Thermo Scientific Apreo User's Operation Manual page 22

loss in the gas. For EDX, samples are scanned at a 10 mm working distance, which is the stage eucentric position
and the collection point of the EDX detector. The cone height is 5.5 mm and restricts the field of view. See Chapter
5 for the cone mounting procedure.
The longer profile of this cone minimizes low voltage beam dispersion and skirting of the
primary beam in the gaseous environment of the chamber, allowing more electrons to interact
with the specimen when focused and increasing the signal to noise ratio.

Note
The PLA also acts as a final or objective aperture, so the pressure over it is considered very low. Any
pollution that accumulates on the aperture edge greatly affects imaging. If it is not possible to correct
astigmatism, it is usually a sign that this aperture needs to be cleaned or replaced (see Chapter 6).

W a r n i n g !
Applying a cone or a detector with an integrated cone scales down the space between the top sample surface and the
cone (at the particular working distance) by the value equivalent to the cone height.
Using gas
The Low Vacuum mode allows a user to image samples in a gaseous atmosphere, which
can be selected in the drop down list box:
• Auxiliary (option) gaseous environment, which is supplied via the inlet placed on the
back of the console.
• Water vapor is supplied from a built-in water reservoir located on the left side of the
microscope console.

Note
On occasion the water reservoir needs to be filled (see Chapter 6).

C a u t i o n !
Maximum overpressure for the Auxiliary gas and Nitrogen inlets is 10 kPa (0.1 bar). The Nitrogen inlet is used only for
venting the chamber with air, or preferably with nitrogen.
When using a particular pressure limiting aperture, there are pressure limits for different gasses.
Maximal Chamber Pressure [Pa (Torr)] under Different Gaseous Environment
Table 2-1:
Working Gas
Water - H O
2
Nitrogen - N
2
Air
Carbon Dioxide - CO
2
Nitrogen Dioxide - NO
Helium - He
70 % He + 30 % H
2
**)
Argon - Ar
*)
C H
x y

C a u t i o n !
The system doesn't watch the limits, and higher overpressure (especially for gasses not listed) set by a user could switch
off the emitter! In some cases, the system would need to be restarted by a service engineer.

Note
*) Combustible gases (acetylene for instance) must always be used with respect to safety issues.
**) Argon use should be minimized to a short time, because the IGP's are not at all optimized for pumping it.
Purging
During this procedure the specimen chamber is automatically pumped down to a lower pressure to remove the old
gas and is then is flooded with the new gas (selected in the Vacuum module) to a higher pressure. This takes place
several times, until the old gas is removed and the chamber is mostly filled with the new gas. This is applied when
the system is in the LoVac mode and the Purge button is pressed (see the Preferences... dialogue / Low Vacuum
section). The procedure is also started automatically with factory presets in these situations:
• when the LoVac mode has not been used for 1 week or more;
• after water bottle venting when refilling the water level (see Chapter 6).

Note
The automatic procedure can take a while (up to 10 minutes). Wait until the Vacuum status indicates Pumped, because the
detectors do not start operation until desired pressure is reached.
2-12 User Manual
System control: Vacuum system
2
Maximum Chamber Pressure [Pa] (Torr)
50 (0.4)
25 (0.2)
20 (0.15)
16 (0.1)
50 (0.4)
C O N F I D E N T I A L – limited rights
Feb 2018

Revision A