Neutron spin echo measurements on dense hydrogen gas

We measured the coefficient of self-diffusion D in high-density hydrogen gas at 120 K and 75 MPa by means of the neutron spin-echo technique. Because of the relatively large value of D the energy window of the spectrometer IN11 of the ILL had to be enlarged by means of additional Larmor coils with a small field integral. We find D = 0.0543 nm2ps-1 with an accuracy of 4%. This result is not inconsistent with time-of-flight incoherent neutron scattering measurements performed previously in Delft, but disagrees with NMR spin-echo data. An explanation might be found in the different time and space scales of the various methods.