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UW- Madison Geology 777. Electron probe microanalysis - Electron microprobe analysis EPMA (EMPA). An Historical Introduction: Merging of discoveries in physics, chemistry and microscopy. Revised 1/21/2012. UW- Madison Geology 777. Overview. Electrons and x-rays

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UW- Madison Geology 777

Electron probe microanalysis - Electron microprobe analysisEPMA (EMPA)

An Historical Introduction:

Merging of discoveries in physics, chemistry and microscopy

Revised 1/21/2012


UW- Madison Geology 777

Overview

Electrons and x-rays

Spectroscopy and chemical analysis

Development of electron and x-ray instruments

Essentials of an electron microprobe


UW- Madison Geology 777

Electrons - 1

1650, Otto von Guericke built the first air pump; 1654 he demonstrated power of vacuum to German emperor (horses couldn’t pull 2 hemispheres apart) in Magdeburg

Guericke built first frictional electric machine, producing sparks from a charged sulfur globe, which he reported to Leibniz in 1672

1705, Francis Hauksbee improved the frictional machine (evacuated glass sphere, turned by crank)

1745 at University of Leiden, the “Leyden jar” (primitive condensor) was built, a metal-lined glass jar with rod stuck in middle thru cork; it stored large quantities of static electricity produced thru friction

1752, B. Franklin flew kite in thunderstorm and charged a Leyden jar (and was luckily not killed)


UW- Madison Geology 777

Electrons - 2

18th Century: Benjamin Franklin described electricity as an elastic fluid made of extremely small particles. Electrical conductivity was observed in air near hot poker (= thermoionic emission of electrons)

Cathode ray effects (glow) noticed by Faraday (1821); named “fluorescence” in 1852 by Stokes

1855 Geissler devised a pump to improve the vacuum in evacuated electric tubes (=Geissler tubes)

1858 Plücker forced electric current thru a Geissler tube, observed fluorescence, and saw it was deflected by a magnet. Some credit him with discovery of cathode rays


UW- Madison Geology 777

Electrons - 3

1875 Wm. Crookes devised a better vacuum tube

1880 Crookes found that cathode rays travel in straight lines and could turn a wheel if it was struck on one side, and by their direction of curvature in magnetic field, that they were negatively charged particles

1887 Photoelectric effect discovered by Heinrich Hertz: light (photon of l < critical for a metal) falling on metal surface ejects electrons from the metal

1894, Philipp von Lenard (student of Hertz) put a thin metal window in vacuum tube and directed cathode rays into the outside air


UW- Madison Geology 777

Electrons - 4

Cathode rays confirmed by J.J. Thomson in 1897 to be electrons, and that they travel slower than light, they transport negative electricity and are deflected by electric field

1900 Lenard, studying electric charges from illuminated metal surfaces (photoelectric effect), concluded they are identical to electrons of cathode ray tube

1905 Einstein explained the theoretical basis of the photoelectric effect using Planck’s quantum theory (of 1900); for this, Einstein received Nobel Prize in physics in 1921


UW- Madison Geology 777

Electrons - 5

1922 Auger electrons discovered (“internal photoelectric effect”)

1927 electron diffraction discovered independently by Davisson (US) and Thomson (Gt. Britain)


UW- Madison Geology 777

X-rays - 1

1885-1895 Wm. Crookes sought unsuccessfully the cause of repeated fogging of photographic plates stored near his cathode ray tubes.

X-rays discovered in 1895 by Roentgen, using ~40 keV electrons (1st Nobel Prize in Physics 1901)

1909 Barkla and Sadler discovered characteristic X-rays, in studying fluorescence spectra (though Barkla incorrectly understood origin) (Barkla got 1917 Nobel Prize)

1909 Kaye excited pure element spectra by electron bombardment


n l = 2d sin q

UW- Madison Geology 777

X-rays - 2

1912 von Laue, Friedrich and Knipping observe X-ray diffraction (Nobel Prize to von Laue in 1914)

1912-13 Beatty demonstrated that electrons directly produced 2 radiations: (a) independent radiation, Bremsstrahlung, and (b) characteristic radiation only when the electrons had high enough energy

1913 WH + WL Bragg build X-ray spectrometer, using NaCl to resolve Pt X-rays. Braggs’ Law. (Nobel Prize 1915)


UW- Madison Geology 777

X-rays - 3

1913 Moseley constructed an x-ray spectrometer covering Zn to Ca (later to Al), using an x-ray tube with changeable targets, a potassium ferrocyanide crystal, slits and photographic plates

1914, figure at right is the first electron probe analysis of a manmade alloy

T. Mulvey Fig 1.5 (in Scott & Love, 1983). Note impurity lines in Co and Ni spectra


l

Z

UW- Madison Geology 777

X-rays - 4

  • Using wavelengths, Moseley developed the concept of atomic number and how elements were arranged in the periodic table.

Moseley found that wavelength of characteristic X-rays varied systematically (inversely) with atomic number

  • The next year, he was killed in Turkey in WWI. “In view of what he might still have accomplished (he was only 27 when he died), his death might well have been the most costly single death of the war to mankind generally,” says Isaac Asimov (Biographical Encyclopedia of Science &Technology).


UW- Madison Geology 777

X-rays - 5

1916 Manne Siegbahn and W. Stenstrom observe emission satellite lines (Nobel to first in 1924)

1923 Arthur Compton discovered effect relating direction taken by X-ray and electron after collision, with the energy of collision

1923 Manne Siegbahn published The Spectroscopy of X-rays in which he shows that the Bragg equation must be revised to take refraction into account, and he lays out the “Siegbahn notation” for X-rays

1931 Johann developed bent crystal spectrometer (higher efficiency)


UW- Madison Geology 777

X-rays - 6

X-rays are considered both particles and waves, i.e., consisting of small packets of electromagnetic waves, or photons.

X-rays produced by accelerating HV electrons in a vacuum and colliding them with a target.

The resulting spectrum contains (1) continuous background (Bremsstrahlung;“white X-rays”), (2) occurrence of sharp lines (characteristic X-rays), and (3) a cutoff of continuum at a short wavelength.

X-rays have no mass, no charge (vs. electrons)


UW- Madison Geology 777

1. X-rays cause many materials to fluoresce besides the original BaPbCN coating observed by Roentgen.

2. X-rays affect photographic emulsions.

3. When exposed to X-rays, electrified objects lose charge.

4. Some materials transparent to X-rays

5. X-rays collimated by pinholes, showing they travel in straight lines.

6. X-rays not deflected by magnetic fields, and so are not streams of charged particles.

X-rays: 9 Features-1 (per Roentgen)


UW- Madison Geology 777

7. X-rays produced by beams of high energy cathode rays striking objects.

8. Heavy elements more efficient producers of X-rays compared to light elements.

9. Reflection and refraction of X-rays (bending of rays at interface) not observed (but later they were found to exist in small degrees.)

X-rays: 9 Features-2(per Roentgen)


UW- Madison Geology 777

Chemical analysis

1859 Kirchhoff and Bunsen showed patterns of lines given off by incandescent solid or liquid are characteristic of that substance

1904 Barkla showed each element could emit ≥1 characteristic groups (K,L,M) of X-rays when a specimen was bombarded with beam of x-rays

1909 Kaye showed same happened with bombardment of cathode rays (electrons)

1913 Moseley found systematic variation of wavelength of characteristic X-rays of different elements

1922 Mineral analysis using X-ray spectra (Hadding)

1923 Hf discovered by von Hevesy (gap in Moseley plot at Z=72). Proposed XRF (secondary X-ray fluorescence)


UW- Madison Geology 777

Electron Microscopy -1

1926 Busch developed theory of magnetic lens to focus electrons, confirmed by Ernst Ruska in 1929 -- at High Voltage Institute, Berlin, under Max Knoll-- all related to need to find a way to study surges in HV cables from lightning

1932 Ruska built the first electron microscope, with prototype by Siemens & Halske Co. Ruska received, belatedly, Nobel Prize for it in 1986.

1930’s, electron microscopes also built in labs in England, Belgium, USA, Canada

1938-44, commercially Siemens delivered 38 electron microscopes; also models built by RCA and Japanese firms.


UW- Madison Geology 777

Electron Microscopy -2

1937 grad students J. Hillier and A. Prebus at Univ. of Toronto built an electron microscope that magnified 7000x

1940 Hillier hired (pre PhD) by Zworykin of RCA to immediately build an electron microscope to sell (and pay back his salary) (Electron microscope, U.S. Patent No. 2,354,263; 1944)


UW- Madison Geology 777

Electron Microscopy - SEM

  • Post WWII, Dennis McMullan at Cambridge (England) began working on SEMs. Culminated in 1965 with first commercial SEM, the Stereoscan by Cambridge Instrument Co.

A scanning electron microscope was built in mid 1930s by Manfred von Ardenne (his Berlin lab was bombed in 1944 and he never returned to SEM development)

1942 at RCA, Hillier built SEM and used it to examine surfaces of specimens

Stereoscan MK-1


Electron Microprobe - Precursors

1898 in Berlin, Starke measured the backscattered fraction of electrons and plotted it against atomic weight. First “electron probe” (not micro).

1909, Kaye built apparatus to bombard moveable specimens with 28 keV electrons and observe gas discharge in ionization chamber using various elemental absorption screens to identify unknown by deduction

1912-13, Beatty built apparatus that showed that the effective depth of production of x-rays was very small (<10 mm), which would have critical implications for development of microanalysis


UW- Madison Geology 777

Electron Microprobe - 1

Hillier 1943 and Hillier and Baker (1944) at RCA Labs at Princeton NJ built an electron microprobe, by combining an electron projection microscope and an energy-loss spectrometer.

They obtained spectra of C, N and O K radiation from a collodion film

U.S. Patent: 1945, Electron microanalyzer (No. 2,372,422)

RCA electron-probe microanalyzer (Hillier and Baker, 1944)


UW- Madison Geology 777

  • Hillier also developed the idea of adding an “x-ray spectroscope” strongly reminiscent of Moseley’s design, with a flat diffracting crystal and a photographic plate as a detector.

  • Electron probe analysis employing x-ray spectography (No. 2, 418, 029; 1947)

  • Unfortunately RCA had no interest in pursuing EPMA!

Electron Microprobe - 2

From Hillier’s 1947 patent


UW- Madison Geology 777

  • “It would appear that, because of post-war difficulties in scientific communication, news of the Hillier Patent had not reached Castaing and Guinier in France in 1947.”

  • “In January 1947 Raymond Castaing had joined the research staff of ONERA and became involved in the setting up of

Electron Microprobe - 3

an electron microscope laboratory for metallurgical and materials research.”

“In 1948 during an investigation into properties of Cu-Al alloys, Professor Guinier asked Castaing about the possibility of making a point by point analysis of a metal sample by bombarding it with electrons and measuring the characteristic x-ray emission.”

Quotes from T. Mulvey (1983) Development of electron-probe microanalysis-an historical perspective”


UW- Madison Geology 777

  • “The idea was to analyze at least qualitatively areas of some hundreds of Å units in diameter although it was realized that the counting rates would be low, perhaps a few pulses a minute. It was a tall order but by early 1949 Castaing had succeeded in producing an electron probe of ~1 mm in diameter with current ~10 nA when everything worked OK.”

Electron Microprobe - 4

  • The first version of his probe used a Geiger counter which could not distinguish elements directly. In 1950 he fitted a quartz crystal prior to the Geiger counter to permit wavelength discrimination, and added an optical microscope to view the point of beam impact.


UW- Madison Geology 777

Electron Microprobe - 5

Castaing, while not the inventor under Patent Law, may be rightly regarded as the father of EPMA

In his Ph.D. (Castaing, 1951), he laid down the fundamental principles of the method and its use as a tool for microanalysis.

He established the theoretical framework for the matrix corrections for absorption and fluorescence effects

  • 1956, commercial electron microprobe production begins with Cameca MS85* (above), followed in 1958 by Hitachi.*MicroSonde=microprobe


Electron Microprobe - 6

In the early or mid-50s, Buschmann at GE built an electron microprobe (right) modelled after Castaing’s that has been called the first operating microprobe in the U.S.

However, the bean counters at GE said there was no market for such an instrument and persuaded management to abandon its commercial development.

Newberry, p. 57


UW- Madison Geology 777

Electron Microprobe - 7

1960: ARL EMX, and MAC EMPs. 1961, first JEOL EMP. Many researchers build “homebrew” electron microprobes

Motivation: space/arms race, semi-conductor and other materials research.

David Wittry built an EMP at Cal Tech, shown to right (Thesis, 1957). He and his advisor Pol Duwez also translated Castaing’s thesis (with Army $).


UW- Madison Geology 777

Developments forSEM-Electron Microprobe

1960, Cambridge Instrument Co produced a rastered beam instrument (SEM) to make X-ray maps.

1968, solid state EDS detectors developed. These are add-ons to SEMs and EMPs.

1970, Microspec develops “add-on” crystal (WDS) spectrometer for SEMs.

By 1970-80s: Scanning coils included on EMPs for SE and BSE imaging.

1984, development of synthetic multilayer diffractors (large 2d), for WDS of light elements.

1990s experimental development of micro-calorimeter EDS detectors.


UW- Madison Geology 777

Selected References

Mulvey, T, 1983, The development of electron-probe micro-analysis--An historical perspective, in Quantitative Electron-Probe Microanalysis (Eds V.D. Scott and G. Love), Wiley, p. 15-35.

Asimov, I, 1972, Asimov’s Biographical Encyclopedia of Science and Technology, Doubleday, 805 pp.

Asimov, I., 1994, Asimov’s Chronology of Science and Discovery, Harper Collins, 791 pp.

Newberry, S. P., 1992, EMSA and Its People: The First Fifty Years, Electron Microscopy Society of America

Clark,G. L., 1940, Applied X-rays, McGraw Hill (Ch.1: Before and after the discovery by Roentgen)

David Wittry, Early history of Microbeam Analysis Society, on MAS website


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