Spectral distributions of primary and scattered 140K vp x-rays.

Abstract

There has recently been a renewed interest in detailed information concerning the spectral distribution of x-rays of various energies. The radiation inside a scattering medium will be made up of two components; the part of the primary beam which has penetrated to the point in question and the radiation which has been scattered to that point from other parts of the medium. During the last few years several measurements have been made of the primary spectra of x-rays produced at potentials up to 400 kv (1–5), with scintillation spectrometers of the total-absorption type. Some measurements have been made, also, of the scattered radiation in a water medium (5–7). The present paper will report the results of measurements of both primary and scattered spectra produced by a Picker Vanguard therapy unit operated at 140 kvp. A brief, preliminary report of this work has already been published (8). Primary Spectra The scintillation spectrometer used to measure the primary spectra was similar to that described previously (4). A NaI (T1) crystal, 2 in. in diameter and 1 5/8 in. long, with an aluminum window 0.001 in. thick, was mounted directly on a Dumont K1234 phototube. Pulses from the phototube were passed through a cathode-follower pre-amplifier and linear amplifier and analyzed with a single channel pulse-height analyzer. In order to reduce the counting rate to a reasonable value, the crystal in its lead shielding was placed about 8 meters from the target of the x-ray machine. The beam was limited by two lead diaphragms, one placed about 12 in. from the x-ray target and the other about 10 in. from the crystal. The diaphragms were made of lead 1/8 in. thick with 0.045-in. holes, backed with lead 1/8 in. thick with 0.078-in. holes. This arrangement limited the integrated counting rate to less than 200,000 counts per minute. Background counts were determined by replacing each of the diaphragms in turn by a blank having the same backing but no inner hole. It was found that the background determined in this way was not appreciably different from the counting rate when the x-ray machine was shut off. The collimator nearest the x-ray target was mounted in a holder which allowed it to be tilted about both vertical and horizontal axes. This facilitated alignment of the collimator with the x-ray beam. The collimator could also be moved across the beam in horizontal and vertical directions so that the spectral distribution from various parts of the target could be measured. The inset of Figure 1 shows the variation of counting rate when the collimator was moved laterally across the target. Figure 1 shows, also, the pulse-height spectra measured at points B and C of the focal spot. The shapes of these spectra are the same within experimental error. The ratio of the heights of the peaks at 4 and 10 volts was measured for several other positions as well and was found to be constant within 5 per cent.