Windowless solid hydrogen target

A windowless solid hydrogen target has been successfully developed for RI-beam-induced nuclear reactions in the RIKEN RI Beam Facility. Hydrogen crystals of high quality were grown reproducibly in a cell bored in a 10 mm thick pure copper plate which was in direct contact with a liquid helium reservoir. Normal hydrogen gas was crystallized directly on the cell wall between 4.7 and 7.3 K. The diameter of the crystal was 25 mm and the thickness was chosen to be either 5 or 10 mm. After crystallization, sidewalls of the cell were separated from both the crystal and cell plate, and removed to a remote position inside a cryostat. Thus, the crystal was self-supported in the cell without any extra material in its neighborhood. No damage was observed in the separating process. The observed hydrogen pressure indicated the crystal temperature of 4.3 K, when the liquid helium reservoir was at 4.2 K, in agreement with the temperature estimated from the heat balance in the crystal. It shows that we can put the crystal temperature close to the reservoir temperature, though we could not confirm the crystal temperature when the reservoir temperature was reduced to 3 K. Hydrogen sublimation rate was calculated from the vapor pressure and pumping condition. The sublimation loss is negligibly small if the crystal is held at 3 K.