Helium mean free path

Research Centers (IUCRC), 24 395 Inelastic mean free path (IMFP), 24 87 Inert fluids, 11 877 properties of, 11 879 Inert gas dilution, 11 456 Inert gases, 13 456 17 376-377. See also Helium- group elements Noble gases narcotic potency and solubility of, 17 377 Inert gas generators, 17 280 Inertial confinement fusion targets, microcapsules as, 16 460 Inertial impaction, in depth filtration theory, 11 339... 

Diffraction, by X-rays or neutrons, has been the standard method for determining the structures of crystals. The mean free path of X-rays and neutrons is very long, and thus is not sensitive to surfaces. To probe the structures of surfaces, the probing particles must have a very short mean free path in solids. Two methods are extensively used for determining surface structures low-energy electron diffraction (LEED) and atomic-beam diffraction. A helium... 

The thermosphere is the thin outer layer of our atmosphere extending from the mesopause near 80 km. altitude out to the exosphere, some several thousand kin. altitude, where the mean free path is sufficiently long to allow escape of atomic hydrogen and helium and atmospheric capture of coronal gas constituents. In the lower thermosphere, heated by solar ultraviolet and x-radiation, the temperature increases rapidly with altitude, with temperatures above 400 km. varying between about 700° and 2100°K., depending on solar activity. 

For helium at room temperature and 1 atm pressure, the average distance between collisions (the mean free path) is only about 2 X 10-7 m, or 1000 atomic diameters, and there are approximately 1010 collisions per second. For a larger O2 molecule, the mean free path is about 6 X 10-8 m. 

Figure 10.10 Mean-free-path X of helium, air and carbon dioxide as a function of the pressure p and at the temperature T = 293 K. From [Ard64].

Calculate the intermolecular collision frequency and the mean free path in a sample of helium gas with a volume of 5.0 L at 27°C and 3.0 atm. Assume that the diameter of a helium atom is 50. pm. 

Optical methods have also been used (cf. section 9.2). Anderson and Sabisky (1968) generated a warm band of phonons by continuous saturation of the thuhum resonance at one end of a crystal of Srp2 containing 0.02%Tm at frequencies between 26 and 38 GHz. Diffusion of the warm phonons through the crystal was monitored at points 2 mm apart by measurements of the spin temperature, deduced from observations of the paramagnetic circular dichroism at 580 nm. The results were interpreted in terms of a phonon transmission coefficient into the helium bath 4% at 1.3 K, and a mean free path determined by diffuse scattering from the crystal surfaces. 

As shown in Fig, 4, the flashover voltage declines with increasing temperature, and this effect is greater the greater the electrode distance is. This is probably due to the decrease of the density of the helium with increasing temperature at constant pressure, and with this, a greater mean-free path. 

For instance, the collision cross-section of a benzene molecule (0.88 nm ) is about four times greater than that of a helium atom (0.21 nm ), and at the same pressure and temperatiue its mean free path is four times shorter. 

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