Intense UCN production in He-II for nEDM*

  We have been developing a new UCN production method in a  0.8 K He-II, which is placed in a 20 kW spallation neutron source. We are aiming to obtain a UCN density of 800 UCN/cm³ in a nEDM measurement cell for a measurement of 10^-27 e cm.

  We need to remove a γ heating of 5 W in the He-II at a proton beam power of 20 kW so that He-II temperature is kept below 0.8 K to suppress UCN loss in He-II. The heat conducts through the He-II to a ⁴He-³He heat exchanger, which is cooled down by a ³He cryostat.  We have studied quantum effects at the heat conduction in He-II, phonons, potential flow and quantized vortices, and also an acoustic mismatch at the boundary between the He-II and the heat exchanger. 

  We have constructed the second generation UCN source of He-II. Here, we will show how the heat in a UCN production volume is removed through a 85 mm diameter He-II by a 0.7 K ³He cryostat. Phonon heat conduction is enough to cool down the UCN production volume below 0.8 K. But, a TRIUMF-KEK group claims the quantum turbulence and the acoustic mismatch have serious effects on the heat conduction. Here, we will show we can control these effects to keep the He-II temperature below 0.8 K.