I use both soft and hard X-Ray generators and have the following comments for you.
1. The GM tube is not the most suitable detection device for X-Ray generators running at much less than 45keV. They will not respond well to the low X-Ray energy levels being emitted from the tube
2. Remember that the excitation voltage applied to the tube produces a far lower keV at the output and so KVp is NOT a direct indicator of keV being emitted. That is to say, if you apply 45kV to an X-Ray tube, it's theoretical maximum output energy is 45keV, but in reality the output is far less.
3. For X-Ray energy levels of 45keV and lower, the most appropriate measuring and detection devices use a scintillation probe, ionisation chamber or semiconductor detector. The GM tube is unlikely to respond and even if it does, it will be hopelessly inaccurate. GM tubes normally have a minimum detection keV specification and the correction factor needed for any meaningful measurement. Below this level of energy, the tube is virtually blind to the energy.
4. I own a Tel-X-ometer 580 that uses a 20 or 30kVp excitation voltage on the X-Ray tube. There is an optional GM tube and rate counter available for the 580 so there are GM tubes that will respond to such low levels of keV. They will likely be on the edge of their detection capability though.
5. If you wish to continue your use of GM tubes in your experiments, you need to find one with a very low keV threshold as anything else will be misleading in its operation.
6. Personal Dosimeters for use with X-Ray need to be accurate and effective at lower keV levels. They often use a scintillator, or more modern versions, a specially doped semiconductor detector. GM tubes are normally found in the units designed for higher levels of Gamma radiation, such as found in the Nuclear industry or high energy X-Ray generation.
If you are interested in X-Ray's and generators, you may wish to search this forum for my posts on the Faxitron MX-20 and Tel-X-ometer 580 machines. I also provide comment on the appropriate personal dosimeters that I use.
Fraser