4.7 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Gaseous detection device | 3/4 | https://en.wikipedia.org/wiki/Gaseous_detection_device | reference | science, encyclopedia | 2026-05-05T10:04:37.957284+00:00 | kb-cron |
At the point in time when the charge arrives at the electrode, there is no current flowing in the circuit since υ = 0, only when the charge is in motion between the electrodes do we have a signal current. This is important in the case, for example, when a new electron-ion pair is generated at any point in the space between anode-cathode, say at x distance from the anode. Then, only a fraction ex/d of charge is induced by the electron during its transit to the anode, whilst the remainder fraction of e(d − x)/d charge is induced by the ion during its transit to the cathode. Addition of those two fractions gives a charge equal to the charge of one electron. Thus by counting the electrons arriving at the anode or the ions at the cathode we derive the same figure in current measurement. However, since the electrons have a drift velocity about three orders of magnitude greater (in nanosecond range) than the ions, the induced signal may be separated in two components of different significance when the ion transit time may become greater than the pixel time on the scanned image. The GDD has thus two inherent time-constants, a very short one due to the electrons and a longer one due to the ions. When the ion transit time is greater than the pixel dwell time, the useful signal intensity decreases together with an increase of signal background noise or smearing of image edges due to the ions lagging behind. As a consequence, the above derivations, which include the total electron and ion contributions must be modified accordingly with new equations for the case of fast scanning rates. The electrode geometry can be altered with a view to decrease the ion transit time as can be done with a needle or cylindrical geometry. This fundamental approach helps also understand the so-called “specimen absorbed current” mode of detection in the vacuum SEM, which is limited only to conductive specimens. Image formation of non-conductive specimens now possible in the ESEM, can be understood in terms of an induced displacement current in the external circuit via a capacitor-like action with the specimen being the dielectric between its surface and the underlying electrode. Therefore, the (misnomer) "specimen absorbed current" per se plays no part in any useful image formation except to dissipate the charge (in conductors), without which insulators cannot be generally imaged in vacuum (except in the rare case when the incident beam current equals the total emitted current).
== SE detector gain == By use of a derivation for the Townsend coefficient given by von Engel, the gain factor G, in the case of SE with total current collection Itot (i.e. for R = 1), is found by:
G
=
I
tot
δ
I
b
=
exp
[
A
p
d
exp
(
−
B
p
d
V
)
]
{\displaystyle \ G={\frac {I_{\text{tot}}}{\delta I_{\text{b}}}}=\exp \left[Apd\exp \left(-B{\frac {pd}{V}}\right)\right]}
where A and B are tabulated constants for various gases. In the diagram supplied, we plot the gain characteristics for nitrogen with A = 9.0 and B = 256.5 valid in the range 75–450 V/(Pa·m) for the ratio E/p. We should note that in ESEM work the product pd < 3 Pa·m, since at higher values no useful beam is transmitted through the gas layer to the specimen surface. The gray-shaded area shows the region of GDD operation provided also that the γ processes are very low and do not trigger a breakdown of the proportional amplification. This area contains the maxima of the gain curves, which further re-enforces the successful application of this technology to ESEM. The curves outside the shaded area can be used with beam energy greater than 30 kV, and in future development of environmental or atmospheric transmission scanning electron microscopes employing very high beam energy.
== General implementations ==