Speed ratios greater than 1000 and temperatures less than 1 mK in a pulsed He beam

Speed ratios v/{Δ}v greater than 1000 and temperatures less than 1 mK have been achieved for the first time in a gas expansion into vacuum. The pulsed-beam source used in these experiments had an open time of 150 {μ}sec, a nozzle diameter of 0.125 mm, and was operated with pure He gas at 294 K and pressures between 34 and 136 atm. Speed distributions for the collimated He beam were measured by time of flight. It is possible that such temperatures may be low enough to probe the low-energy limiting behavior predicted by Wigner in 1948 for the single-collision quantum dynamics of atoms and molecules.