A Vacuum Cold Bore Test Section at the CERN ISR
A 2 m helium cooled vacuum test section has been inserted into the CERN ISR to investigate problems which could be encountered in future cold bore proton machines. The UHV bore is cooled from the helium bath via a variable pressure gas filled space which enables operation at any temperature between 2.5 K and ambient. Temperature and degree of surface contamination can be remotely controlled. First observations from nominally clean and from hydrogen contaminated surfaces operating at temperatures close to 4.2 K are presented.
Influence of thermal radiation on the vapor pressure of condensed hydrogen (and isotopes) between 2 and 4.5 K
Hydrogen molecules, physisorbed or condensed on cryosurfaces at liquid‐helium temperatures, can be desorbed by thermal radiation. The probability of the process depends on the nature of the cryosurface, the degree of H2 coverage, and the spectrum of the radiation. For a given spectrum the desorption rate is proportional to radiation intensity, and, with a few exceptions, to the absorptivity of the cryosurface. The desorption efficiency of the absorbed 300‐K radiant energy is approximately 7×10−6 for most of the 20 bakeable cryosurfaces tested. A decrease of radiation temperature below 130 K causes a progressive reduction of the desorption efficiency which amounts to a factor of three at 80 K. This variation of efficiency can be fitted to a curve obtained by assuming that a threshold wavelength for desorption exists in the radiation spectrum at 45 μm. The energy carried by a photon of this threshold wavelength is about three times that required for the desorption of a condensed H2 molecule. Precondensed su...
Unidirectional hexagonal rare-earth disilicide nanowires on vicinal Si(100)-2×1
Rare-earth disilicide nanowires grown on vicinal Si(100) with a miscut of 2–2.5° toward the [110] azimuth at 600 °C were studied by scanning tunneling microscopy and compared with those grown on flat Si(001). In contrast to rare-earth disilicide nanowires grown on flat Si(100) surfaces, the nanowires grow unidirectionally along the [01̄1] direction of the vicinal Si(100) surface. Rare-earth disilicide nanowires form bundles composed of single nanowire units on both flat and vicinal surfaces. Yet, on the vicinal surface, the bundle width is comparable to the width of the terrace. The average nanowire length on the vicinal substrate is longer than that on the flat substrate. Scanning tunneling spectroscopy shows that the rare-earth disilicide nanowires have metallic properties.