ib

Ifbf

12

2

12

1,

Here, Here, bb are the are the coherent lengthscoherent lengths for the correspondent spin states. for the correspondent spin states.

This distinctions means difference between neutron This distinctions means difference between neutron refraction coefficientsrefraction coefficients::

bbII

k

Nrrr

124

2

From other hand, it means rotation of transverse neutron polarization around From other hand, it means rotation of transverse neutron polarization around nuclear one like around magnetic field. The frequency of such nuclear one like around magnetic field. The frequency of such pseudomagneticpseudomagnetic rotation isrotation is

bb

I

I

m

Nw

12

40

s

s

I

Neutron spin is a single direction can be associated with dipole moment:Neutron spin is a single direction can be associated with dipole moment:    If P If P –– invariance holds, then: invariance holds, then:  

Thus, Thus, 0 0. . In case of PNC, In case of PNC, 0 0 if holds CP if holds CP –– symmetry (which implies T symmetry (which implies T –– invariance invariance via CPT via CPT –– theorem). Indeed: theorem). Indeed:    Thus, nonzero neutrons EDM means simultaneous Parity nonconservation Thus, nonzero neutrons EDM means simultaneous Parity nonconservation and Time Reversal Invariance violation.and Time Reversal Invariance violation.Present experimental limit on neutron EDM (ILL):Present experimental limit on neutron EDM (ILL):  

Neutron Electrical Dipole Neutron Electrical Dipole Moment (EDM)Moment (EDM)

nsD

n

P

n

PssandDD

n

CP

n

CPssandDD

cmeD 2610

Space parity nonconseravation (PNC) Space parity nonconseravation (PNC) and time reversal invariance (TRI) test and time reversal invariance (TRI) test

in neutron transmissionin neutron transmission Neutron Neutron forward scattering amplitudeforward scattering amplitude on polarized nuclei on polarized nuclei has a form: has a form:

nnnn kIEksCsIBAf

AA:: P,T P,T –– even strong interaction even strong interaction BB: : P,T P,T –– even strong spin dependent interaction even strong spin dependent interaction (pseudomagnetism)(pseudomagnetism)C C andand E E: P: P––odd, T-even PNC weak interactionodd, T-even PNC weak interaction

IksD nn

DD: : P P––odd, T-odd TRI violation - weak interactionodd, T-odd TRI violation - weak interaction

PNC effects in nuclear reactions are explained using the assumption of mixing of PNC effects in nuclear reactions are explained using the assumption of mixing of compound - states with opposite parities by the compound - states with opposite parities by the weak nucleon - nucleon interactionweak nucleon - nucleon interaction. .

From nowadays experimental data:From nowadays experimental data:

eV10 3 pwsw pp

and expected value ofand expected value of 410P

PT

w

w

Experimental DesignExperimental Design

kI

s

s