Minimizing pressure inhomogeneities across large samples under helium pressure media at low temperatures

When using gas pressure media to achieve hydrostatic pressure under a constant pressure and volume, passing through the melting curve ( P × T) of the gas to achieve low temperatures (1.5 K ⩽ T < 30 K) will cause an overall pressure reduction at base temperatures of at most 25 percent of the pressure read while under hydrostatic conditions at the P × T. In-situ inert gas high pressure vessels for neutron scattering for pressures of up to 1.0 GPa and temperatures as low as 1.5 K pose a particular problem due to the P × T phase diagram of the pressure media. A methodology for pressurization to ensure minimal pressure loss as temperature falls below the pressure media phase change, at the same time minimizing pressure inhomogeneity throughout the length of the sample; as well as calibration data are presented within this work.