Potassium Beam Research Articles

XII.—The Hexapole Magnet as a Velocity Selector and Polarizer for Neutral Spin-½ Particles

SynopsisThe general paraxial particle-optic properties of the hexapole field are summarized and lens formulae derived. Formulae are given for the transmission as a function of particle velocity and system geometry for geometries appropriate to the case of neutral atom beams and of slow neutron beams respectively. These formulae are used to evaluate the hexapole magnet as a velocity selector and polarizer for atomic beams and as a spin polarizer for neutron beams. Some experimental observations on potassium beams are quoted in support of the theory.

Atomic Beam Source for the Energy Range of 0.2–45 eV

An apparatus has been developed for producing an atomic beam of potassium in the energy range of 0.2 to 45 eV (1000–15 000 m/sec). The technique used here is cathode sputtering. With a velocity selector (spread, 6.7%; maximum rotational speed, 48 000 rpm), both the signal of a Pt-W detector and an energy independent one are measured as a function of the velocity of the beam particles.

Laufzeitanalyse eines an einem CO2-Molekularstrahl gestreuten K-Atomstrahls

A method is described to determine the velocity dependence of scattering processes from timeof-flight measurements with crossed molecular beams. Scattering of a potassium atomic beam by short pulses of a carbon dioxide molecular beam generates attenuation pulses in the intensity of the potassium beam. The time-of-flight broadening of these attenuation pulses is measured employing signal averaging to improve the signal-to-noise ratio from 1 to 100. The experimental results are in accordance with computed time-of-flight distributions based on the additionally measured velocity distributions and geometries of the two molecular beams, and using the known velocity dependence of the K-CO2 scattering cross section.

289. A potassium beam probe for the measurement of electron density: J W Hill et al, Rep CLM-R-68, 1967, 9 pages ( Sci Tech Aerospace Reps, 5 (15), 2714, N-67-28855)

289. A potassium beam probe for the measurement of electron density: J W Hill et al, Rep CLM-R-68, 1967, 9 pages ( Sci Tech Aerospace Reps, 5 (15), 2714, N-67-28855)

Bestimmung der mittleren Agglomeratgröße und des Restgasanteils kondensierter Molekularstrahlen durch Streuung eines K-Atomstrahls

The mean size of the clusters in a condensed nitrogen molecular beam is evaluated from the attenuation of a potassium atomic beam crossing the nitrogen beam at right angles. The method is based on the fact that molecules in a cluster will shield each other and therefore contribute the less in the scattering processes the larger the cluster is. The percentage of unclustered molecules in the condensed beam is determined by measuring the decrease of the potassium beam attenuation resulting from filtering the condensed nitrogen beam with a scattering chamber. Assuming a spherical drop as a model for the clusters the average number of molecules per cluster can be estimated from the measured effective cross sections. It increases from 160 to 6500 when the nitrogen source pressure is increased from 100 to 700 mm Hg. At the same time the percentage of single molecules in the original condensed beams decreases from 30% to 0.15% of the total beam intensity.

Total Cross Sections for the Scattering of Potassium by Helium, Neon, Argon, Krypton, and Xenon

Abstract Total cross sections for the scattering of potassium by He, Ne, Ar, Kr, and Xe have been measured in the thermal energy range. The potassium beam and the scattering gases had Maxwellian velocity distributions. The angular resolution of the apparatus was 2.62 min of arc, so that quantum mechanical calculations could be applied. If a van der Waals potential, V=−C⁄r6, is assumed for the long range intermolecular forces, the total cross section may be related to the constant, C, as follows;(Remark: Graphics omitted.), where p is a constant. In the present experiments care was taken to avoid the pumping effect of a McLeod gauge in the measurement of the pressure in the scattering chamber. The Ga0 function denned by Berkling et al. was used to derive an absolute total cross section from a measured effective total cross section. The van der Waals constants calculated from the measured total cross sections were compared with the theoretical values. The present values were about 25 percent smaller than the theoretical values for the Ar, Kr, and Xe systems. The differences for the He and Ne were considerably larger, with the theoretical values larger than the experimental by a factor of two or three.

Velocity Dependence of the Differential Cross Sections for the Scattering of Atomic Beams of K and Cs by Hg

Measurements of the velocity dependence of the angular intensity distribution of potassium and cesium beams scattered by a crossed beam of mercury are presented. The alkali beam was velocity selected, with a triangular velocity distribution (half-intensity width 4.7% of peak velocity); the velocity was varied over the range 185–1000 m/sec. The Hg beam had a thermal distribution; the average Hg speed was ∼235 meters per second. The scattering data have been converted to the center-of-mass system. The angular distributions show the expected strong forward scattering and evidence the phenomenon of rainbow scattering. The energy dependence of the rainbow angle is used to evaluate the interatomic potential well depth, interpreted as the dissociation energy De of the 2Σ+ molecular ground state. Values (in erg×1014) thus obtained (±5%) are 7.46 for KHg and 7.72 for CsHg. Absolute values of differential cross sections could not be obtained; only relative cross sections D(θ) are reported. The observed low-angle behavior D(θ) ∝θ- 7/3 serves as direct experimental confirmation of the r—6 dependence of the long-range attractive potential for K–Hg and Cs–Hg systems.

Adsorption of Potassium Beams on Surfaces

The adsorption probabilities have been measured for a thermal potassium beam impacting on 20 different target materials. These measurements when included in the general body of previously published data have permitted the derivation of several empirical relations involving binding energies, adsorption coefficients, and heats of formation. These relations may find application in predicting other coefficients, and binding energies. Thus the tedium of time-consuming experiments usually required in the search for suitable beam-collector materials may be significantly reduced.

Double resonance measurements of hyperfine structures in potassium

Radio-frequency magnetic resonances have been studied between the states of the atomic level 5 2 P , 3/2 excited in a coarse atomic beam of potassium by optical resonance radiation. The resonances were detected by observing changes in the intensity of the scattered resonance radiation. Hyperfine structure was observed in the Paschen—Back region, and in magnetic fields down to zero. Values of a and b , the magnetic dipole and electric quadrupole interaction constants, derived independently from measurements in the strong and zero field regions, are in agreement. The values a = 1.97 + 0.1 Mc/s, b = 1.7 + 0.3 Mc/s have been obtained for the interaction constants of 39 K. Double quantum transitions between hyperfine structure levels were observed in zero field. The nuclear magnetic moment derived from a agrees with molecular beam magnetic resonance measurements; the nuclear electric quadrupole moment derived from b is 0.11 3 + 0-02 x 10 -24 cm 2 . Hyperfine structure was also observed in the level 5 2 P 1/2 . A preliminary value for the a factor is 9.3 + 0.5 Mc/s.

Velocity Distributions in Potassium and Thallium Atomic Beams

A high-resolution, high-intensity, spiral velocity selector has been designed for the study of the velocity distributions of the components of atomic and molecular beams. It has been found possible to design oven slits which closely approximate the ideal aperture of kinetic theory. An analysis has been made of the velocity distributions in beams of potassium and thallium over a range of velocity from 0.3 to 2.5 times the most probable velocity in the oven. The agreement between the observed distribution and that deduced, on the basis of the assumptions that the distribution in the oven is Maxwellian and that the aperture is ideal, is very good.

The Free Fall of Atoms and the Measurement of the Velocity Distribution in a Molecular Beam of Cesium Atoms

The free fall of atoms is observed in long molecular beams of potassium and cesium atoms. The measurement of the intensity distribution in a beam deflected by gravity represents the velocity distribution of the beam atoms and permits an accurate determination of this distribution. The results show that the measured values agree in general with those calculated for a modified Maxwellian velocity distribution in the beam. At larger deflections, i.e., for slow atoms, a deficiency of intensity was observed, which increased both with increasing deflection and with increasing pressure in the oven where the beam originates. This deficiency is explained by collisions in the immediate vicinity of the oven slit.

Intensity Ratios of the Hyperfine Structure Components of the Resonance Lines of Potassium

IN view of conflicting evidence as to the sign of the nuclear spin of potassium 39, it was decided to make a quantitative investigation of the intensities of the hyperfine structure components of the resonance lines. The method used was similar to that used by the authors for the hyperfine structure of the resonance lines of silver1. The absorption produced by an atomic beam of potassium in the resonance lines given by a potassium lamp similar to that previously described was observed by means of an etalon of 5 cm. plate separation, with heavily silvered plates. Under these conditions, the adjacent orders of the emitted lines are well separated from each other, and the hyperfine structure can be observed as a narrow absorption doublet. In order to measure intensities, a photograph is made first of the emitted light without absorption, and then with absorption ; the rednctions in the logarithmic intensities by the absorption lines give their respective absorption coefficients, and the ratio of these the required intensity ratio.

The atomic work function of tungsten for potassium

In a recent paper some experiments on the rate of evaporation of positive ions of the alkali metals from a hot tungsten surface were described, and, from the temperature coefficient of the evaporation rate the positive ion work function, the work required to remove a positive ion from the surface, was derived. In the experiments described here the method has been extended to investigate the rate of evaporation of potassium atoms from a tungsten surface, and the atomic work function has been measured. Theory . Consider a hot tungsten surface on which an atomic beam of potassium is incident, and let a negative field be maintained at the surface to prevent positive ions from evaporating. Under these conditions potassium can leave the surface only in the atomic state and the surface concentration will thus go on increasing until the rate of evaporation is equal to the supply in the incident beam, when an equilibrium state will be set up. The equilibrium surface concentration N a ∞ will be given by N a ∞ = Q/ a , where a is the atomic evaporation rate for unit surface concentration, and Q is the intensity of the incident beam. The corresponding ionic surface concentration is N p ∞ = f N a ∞ / g = f Q/ ag , where f and g are quantities such that f N a ∞ is the number of adsorbed atoms which are converted into ions and g N p ∞ is the number of adsorbed ions which are converted into atoms on unit area of the surface per second. We shall, therefore, have f / g = ½ exp(—I s e / k T), where I s is the surface ionization potential, and the numerical factor ½ arises from the different weights to be attributed to the potassium atom and ion.

The Reflection of Beams of the Alkali Metals from Crystals

Beams of lithium, potassium, and caesium were reflected from crystals of sodium chloride and lithium fluoride, as a means of studying the wave nature of these atoms. Incident angles from 2\ifmmode^\circ\else\textdegree\fi{} to 60\ifmmode^\circ\else\textdegree\fi{} were investigated. Although one one-hundredth percent of specular reflection could have been detected, no trace of such a reflection or of diffraction was found. The measured angular distribution of reflected atoms followed closely the cosine law. Apparatus, and the detecting device depending on positive ion emission by which this sensitivity of measurement was obtained, are described.