Determination of the Practical Peak Voltage from the Bremsstrahlung Damping Rate in a Semiconductor Detector

Abstract

The spectrum of the bremsstrahlung depends on X ray anode tube voltage, anode voltage curve, anode mate rial, anode slope angle, and equivalent total filtration rate. A new standard of X ray dosimeters is based on non invasive monitoring of X ray tube voltage with detection of practical peak voltage (PPV) [1]. According to the standard, PPV is determined as the constant potential providing the same contrast of air kerma behind the phantom as the voltage under monitoring.In practice, X ray tube anode voltage is measured using two or more PPV detection channels. Such chan nels have different spectral sensitivities. Air kerma asso ciated methods of PPV measurement are rather sophisti cated and can be implemented only in the laboratory.Determination of actual peak voltage from recovered spectral distribution of bremsstrahlung is described in this work. The bremsstrahlung spectrum is restored from absorption curves detected using a linear discrete semi conductor detector. The PPV value is calculated from the calibration curve.The absorption curve was plotted using a linear dis crete semiconductor detector in which the detection channels also provide a filtration function (Fig. 1).In the photon count mode of the detector, the absorp tion curve is a hyperbola with three segments: quasi linear attenuation segment determined by low energy photons; saddle segment determined by radiation rigidity; below saddle segment with constant radiation energy [2].The family of the absorption curves detected using the gallium arsenide detector is shown in Fig. 2. The abscissa is channel number and the ordinate is logarithm of brightness signal in each detection channel.If the abscissa is the product of linear density of detector material ρ·d (g/cm 2 ) multiplied by detector channel number i, the dimension of the absorption curve slope coefficient is the dimension of the mass extinction coefficient (cm 2 /g). The absorption curve shape is deter mined by the spectrum of the incident radiation.Approximation of this absorption curve is imple mented within the initial segment.The mathematical model of bremsstrahlung damp ing rate was constructed. This model was implemented for MathCAD software. The effect of anode voltage pul sation on test results was measured.Cramer's equation describes the X ray tube bremsstrahlung spectrum. The X ray tube anode in this case is made of tungsten (Z = 79). This equation takes into account attenuation in the filter [4]:Determination of the practical peak voltage from the bremsstrahlung damping rate in a linear discrete semicon ductor detector is described. The method of formation of absorption curves is given. Results of mathematical modeling of an absorption kilovoltmeter are presented. Fig. 1. Absorption curve formation scheme. Filter Emitter Detection channel Detector