Transport of 3He for the nEDM experiment at the SNS
The neutron electric dipole moment (nEDM) experiment at the Spallation Neutron Source (SNS) will measure the electric dipole moment (EDM) of the neutron with a precision of 5.4\times10^{-28}\,\mathrm{e\cdot cm}. This experiment utilizes polarized 3He in superfluid 4He to detect neutron precession in a 30 mG magnetic field and 75 kV/cm electric field via the spin dependent reaction \mathrm{n}\mathrm{+^{3}He}\rightarrow\mathrm{p}\mathrm{+^{3}H}+764\mathrm{\,KeV}. Polarized 3He is produced by an atomic beam source (ABS) and incident on a free surface of superfluid helium in the “injection volume”. A film burner and baffle is used to contain superfluid film climbing the walls of the volume, preventing helium vapor from scattering the incoming beam. From the injection volume 3He diffuses into an intermediate volume. From this point heat flush is used to transport the polarized 3He into and (once depolarized) out of the measurement cells. The heat flush mechanism transports the 3He by utilizing collisions between phonons, generated in the superfluid helium by establishing a thermal gradient, and the 3He. Since phonons have no spin these collisions preserve the polarization of the 3He. A test of the heat flush mechanism has been conducted using a capacitive pressure sensor to measure the change in concentration resulting from using heat flush to transport 3He from one volume of superfluid to another connected by a small diameter pipe. The system initially had an uniform 3He concentration of approximately 0.8 ppm. This test demonstrates that heat flush can be used to transport 3He in the temperature range of 350-550 mK, as well as providing a measurement of the phonon dominated diffusion constant of 3He in this temperature region. A value of D=\frac{2.56_{-.09}^{+.14}}{T^{7}}f(T) was found. The measured constant is 3 times larger than what is calculated by Baym, Beck and Pethick in [27, 28]. Also it has been demonstrated that a film burner can be used to control superfluid film, and prevent vapor that would otherwise scatter the polarized 3He beam from the ABS into the injection volume. At 310 mK, 3.4 mW is required to contain the vapor within the baffle.
Helium 3, neutron, electric dipole moment, heat flush
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign