A transportable spectrometer and collimating system have been developed for the measurement of photon spectra from a variety of x-ray machines in different locations. The photon spectrum is required for the assessment of dose and radiographic effect in diagnostic radiography. The accuracy of calibration procedures using filtered x-ray beams is also improved if the spectrum produced is known precisely.The equipment can be set up and the measurement of a large range of operating conditions completed in about 8 hours. A computer program has been written to convert the observed pulse height distributions to photon spectra. The mean energy and width are given for a range of calibration spectra, and examples are shown of the spectra produced by the diagnostic machines. Some effects of tube current and potential waveform are discussed.
The energy response of lithium fluoride (LiF) thermoluminescent dosemeters has been determined at two temperatures. Glow peaks 3,4, and 5 are measured at 240 degrees C and peaks 6 and 7 at 300 degrees C. In studying the response to X-radiation it was observed that the response of LiF chips to photons below 150 keV is greater, relative to cobalt-60, than that expected from the energy absorbed in the material. There is an enhancement of 10% at 240 degrees C and of about 100% at 300 degrees c for the same energy absorption. This effect is particularly important where precise measurement of X-ray dose is required and corrections of from 10 to 15% above the correction from rads in LiF to rads in water will be necessary depending upon the maximum read-out temperature. Qualitatively the enhancement may be explained in terms of the higher mean rate of linear energy transfer at low photon energies relative to cobalt-60 gamma rays.
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.
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