Relaxation of spin polarizedH3ein mixtures ofH3e

Abstract

We report a first study of the depolarization behavior of spin polarized 3 He in a mixture of 3 He-4 He at a temperature below the 4 He λ point in a deuterated TetraPhenyl Butadiene-doped deuterated PolyStyrene (dTPB-dPS) coated acrylic cell. In our experiment the measured 3 He relaxation time is due to the convolution of the 3 He longitudinal relaxation time, T1, and the diffusion time constant of 3 He in superfluid 4 He since depolarization takes place on the walls. We have obtained a 3 He relaxation time of ∼ 3000 seconds at a temperature around 1.9 K. We have shown that it's possible to achieve values of wall depolarization probability on the order of (1 − 2) × 10 −7 for polarized 3 He in the superfluid 4 He from a dTPB-dPS coated acrylic surface. Hyper-polarized 3 He based on the technique of optical pumping [1,2] has found applications in diverse fields such as in the study of quantum phenomena in low temperature fluids [3] and in the search for violations of fundamental symmetries [4,5,6]. They are also routinely used as polarized neutron spin-filters [7], as effective polarized neutron targets for nuclear and particle physics experiments [8] and for low magnetic field magnetic resonance imaging [9]. All such applications have motivated, as well as benefited from, studies of the relaxation mechanisms of polarized 3 He in gas, liquid and superfluid phases and under different surface conditions. The study of relaxation of polarized 3 He in 3 He -4 He mixtures at low temperatures is, however, of longstanding interest in its own right [10]. The simple atomic structure of 3 He makes a system of 3 He atoms ideal to model correlated fermions.Super-thermal production of Ultra-Cold Neutrons (UCN) from superfluid 4 He has been demonstrated [11] as an efficient way of producing a large number of UCNs. The capability of storing a large number of UCNs following their production is important for experiments studying fundamental properties of the neutron, for example the experiment on the search of the neutron electric dipole moment [6]. In this experiment, the deuterated TetraPhenyl Butadiene-doped deuterated PolyStyrene (dTPB-dPS) coated acrylic surface is chosen for such an application because of the small neutron absorption rate on the surface and its wavelength shifting property. The focus of this work is a first study of polarized 3 He relaxation time in a mixture of 3 He-4 He below the 4 He λ point in a dTPB-dPS coated acrylic cell. Such a study may find applications in the development of cryogenic 3 He magnetometers for experiments where trapping of polarized UCNs is involved as well as in other types of applications where polarized 3 He atoms are employed at low temperatures. At present the feasibility of 3 He magnetometers for UCNs has been studied only at room temperature [12].While a number of experiments [15,16,17,18,19] have reported 3 He longitudinal relaxation times (T 1 ) in mixtures of 3 He-4 He at temperatures similar to our work, the measurements most relevant to ours are [16,18,19]

Relaxation of Spin Polarized $^3$He in Mixtures of $^3$He and $^4$He at $\sim$330 mK

“…The polarized 3 He in our cell is in the bulk liquid 4 He with layers of superfluid 4 He film on the wall surfaces, while in [4,5,6], 3 He atoms are mostly in vapor phase. Our data show that the DPs from the dTPB-dPS coated acrylic cell surface covered by superfluid 4 He remain around 1.0×10 −7 in the temperature range between 0.33 K and 0.65 K, which is close to the value at 1.9 K [24]. Our experiments probe the DPs down to the lowest temperature of 330 mK and the dTPB-dPS coated acrylic surface outperform the surfaces in open geometry systems (Figure 3 (a)), including bare pyrex glass surfaces (Fig.…”

“…In our previous depolarization study of polarized 3 He in superfluid 4 He at 1.9 K [24], we reported a depolarization probability ∼ 1.6 × 10 −7 from a dTPB-dPS coated and cylindrical-shaped acrylic cell. This number may change at lower temperatures.…”

“…The relaxation times are obtained from the exponential decay of the FID signal strength as a function of time corrected by the FID inefficiency. The NMR-AFP system uses an RF amplifier to power the RF coils for the spin flip during the measurement [24]. The RF amplifier is left off during the time intervals between AFP measurements to prevent the polarized 3 He atoms from depolarizing due to the wide band amplified RF noise.…”

“…Since our cell is full of superfluid 4 He during the measurement, the surface to volume ratio, S/V = 2.1, is a constant at different temperatures. Figure 3 (a) also includes the data at 1.9 K [24]. The other DPs in the figure are extracted from the data in [4,5,6] by using the 3 He mean speeds at the corresponding temperatures, the relaxation times and the surface to volume ratios in the open cell geometries and sealed cells.…”

Optically polarizedHe3

“…Motivated by an experiment to measure the neutron electric dipole moment, new relaxation studies in liquid mixtures of 3 He and 4 He have been performed for the relevant polymer-coated materials at cryogenic temperatures (Ye et al , 2008, 2009). …”