The measurement of 140 kvp spectra
Burke, Dennis G.
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In order to calculate the amount of energy absorbed in a material which is being irradiated, it is necessary to know the spectral distribution of the radiation. Hence it is desirable to have a knowledge of the spectra of x-rays coming from commercial machines. A radiation chemist, for instance, who irradiates a small sample in a beam of x-rays with a large field area, is interested in the spectral distribution of the primary radiation from the target. On the other hand, when the object being irradiated is large in area compared to the field area of the x-ray beam, as in cancer therapy, the scattered radiation within the medium must also be considered. In such a case it is also interesting to know the rate of energy absorption at various depths below the surface, and so the effect of depth on the spectral distribution is also desired. Of the various types of spectrometers that can be used in this energy range, a total absorption scintillation spectrometer was chosen. Such a system has an advantage over a crystal spectrometer or a Compton spectrometer in that the measured spectra are not altered as much by the necessary corrections. Another advantage, especially when measuring scattered spectra, is that much better counting efficiencies can be obtained. Also, since the detector consists only of a scintillator crystal with photomultiplier and impedance matching circuit and can be located remotely from the counting circuitry, a small and easily manoeuvred spectrometer can be constructed. Using such an apparatus, Cormack et al (1) have measured primary and scattered radiation from an x-ray machine operated at 400 kvp. Similar measurements were later made with 280 kvp radiation (2). This discussion covers work done using a Picker Vanguard x-ray machine operated at 140 kvp. It was found that since the lower end of the energy region used by previous workers was particularly important for this radiation, some refinements were required in the correction and calibration procedures. Also, effects such as background and attenuation which are more important at low energies, had to be treated more carefully.