study of T 1 relaxation time A proposal to test T 1 using a dilution fridge and SQUID NMA at Royal Hollow University,London

neutron EDM temperatures, concentrations and magnetic field  Temperature : K  Relative Concentration of 3 He : x=  Magnetic field :  T, Hz T1 relaxation time 1/T 1 = 1/T 1 Binh + 1/T 1 bulk + 1/T 1 wall

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

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 3 He 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 T 1 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, H 2 coatings were used with a dilute mixture and here wall relaxation dominated, the measure total T 1 being around 400 s at 450 mK.”

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 -> less sensitivity to the local fields of the wall magnetic/paramagneic impurities C.Lusher,PhD

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 3 He concentrations 0.1, 0.01 and At 1.5 K : T 1 = 50 sec, no dependence on concentration -point dependence relaxation rate is pure wall effect that is defined by diffusion time  =R 2 / D

What has been measured with plastic?  Nylon cell, open geometry “The spin diffusion Coefficient of 3 He in 3 He- 4 He solutions” D.C. Chang and H.E. Rorschach, JLTP 10, 1973 T 1 and spin diffusion coefficient were measured by spin-echo technique Field: 30.3 MHz, 1 T, T 2 =100 sec cell volume 0.1 cc Gas handling system: glass and stainless steel with LHe trap for Helium sample 3 He concentrations 5, 9, 14 and 24% At 0.9 K and 5 % : T 1 = 25 sec, Below -point T 1 is sensitive to concentration relaxation rate is due to wall effect defined by diffusion time  =R 2 / D

Possible Experiments at Royal Holloway University proposed by Chris Lusher We would propose to measure T 1 under conditions closer to that in the final EDM experiment. By using DC SQUIDs we can measure T 1 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 , however in the low concentration regime (where wall relaxation dominates over bulk effects) the observed relaxation time is expected to be concentration independent, since (n 3B /n 3W ) should be concentration independent. It should also be relatively temperature independent since there should be no bound state for the 3 He near the surface. The x 3 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. T 2 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.

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 = 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 T 1. However the possibility of looking at smaller concentrations using a tuned spectrometer exists.

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 don’t know  Wall cleaning: can’t be baked  can be only pumped out in vacuum Best result with glass is 500 sec at 0.5K, 0.5 cm 3 Nylon demonstrate 25 sec at 1K

Open geometry and wall contamination  Open geometry and “dirty” cell Both demonstrate a drop at -point Both demonstrate T 1  10 3 sec at 4.2K while the “clean” cells usually have T 1  10 5 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

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?