No abstract

No abstract

No abstract

ENERGY-sensitivity studies of y-ray instruments have indicated strong variations in sensitivity in the energy range below 200 keV (Da51; He52). These studies have been made using heavily filtered X-rays to provide approximately monoenergetic photons. Despite the heavy filtration, there remains a considerable spread in energy of these rays; consequently, their use in an energy range in which the detector sensitivity varies rapidly with energy can be misleading. The "effective energy" of such filtered beams is determined by absorption measurements. In this way, an average is obtained that may represent a different weighting of the energy spectrum than that made by the instrument. We have avoided these difficulties with nearly monoenergetic X-rays of less than 200 keV derived from K-fluorescence.Nearly monoenergetic sources for microradiography of histologic sections (Ro52) and for metallurgical microradiography (Sp32) have already been developed, using the Kfluorescence X-rays produced by irradiating selected secondary targets with ordinary Xrays. These devices are not suitable for instrument calibration because of the small beam size available. It was possible to increase the size of an apparatus of this sort so that a beam uniform in intensity to within a few percent over a 6-in.-diameter circle could be obtained with dose rates adequate for instrument calibration and similar studies.A 220-kvp industrial X-ray machine was used as the source of primary radiation. The minimum voltage to which this machine could be adjusted was 60 kvp. This did not permit selecting a voltage that minimized scattered radiation for characteristic X-rays whose energies were below 60 keV. A lower-voltage machine, particularly one with a beryllium window for greater intensity, would be better. However, it was possible to achieve very satisfactory results with the machine described.The source is shown in the illustration.FIG. 2. X-ray source is constructed of standard 6-in.steel pipe convered with :-in. lead sheet. Standardbrass-tubing sections slide into pipe to support collimators and filters. Second collimator captures photons scattered by first one; third collimator minimizes scattering from 30-cm beam trap into fluorescent beam emerging from side portal. 1075

A number of experiments made in a B-29 on the properties of cosmic rays are herein reported. These include the latitude effect of the following, performed along the 80th geographic west longitude from 64° geomagnetic north to the geomagnetic equator, all at a constant pressure altitude of 30,000 ft. (3.10 m of water equivalent): (1) Intensity at the vertical without and with lead absorber; (2) intensity at 45° West and 45° East both without and with lead absorber; (3) total ionization intensity; (4) extensive showers. Also measured at 48° North and at the geomagnetic equator were the following: (1) Zenith angle effect both without and with lead absorber; (2) intensity vs. air pressure without and with lead; (3) extensive showers vs. air pressure; (4) local showers. In addition to the above the variation in azimuth of the cosmicray intensity was measured with the various telescopes tipped at 22i°, 45°, and 67i° to the vertical at an elevation corresponding to 2.35 m of water equivalent at the geomagnetic equator. Two lead thicknesses were used, 10 and 20 cm, inserted between the counter trays. These experiments throw new light on the nature of the primary radiation and on the interaction of the primary particles with the matter through which they pass. ! z=o* Z«48* Z»67.5* -5 J -« J eo CM PB