Special Considerations for Helium-Filled Ion Chambers

As the flux at synchrotron radiation beamlines increases, helium-filled ion chambers are more common to avoid saturation and non-linearities. For helium, this paper will show that the conversion of the current to flux is best performed using the mass-energy cross-section but is complicated by a strong interaction of scattered photons with the ion chamber plates. This makes the conversion highly dependent on the ion chamber geometry, and an accurate flux determination will typically require a calibration. To minimize the sensitivity of the calibration to external influences, it is proposed to add internal scatter baffles to the standard chamber configuration.