A method of calibration for low energy X-ray beams

The measurement of very soft X-ray beams with conventional thimble ionization chambers can be very difficult because of extremely high exposure rates. Simple inverse square calculations do not suffice, even if the X-ray target is small, because of significant attenuation by the air. A procedure sometimes employed is to plot the inverse square root of several readings taken at various distances and extrapolate this curve down to the desired distance. The trouble here is that a small error in plotting or reading the graph can result in a large error in exposure rate. The method described below is basically an inverse square calculation which takes into account the attenuation due to the air.