An attempt toward dynamic nuclear polarization for liquid 3He

1
An attempt toward dynamic nuclear polarization
for liquid 3He
17/Nov./2005 _at_ PST2005
Takahiro Iwata Yamagata University

• 1. Motivation of the study
• 2.Polarizing 3He in dense form
• 3. DNP for liquid He3
• 3.Doping with free radical
• 4. ESR signals
• 5. Summary

2

• Polarized 3He targets have been employed in
  various scattering experiments
• Only neuron is polarized in 3He
• Good target for the study of neutron
  characteristics
• Realized by the optical pumping technique
• applied only for gas
• Due to its gaseous form, the density is limited.
• Its application is also limited
• Polarized 3He in dense form will open a door to
  extended applications.
• not only in particle physics, but also in other
  fields ( e.g. medical applications, condensed
  matter physics, chemistry, )

3
Possible ways for polarizing 3He in dense form

• Brute force method
• 55 polarization obtained in solid at 6.6T, 6mK
  and 30 bar, G.Bonfait et al., Phys.Rev.Lett. 53
  (1984) 1092
• Polarized liquid is also obtained by quickly
  melting the polarized solid.
• However, its application is limited due to the
  extreme condition.
• Dynamic Nuclear Polarization (DNP)
• Direct coupling from electron system to 3He,
  Delheiji et al. in 1990
• diluted paramagnetic centers in liquid He3
• no polarization enhancement obtained
• Coupling between 3He and polarized material with
  large surface area

4
Coupling between He3 and polarized material

• Nuclei polarized by DNP in material with large
  surface area
• Polarization transfer to 3He in solid or liquid
  on the surface
• A.Shuhl et al.,Phys.Rev.Lett. 54 (1985) 1952,
• Coupling to 19F in Teflon beads(d2000A)
  polarized by DNP,
• originally existing paramagnetic centers in
  Teflon
• enhancement factor 2.0 for 3He
• L.W.Engel and K.Deconde Phys.Rev. 33 (1986) 2035,
• DNP of Liquid 3He in powdered charcoal
• originally existing paramagnetic centers in
  charcoal
• enhancement factor 1.18 for 3He
• B.van den Brandt et al. (PSI-group), NIM A 356
  (1995) 138-141
• beads of Polyethylene, Teflon, Zeolite
• Doping of free radical (TEMPO)
• small polarization (P2.5) obtained with Teflon
• 3He NMR signal changed (PE case)
• Doping was not successful for Teflon and Zeolite

5
Development of DNP for polarizing He3 in liquid

• Our idea
• Direct coupling between a free radical and 3He
• The free radical is embedded into porous
  material.
• The porous material is filled with Liquid 3He
• Coupling between the free radical and the 3He is
  induced by microwave.
• Diffusion of 3He in the material would help the
  spin diffusion of 3He.

6
The key issue

• One of the key issues
• Embedding a free radical in porous materials
• The free radical molecules
• should be firmly trapped
• should be well dispersed
• Matching the cavity size of the porous material
  to the free radical molecule.
• NaY type zeolite with a combination of TEMPO
  free radical.

7
Zeolite and TEMPO

• NaY type zeoltie
• Cavity(supercage)
• 13A(max. dia.)
• 7.4A(window dia.)
• 4.7x1019 cavities/g
• porosity 6
• TEMPO (2,2,6,6-tetramethyl-piperidinyl-1-oxyle)
• Melting point 36 oC.
• Boiling point 67 oC
• Molecule size 68A
• 3He
• atomic radius 1.5A

NaY zeolite (Na56) AL56Si139O384
supercage
Si or Al
double T6-ring
sodalite cage
TEMPO
8
Doping process
The amounts to give 1.6 x 1019 spins/cc, ¼ of
super cages occupied with TEMPO
zeolite(7510mg) activated at 500 oC for 8
hours TEMPO(24.8mg) n-pentane(300ml) boiling
point 36 oC
evaporating n-pentane
in draft chamber
stirred for 8 hours
stirrer
This method is used in studies of unstable
radicals
9
ESR signals
TEMPO in zeolite

• ESR signal of TEMPO in zeolite
• a little broader than that in ethanol
• peaks still separated
• TEMPO molecules are dispersed at some level

TEMPO in ethanol
10
Stability of TEMPO in zeolite
Measurements of intensity variation of the ESR
signal
Intensity of the ESR signal in the air at room
temperature
Intensity of the ESR signal in vacuum at room
temperature
in the air
in vacuum
TEMPO is firmly trapped in zeolite
11
Stability of TEMPO in PE

• PE foil(0.1 mm thick) doped with TEMPO by
  diffusion
• The ESR intensity decreases with a time constant
  of 5 hours
• Reasons
• TEMPO trapped in the amorphous part
• PE molecule movable at room temp. (Tg205K)
• ? enhance diffusion
• evaporation from surface
• Zeolite case
• TEMPO trapped in the supercage which is a part
• of firm structure.

Intensity of TEMPO in PE in the air at room
temperature
12
Next step

• Filling the zeolite with liuid 3He and trying to
  do DNP
• _at_2.5T, 0.6K
• jobs
• installing the sample cell into the cryostat
  (final assembly needed)
• setting the 3He gas handling system (almost
  ready)
• NMR system for 3He (tuning required)
• system for DNP (ready)

13
Summary

• DNP for 3He in dense form will open the door to
  various applications.
• DNP for liquid 3He is pursued through the direct
  interaction between 3He and a free radical
  molecule embedded in cavities of zeolite.
• We have prepared the zeolite doped with TEMPO
  free radical.
• Being well dispersed, the TEMPO molecules are
  firmly trapped in zeolite.
• We are ready to make DNP for liquid 3He in the
  zeolite.

14
Backup Slides
15
polarized 3He circulation scheme
16
Characteristics of the zeolite for the test

• 320NAA
• Cation type Na
• SiO2/ALO3(mol/mol)5.5
• Na2O(wt)12.5
• U.C.C. by ASTM 24.63
• NH3-TPD(mmol/g) -
• Surface Area (BET,m2/g) 700
• Crystal Size 0.3 micro meter
• Mean Particle Size 6 micro meter
• density 1.38g/cc
• lattice constant 24.2-25.1 A
• supercage density 6.6x1019 cages/cc