Accurate, safe and cheap hydrogen detectors that can function under a wide range of conditions are required to detect hydrogen concentrations well below this explosive limit are a vital ingredient for the future hydrogen economy. A Novel way to detect hydrogen that is cheap and safe is using optical detectors based on thin film hydrides. Recently, we considered the use of hafnium as a potential sensing material. We find that it is capable of detecting hydrogen under an extremely wide sensing range. However, the relation between optical transmission and the applied hydrogen pressure is not well understood. Additionally, we find large differences between the use of hydrogen and deuterium as a loading gas. To acquire more knowledge, we propose to study a hafnium nanolayer with neutron reflectometry to get more insights in layer thicknesses and hydrogen content of the hafnium layer.