| Summary: | 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.
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