R. C. MILLER AND P. KUSCH In 1947, Estermann, Simpson, and Stern measured of the beam by placing a 0.005-cm slit at each end of velocity distributions in cesium beams by use of the velocity selector; but it was impossible to observe the free fall due to gravity as a velocity selector. They a velocity distribution with this arrangement, since observed a pronounced deficiency of atoms on the low- vibration caused by the rotating velocity selector pro- velocity side and noted that the discrepancy increased duced serious unsteadiness in the beam. Sufficient en- deficiency to collisions near the oven slit. An order of t gement of the collimation slits to reduce variations the beam intensity consequent to vibration reduces magnitude calculation is given which indicates that the collimation of the beam to the point where the the deficiency might be explained on the basis of slits no longer serve to prevent the detection of atoms Cs- Cs collisions in the neighborhood of the slit. which may have undergone reflections from the wal The most recently reported results on velocity of the slots and the slits were eventually removed distributions are those of Bennett and Estermann who However, before the slits were removed, an attempt measured potassium velocity distributions. Their ex- was made to reflect potassium atoms from the wall of perimental velocity distributions were in good agree- one of the two straight slots when the rotor was at rest ment with theoretical distributions on the high-velocity There was no evidence at all of specular reflection. The side of the intensity maxima, but there was always a diffused reflection and scattering of atoms was so small deficiency of atoms on the low-velocity side, a deficiency that the intensity intercepted on the detector when which increased with increasing oven pressure and with not in the direct line of the beam was less than 0.1 a decreasing velocity. Even at the lowest oven pressure percent of the beam and hence not important in the at which experimental results were obtained, 1.8X10-3 present work. mm of mercury, the intensity was about 90 percent of the theoretical intensity for atoms at a velocity one-half SCATTERING FROM RESIDUAL GAS the velocity of the intensity maximum. The deficiency The theoretical velocity distribution to be observed of atoms on the low-velocity side was greater if the oven at the detector has been derived under the assumption slits were rectangular than if they were knife edges. that a Maxwellian distribution occurs in the oven and The velocities at which the experimental intensity that the slit of the oven is an ideal aperture. It is inter maxima occurred were usually found to agree within esting to investigate the occurrence of a possible distor tion of the velocity distribution due to velocity de- In the present work, experimental velocity distribu- pendent scattering of the beam atoms by the residual gas tions of both potassium and thallium were found to be in good agreement with theoretical velocity distribu- velocity b in a single Maxwellian gas islmolecule with in the apparatus. The mean free path of tions. It has been shown that the experimental and theoretical distributions, for small beams, do not differ L,=a1/nIS by more than one percent of the maximum intensity over the entire range of the measured distributions extending from as low as 0.2 to as high as 1.8 times ep(-)+(2+x)Je(-y)y,⑨ the velocity of the intensity maximum. Furthermore the velocities at which the intensity maxima occur where a1=(1/2)m/(kTi)in which the subscript 1 agree within 0.5 percent with velocities calculated refers to the scattering gas, n is the density of the gas from the measured oven temperature. These results a is equal to a1v and S is the collision cross-sectional area provided the best agreement to date between observed To make any calculations from Eq. (9), one has to distributions and theoretical distributions calculated assign a value to an and the cross-sectional scattering from the assumption that the beam is a consequence area. In the case at hand, the scattering gas probably of a Maxwellian distribution within the oven which consists of air plus all sorts of condensable vapors including pump oil dissociation products. The tempera- ture of the residual gases is not defined but an"effective REFLECTION FROM THE ROTOR SLOTS temperature may range from slightly above room tem- It has been assumed that all molecules which strike perature to liquid nitrogen temperatures. Since the the wall of the slots in the velocity selector are removed from the beam. If the beam were very narrowly colli- increasing rotor speed, there may be a change in com- mated so that the detector would intercept only atoms postion of the gas at different values of xo. Thus it is virtually impossible to calculate Ly for this situation which arrive along a line parallel to the rotor axis and To estimate the magnitude of the effects due to scatter which leave the oven along the same line, the assump- ing, it will be assumed that the residual gas approxi tion would be justified. However, no collimation other mates air at room temperature and that the cross than that provided by a source and detector of small sectional scatter a is 4.0X10-14 cm". This area width was provided in the original design of the appa-I6Eh.Ke ratus. An attempt was made to improve the collimation Company, hanard. kinetic ases(McGraw-Hill Book