I am using velocity map imaging (VMI) for analyzing the electrons generated by soft x-ray beam. The target gas is Neon and is injected through a thin capillary, I get pressure of 1.3 E-4 mbar in my chamber. The beam is p-polarized and is incident normal to the VMI detector. The problem is even at maximum MCP gain the count rate is 4 electrons per shot maximum which is very low as compared to values cited in literature. Can somebody give a explanation and/or solution to this problem? The sketch from the setup is attached.
The count rate depends on (1) gas density (2) light intensity (3) atomic cross section.
To increase (1) take a look at the design of a repller integrated source in: Ghafur et al. Rev. Sci. Instr. 80, 033110 (2009). You might also try a smaller diameter capillary.
To increase (2), you will need have to work on the XUV source. Is it a high-harmonic source?
To increase (3), choose a target with larger cross section e.g. xenon.
Thanks a lot for taking time to answer my question. As You can see in the sketch of my setup I am using a thin capillary already the outer diameter is 0,3 mm and inner is 0,2 mm (should give a high target density) and it is coming transversely between VMI electrodes therefore in principle I should get even higher count rate than cited for VMI setups with no capillary involved. The light is coming from a high harmonic source and photon flux at the interaction point is 10E4 which is comparable to highly efficient sources cited. I have used Xenon as well but even there the count rate is not more than 4 counts per shot which is far less than the conventional value of 20 counts per shot.
Another suggestion: think about focusing conditions of the XUV beam. A tight focus will allow you to reach high intensity but will still produce fewer total counts.
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