page_title: Environmental scanning electron microscope
section_title: Gaseous detection device
hierarchical_section_title: Environmental scanning electron microscope / Microscope / Signal detection / Secondary electrons / Gaseous detection device
caption_attribution_description: Principle of gaseous detection device (GDD) used for detection and amplification of signals in ESEM. Secondary electrons (SE) from specimen are acted upon by electric field E between cathode and anode electrodes at distance d. Electrons drift towards the anode biased with voltage V, multiplying in an avalanche process by collisions with gas while they diffuse laterally by thermal collisions with gas molecules.
context_page_description: The environmental scanning electron microscope is a scanning electron microscope that allows for the option of collecting electron micrographs of specimens that are wet, uncoated, or both by allowing for a gaseous environment in the specimen chamber. Although there were earlier successes at viewing wet specimens in internal chambers in modified SEMs, the ESEM with its specialized electron detectors and its differential pumping systems, to allow for the transfer of the electron beam from the high vacuums in the gun area to the high pressures attainable in its specimen chamber, make it a complete and unique instrument designed for the purpose of imaging specimens in their natural state. The instrument was designed originally by Gerasimos Danilatos while working at the University of New South Wales.
context_section_description: In a simple form, the gaseous detection device (GDD) employs an electrode with a voltage up to several hundred volts to collect the secondary electrons in the ESEM. The principle of this SE detector is best described by considering two parallel plates at a distance d apart with a potential difference V generating a uniform electric field E = V/d, and is  shown in the accompanying diagram of the GDD. Secondary electrons released from the specimen at the point of beam impingement are driven by the field force towards the anode electrode but the electrons also move radially due to thermal diffusion from collisions with the gas molecules. The variation of electron collection fraction R within anode radius r vs. r/d, for fixed values of anode bias V, at constant product of (pressure·distance) p·d = 1 Pa·m, is given by the accompanying characteristic curves of efficiency of the GDD. All of the secondary electrons are detected if the parameters of this device are properly designed. This clearly shows that practically 100% efficiency is possible within a small radius of collector electrode with only moderate bias. At these levels of bias, no catastrophic discharge takes place. Instead, a controlled proportional multiplication of electrons is generated as the electrons collide with gas molecules releasing new electrons on their way to the anode. This principle of avalanche amplification operates similarly to proportional counters used to detect high energy radiation. The signal thus picked up by the anode is further amplified and processed to modulate a display screen and form an image as in SEM. Notably, in this design and the associated gaseous electron amplification, the product p·d is an independent parameter, so that there is a wide range of values of pressure and electrode geometry which can be described by the same characteristics. The consequence of this analysis is that the secondary electrons are possible to detect in a gaseous environment even at high pressures, depending on the engineering efficacy of any given instrument.
  As a further characteristic of the GDD, a gaseous scintillation avalanche also accompanies the electron avalanche and, by detection of the light produced with a photo-multiplier, corresponding SE images can be routinely made. The frequency response of this mode has allowed the use of true TV scanning rates. This mode of the detector has been employed by a latest generation of commercial instruments.
  The novel GDD has become possible first in the ESEM and has produced a practically 100% SE collection efficiency not previously possible with the Everhart-Thornley SE detector where the free trajectories of electrons in vacuum cannot all be bent towards the detector. As is further explained below, backscattered electrons can also be detected by the signal-gas interactions, so that various parameters of this generalized gaseous detector must be controlled to separate the BSE component out of the SE image. Therefore,  care has been taken to produce nearly pure SE images with these detectors, then called ESD (environmental secondary detector) and GSED (gaseous secondary electron detector).
caption: ESEM gaseous detection device (GDD)-principle
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