Introductory Comments

Microbeam Techniques in Geology

A large number of microbeam analytical techniques have been developed for the analysis of geological materials. These techniques use some manner of sharply focused incident beams of particles or energy to determine chemical or isotopic composition in the microscopic scale (diameters ranging from about 100 pm to 50 µm). The table below lists the more familiar techniques; this class will focus on the ones in red.

Most techniques are non-destructive, but several result in sample damage, usually in the form of pitting where material has been removed. For example, incident pulsed laser light volatilizes material from the sample in laser-ablation ICP-MS, and low-energy ions ablate the sample leaving holes in SIMS. Nondestructive techniques are more numerous. Many use characteristic x-rays produced from the sample to analyze composition. As indicated, there are several ways to produce x-rays from the sample, including incident x-rays, electrons, and protons.

This course will focus on effects that are produced by electron bombardment (in red above). Using these effects to examine geological materials one may (among other things):

1. Identify minerals; this is especially useful for opaque and micrometer-sized grains;
2. Determine phase compositions, which are required for gothermometry and geobarometry calculations;
3. Document chemical zoning within minerals for petrologic, growth, and diffusion studies; and
4. Locate rare phases, such as zircon, monazite, and badellyite, which often have distinctive chemistries.

Incident Beam	Technique	Effect or Measured Signal
Visible light	Reflected light microscopy Polarized light microscopy UV-IR microspectrometry	Reflected light Transmitted light Transmitted light
Continuous laser light	Micro-Raman spectrometry Selected-area gas release mass spectrometry	Scattered light Heat and released gas
Pulsed laser light	Laser ablation - Inductively coupled plasma mass spectrometry (LA-ICP-MS)	Volatilized material
X-rays	X-ray microscopy Micro x-ray fluorescence analysis (Micro XRF)	Transmitted x-rays Characteristic x-rays
Electrons	Transmission electron microscopy (TEM) Scanning electron microscopy (SEM) Electron microprobe analysis (EMPA) Cathodoluminescence microscopy Auger microprobe analysis	Transmitted electrons Backscattered electrons (BSE) Secondary electrons Characteristic X-rays Visible light Auger electrons
High-energy protons	Proton-excited X-ray emission analysis (PIXE)	Characteristic X-rays
Low-energy ions	Secondary Ion mass spectrometry (SIMS) Sensitive high resolution ion microprobe (SHRIMP)	Sputtered secondary ions

Units

Before proceeding, it will be useful to define the units we will use in the following materials. All units (with one prominent exception) will be in the International System of Units (abbreviated SI from the French language name Le Système international d'unités). The SI system was developed in 1960 from the meter-kilogram-second (mks) system and uses a series of prefixes that are attached to seven base units that are nominally dimensionally independent:

• meter (m), length
• kilogram, (kg), mass
• second (s), time
• ampere (A), electrical current
• kelvin (K), temperature
• mole (mol), amount of substance
• candela (C), luminous intensity

Other units are derived from these seven base units; for example, a pascals (Pa) is 1 N/m², a Newton (N) is 1 kg m/s². Prefixes are attached to base units to denote multiples (or fractions):

• giga (G), 10⁹
• mega (M), 10⁶
• kilo (k), 10³
• centi (c), 10^-2
• milli (m), 10^-3
• micro (µ), 10^-6
• pico (p), 10^-9
• nano (n), 10^-12

One non-SI unit very frequently used is the Ångstrom (Å), which is 1 x 10^-10 m. X-ray wavelengths are typically 1 to 100 Å (0.1 to 10 nm). Visible light has wavelengths from 400 (violet) to 750 nm (red). Typical distances of interest are µm (sizes of mineral grains, interaction volumes, beam diameters, etc.) and pm (atomic and ionic sizes).

Historical Background

Early Work with Cathode Rays

The English physicist Sir William Crookes (1832-1919) created a vacuum tube around.1875, which he used to study gases. The glass tube contained negative and positive electrodes across which high-voltage electrical currents. Crookes primarily worked with the conductivity of gases, which he placed inside the glass. Although he did not know it, these cathode ray tubes also produce X-rays and Crookes unsuccessfully sought cause of repeated fogging of photographic plates that he had stored nearby.

William Crookes and the "Crookes Tube. In a "Crookes" tube, a negatively biased electrode, called the cathode, emits cathode rays (electrons) which accelerate toward the anode. Many cathode rays miss the anode and instead strike the glass end of tube, causing it to fluoresce.

Crookes also discovered that some minerals would glow if placed in the tube. This is the phenomenon of cathodoluminescence (production of visible light by electron bombardment), which we will consider in detail later. Different minerals yield different colors: calcite (red), apatite (yellow), willemite (bright green), scheelite (bright blue), dolomite (brown), and magnesite (violet).

Mineral Tube. When activated the minerals sample (calcite in this case) glows as it is struck by electrons. Images source: http://members.chello.nl/~h.dijkstra19/page7.html.

Later variations on the mineral tube used colored phosphors to produce dramatic displays. In the eponymous Crookes flower tube, copper flowers are covered with different phosphors. Rotating vanes made of mica plates are situated above them. When the tube is activated, the cathode rays turn the vanes, resulting in a moving shadow on the flowers below.

Flower Tube. When activated the flowers painted with phosphors glowed in different colors. Images source: http://members.chello.nl/~h.dijkstra19/page7.html.

In 1858, Julius Plücker (1801-1868) and Johann Wilhelm Hittorf (1824-1914) discovered that cathode ray beams were deflected by a magnetic field. They used a modified Crookes tube, with an insert consisting of an aluminum sheet covered with phosphor.

Deflection Tube. A magnet brought near the tube causes the cathode rays to deflect from a straight path. Images source: http://members.chello.nl/~h.dijkstra19/page7.html.

In 1869, Hittorf demonstrated that cathode rays go in a straight line and are blocked by metal. He used the Maltese Cross arrangement shown above. When the cross was down, the glass face of the tube emitted a green glow, which faded over time. When the cross was up, intercepting the cathode rays, its shadow was visible on the end of the tube.

Maltese-cross Tube. When activated the metal cross intercepts the cathode rays casting a shadow on the tube's end. Images source: http://members.chello.nl/~h.dijkstra19/page7.html.

The German scientist Philipp Lenard (1832-1947) added a thin aluminum window to the basic Crookes tube. The window's foil was thick enough to maintain the vacuum inside the tube, yet thin enough to allow the cathode rays to pass out of it. Lenard concluded from subsequent experiments that cathode rays propagated through air for distances of about 100 cm, but that in a vacuum they traveled for several meters without being weakened. These and other early findings were published by Lenard in 1894. During one of his experiments, the fluorescence from a dissipated ray caused a few pieces of paper that had been soaking a barium platinocyanide solution to glow. Lenard had unknowingly discovered the first evidence for X-rays, but failed to investigate the strange phenomenon further. Lenard later claimed that he, rather than Roentgen, should be honored as the discoverer of X-rays. Although he made significant contributions to the field of physics, Lenard was a fervent Nazi and condemned Albert Einstein and other individuals with Jewish backgrounds.

Schematic diagram of the Lenard Window. Source: http://nobelprize.org/nobel_prizes/physics/laureates/1905/lenard-lecture.pdf

Discovery of X-rays

A few months later, on November 8, 1895, Wilhelm Conrad Röntgen (re)discovered X-rays at the University of Würzburg in Germany. Röntgen used ~40 keV electrons to bombard inert gas in tubes, and noticed that a screen coated with barium platinocyanide (BaPbCN) across room began to glow. He placed a deck of cards and a two inch book between the tube and the screen and discovered the rays penetrated these materials. He called them X-rays after the algebraic symbol of the unknown, x. Once while holding lead pipe to the rays, he noticed that the bones of his fingers were shadowed on the screen. On December 22^nd, Roentgen decided to show his wife what had been occupying his time and took an X-ray image of her hand.

Röntgen with X-ray photographs of this wife Bertha's hand (left) and of von Kölliker's hand (right). Sources: http://www.deutsches-museum.de/sammlungen/ausgewaehlte-objekte/meisterwerke-ii/roentgen/ (left and center), http://www.vmas.kitasato-u.ac.jp/radiology/CALS/von-Kolliker.htm (right)

Röntgen also demonstrated that X-rays:

• were produced from the fluorescence part of the wall of the discharge tube
• traveled in straight lines
• were not deflected by a magnetic field
• were absorbed more by denser metals
• were scattered when passing through a body
• and could ionize gases.

He submitted the first paper describing his work, Uber eine neue Art von Strahlen, to the Würzburg Physical Medical Society on 28 December 1895. He gave a public lecture on 23 January 1896, after which he made a plate of hand of Alfred von Kölliker, a famous anatomist. Kölliker proposed that the newly discovered rays be named Röntgen's Rays; X-rays are still called Röntgenstrahlen in Europe. Röntgen received the first Nobel Prize in Physics in 1901 for his discovery, donating the prize money (then about $40,000) to the University of Würzburg.

In addition to Lenard, there were others who had observed the effects of X-rays before Röntgen. In the United States, A. W. Goodspeed (1860-1943) and William Jennings (1860-1945) made an accidental photograph of coins stacked with photographic plates using X-rays from Crookes tube on 22 February 1890 in Philadelphia. However, unlike Lenard, neither claimed priority for discovery of X-rays, noting that they had ignored the plates until Röntgen's announcement caused them to review the photographs.

Duplicate of Goodspeed and Jenning's 1890 photograph. Source: http://medinfo.ufl.edu/other/histmed/klioze/slide16.html

X-rays rapidly became the latest rage. The public was fascinated with this unknown phenomenon; people were amazed at seeing through human flesh, however, they were commonly ill at ease because bones were associated with death. Indeed, people occasionally fainted when first they saw an X-ray image! Thomas Edison capitalized on the American public's intoxication with the X-rays and announced in March 1896 that he would be the first to photograph the living human brain; however, he never accomplished this feat. American physicians quickly recognized the value of X-rays for examining bone fractures and locating foreign objects in the body. The first diagnostic X-ray was taken at Dartmouth Hitchcock Hospital in 1896; Gilman Frost and his Dartmouth physicist brother, Edwin, used X-ray imaging to help set a boy's broken arm.

Humorous postcard and Poster. The card, titled "Beach idyll á al Röntgen," is dated 20 August 1900. A poster for a British Exhibition reflects the sentiment of the times. Sources: (left) http://www.gamlavykort.nu/artiklar/rontgen1900.htm, (right) www.slac.stanford.edu/pubs/beamline/25/2/25-2-assmus.pdf

Further Investigations of X-rays

Another major figure in the investigation of the properties of X-rays was Charles Glover Barkla (1877-1944). He demonstrated in 1905 that X-rays could be polarized, thus having properties identical to visible light. He thus eliminated the possibility that X-rays might be a type of longitudinal wave (like sound) rather than a transverse wave.

In 1909, Barkla discovered what are now termed characteristic X-rays. He found that under certain conditions, the emergent X-rays contained one strong homogeneous component with a constant absorption coefficient. Barkla documented that the absorption coefficient decreased with increasing atomic weight of the anode material. Since no one could yet measure the wavelengths (or frequencies) of X-rays, Barkla measured the absorption of the secondary radiation by directing the secondary radiation through a 0.01 cm-thick layer of aluminum and measuring how much of the beam was absorbed. Barkla next discovered that his homogeneous X-rays were, in fact, heterogeneous. He wrote: "... the radiations from Sn, Sb, I ... emit ... radiation of variable penetrating power."

Plotting his results yielded two monotonic curves, one for the lighter elements and one for the heavier ones, with some elements showing two absorption values. At first he labeled the two curves with by letters B and A, but in 1911 changed his notation remarking that, "[t]he letters K and L are, however, preferable as it is highly probable that series of radiations both more absorbable and more penetrating exist."

George Glover Barkla and his Plot of "K" and "L" Series X-rays. Image sources: (left) http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/Barkla-Graph-1909.GIF, (right) http://www.nndb.com/people/549/000099252/charles-glover-barkla-1-sized.jpg.

In 1912-13, R. T. Beatty demonstrated that electron bombardment of the anode produced two types of x-rays: the characteristic X-rays described by Barkla and continuum radiation, which is also called bremsstrahlung. He noted that characteristic X-rays required a minimum threshold electron energy; below this energy only continuum X-rays were produced. Beatty also showed that depth of x-rays production by electron bombardment was very small (<10 mm).

Diffraction of X-rays

Max von Laue (1879-1960), a junior colleague of Röntgen, considered passage of waves of light through crystalline arrangement of particles and realized that short wavelength electromagnetic rays, such as X-rays, should cause diffraction or interference phenomena in crystals. In 1912, he and his lab assistant, Walter Friedrich and doctoral student, E. Paul Knipping, confirmed diffraction of X-rays by systematic crystal. Von Laue worked out a mathematical formulation for this behavior and published this discovery.

Laue Diffraction Method. Image source:http://www.matter.org.uk/diffraction/x-ray/laue_method.htm

Von Laue took diffraction as positive proof that X-rays are electromagnetic radiation (waves), not particles. However, he was forced to suggest that the incident X-rays must contain only certain wavelengths to account for missing diffracted beams from the crystal. He received Nobel Prize in 1914 for his work. X-ray diffraction work by Rosalind Franklin (1920-1958) was key in determining the structure of DNA.

Max von Laue and the First Diffraction Pattern.. Image sources: (left) W. Friedrich, P. Knipping, & M. von Laue, Interferenzen-Erscheinungen bei Röntgenstrablen, Sitzungsberichte der mathematisch-physikalischen Klasse der K. B. Akademic der Wissenschaften zu München (1912), Heft II, fig 1., (right) http://nobelprize.org/nobel_prizes/physics/laureates/1914/laue.gif

In 1896, after William Henry Bragg learned of W. K. Röntgen's discovery of X-rays, he set about producing the new radiation. On 13 June 1896, he photographed his son's broken elbow using the primitive equipment. W. H. Bragg was proponent of the idea that X-rays were particles, not waves. He supported this view with the evidence that X-rays produced ionization. Bragg engaged in controversy with Barkla, who advocated the wave theory; however, in retrospect, the source of their differences was that he was studying high energy (hard) X-rays, whereas Barkla was studied low-energy (soft) X-rays. Bragg developed an X-ray spectrometer that allowed many different types of crystals to be analyzed, precisely measuring the diffraction angles, but perhaps Bragg's greatest accomplishment was developing an X-ray detector based on ionization of gas. Detectors of this type are still used to day in many applications.

Bragg Spectrometer.. Labels: L, lead box; A, B, D, slits; C, crystal; I, ionization chamber; V', vernier of ionization chamber; K, earthing key; E, electroscope; M, microscope. Image sources: (left) http://www.chemheritage.org/classroom/chemach/pop/07pharma/bragg2.html, (right) http://nobelprize.org/nobel_prizes/physics/laureates/1915/wh-bragg.jpg

In August 1912, William Lawrence Bragg (1890-1971), WHB’s son who had just graduated from Cambridge, realized that the pattern of spots in the Laue diffractogram could be explained by reflection of waves from crystal planes. L. W. Bragg used his father's spectrometer to observe the diffraction of Pt-Lα X-rays by NaCl crystal. He published his results in 1913, The diffraction of short electromagnetic waves by a crystal, in the Proceedings of the Cambridge Philosophical Society. In it, WLB concluded that salt consisted of a three dimensional lattice of Na⁺ and Cl^- ions. For some time after, chemists refused believe that NaCl contains no NaCl molecules (just alternating array of Na⁺ and Cl^- ions! The collaboration between father and son led many to believe that WHB had initiated the research, a fact that upset WLB and continued to haunt him throughout his life. The Braggs shared the 1915 Nobel Prize in Physics.

Bragg's Law and the Structure of NaCl.. Labels: Image sources: (left) http://www.bruker-axs.de/fileadmin/user_upload/xrfintro/images/Abb14_e.jpg, (right) http://nobelprize.org/nobel_prizes/physics/laureates/1915/wl-bragg.jpg

While studying scattering of X rays by crystals in 1913, the Braggs noticed a different distinctive pattern of peaks appeared for each of the different anodes used to produce X-rays. These occurred when there was constructive interference of the characteristic X-rays. This method is used today in wavelength-dispersive spectrometers to examine elements of interest.

Moseley used a simple flat diffracting crystal, but it was later shown that curved crystals would provide higher intensities by better "focusing" the diffracted X-rays. In 1931, H. H. Johann showed that bending the crystal would produce a better focus, however, this results in some broadening and asymmetry of the focus point. In 1933, T. Johannson proposed grinding the surface of a bent crystal to achieve a perfect focusing. However, the machining difficulties of such a procedure are tremendous and most modern microprobe use Johann crystals.

Wave-Particle Duality

The apparent conflict between those who considered X-rays particles and those who considered them waves reflects the manner in which they were studies. All electromagnetic energy and particles depending upon the method of examination will show either wave-like behavior (diffraction) or particle like behavior (discrete energies, ionization, scattering, photoelectric effect). In 1924, Louis-Victor de Broglie (1892-1987) formulated the de Broglie hypothesis, claiming that matter, not just light, has a wave-like nature. He related wavelength, λ, and momentum, p:

where λ = the particle's wavelength, h = Planck's constant, p = the particle's momentum, m = the particle's rest mass, v = the particle's velocity, and c = the speed of light in a vacuum. For example, consider the wavelength of a baseball (0.15 kg), thrown at 90 mph (40.2 m/s):

This wavelength is considerably smaller than the diameter of a proton (about 10^-15 m) and approaches the Planck length (1.61 x 10^-35 m). The wave-like properties of this baseball are too small to be observable. In contrast, electrons have much smaller mass and move much faster, so much faster that relativistic effects (mass increase) must be considered. The wavelength of 10 keV electrons is 12.3 x 10^-12 m, permitting diffraction to be observed.

Electron Interference. Electrons are sent through the slits one at a time, but still build up an interference pattern like that expected of waves (time elapses a → b → c → d). Image sources: (left) http://www.blacklightpower.com/theory/DoubleSlit/Fig_37-1_Two_Slit_Particles.jpg, (right) http://www.hqrd.hitachi.co.jp/em/emgif/fig2.gif

Wikipedia reports that the diffraction of C₆₀ fullerenes was reported by researchers from the University of Vienna in 1999. Fullerenes have an atomic mass of about 720 amu; their de Broglie wavelength is 2.5 x 10^-12 m. The diameter of the molecule is about 400 times larger. As of 2005, this is the largest object for which wave-like properties have been directly observed.

Image source: http://abyss.uoregon.edu/~js/images/wave_particle.gif

X-rays and Elements

In late 1913, Henry Gwyn Jefferys Moseley constructed X-ray spectrometer using a potassium ferrocyanide, K₄Fe(CN)₆·3H₂O, diffracting crystal. Moseley had been working on radioactivity since 1910, but decided to study X-ray diffraction since it was the hottest new field in physics. With his new spectrometer, he measured the frequency of characteristic K-series X-rays produced from tubes with different anode materials (Ca to Zn). His results showed that the ordering of the wavelengths of the X-ray emissions of the elements coincided with the ordering of the elements by atomic number. This relationship is called Moseley's Law and will be discussed in detail later in this course.

Henry Moseley and X-ray Spectra. Moseley in the lab and aligned photographs of X-ray spectra from different anodes. Note impurity lines in the Co and Ni spectra and the unidentified Zn line in the spectrum of brass. Images sources: (left) http://photos.aip.org/history/Thumbnails/moseley_henry_d9.jpg, (right) http://www.geology.wisc.edu/~johnf/g777/Moseley.jpg

Prior to Moseley's work, atomic numbers were considered semi-arbitrary, based on atomic masses but altered when necessary to put an element in the appropriate place in the periodic table. For example, cobalt and nickel had been assigned atomic numbers of 27 and 28, respectively, based on their chemical properties, since they have nearly identical atomic mass. Moseley's experiments showed that the order of these elements should be reversed. Moseley showed that there were gaps in the atomic number sequence at numbers 43, 61, 72, and 75 and suggested that these would be filled by undiscovered elements. After this work, which was done in Manchester, Moseley moved to Oxford in 1914 and continued his experiments by studying the L-spectra of heavier elements.

Moseley's Results. The linear relationship between atomic number and the square root of frequency is Moseley's Law. Notice in this diagram that the lighter elements have two distinctive characteristic X-ray lines plotted and the heavier have up to three. Image source: http://photos.aip.org/history/Thumbnails/moseley_henry_d9.jpg

However, with the onset of war, Moseley enlisted in the Royal Engineers and was killed on the Gallipoli Peninsula (Turkey), shot through the head while telephoning an order on 10 August 1915. He was just 27. Belatedly, recognizing this great loss to science, the British army changed its policy in World War II, no longer allowing scientists to enlist for combat.

In 1923, Georg Charles von Hevesy proposed using X-rays to excite characteristic X-rays from a sample and use diffraction to identify its constituent elements. This is termed X-ray fluorescence. Later he introduced a method of activation analysis based on neutron bombardment (neutron activation); this method yields better detection limits than X-ray analysis with fluorescent X-rays. In 1923, he and Dirk Coster discovered the element hafnium in X-ray spectrum emitted from zircon. He received the 1943 Nobel Prize in Chemistry for his work on the use of isotopes as tracers in studying chemical processes. Hevesy was the first to apply the radioactive tracer technique to biology, and he later used it in medical research.

Von Hevesy and X-ray Fluorescent Spectra. Typical wavelength dispersive XRF spectrum; note the systematic increase in atomic number toward the left. Image sources: (left) http://www.britannica.com/nobel/art/ohevesy002p1.jpg, (right) http://upload.wikimedia.org/wikipedia/en/2/28/XRFScan.jpg

Development of Instrumentation

Magnetic Lenses

Hans Busch showed theoretically in a 1927 paper that coaxial magnetic field produced by an electric coil could be expected to focus a beam of electrons. He also predicted that the focal length of such a magnetic electron lens could be changed continuously by varying the coil current. This theory was confirmed in 1929 by Ernst Ruska (1906-1988) at the High Voltage Institute, Berlin, under the direction of Max Knoll (1897-1969). In 1931, Ruska constructed a magnetic lens of the type that has been used in all magnetic high-resolution electron microscopes since then. Further work, conducted with Knoll, led to the construction in 1933 of an electron microscope that for the first time gave better definition than a light microscope. Ruska's 1934 Ph. D. thesis investigated the properties of electron lenses with short focal lengths.

Ruska and Knoll's Electron Microscope. The first electron microscope was a transmitted electron instrument shown here in a wonderful mad-scientist photograph (Knoll is at left). Image source: http://www.microscopy.ethz.ch/history.htm

Major limitation of their microscope was that electrons are unable to pass through thick specimens. Thus it was impossible to utilize the instrument to its full capacity until the diamond knife and ultra-microtome were invented in 1951. Ruska was awarded half of 1986 Nobel Prize for Physics; the other half was divided between Heinrich Rohrer and Gerd Binnig for their invention of the scanning tunneling microscope (STM).

Scanning Transmission Electron Microscope

The earliest known paper presenting the concept of a scanning electron microscope was by Knoll (1935). Subsequently, in 1938, Manfred von Ardenne (1907-1997) constructed a scanning transmission electron microscope (STEM) by adding scan coils to a transmission electron microscope. The first micrograph was of a ZnO crystal imaged at an operating voltage of 23 kV at a magnification of 8000 times. The spatial resolution was between 50 and 100 nm. The micrograph contained 400 x 400 scan lines and took 20 min to record, because the film was mechanically scanned in synchronization with the beam. The instrument also had a viewing CRT, but it was not used to record the image.

Von Ardenne and the Scanning TEM. The scan coils (blue star) are located between the electrostatic lenses (red stars). Image sources: (left) http://www-g.eng.cam.ac.uk/125/achievements/mcmullan/images/fig4.jpg; (right) http://www.dhm.de/lemo/objekte/pict/f64_1025/200.jpg

Scanning Secondary Electron Microscope

The first, true scanning electron microscope (SEM) was developed and described in 1942 by Russian immigrant Vladimir Kosmo Zworykin (1889-1982), J. Hillier and R. L. Snyder, working in the RCA Laboratories in the United States. Zworykin also invented the cathode-ray tube called the kinescope in 1929 the precursor of modern television picture tubes; he is sometimes called the "father of television."

The instrument consisted of an inverted column (electron gun at the bottom), three electrostatic lenses and electromagnetic scan coils placed between the second and third lenses. A photomultiplier tube detected the scintillations on a phosphor screen caused by the secondary electron emissions. This detector was an early version of the combination of phosphor and photomultiplier that Everhart and Thornley used nearly twenty years later. The electron gun was located at the bottom so the specimen chamber was at a comfortable height for the operator. However, this arrangement had the slight disadvantage that the specimen could fall down the electron column! The instrument achieved a resolution of about 50 nm, but this figure was considered unexciting compared to results achieved by the rapidly developing TEM, and further development lapsed.

Vladimir Zworkyin and a Schematic of the first SEM. Note that the column is inverted with the electron gun located at the bottom. Image sources: (left) http://www-g.eng.cam.ac.uk/125/achievements/mcmullan/images/fig7.jpg, (right) http://www.acmi.net.au/AIC/ZWORYKIN_1929.html

The early history of the SEM is very well described at the Cambridge University, Department of Engineering SEM site. The following summary is based on this information.

In the late 1940s, Charles Oatley at the Engineering Laboratories of the University of Cambridge decided that another look at the SEM might be worthwhile. He had decided that "Zworykin ... had shown that the scanning principle was basically sound" and believed that the improvements in electronics that had resulted from work during the war would allow better results. Specifically, Oatley thought that the RCA detector had a too low efficiency and thus that the images were noisy in spite of the long recording time.

Oatley selected Dennis McMullan build an SEM as his Ph.D. project! McMullan first finished building a 40 keV electrostatically focused TEM, which had been begun by another PhD student, converting it into an STEM. Next he converted the STEM into an SEM by adding scan coils, an electron multiplier detector and a long persistence cathode-ray tube.

Schematic Diagram and Photograph of McMullan's SEM1. Note that the electron gun is at the top of the column. Image sources: (left): http://www-g.eng.cam.ac.uk/125/achievements/mcmullan/mcm.htm. (right) http://www2.eng.cam.ac.uk/~bcb/cwo2.htm.

Many improvements were incorporated in subsequent SEM2, SEM3, and SEM4 models. In 1952, K. C. A. Smith developed a way to efficiently detect low-energy secondary electrons (previously images were made using high-energy electrons), allowing vastly improved surface imaging. In 1955, T. E. Everhart: devised a new electron detector; the "Everhart-Thornley" detector is still used today. Magnetic lens were added to the SEM4 in 1961. Eventually, Oatley persuaded Associated Electrical Industries (AEI), a company that manufactured both transmission electron instruments and electron probe microanalyzers, to take an interest in the SEM. The first four production models, sold under the trade name "Stereoscan", were delivered in 1965.

Electron Microprobe

In 1944, James Hillier (1915-2007) and Baker at the Radio Corporation of America (RCA) Labs at Princeton, New Jersey, built an electron microprobe, combining an electron microscope and an energy-loss spectrometer. Electron energy-loss spectrometry is very good for light element analysis and they obtained spectra of C-Kα, N-Kα and O-Kα radiation. In 1947, Hillier patented the idea of using an electron beam to produce analytical X-rays, but never constructed a working model. His proposed design used Bragg diffraction from a flat crystal to select specific X-ray wavelengths and a photographic plate as a detector.

Hillier's Electron Microprobe. a) Schematic of microprobe constructed. b) Patented design of the unconstructed microprobe. Note that the electron gun is at the bottom of the column in both designs. Image sources: (left) http://www.geology.wisc.edu/~johnf/g777/Hillier-EELS.jpg, (center) http://www.geology.wisc.edu/~johnf/g777/Hillier_xtal.jpg, (right) http://www.cedmagic.com/mem/whos-who/hillier-james.html

In 1948-1950, Raimond Castaing (1921-1999), supervised by André Guinier built the first electron microprobe ("microsonde electronique") at the University of Paris. He was apparently unaware of Hillier's ideas and patents and worked independently. The resulting instrument produced an electron beam diameter of 1-3 mm with beam current of ~10 nA and used a Geiger counter as a detector. In 1950, Castaing added a fully-focused Johannson quartz crystal between sample and detector to permit wavelength discrimination and an optical microscope to view point of beam impact.

Castaing's 1951 Ph.D. thesis laid the foundations of the theory and application of quantitative analysis by electron microprobe. Castaing recognizing that unknown X-ray intensities measured relative to a pure element could be used as a first approximation for quantifying the chemical composition of the specimen being bombarded. He soon realized that X-ray generation in a multi-element specimen was complex. For example, the interaction volume varies with composition, as does the path for the x-rays leaving the specimen. These complications are known as atomic number (Z) and absorption (A) effects. The interaction is further compounded by a secondary fluorescence effect (F), which addresses the generation of secondary x-rays by absorption of primary x-rays within the specimen. Castaing also discussed effects such as instrumental drift and X-ray background. Because of the enormous breadth of his contributions to the field, Castaing is often described as the "father" of electron microprobe analysis.

Schematic diagram of Castaing's microprobe and photograph of the MS85 instrument. Note that the electron gun is at the top of the column. Image sources: (left) <unknown>, (center) http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Microsonde_castaing_MS85.jpg/85px-Microsonde_castaing_MS85.jpg, (right) <unknown>.

Cameca (France) produced the first commercial microprobe, MS85, in 1956; based on Castaing's design. Later in his distinguished career, Castaing was involved in the development of the ion beam microprobe.

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