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The cross sections for the production of characteristic x rays emitted by targets bombarded with lowenergy protons have been measured for a number of elements. Scintillation counters capable of resolving x rays with quantum energies as low as 2 keV were used as detectors. X rays from K, L, and M shells were observed and the behavior of the production cross section for these was measured as a function of incident proton energy. Proton beams with energies between 100 and 500 keV produced by the Lawrence Radiation Laboratory (LRL) Cockcroft-Walton accelerator were used for the bombardments. The ionization cross sections for the shells have also been computed using measured or extrapolated values of the appropriate fluorescence yields. The experimental results for i£-shell ionization are in good agreement with previous measurements for a number of elements. The dependence of the iT-shell ionization cross section on atomic number (Z) and on the energy of the bombarding protons is also in accord with the theoretical predictions provided that the deflection of the bombarding particle by the Coulomb field of the nucleus is taken into account. The measured Z-shell ionization cross sections are smaller than the values predicted by a Born approximation calculation in which Coulomb effects are neglected. No calculations are available for If-shell ionization cross sections so only a qualitative explanation of their behavior is presented.

An important question which must be answered about the recently discovered stellar x-ray sources is the nature of the emission spectrum of these sources. Several experiments 1 " 5 have been performed in the past few years and a number of theoretical models 6 ' 7 have been proposed to explain the observations. To obtain more precise information about the spectrum of the x-ray source in Scorpius (SCO-XR-1), 1 a proportional counter sensitive to photons with quantum energies between 2 and 20 keV was flown on a rocket. With this detector and a special telemetry system it was possible to measure the spectrum in this energy region with high resolution.The counter employed in this experiment was a proportional gas counter filled with a 90%-10% xenon-methane mixture at atmospheric pressure; it had a resolution of 20% full width at half-maximum, at 5.9 keV. The counter window was made of 3-mil beryllium and had a rectangular shape with an area of 8.67 cm 2 . Surrounding the counter (except over the window and over one end) was -| in. of plastic scintillator which was viewed by an RCA 4440 photomultiplier. This scintillator functioned as an anticoincidence shield against high-energy charged particles in cosmic rays. In addition, |-mil aluminized Mylar was placed over the counter window to protect the scintillator from light. The proportional counter was collimated to a transmission half-angle of ±10° in azimuth and ±45° in elevation, and it had a geometric factor of 3.41 cm 2 sr. The calculated efficiency of the proportional counter as a function of quantum energy is shown in Fig. 1.The counter was mounted on an Honest John-Nike-Nike rocket which was launched on 12 June 1965 from Kauai, Hawaii. The launch time was 1515 hours UT, which was 39 minutes before local sunrise. At that time, the zenith coordinates were 22 hours right ascension and +22° declination. The rocket was launched to-wards an azimuth of 340° and 5° from the zenith and reached an altitude of 170 km. Apogee of the flight occurred at 160.0° W and 22.5° N at 1518 UT.An Fe 55 , Cd 109 source was mounted on the inside of the nose cone for continual calibration during launch. The nose cone and this source were detached from the vehicle at an altitude of 88 km. Data were taken for the next 275 sec. The payload was spin-stabilized with a spin rate of 6.0 rev/sec, and was observed to precess in a cone of half-angle 3° with a frequency of 0.11 rev/sec. Therefore, the rocket spin vector pointed at a spot on the celestial sphere which traced out a circle of radius 3°, the center of which was located at 21 hours 50 min. right ascension and 27° declination. Scorpius was scanned every revolution during this time, and the x rays from the sun were detected during the 70-sec period centered on apogee during which the sun was high enough above the rocket's horizon so that atmospheric absorption did not completely attenuate the x rays. At apogee there was approximately 6xl0~3 g/ cm 2 of atmosphere between the counter and the sun.Signals from the proportional counter were ampl...