Conversion from ionization chamber current to photon flux
The ionization chamber is assumed to be filled with Ar (1atm, 295K).
attenuation coefficents and energy-mass absorption coefficients were obtained
from NIST for dry air near sea level
conversion is very inaccuate when the energy is high and the absorption by air
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.
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