Cryogenic system for X-ray Compton scattering measurements of superfluid helium below 2Â K

A cryostat was constructed for high-resolution X-ray Compton scattering measurements at temperature down to 1.7 K, in order to investigate superfluid helium-4. Compton profiles of helium were measured using synchrotron X-rays for gas and liquid phases, respectively. In the measurement of the liquid phase, we succeeded in measuring the Compton profile of the superfluid helium at 1.7 K. Comparison of the results with theoretical calculation reveals importance of many-body effects beyond the mean-field treatment of electron systems.