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Charging effect in electron‐irradiated ice
Author(s) -
Fuchs Wolfgang,
Brombach Jobst D.,
Trösch Walter
Publication year - 1978
Publication title -
journal of microscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.569
H-Index - 111
eISSN - 1365-2818
pISSN - 0022-2720
DOI - 10.1111/j.1365-2818.1978.tb01154.x
Subject(s) - space charge , electron , irradiation , microanalysis , conductivity , cathode ray , acceleration voltage , electron microprobe , materials science , analytical chemistry (journal) , coating , beam (structure) , chemistry , atomic physics , optics , mineralogy , composite material , physics , organic chemistry , chromatography , quantum mechanics , nuclear physics
SUMMARY To maintain the original distribution pattern of diffusible elements in biological samples, electron probe microanalysis is carried out with frozen hydrated bulk specimens and cryosections, analysed at temperatures below 130 K. Ice has a very low intrinsic conductivity at this working temperature and surface‐ and space‐charging appears, when uncoated specimens are irradiated with non‐penetrating electrons. Although coating with a grounded conductor abolishes the surface potential, the build‐up of an internal space‐charge field is possible, depending on the sample thickness and beam voltage used. Consequently, the geometry of the X‐ray source volume and the spectral distribution of the emitted continuous and characteristic X‐rays are affected. To simulate the situation for microanalysis of frozen hydrated specimens the charging process in electron irradiated ice is studied by recording simultaneous specimen currents from the top and bottom of ice layer preparations. The external currents yield information on the build‐up of internal space‐charge fields which result from the balance of charge injection, storage, and transport. Irradiation of uncoated bulk specimens with a finely focused beam results in the build‐up of a space‐charge field close to the surface, which causes a reduction of the depth of microprobe analysis. In coated bulk specimens the induced conductivity renders possible a current flow to the front electrode, thereby limiting the space‐charge field. Sections with an effective rear electrode will not charge appreciably if the electron range is larger than about half the section thickness.