High Voltage Electron Microscope

High voltage Electron Microscopes (HVEM) are characterized by their high acceleration voltage (>300KV) and great penetration depth. Virtually all HVEM's are of the transmission type. Since the interaction volume of the beam is of such size that useful resolution beyond that of the light microscope can not be achieved by means of probe surface interactions as in the scanning electron microscope. At this point in time there is a new term for microscopes which exceen acceleration voltages of 1.5MV, these are known as Ultra High Voltage Electron Microscope

Charactaristics

HVEM's are capable of penetrating thick samples due to their very large beam energy in comparison to their medium and low voltage brethren, this is useful for 3D imaging and nano tomography.

It is also very common for the focal length of these high voltage microscopes to be rather large in comparison to lower voltage microscopes, this is due to there being an upper Limit to flux density that can be achieved in a magnetic lens of normal design as well as with those featuring superconducting lenses.

Due to the above mentioned large interaction volume, the image on the view screen is quite dim in comparison to lower voltage microscopes, since most energy is deposited below the phosphor layer, thus not contributing to the conversion to light.

Radiation shielding

Due to the high voltage beam used, the energy spectrum of the Bremsstrahlung ist well into that of Gamma Rays, but since its created by artificial means its called Hard X-Rays. Shielding of these is of rather high importance for user safety. This necessitates the use of thick lead glass windows for the view screen, typically with thicknesses of 20cm or more, as well as thick lead shielding. Normally the user also wears a Dosimeter to track the unavoidable radiation leakage from the microscope while in use.

A notable example for a microscope that did not feature heavy shielding, but instead relied on the use of a dosimeter and cutout circuit was the 650KV HREM in Cambridge. This was chosen so as to facilitate easier maintenance and disassembly of the column, owing to it being an experimental microscope.

High Voltage

These microscopes require the generation of exceptionally high voltages, this is done in one of two ways. Either by means of a Van de Graff belt generator, or by means of a Voltage Multiplier. The latter being the more common. To limit the size of the microscopes, the high voltage generator is often housed within a pressure vessel filled with a insulating gas, such as Freon or SF6 at elevated pressure.

The Electron guns used for these microscopes are exclusively of the multi anode variety, forming a electrostatic linear accelerator. In order to feed the cathode heater, either Accumulators or a Isolation transformer cascade where used. The latter being several transformers of the same winding ratio connected to each other in order to allow one transformer to isolate only a fraction of the total system voltage. Each transformer is raised to a potential higher then the last, along the multiplier circuit.

In the case of accumulators, a telescoping charging connector was used to charge them, usually over night, for use during the day. An example of this design is the AEI EM7. The Accumulator design originates directly from the principle used by early electron microscopes, which likewise featured accumulators at potential close by the cathode.

The cascade type generators are normally fed by high frequency AC, either generated by inverters, either Vacuum Tube, or semiconductor based. One notable exception is the JEOL JEM-1000, which used a rotary converter to generate its high frequency AC.

Commercial Units

Below is a incomplete list of commercially available HVEM throughout history. Most such microscopes where either custom made by a manufacturer or by research institutes themselves.

- AEI EM7/EM7A - Hitachi HU650 - JEOL JEM1000 - JEOL JEM-4000EX/FX - JEOL JEM-ARM-1250

Experimental Units

- 650KV HREM (Cambridge) - 600KV Superconducting Microscope (Siemens) - 1MV Van De Graff Microscope (Tübingen) - 1MV Holographic Microscope (Hitachi Japan) - 1MV Gaston Dupouy Microscope (CNRS France) - EX-1000 Phillips Endhoven - 1MV Folded column microscope - 1MV TEM (Australia) - 1MV RCA TEM