Hi Drilbert
Here some answers to your questions:

1) I think focal point and focal spot diameters are the same. This diameter
is a very important characteristic of the Xray equipment. But not only.
Another point is the minimum distance between specimen and focal spot.
These two quantities determine the resolution and the maximum magnification
of your equipment. This comes from the fact that a Xray equipment works
with the principle of casting a shadow. Therefore, the smaller the distance
between focal spot and specimen the bigger the magnification and the
diameter of the focal spot is the maximum magnification you get. E.g. if
you have a focal spot of 10um diameter you may resolve two points with a
distance of 10um as two points.

2) If you want to work with Xray you must generate Xrays. This is done by
sending a electron beam on a target. This has two effects:

- There electrons become for a short time part of the top atoms of the
target. The atoms are now in a exited state which is not stable. The excess
electrons are released again by emitting Xray .
- The electrons are deflected. This produces Xrays as well.

To establish an electron beam you need a vacuum. In a sealed tube the
vacuum is always there. As soon as you start your equipment you can work. A
not sealed tube must be evacuated before a electron beam can establish.
This takes approx. 5-10min. for a vacuum good enough to work with 80kV
acceleration voltage.

Both tubes have a filament as a source for the electron beam. Both
filaments burn out with time when used. In my opinion this is a just a
question of money and time. To my knowledge, a not sealed filament is
cheaper than a evacuated tube. It might be that changing a sealed filament
is easier than opening a vacuum tube to change a filament. However, this
takes me not more than 1/2 h.

Best regards

Guenter Grossmann



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