The α–β Transition of Nitrogen

It is well-known that, aside from its triple point, nitrogen exhibits a solid-to-solid transition at about 35 K that is of some interest as a secondary reference temperature. During the recently published highly accurate measurements of the triple point of nitrogen (Metrologia 43, 435 (2006)), an extensive study was made also of the solid alpha-beta transition of nitrogen, using both the continuous heating method and the pulse-heating method. This transition is of significantly lower quality than the triple point of nitrogen. A very high thermal resistance and a large time constant characterize the transition. Therefore, even the determination of the self-heating of the thermometer requires a very long time. A value of T 90 = 35.620 K with an expanded uncertainty U = 8 mK for the coverage factor k = 2 was found, differing by +6 mK from the published CCT-recommended value. The reproducibility of the value was better than +/- 5 mK. In addition to the temperature value found for the transition, a comparison is made with previous measurements on this point, and an overview is given of the available information about it.