A method of computing theoretical X-ray spectra in the range 30-150 kV is presented. The theoretical spectra are compared with constant potential, high resolution spectra from a tungsten target measured with a Ge(Li) detector, for a range of target angles, tube voltage and filtrations. Above 100 kV the spectra were also measured with a NaI detector but, as there was good agreement between the Ge(Li) and NaI detectors, only the former are presented. Spectra computed using Kramers' theory are also included for comparison, giving fairly good agreement at large target angles (30 degrees) but becoming gradually worse as the target angle decreased. Spectra may be computed by this method for any desired filtration, target angle, and tube voltage between 30 and 150 kV, in excellent agreement with the measured data.
The relative merits of tungsten and molybdenum targets for mammography have been the subject of much discussion. Therefore the spectra and outputs (at constant potential) from molybdenum and tungsten targets, interchangeable in the same tube, have been measured with a Ge(Li) detector and ion chamber respectively. All conditions apart from the target material were unaltered. The spectra have been corrected for the distortions produced by the detector. The effects of filtration on spectra and exposure rates have been calculated and are in agreement with measured values. The spectra and outputs from molybdenum and tungsten targets filtered by aluminium and molybdenum have been investigated and the results are discussed with reference to mammography and the radiography of specimens.
The variation in current and accelerating voltage across an X-ray tube, that occurs over the mains voltage waveform cycle, produces changes in photon flux and spectrum shape. A knowledge of these changes is required to provide an understanding of the parameters affecting X-ray output. Variation in the photon flux will cause distortion of a measured 'mean' spectrum if the dead time of the spectrometry system employed varies over the waveform cycle. A system has been developed that enables the spectrometer to be synchronized to the mains voltage cycle so that variations in photon flux and the instantaneous spectra at selected parts of this cycle can be measured. The variation in photon flux is dependent upon the type of power supplies (both high and low tension). the tube current and the degree of filtration employed. Examples are given of the ripple of photon output due to the voltage and current ripple for a nominally constant potential generator and a half-wave rectified type. The way in which they interact to produce the measured variation is shown.
The L x-rays of tungsten form a significant fraction of an x-ray spectrum at tube voltages in the range 20-50 kV for an x-ray tube with a tungsten target and low inherent filtration. It is necessary to know the complete x-ray spectrum in order to understand the various stages in the production of a diagnostic image, with the aim of reducing patient dose and optimising image quality. Measurements of the intensities of the L,, L,, and L, x-rays have been made with a germanium detector over this voltage range and compared with theoretical values. It is estimated that 12 to 16% of the characteristic L x-rays originate from interactions of x-ray photons with the L shells of the tungsten atoms. The majority are produced by ionisation of the target atoms by the incident electrons. The theoretical cross-sections for ionisation of the L shells by electrons are unreliable; however, the measured results presented enable the intensities of the L x-rays to be calculated for any desired tube voltage and filtration. Good agreement has been found between calculated and measured bremsstrahlung spectra for conditions of low tube voltage (<50 kV) and low filtration.
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