An apparatus for determining fluid phase fraction of a multiphase fluid mixture (13) comprises: —a x-ray generator (20) arranged to emit a x-ray radiation spectrum comprising a low energy region and a high energy region, the high energy region including a Bremsstrahlung spectrum; —a pipe section (27) through which the multiphase fluid mixture (13) flows comprising a measurement section (28), said measurement section (28) being coupled to said x-ray generator (20); —a detector (30) coupled to said measurement section (28) and arranged to detect x-ray radiation that has passed through said multiphase fluid mixture (13), the detector (30) being coupled to a multichannel analyzer (32) producing a measurement output comprising a low energy (LE) and high energy (HE) measurement counts; wherein the measurement output further comprises a low energy (LV) and high energy (HV) control counts, in a low energy and high energy control windows located on an edge of the Bremsstrahlung spectrum, respectively; and wherein the apparatus further comprises an electrical parameter control arrangement (33) coupled to the x-ray generator (20) and the detector (30), the electrical parameter control arrangement (33) being arranged to calculate a first ratio of the high energy control count relative to the low energy control count (RV=HV/LV) and a second ratio of the high energy measurement count relative to the low energy measurement count (RE=HE/LE), and to adjust the electrical operation of the x-ray generator (20) based on an electrical parameter control function (FC(V)) of said ratios that minimize a dependence of the electrical operation of the x-ray generator on the fluid phase fraction of the multiphase fluid mixture (13) flowing in the measurement section (28).