Conversion from ionization chamber current to photon flux
The ionization chamber is assumed to be filled with air (1atm, 295K).
Mass attenuation coefficents and energy-mass absorption coefficients were obtained from NIST for dry air near sea level (http://physics.nist.gov/PhysRefData/XrayMassCoef/ComTab/air.html).
The conversion is very inaccuate when the energy is high and the absorption by air is small.
There can be a large error due to re-combination of charges when the x-ray beam is small.
Probably, most people use the value based on the mass attenuation coefficient, especially with low energy x-rays.
You can copy this page and run this program on your PC!