Transcript Applications of polarized neutrons

Applications of polarized
neutrons
V.R. Skoy
Frank Laboratory of Neutron Physics, Joint Institute for Nuclear
Research141980 Dubna, Moscow Region, Russia
Currently at
Pohang Accelerator Laboratory, Pohang University of Science and
Technology, Pohang, 790 – 784, Korea
Main fields of Application
• Nuclear structure parameters
Angular and polarization correlations in neutron scattering and capture reactions.
Scattering lengths and nuclear pseudomagnetism.
• Tests of fundamental symmetries
Space parity nonconservation in transmission, fission and capture reactions with
unpolarized nuclei. Time reversal invariance test in neutron reactions with polarized or
aligned nuclei.
• Fundamental properties of neutron
Anomalous neutron dipole moment.
• Magnetic properties of matter
Investigations of domen structure of ferromagnetic. Dynamics of the phase transitions.
Definition of Polarization
P
N  N
N  N
P=0
P = 6/12 = 0.5
Polarized neutrons
production
• Transmission through polarized targets
Polarization via Scattering (n,p) reaction
Polarization via Capture 3He(n,p) reaction
• Reflection from magnetic single crystals or magnetic mirrors
(cold and thermal neutrons)
Polarization via Scattering
For (n,p) scattering cross section
below 100 keV
  20  

I

k
Proton Target
Dynamical Nuclear
Polarization
For polarization via
scattering LMN single
crystal was initially
used.

 we  w p
Now some sorts of
alcohol and
polyethylene are used.
Dynamical Nuclear
Polarization method
which requires:
T<1K

w p
we
we  w p 
RF - Field
H > 1 Tesla
This method can be used for
polarization of other nuclei.
RF - pumping
For example, La in LaAlO3
Polarization via Capture
For 3He(n,p) capture cross section

I
tot  
and
tot  1/ En

k
3He
Target
Optical polarization of the
Noble Gases
The high nuclear polarization of odd isotopes of any noble gases can be built
by means of the following by two – stage process
1.
The circularly polarized resonance laser light optically pumps an alkali
– metal vapor (usually Rb), aligning the optical electron spins along the
quantization axis (direction of laser beam).
2.
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 these nuclei via the hyperfine interaction during the
collisions (the spin – exchange process).
Optical Pumping of Rb
atoms
But in presence of buffer gas…
1.
Helmholtz Coils (20 – 40 Gauss)
2.
RF – Coils (NMR)
3.
Glass Cell with 3He (1 – 10 atm.)
4.
Pick – Up Coil (NMR)
5.
Laser Beam (795 nm, > 15 Watt)
6. Photodiode
Typical Installation
Design
Relaxation (decay) of 3He polarization
The origin of 3He polarization relaxation are:
1.
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.
2.
Inhomogeneity of the external magnetic fields.
The last characteristics time can be expressed as:
 H  
1

 Dng 

Tm
 H || 
2
If one put a cell inside the cylindrical magnetic shield, then
2
1
1  S|| 
1







Tmsh Tm  S   Tm
Here, S is transverse field attenuation factor, and S|| is longitudinal one.
Usually: S > S||
Neutron Spin Filter Based On Optically Polarized
3He In a Near – Zero Magnetic Field
Frank Lab. of Neutron Physics, JINR 2001
0.30
Neutron polarization with 3.4 atm. cell
with guide field
witout guide field
fit for PHe=0.30 ± 0.03
0.25
polarization
0.20
0.15
Results:
0.10
0.05
0.00
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
neutron energy (eV)
0.9
1.0
1.1
1.2
1.3
Reflection from magnetic single
crystals or magnetic mirrors
Polarization in outgoing neutron beam after reflection from magnetic single crystal
arise due to interference between nuclear and magnetic scattering amplitudes:
1
 
2
   f n  sn  M  f m   f n2  f m2  Mf m f n
2
Polarization in outgoing neutron beam after reflection from magnetic mirror arise
due to the difference of the refraction coefficients n for two neutron spin states:
n  1  n0    H 
Thus, by adjustment of field H, for one of spin states the mirror reflection can be
realized.
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
crystal planes or the thickness of mirror surface coating.
Angular and polarization correlations
in (n,) reactions
Differential cross section of neutron capture by unpolarized nuclei (17 terms):
 
dn,  
  
 
 
 a0  a1 k n  k    a2 sn  k n  k    a3 sn  k n   a4 sn  k    ...
 d
kn : unit vector along incident neutron momentum
k  : unit vector along outgoing  - ray momentum

sn : unit vector of neutron polarization

Terms:

ai  ai E s ,  ns ,  s , s ; E p ,  np ,  p ,  p ; w P 
Indexes “s,p” refer to capture of neutron with orbital momenta 0 (s – wave) and 1
(p – wave) respectively. Some of terms depend from space parity nonconservation
weak matrix element wp.
Simultaneous investigation of above correlations can give
information about the parameters of the s – and p - wave
resonances of nuclei and matrix elements of the weak interaction.
Nuclear Pseudomagnetis
Let neutron momentum coincides with nuclear polarization. Then, neutron
forward scattering amplitudes for parallel (+) and antiparallel (-) directions of
neutron and nuclear spins would be:
f   b ,
2I
 1

f   
b 
b 
2I  1 
 2I  1
Here, b are the coherent lengths for the correspondent spin states.
This distinctions means difference between neutron refraction coefficients:
r  r  r 
4N
k2
I
b  b 
2I  1
From other hand, it means rotation of transverse neutron polarization around
nuclear one like around magnetic field. The frequency of such pseudomagnetic
rotation is 
s

s

I
w0 
4N I
b  b 
m 2I  1
Neutron Electrical Dipole
Moment (EDM)
Neutron spin is a single direction can be associated with dipole moment:


D    sn
If P – invariance holds, then:
 P

D  D
and
 P 
sn  sn
Thus,   0.
In case of PNC,   0 if holds CP – symmetry (which implies T – invariance
via CPT – theorem). Indeed:
 CP 
D  D and
 CP 
sn   sn
Thus, nonzero neutrons EDM means simultaneous Parity nonconservation
and Time Reversal Invariance violation.
Present experimental limit on neutron EDM (ILL):
D  10 26 e  cm
Space parity nonconseravation (PNC)
and time reversal invariance (TRI) test
in neutron transmission
Neutron forward scattering amplitude on polarized nuclei has a form:
 
 
  
 
f  A  BI  sn   C sn  kn   E I  kn  Dsn  kn  I 
A:
P,T – even strong interaction
B:
P,T – even strong spin dependent interaction (pseudomagnetism)
C and E: P–odd, T-even PNC weak interaction
D:
P–odd, T-odd TRI violation - weak interaction
PNC effects in nuclear reactions are explained using the assumption of mixing of
compound - states with opposite parities by the weak nucleon - nucleon interaction.
From nowadays experimental data:
w p  s w p p  103 eV
and expected value of
w PT
 10 4
wP
Experimental Design

s

s

I

k