1316 R, C. MILLER AND P. KUSCH hamber and the other two chambers are the slits shown in fig. 2 A large liquid nitrogen cooled surface is provided in each chamber, to increase the pumping speed of the cold traps for condensable gases. Pressures of 1. 2X10-7 and 1.3X10-7 mm of mercury were obtainable in the detector and rotor chambers respectively, with the rotor at rest The velocity selector was provided with a four-to-one tepup gear built into the mount. Power was trans- mitted to the rotor from the outside of the vacuum envelope by a shaft which rotates in a long, lubricated phosphor bronze bearing. As shown in Fig.2,two holes were drilled perpendicular to the axis of the bearing, one on the high pressure side so the lubricant FIG. 2. Schematic diagram of the apparatus designed could be replenished and one in the middle which was pumped by a mechanical pump. The vacuum seal some values of I, calculated from Eqs. (5)and(6) functioned very well, though there was some tendency Both sets of values have been multiplied by a constant The velocity selector was seldom run over 4000 rpm Eq.(5)equal to 20.00. Table I demonstrates that the at which speed the vacuum in the rotor chamber simple expression, Eg.(5), can be used for calculating creased to about 10-6 mm of mercury. theoretical distributions to be compared to the experi OVENS mental data in the present work, as the average experi mental uncertainty is about one percent of Iu(max) The ovens used in these experiments were much like The intensity at the detector, Io, may be expanded as conventional molecular beam ovens, the main differ greater than or equal to two. Then the leading terms are Since the interpretation of velocity distribution data requires that the beam produced at a wn equilib- I=2yxo4exp(-x02)[1+2 rium temperature, the ovens must be made of high con 137xo /6+730 / 3+...](7) ductivity material to avoid significant temperature gradients. Thus the present ovens were constructed than 2 ae he x terta in values af te catenated rgen of oxygen-free, high-conductivity copper, instead of the customary iron or nickel. Preliminary experiments Finite source and detect in the present work have shown that in the beam not all of whose elements are parallel to the axis hood of 900K, temperature differences of 30Coccur of the rotor. The effect of the finite vertical dimension oven. This temperature difference is roughly six times of the beam is negligible since for an angle of elevation, the estimated accuracy of the measured temperatures B, of a beam element with respect to the axis, the When the same measurements were repeated with a analysis of v cose is made and coso differs only trivially copper oven, the temperature difference was reduced from 1. The center of the admittance curve for non- to 3.5C parallel beam elements which results from the finite Kinetic theory considerations indicate that the horizontal dimensions, or widths, of the oven slit and width of the slit in a direction parallel to the direction detector wire is at either higher or lower velocities of the propagation of the beam must be much smaller than the center of the curve for parallel beam elements. than the mean free paths of the atoms in order to The solution of this problem has been discussed else- eliminate scattering in the neighborhood of the slit where. In the present apparatus, the detector and The defining slits were made of 0.0038 or 0.0025cm source widths are sufficiently small, 2.5X10- cm, so thick steel strips held on the face of the oven with copper hat the widths have a negligible effect on the shape of strips, whose edge extended to within about 0.25 mm the velocity distributio of the slit itself so that the orifice was determined b GENERAL DESCRIPTION OF APPARA the thin steel strips. At a nominal temperature of 900K, a thermocouple inserted in one of the copper strips The vacuum envelope consists of the oven, rotor and indicated a temperature 0.7 C lower than the tenlfthe detector chambers. Other than the pumping connec- ture measured with a thermocouple mounted tions, through which a free flow of gas between chambers front part of the oven as shown in Fig 3. This tempera- cannot occur, the only openings between the rotor ture difference is small compared to the estimated