study of T1 relaxation time

1
study of T1 relaxation time

• A proposal to test T1 using a dilution fridge and
  SQUID NMA at Royal Hollow University,London

2
neutron EDM temperatures, concentrations and
magnetic field

• Temperature 0.3 -0.5 K
• Relative Concentration of 3He x10-10
• Magnetic field 1 - 0.1 ?T, 32 - 3.2 Hz
• T1 relaxation time
• 1/T1 1/T1 Binh 1/T1bulk 1/T1wall

3
Magnetic field inhomogeneities

• Relaxation of spins due to field inhomogeneities
  in gaseus samples at low magnetic fields and low
  pressures
• G.D.Gates,S.R.Shaefer,W.Happer, PR A 37, 1988,
  pp.2877
• relaxation rate at high pressure is scaled as
• That is proportional to the diffusion
  constant D
• In contrary, at low pressures it is inversely
  proportional to the diffusion constant D

4
Measurements of bulk relaxation

• Pyrex cell, Cs coating,optical pumping at room
  temperature
• This is a most recent result
• N.Piegay,G.Tastevin, JLTP 126 (2002)157
• Field 2 mT, 64 kHz
• cell 0.5 cc
• 3He concentrations from 0.3 to 34
• T 1.1 K
• Linear dependence on concentration
• Claim No dependence on the surface/volume ratio
• Claim relaxation rate is pure bulk effect
  extrapolation to the zero concentration gives 10h
• But the actual T1 measured at 0.3 were 3.5 h,
  2.5 h and ? 1.5h in different cells

In this paper they do not explicitly mention how
the cells were cleaned prior to filling, however
in earlier work the cells were evacuated and
degassed above 150?C for several days before
introducing the cesium and sealing in the gas
mixture. Piegay and Tastevin say that they
present data from the three best cells. This
suggests that wall relaxation can vary. In the
paper cited for experimental detail, H2 coatings
were used with a dilute mixture and here wall
relaxation dominated, the measure total T1 being
around 400 s at 450 mK .
5
Wall relaxation

• Theory predicted no substrate states for He-3
  impurities
• Impurity dinamics in boson quantum film,
  B.E.Clements, E.Krotscheck,M.Saarela,
• Phys Rev B 55 (1997),5959
• Ballistic movement of He-3 implies a very short
  correlation time -gt less sensitivity to the local
  fields of the wall magnetic/paramagneic
  impurities
• C.Lusher,PhD

6
What has been measured with plastic?

• Stycast 1266
• This is a most pessimistic result
• Los Alamos group, PR B 37,N4, 1988
• Field 3T
• cylindrical cell 0.2-in r. x 0.62-in
• 3He concentrations 0.1, 0.01 and 10-3
• At 1.5 K T1 50 sec,
• no dependence on concentration
• ?-point dependence
• relaxation rate is pure wall effect that is
  defined by diffusion time
• ?R 2/ D

7
What has been measured with plastic?

• Nylon cell, open geometry
• The spin diffusion Coefficient of 3He in 3He-4He
  solutions
• D.C. Chang and H.E. Rorschach, JLTP 10, 1973
• T1 and spin diffusion coefficient were measured
  by spin-echo technique
• Field 30.3 MHz, 1 T, T2100 sec
• cell volume 0.1 cc
• Gas handling system glass and stainless steel
  with LHe trap for Helium sample
• 3He concentrations 5 , 9 , 14 and 24
• At 0.9 K and 5 T1 25 sec,
• Below ?-point T1 is sensitive to concentration
• relaxation rate is due to wall effect defined
  by diffusion time
• ?R 2/ D

8
Possible Experiments at Royal Holloway
Universityproposed by Chris Lusher

• We would propose to measure T1 under conditions
  closer to that in the final EDM experiment. By
  using DC SQUIDs we can measure T1 down to very
  low magnetic fields.
• The sample cell, prepared at HMI, would be
  mounted on a cryostat at RHUL, allowing
  measurements down to 300 mK.
• We will use an open geometry with a high vacuum
  gas handling system.
• There are no optical pumping facilities at RHUL,
  however since the DC SQUIDs are extremely
  sensitive detectors of magnetic flux we can
  detect lower concentrations of thermally
  polarized 3He than can be obtained using
  conventional NMR techniques.
• We would use much higher 3He concentrations than
  10-10, however in the low concentration regime
  (where wall relaxation dominates over bulk
  effects) the observed relaxation time is expected
  to be concentration independent, since (n3B/n3W)
  should be concentration independent.
• It should also be relatively temperature
  independent since there should be no bound state
  for the 3He near the surface.
• The x3 dependence should be checked since Piegay
  and Tastevin say they observed some concentration
  dependence of wall contributions in their
  experiments. The concentration dependence could
  relatively easily be measured given the open
  geometry of our proposed set-up. The sensitivity
  of the spectrometer is such that a signal to
  noise of 1 in a single shot could be obtained at
  1 K and 50 k Hz for a concentration of 10-3 for
  a right circular cylinder of 10 mm length and 10
  mm diameter.
• Our dilution refrigerator is conventional and
  therefore it is not non-magnetic. However we do
  not expect field inhomogeneities to be a problem
  in low fields. T2 might well be limited by
  diffusion in a gradient, but the upper limit on
  T2 will be T1.
• We would measure the longitudinal relaxation
  times as a function of frequency, temperature and
  concentration in order to put a limit on the
  relaxation times that could be achieved with the
  deuterated plastic sample chamber in the EDM
  experiment. Cryogenic coatings e.g D2 could also
  be investigated.

9
Predicted sensitivity of the DC SQUID
spectrometers

• We have calculated the expected signal to noise
  in a dilute solution for both a tuned and a
  broadband SQUID system. The sample is considered
  to be a right circular cylinder. Numbers as
  follows for thermal polarizations
• Broadband system at 500 kHz, concentration 10-5,
  d 10 mm (right circular cylinder), T 1K,
  signal to noise of 3.6 (T2)1/2 in a single shot.
  Proportionally higher concentrations would be
  required to go to lower magnetic
  fields/frequencies. For example at 50 kHz with a
  T2 of 1 ms a signal to noise of 1 would be
  obtained in a single shot from x3 10-3. Since
  the surface area would be made large to reduce
  the observed T1 then significant improvement
  could be obtained by signal averaging.
• For the tuned system at 1 MHz, concentration of
  10-5, d 10 mm (right circular cylinder), T
  1K, a signal to noise is 38 (T2)-1/2 should be
  obtained in a single shot.
• We would probably use a broadband set-up in order
  to measure the frequency dependence of T1.
  However the possibility of looking at smaller
  concentrations using a tuned spectrometer exists.

10
Comparison of glass with Plastic walls

• Substrate potential is not homogeneous, surface
  is rough, density is lower then glass - probably,
  He-3 can be trapped?
• Gases easy diffuse in at room temperature,
  therefore plastic can be used only at low
  temperatures, porosity and diffusion at low
  temperatures- we dont know
• Wall cleaning cant be baked
• can be only pumped out in vacuum

• Best result with glass is 500 sec at 0.5K, 0.5
  cm3
• Nylon demonstrate 25 sec at 1K

11
Open geometry and wall contamination

• Open geometry and dirty cell
• Both demonstrate a drop at ?-point
• Both demonstrate T1 ? 103 sec at 4.2K while the
  clean cells usually have T1 ? 105 sec
• Therefore, preliminary tests at 4.2K could be
  very useful to give some ideas about quality of
  the sample and cleaning procedure but real
  estimation for EDM could come only from low
  temperature measurement

12
Summary

• August, 2003 we can provide some samples to
  TUNL. Samples can be plastic foil coated with d-
  and h-coatings
• September-Octobe-November - development of the
  coating technic for a small cell and gas handling
  system
• We can do test of the chemical content (element
  traces) at HMI
• Magnetic susceptibility?