Contains research objectives and reports on one research project.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E)Sloan Fund for Basic Research (M.I.T. Grant 99

King, J. G.

Oates, Daniel E.

Tinker, R. F.

Zacharias, J. R.

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GENERAL PHYSICSI. MOLECULAR BEAMSAcademic and Research StaffProf. J. R. Zacharias D. K. Fokkens Dr. S. G. KukolichProf. J. G. King Dr. R. C. Pandorf F. J. O'BrienGraduate StudentsW. R. Bigas G. A. Herzlinger D. E. OatesT. R. Brown J. W. McWane R. F. TinkerRESEARCH OBJECTIVES AND SUMMARY OF RESEARCHWe have started a series of experiments in which we apply newly developed atomicbeam techniques to certain problems in low temperature physics. These experimentscan be classified as follows.(i) Studying the scattering of cold He beams by gas, by other beams and from sur-faces, both liquid and solids.(ii) Observing the diffraction of atoms from gratings and apertures.(iii) Investigating various properties of liquid helium as exhibited by the emittedatoms.(iv) Using beams of magnetic atoms as microscopic probes of superconductors.We are also constructing a first model of a molecular microscope in which we shallstudy the evaporation of K and K20 from a potassium feldspar matrix containing alu-mina grains. The principles and possibilities of this device were described in Quar-terly Progress Report No. 84, January 15, 1967, pp. 1-2.During the past year we have been equipping the Molecular Beam Laboratory so thatwe can perform experiments at low temperatures. Three cryostats of standard designwith associated pumping and gas lines have been installed. A helium dilution refrigera-tor intended to attain temperatures below 0. 1*K has nearly been completed. Low tem-perature molecular beam apparatus is being constructed to go into the cryostats.Preliminary results have been obtained in experiments under headings (iii) and (iv)above.J. G. King, J. R. ZachariasA. MOLECULAR BEAM UNIVERSAL IONIZERWe are developing a new type of universal ionizer as a molecular beam detector(see Fig. I-1). The detector has 20 tungsten filaments arranged cylindrically outside aconcentric accelerating grid. The molecular beam that is to be detected is directedalong the axis of the cylinder and is ionized by the electrons accelerated inwardly bythe grid. Detection of the ions takes place at the end of the cylindrical region. Inorder to overcome the low efficiency of conventional ionization detectors, weThis work was supported by the Joint Services Electronics Programs (U. S. Army,U.S. Navy, and U.S. Air Force) under Contract DA 28-043-AMC-02536(E), and in partby the Sloan Fund for Basic Research (M.I.T. Grant 99).QPR No. 88(I. MOLECULAR BEAMS)GRIDG I D- -IO N IZ A T IO N R E G IO NFILAMENTSCOLLECTORIONIZATIONMOLECULAR BEAM I REGIONGRIDELECTROMETERFig. I-1. Diagram of the ionizer.hope that the electrons entering the central region will produce a space charge largeenough to provide a focusing of the positive ions in a region close to the axis of the cyl-inder. This idea was first used by R. Weiss,1 but the present construction is simpler.Calculations have been made to determine if such a space charge would exist and ifit would affect the ionizing efficiency of the electrons.Using the facilities of the R.L.E. computation group, we have solved the differentialequation for the electron distribution inside the ionizer. Poisson's equation V2V = -4Trpis to be solved. Assuming cylindrical symmetry, we can eliminate all but the depen-dence on r. We also assume monoenergetic electrons.We use following definitions:n = number of electrons entering the ionizer per unit angle per unit length perunit timev = velocity of the electronsR = radius of the gridV = potential.The charge density of electrons in the central region is2nervrvBy choosing the total energy to be zero, Poisson's equation becomes1 a aV 2ne mr r-= 4T-rr r ar r 2 VWe use the dimensionless variablesrX =R2eVmyv = v(R).oThen the equation becomesQPR No. 88(I. MOLECULAR BEAMS)a a -1/2ax-x axwhereC 16ne2 R3 'myand the boundary conditions are 4(1) = 1, 4(O) is finite.A program that supplied a value of 8 at X = 1 and adjusted this to give a finite value0 01 02 0.3 0.4 0.5 0.6 0.7 0.8 0.9Fig. I-2. Potential.0.400 0.1 0.2 03C = 0.750.7 0.8 0.9Fig. 1-3. Charge density.QPR No. 88(I. MOLECULAR BEAMS)of 4 at X = 0 was run for various values of C. Two families of solutions occur. ForC < 1/2, t(O) is positive, which indicates that electrons traverse the center with a non-vanishing speed. For C > 1/2, d(X ) becomes zero for Xo > 0, which indicates that theelectron space charge is sufficient to exclude electrons from the central region X < X oFigures I-2 and I-3 are plots of c and p for C = 0.4 and C = 0.75.We would like to thank Eloise Kleban for her help with this problem. We also thankProfessor George G. Harvey for his help.D. E. Oates, R. F. TinkerReferences1. R. Weiss, Rev. Sci. Instr. 32, 397 (1961).QPR No. 88

Molecular Beams

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Molecular Beams

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