APPARATUS AND METHODS FOR ELECTRON SPIN POLARIZATION DETECTION

Abstract: Provided herein are a device and a method for determin­ ing the spin polarization of an electron beam where the device and method contemplate diffusely back-scattering the beam from an electron opaque target, at a kinetic energy less than 10,000 electron volts, collecting the scattered electrons which may be of a preselected energy range and within a predetermined solid angle relative to the target and the collector, and measuring the number of scattered electrons which were collected.

Description: This invention relates to electron spin polarization and more particularly, to the measurement of electron spin polarization via low energy diffuse scattering. An electron, in addition to being characterized by its energy and momentum, is characterized by a quantity known as its spin. Quantum mechanical principles limit the possible values of the spin of an electron to plus(+) or minus(-) h/(4pi), where h= Planck's constant. The plus or minus designations depend on whether the electron is oriented parallel (or "up") or antiparallel ("down") to a pre-selected arbitrary direction. A beam of electrons necessarily contains a large number of electrons where each individual electron contributes a spin, either up or down. If a greater population of one electron spin type exists in the beam, then the beam is spin polarized. The spin polarization of the beam is quantifiable by the following equation: P = (N up - N dn )/(N up +N dn ), where N up electrons have their spins in the up direction and N dn spins are pointing down. P represents the net orientation of the beam or ensemble of electrons. P values may range from +1 (all spins up), to 0 (an equal number of spins up and down), to -1 (all spins down). As noted, the preselected "up/down" polarization axis is arbitrary as to where it is oriented relative to the coordinate system used for measurement. It is preferred, for purposes of interpretation, to employ a cartesian coordinate scheme where the determination of the cartesian components of the polarization of the electron beam, P x ,P y , and P z , is equivalent to the determination of the magnitude and direction of the polarization vector. A beam with vector polarization is further characterized as transversely polarized when the polarization is perpendicular to its velocity and longitudinally polarized, when the vector polarization is parallel to its velocity direction. Known sources for producing beams of polarized electrons include photoemission from GaAs, described in United States Patent 3,968,376; and secondary emission from a ferromagnet, as discussed by Unguris et al in The Physical Review Letters 47,72(1982). Because an electron has a spin of h/(4pi) and this spin has an associated magnetic moment of one Bohr magneton, a measurement of the electron spin orientation yields information about, for example, the origin of the electron or its interactions. An example of the uses of electron spin polarization for analytical applications includes the scattering of very high energy electrons (10¹⁰ eV) from other elementary particles to test the violation of the physical law of parity conservation. Another use involves measurement of the interaction characteristics of an electron with an isolated atom, including measurement of the polarization of the electron before and after scattering. Such measurement demonstrates the extent of electron substitution within the atom and/or the degree of electron deflection from the atomic charge cloud. Additionally, spin polarization measurements may be used for probing solid materials in the case of ferromagnetic materials due to the fact that electrons emitted from a ferromagnetic material either by photoemission, field emission, or secondary emission can be used to characterize the magnetic nature of the material. Information storage mechanisms now exist in which magnetic domains in ferromagnetic materials are oriented parallel or antiparallel relative to particular reference coordinates. The development of high density, magnetic storage mechanisms is contingent upon the microscopic investigation of magnetic domains which is enhanced by the use of electron polarization detection. Information so stored can be "read" by measuring the spin polarization of electrons emitted therefrom. Where a finely focused electron beam, e.g. a scanning electron microscope, generates electron emission from the magnetic domain of the information storage device, the spin polarization of the emitted electrons can be measured. Hence, it is possible to read information stored in much greater density, than, for example, can be used with optical microscopy. The traditional means of measuring electron spin polarization is known as Mott scattering, after Sir Neville Mott. In 1932, Mott predicted theoretically that the scattering of an electron from an atom would depend upon the direction of the incident electron's spin if 1) the electron was scattered through an angle greater than 90 degrees as measured with respect to the incident beam direction, (0 degrees = forward scattering, 180 degrees = backscattering along the beam); 2) the nucleus from which the electron scattered had a high atomic number Z; and 3) the speed of the electrons approached the speed of light, that is, electrons with energies greater than 50,000 electron volts. Under these conditions, there is an interaction between the spin of the incident electron and the angular momentum associated with the trajectory of the electron scattering from the nucleus. If the incident electron beam has a spin polarization normal to the scattering plane, there is an observable difference in scattering probability for scattering at the same angle to the right or the left. From this theory, the Mott polarization detector emerged. The Mott detector measures the backscattered intensity for equal angles to the right and left of the beam's incident direction to determine the degree of polarization normal to the scattering plane. The normal component to the scattering plane is defined as P n = A/S, with A = (I L -I R )/(I L +I R ), where A is the scattering asymmetry; I R and I L are respectively, the scattered electron intensities to the right side of the beam and the left, and S is the analyzing power which depends on the scattering target and the particular scattering geometry. Accordingly, the Mott detector employing a thin gold foil target in the thickness range of 300-1500 Angstroms, is calibrated to determine the effective analyzing power, S, by using theoretical calculations of the spin asymmetry in scattering 100,000 to 120,000 electron volt electrons. To avoid the multiple scattering or absorption of electrons within the foil, since such effects are not accounted for in the theoretical model used to calibrate the detector, the Mott detector employs very thin gold foils for the target. The calibration of the detector involves a series of measurements of polarized electron beam scattering for well-defined geometries as a function of the target foil thickness. The Mott device suffers reduced efficiency as a result of many of the incident electrons passing through the scattering target, i.e. the target is not opaque to the electron beam. At 100,000 electron volts, the target is not opaque to the high energy electrons where the mean free path of the electrons is sufficiently long so that many pass through the thin gold foil. As a result of using thin foils to minimize scattering typically only about 0.01% of the electrons entering the device are back scattered into the detectors actually used in the measurement. In contrast, low energy electrons (with energies less than a few thousand electron volts) are much more strongly interacting and are stopped by a gold foil of 300 Angstroms. The target opacity depends on both the electron energy and the target density. There are a number of additional disadvantages of the Mott spin analyzer for measurement of electron spin polarization: 1) the intensity asymmetry occurs over a range of scattering angles where the scattering cross section is quite small, resulting in small signals to be detected. 2) The apparatus is very bulky owing to the high voltage isolation and safety region required for 100,000 volt operation, and 3) The analyzer and detectors are not readily movable to allow measurement of polarization of electrons being emitted by different sources or a source in different directions. The next advances in this art evolved during the 1960's when it was discovered electron scattering at low energies (<1000 electron volts) from a mercury atomic beam was possible. At such energies, the scattering is from the entire atom (nucleus plus electrons) rather than from the nucleus alone. Although avoiding the apparatus problems associated with the high voltage Mott Detector, the mercury atomic beam provides a target of significantly lesser density than that provided by the Mott gold foil target. Consequently, the scattering efficiency is very small. Furthermore, these devices generate a small angular range over which significant scattering asymmetry occurs and a minimal scattered intensity. An additional physical consideration, somewhat disadvantageous, is based on the necessity to employ large vacuum pumps to maintain sufficiently low vacuum in the presence of a mercury atom beam. Moreover, depending on the application, the mercury beam is often corrosive. Also, like the Mott Detector, the mercury atomic beam suffers detection inefficiencies caused by the target not being opaque to the electron beam. These inefficiencies are caused, in the case of the mercury atomic beam, by low target density. A more recent development in spin polarization analysis was introducted by Kirschner and described in United States Patent 4,153,844. The Kirschner detector is distinguishable from the above-described devices as it employs an opaque monocrystal scattering target for determining electron spin dependent interactions. The spin dependent interaction is the spin orbit interaction in the scattering of the incident electrons from the monocrystal. The monocrystal presents an orderly arrangement of atoms in crystal planes which, upon electron beam impingement, causes the electrons to be diffracted backward into well defined beams. These beams of scattered electrons are symmetrically spaced relative to the electron beam incident on the monocrystal surface. Significant multiple scattering occurs which cannot be reliably calculated thereby necessitating experimental calibration. While the Kirschner device is compact and allows operation at low voltages, it has several disadvantages: 1) The actual intensity of any two symmetrically diffracted beams which would be used for a polarization measurement is small, typically 0.1% of the incident beam. 2) The angular spread of the incident electron beam at the monocrystal target is required to be less than two degrees, which in turn limits the variety of electron beams which can be analyzed. Being dependent on diffraction from the crystal planes, the device severely constrains the relative alignment of the incident electron beam, the planes of the atoms in the monocrystal, and the position of the detectors for the backscattered electrons. Where impingement of electrons is not perpendicular to the monocrystal, those electrons are diffracted in different directions, and consequently, are no longer symmetrically displaced about the incident beam. Thus, the Kirschner detector provides a minimal angular range for significant scattering asymmetry suitable for spin polarization detection. 3) The angle of diffraction also changes with incident electron energy and at an incorrect energy, the diffracted beams may be shifted off the detector. In order to employ the monocrystal as a detector, the spread of energies of the electrons in the incident beam, is necessarily less than 5 electron volts. Furthermore, the detector's analyzing power decreases rapidly when the beam energy changes from the optimum energy and may even change sign leading to inaccurate or erroneous results. 4) Due to very sensitive, surface diffraction, a Kirschner monocrystal surface must be cleaned to be free from any contaminants and maintained in ultrahigh vacuum. Such cleaning a surface involves heating to high temperatures in ultrahigh vacuum (<10⁻⁹ Torr 1 Torr = 1.33x10²Pe), treating with specific gases at specific temperatures, or bombarding with rare gas ions followed by an annealing process. For example, in Kirschner, a tungsten monocrystal is cleaned by heating to 1800 o K in 10⁻⁶ Torr of oxygen for a period of order 10 minutes every few days. Between these treatments the crystal surface must be cleaned periodically (every 15-30 minutes) by heating rapidly to 2500 o K in vacuum. The analyzing capability of the above-described detectors may be arithmetically quantified in a figure of merit, FM=S²I/I o , when the statistics of the measurement are the limiting factor. Thus, it is desirable to maximize S, the analyzing power, as well as to maximize the fraction representing the intensity of the scattered electrons I measured in the collector divided by incident electron beam intensity I o . With reference to the figure of merit, the angular and energy spread are not included in the conventional calculation, FM= S²I/I o , because only the I o reaching the target is applicable. In general applications with the Kirschner device, it is necessary for the electron optics to reject much of the electron beam before it reaches the target surface so as to satisfy the severe constraints on spread in angle and energy. Consequently, the I o at the target is much smaller and the measurement signal is severely decreased. Even though a figure of merit of 10⁻⁴ can be achieved for the Kirschner monocrystal analyzer, under optimal conditions, the monocrystal device is efficient only for well collimated beams with narrow energy spread. Moreover, the monocrystal device requires the elimination of any inelastically scattered electrons, (usually those which have lost over 10 electron volts of energy in the scattering process) which results in a loss in useful scattering intensity. In view of the foregoing, it is evident that there has long been a need for a simple, efficient and compact spin polarization detector. An object of this invention is to provide an efficient means and method to measure electron spin polarization that is of compact and elegant construction. It is a further object of this invention to minimize constraints on the angular and energy distributions of the electron beam to be measured. It is yet another object of the invention to achieve a polarization detection means in which the scattering target surface is easily prepared, is subject to minimum constraints on its alignment, and can be chosen to provide stable spin dependent characteristics for relatively long periods of time. According to the present invention, there is provided apparatus for determining the spin polarization of a beam of electrons comprising: a) beam energy control means for controlling the kinetic energy of the electron beam to less than 10,000 electron volts, b) target means positioned in the path of the resulting energy-controlled electron beam, said target means being substantially opaque to the electron beam and causing diffuse scattering of electrons therefrom, c) collecting means for collecting said electrons diffusely scattered from said target means within a selected large solid angle, and d) signal-deriving means for generating signals representative of numbers of electrons collected by the collecting means, said collecting means and signal-deriving means being adapted to determine positional data in respect of the collected electrons thereby to measure spin dependent asymmetry. The invention further provides a method for determining the spin polarization of a beam composed of electrons, comprising the steps of; a) controlling the electron beam to have an energy of less than 10,000 electron volts, b) causing the energy controlled beam to impinge on a substantially opaque target that diffusely scatters electrons from the electron beam, c) collecting the diffusely scattered electrons within a selected large solid angle, d) measuring numbers of electrons collected, and e) deriving positional information in respect of the electrons collected, thereby to determine the spin dependent asymmetry of the scattered electrons. Turning now to practical embodiments of the invention, the target surface can be easily prepared from polycrystalline or amorphous metals such as gold, mercury, platinum, lead, bismuth, uranium or a semiconductor such as mercury telluride. The film may be deposited on an appropriate substrate by sputtering or evaporation. Alternatively, the surface of a piece of polycrystalline or amorphous material can be cleaned by conventional means to provide the target surface. In a preferred embodiment, the scattering surface is an evaporated gold film. Gold, being a noble metal, is desirable as a stable target due to its especially nonreactive characteristics. A scattering target, composed of a polycrystalline film, provides sizeable spin dependent asymmetry in the scattering, and this analyzing power is useful over a broad range of incident beam energies. The combination of the enhanced energy range and relaxed constraints on the angular divergence of the incident beam gives the present arrangement a great advantage over the monocrystal spin analyzer as it provides an acceptable detector for a broader range of electron beams. A key feature is to scatter electrons diffusely at low energies from the polycrystalline or amorphous metal or semiconductor target surface and collect those electrons as well as inelastically scattered electrons. Such a target surface is of sufficient density so as to be substantially opaque to the incident electron beam - electrons do not pass through it. In the absence of coherent diffraction generated by the continuous planes of atoms in the monocrystal, scattering from the target surface is diffuse. As a result, scattering is generated from individual atoms, rather than from the crystal planes as in the diffraction from a monocrystal. Because the electron scattering depends not on the orientation of the surface but on the scattering angle, the constraints on the angular divergence of the incident electron beam and on the alignment of the sample surface are minimized. The relative weakness of the scattered intensity at a particular scattering angle is compensated by collection of the scattered electrons over a large solid angle. The electron scattering may take place at electron kinetic energies in the order of 10¹-10³ electron volts. At these energies the spin analyzer can be constructed into a very compact unit (fist -sized or less). In view of its relatively diminutive size, it is small enough to be attachable and movable. For example, it may be employed as an energy analyzer inside the vacuum chamber of an angle resolved photoemission apparatus or in other applications the spin analyzer can be enclosed in its own small vacuum chamber. The apparatus also detects inelastically scattered electrons which still contribute significantly to the spin dependent asymmetry and can be used to enhance the signal. Returning to the preferred embodiment contemplating scattering from a gold film at, say, 150 electron volts kinetic energy, all scattered electrons which have at least 40 electron volts of kinetic energy in the direction normal to the collector can be used leading to a significant increase in efficiency. A figure of merit of 10⁻⁴ is readily achieved by this apparatus when employing a polycrystalline gold film and electron kinetic energies of 150 ± 20 electron volts. The device even operates using a larger energy deviation (± 75 electron volts) from the mean incident kinetic energy. In this case the figure of merit is reduced only by a factor of two which in certain applications may be justified because an energy selector is no longer necessary. Control of the electron beam is readily accomplished by conventional electron optical means such as electrostatic or magnetic lenses and deflection plates which transport, change the energy, and focus the beam whereby the beam achieves the appropriate angular divergence and mean energy. As noted above, the use of an electron energy selector assists to limit the energy spread of the electrons about the mean energy. After the electrons of the preselected energies impinge approximately perpendicular on the target, the diffusely back-scattered electrons are collected in a collector. The addition of an electron multiplier assembly can enhance the electron signal to the detector. Low energy secondary electrons can be prevented from reaching the target by establishing a retarding electric field between the target and the collector with appropriate grids. An additional electron beam deflecting means may be employed to change any longitudinal component of electron spin polarization to a transverse component. A further advantage is that the apparatus is fully compatible with ultra-high vacuum but only high vacuum is required. For example, the analyzing power S of a polycrystalline gold film scattering target was found to decrease only 8% on exposure to air at a pressure of 10⁻⁷ Torr for 3.5 days. This suggests that a similar decrease could be expected only after more than a month of use at typical operating pressures of 10⁻⁸ Torr or less. It is sometimes advantageous to detect the signal composed of all the electrons incident on the target without making a polarization measurement. This might occur when an adjustment of experimental parameters needs to be made where a signal whose intensity has not been degraded by scattering from the target is required. In this case the target may be biased negatively. Thus, the potentials of the electrodes surrounding the scattering region can be adjusted to prevent the incident electrons from reaching the target and to guide them to the entrance of the backscattered electron collector. In this way, it is easy to switch from making a spin resolved measurement to a non-spin resolved measurement and still use the same signal acquisition system. Other features and advantages of this invention will become more apparent from the following description taken in connection with the illustrated embodiments in the accompanying drawings. Figure 1 is a schematic and block diagram of one embodiment of the invention. Figure 2 is a diagramatic representation of a four quadrant collecting anode. Figure 3 is a schematic and block diagram of an alternative embodiment of the invention which measures all three components of the polarization vector. Figure 4 is a graph representing the relationship between the analyzing power and the electron beam kinetic energy at an evaporated gold film target according to this invention. Figure 5 illustrates the relationship between the average figure of merit of this invention as a function of the energy spread ▵E of the incident beam. Figure 6 is a graphic representation of the relationship between the analyzing power and the collected intensity according to this invention. Figure 7 illustrates the figure of merit S²I/I o representative of the present invention. Referring to Figure 1, electrons in free space 10, may be generated by any source such as field emission, photoemission, secondary emission, or an electron scattering process (not illustrated). The electrons enter the electron optics 12 where they are transported, accelerated or decelerated, focused and guided by conventional electrostatic and/or magnetic electron lenses and deflection plates designated electron optics 12. In one embodiment, optics 12 consist of a one inch (25.4 mm) (interior diameter) copper cylinder with 0.1 inch (2.54 mm) gaps at the cylinder ends. Each of the copper lens elements are electrically insulated from each other by alumina spacers. Because the spread in energies of the electron beam acceptable at the scattering target is very large, 100 electron volts, the electron energy selector 14 need not have high energy resolution. Any of a number of conventional types such as a cylindrical or spherical condenser type or a plane or cylindrical mirror type is suitable. One example of a suitable energy selector consists of a spherical deflector with a mean radius of 10 centimeters and potentials of 1811 and 1244 volts respectively to provide a pass energy of 1500 volts. When selector 14 is operated at a mean energy of 1500 electron volts with 4 mm slits, electrons 10 which have an energy spread of 30 electron volts full width at half maximum pass through selector 14. Electrons 10 escaping energy selector 14 move to electron optics 16 which changes the kinetic energy of electrons to 150 electron volts. Also electrons 10 are then focused to pass through a copper drift tube 15 to guide the electrons through central holes 17 in collecting anode 18, channel multiplier assembly 20, grids 22 and 24, and onto polycrystalline gold film target 26. The drift tube 15 prevents the electrons from seeing the potential on elements 18, 20, 22, and 24 and is insulated from these elements by a 0.005 inch (0.127 mm) thick Kapton spacer (Dupont trademark). The drift tube 15 limits the cone angle of the electron beam to approximately 18 o . Referring briefly to collecting anode 18 in the described embodiment, it is composed of a 2.5 inch (63.5mm) diameter, 0.125 inch (3.18mm) thick pyrex plate and coated by vacuum evaporation with a 300 Å chromium film, which in turn was evaporation coated with a 5000 Å gold film. The intermediate chromium layer is employed to improve the adhesion of the gold to the glass. A half-inch (12.7mm) diameter hole is provided for insertion of drift tube 15 and six holes are provided on the outer periphery to permit the anode to be attached to channel plate assembly 20. Preferably, alumina spacers separate anode 18 from assembly 20 by approximately 0.15 inches (3.81mm). Channel plate assembly 20 has a circular chevron configuration with a 2.5 inch (63.5mm) outer diameter and a 0.25 inch (6.35mm) central bore. One such assembly is the Model 3040 available from Galileo Electro-Optics Corp. of Sturbridge, Mass. In the particular arrangement described here, anode 18 and assembly 20 are biased by a resistor divider which provides anode 18 with a voltage of 0.9 V o , the back of the channel plate with a voltage of 0.8 V o and the front of the channel plate with a voltage of 0.15 V o where V o ranges from 1500 to 3000 volts with respect to the target 26. Grids 22 and 24, approximately 2 inches (50.8mm) in diameter, are constructed from 100 line/inch (39 line/cm), 0.001 inch (.0254mm) diameter stainless steel wire mesh mounted by spot welding on to a circular frame. Each grid has an approximately 6mm hole punched through its center. Grid 24 is separated from target 26 by 0.24 inches (6.1mm), and grid 22 by 0.07 inch (1.78mm) and channel assembly 20 is separated from grid 22 by 0.11 inches (2.79mm). Both grids 22, 24 and channel plate 20 may be flat, curved or otherwise conform to any desired geometry. Returning now to the function of the apparatus, only electrons 10, diffusely scattered in the field free region created by the shield 28 and the grid 24 at the potential of a gold target 26, reach grid 24 in an angular range of from 125 to 155 degrees. The provision of electrode 30, concentric with the scattering region, permits adjustment of the solid angle of the scattered electrons that reach collector 18. In this embodiment, both shield 28 and electrode 30 are constructed from 0.02 inch (0.51 mm) molybdenum sheeting. The electrons scattered from the target 26 pass through the grid 24 to the grid 22. Grid 22 having a negative potential of -40 volts with respect to the potential of target 26, serves to limit the range of energy loss and accordingly, retards the low energy secondary (inelastic) electrons created upon electron beam scattering from the target. Only electrons with sufficient energy to pass grid 22, i.e. all electrons leaving the target with kinetic energy greater than 40 electron volts, reach channel multiplier assembly 20 which emits a pulse of 10⁶ electrons from its opposite surface per each electron incident on the assembly. These electrons are accelerated through a potential of approximately 0.1 volts between the exit side of the channel plate assembly 20 and the anode 18 where they are collected and detected through the use of conventional digital pulse counting assembly 32. When the electron population passing grid 22 is large, the gain of the channel multiplier assembly 20 can be reduced, or alternatively, assembly 20 can be omitted entirely, and the number of electrons reaching sectored anode 18 can be measured by conventional analogue means 34. Gold film target 26 is formed by evaporation of gold onto an abraded 0.020 inch (0.51 mm) thick tantalum blank. Gold is deposited from an evaporator 36 incorporating a 0.01 inch (0.25 mm) tungsten wire filament by turning the gold film target substrate 26 toward the evaporator 36 by rotating a vacuum feed pipe entering the chamber at 38. A 5 amp current is passed through the filament for 15 seconds followed by 8.5 amps for 10 seconds. Evaporator 36 is connected to an electrical supply 42 which passes through the chamber wall 13. Referring to Figure 2, the particular arrangement of collector anode 18 is illustrated. The four discrete quadrants of 46, 48, 50 and 52 of the collector anode 18 consist of metallic films deposited on an insulating substrate as described above. Each anode is electrically isolated from the others and connected independently to the signal processing electronics through chamber wall feed-through junction 44 (see Figure 1). Quadrants 46 and 48 oppose each other along the X direction and, likewise, quadrants 50 and 52 in the Y direction. The asymmetry of the diffusely scattered currents I(Q₁) and I(Q₂), at quadrants 46 and 48, respectively, determine the Y component of the transverse polarization according to, where S is the effective analyzing power of the gold target for the given geometry. Correspondingly, the asymmetry in the diffusely scattered current, measured in quadrants 50 and 52 of the sector collector, determine the X component of the transverse polarization. Alternatively, any conventional position-sensitive detector can be employed. Moving now to Figure 3, an alternative embodiment contemplating measurement of the third component of the polarization vector is depicted. Transport optics 60, an energy selector 62 and further transport optics 64 are equivalent to elements 12, 14 and 16 in Figure 1. An entering electron beam then encounters a 90 degree sector of a spherical condensor 69 but if the inner and outer sections 66 and 68, respectively of this condensor have a zero potential difference therebetween, the electron beam passes undeflected through an aperture 70 formed in the outer section 68. Once the electron beam exits from aperture 70, optics 72 change its energy and focus it to impinge on to target 74. As this apparatus and sequence correspond to that described above in Figure 1, the X and Y components of polarization are measured. However, if a non-zero potential is applied between the condensor sections 66 and 68 sufficient to deflect the beam 90 degrees toward optics 76 and on to corresponding gold foil target 78 (disposed at right angles to gold foil target 74), the deflection changes the direction of the electron beam without changing the direction of the electron spin. The Z component of the polarization, formerly in the direction of the electron beam (longitudinal polarization), is now transverse to the electron velocity. Thus, the Z component can be measured by scattering in the same manner as that described above. The Y component is also measured again redundantly by such scattering which is useful for balancing the sensitivity of the signal detection channels associated respectively with scattering from gold film targets 74 and 78. In reference to the hardware employed for switching condensor 69, it is comprised of a 0.6 inch (15.2mm) radius inner quarter sphere 66 and matched 1.0 inch (25.4mm) radius outer quarter sphere 68 with 0.2 inch (5.1mm) diameter aperture 70 centrally disposed therethrough. With this arrangement, in order to achieve orthogonal beam deflection at the 1500 volt pass energy, 2501 volts must be applied to member 68. Turning now to Figure 4, there is illustrated in graph form the analyzing power of this invention for different kinetic energies of the electrons hitting the target. The analyzing power is useful for electrons over a wide energy range, and is, in fact, greater than one-half its maximum value over a 120 electron volt range. One important aspect illustrated by Figure 4 is that the analyzing power does not change sign. As long as the sign of S does not change at a given energy, then electrons at that energy can add usefully to the signal of the polarization measurement. For the purpose of comparative illustration, the analyzing power of the Kirschner device is represented by the shaded area. Although at optimum conditions that device performs well, it exhibits an extremely narrow functional range particularly, in contrast to that provided by this invention. Referring now to Figure 5, there is shown the figure of merit averaged over an energy spread, ▵E, of an incident beam of constant total intensity. The average figure of merit for a beam with a 40 electron volt energy spread is 1x10⁻⁴, while that for a beam with a 120 electron volt energy spread is 0.64x10⁻⁴. If the incident beam was not of constant total intensity, but rather of constant intensity as a function of energy, the intensity in the 120 electron volt energy spread would be three times that in the 40 electron volt energy spread. Figure 5 illustrates the advantageous increase in signal possible in many situations because the invention can measure an electron beam with wide energy spread with high efficiency. Once again, the limitations of the Kirschner device are underscored by its relatively narrow functional range (see shading). The graph presented in Figure 6 illustrates the relationship between the analyzing power and the collected intensity for a nominal beam kinetic energy of 150 electron volts, where both quantities are a function of the potential on grid 22 as measured with respect to the target 26, which as detailed above, determines which energy scattered electrons are collected. The log of the analyzing power, log (S), is located on the right and on the left is the log of the intensity ratio, log (I/I o ), where I is the intensity scattered into two selected quadrants, e.g. 46 and 48 (see Figure 2) and I o is the intensity of the incident beam. The potential on grid 22 ranging from -140 to 0 volts comprises the abscissa. For a grid 22 potential of -140 volts, all electrons leaving target 26 with an energy of motion normal to grid 22 greater than 140 electron volts are collected and the analyzing power is high but the intensity is low. At a grid potential of -40 volts, all of the electrons which have left the target 26 with greater than 40 electron volts of energy of motion normal to grid 22 are collected. The analyzing power decreases only slightly from 0.13 to 0.10, but the intensity has increased by a factor of 10³. When grid 22 is at zero volts, the same potential as the target 26, all electrons are collected. Finally, in Figure 7, the relationship between the figure of merit S²I/I o and the potential of grid 22 is illustrated. The figure of merit, which can be computed from Figure 6, rises with increasing grid potential as a result of the increase in I/I o (see Figure 6). Thus, for the 150 electron volt incident beam kinetic energy of this case, and at the preferred operating potential for grid 22, which is -40 volts with respect to target 26, the figure of merit is 1.04x10⁻⁴. Returning to Figure 1, if it is desirable to detect the entire incident electron beam 10 undiminished by scattering from the gold target 26, the target 26, the shield 28, and the concentric electrode 30 are biased negatively to a voltage such as to retard the electron beam. Thus, a corresponding retardation of the progress of electrons 10 from electron optics 16 to target 26 is achieved. These electrons are then collected by anode 18 after being reflected backward without scattering from the gold film target 26 and pass through grids 22 and 24, and multiplier assembly 20. The beam intensity is thereby measurable by the digital pulse counting assembly 32 or the analogue counter 34 (depending on the size of the signal to be measured).

Claims: Apparatus for determining the spin polarization of a beam of electrons comprising: a) beam energy control means (14,62) for controlling the kinetic energy of the electron beam to less than 10,000 electron volts, b) target means (26;74,78) positioned in the path of the resulting energy-controlled electron beam, said target means being substantially opaque to the electron beam and causing diffuse scattering of electrons therefrom, c) collecting means (18) for collecting said electrons diffusely scattered from said target means within a selected large solid angle, and d) signal-deriving means (32,34) for generating signals representative of numbers of electrons collected by the collecting means, said collecting means and signal-deriving means being adapted to determine positional data in respect of the collected electrons thereby to measure spin dependent asymmetry. Apparatus according to Claim 1, further comprising signal enhancing means (20) for enhancing the signal information provided by the scattered collected electrons, said enhancing means being disposed between the collecting means and the target. Apparatus according to Claim 2, wherein said enhancing means is an electron multiplier plate. Apparatus according to any preceding claim, further comprising angle control means (28,30) for defining the solid angle of the diffusely scattered electrons collected by the collecting means. Apparatus according to any preceding claim, wherein said collecting means is divided into a plurality of sectors (46,48,50,52) for determining the scattering asymmetry of the collected electrons scattered from the electron beam. Apparatus according to any preceding claim, further comprising means (22) to prevent low energy secondary electrons from reaching said collecting means. Apparatus according to Claim 6, wherein said prevention means comprises at least one electrified grid disposed between said target means and said collecting means. Apparatus according to any preceding claim, wherein said target means comprises a polycrystalline metal. Apparatus according to any of Claims 1 to 7, wherein said target means comprises an amorphous metal. Apparatus according to any of Claims 1 to 7, wherein said target means comprises a polycrystalline semiconductor. Apparatus according to any of Claims 1 to 7, wherein said target means comprises an amorphous semiconductor. Apparatus according to any preceding claim, further comprising a second target means (78) and collecting means disposed perpendicularly to the first target means (74) and collecting means, and switching means (69) for selectively switching the electron beam to impinge on said second target means, whereby all three components of the spin polarization vector can be determined. Apparatus according to any preceding claim, comprising means (14) for selecting a desired range of kinetic energy of said electron beam before impingement of the beam on the target means. A method for determining the spin polarization of a beam composed of electrons, comprising the steps of; a) controlling the electron beam to have an energy of less than 10,000 electron volts, b) causing the energy controlled beam to impinge on a substantially opague target that diffusely scatters electrons from the electron beam, c) collecting the diffusely scattered electrons within a selected large solid angle, d) measuring numbers of electrons collected, and e) deriving positional information in respect of the electrons collected, thereby to determine the spin dependent asymmetry of the scattered electrons. A method according to Claim 14, further including rejecting low energy secondary electrons emitted from the target. A method according to Claim 14 or Claim 15, further including enhancing the signal information produced by the scattered electrons emitted from the target. A method according to any of Claims 14 to 16, further including controlling the solid angle within which the diffusely scattered electrons are collected. A method according to any of Claims 14 to 17, further including selecting a predetermined range of kinetic energy of said electron beam before impingement of the beam on the target. A method according to any of Claims 14 to 18, further including changing longitudinal beam polarization to transverse beam polarization by deflecting the electron beam through 90° before impingement of the beam on the target.

Inventors: CELOTTA, ROBERT J., PIERCE, DANIEL T., UNGURIS, JOHN
Applicants: CELOTTA, ROBERT J., PIERCE, DANIEL T., UNGURIS, JOHN
Application year: 1986
Main classification: G01T1
IPC codes: G01T1, G11B11, G01N23, G21K1