A 3-MeV beam of protons of 2to 150-nanoampere intensity has been used to excite X-ray emission from a wide range of biological and environmental samples-e.g., human tissue, body fluids, soil extracts, leaves, coal, fly ash, ion-exchange membranes, and proteins. The X-rays have been detected using a Si (Li) solid state detector for the elements P (Z = 15) through Pb (Z = 82). Linear response has been demonstrated for the typical elements of Pb, Cu, Zn, Co, and Mn from 5 ng to greater than 2 Mg. A lower limit of sensitivity of approximately 200 programs in the irradiated area has been attained with the more responsive elements when they are deposited on very thin substrates. The proton-induced X-ray emission technique seems especially suited to rapid and economical multielement analyses for samples of clinical and environmental interest. Numerous examples of data ob-
Table I were obtained. The absolute cross section for production of atoms in the 3S state was determined from the target pressure, the detection efficiencies, and the 3S population parameter found in the least-squares fits. Table I also summarizes the results from other experiments. 9 ' 10 The measurements show that electron capture into the 3S state dominates and that the cross sections decrease with increasing L. The cross sections for capture into the different M L states decrease with increasing \M L \. The partial cross sections for production of atoms in the P and D states agree satisfactorily with the measurements of Hughes et al? and the extrapolated measurements of Ford and Thomas. 10 There are no theoretical predictions for the partial cross sections for electron capture from a nitrogen target. The reported calculations for electron capture from an atomic hydrogen target vary by more than a factor of 2 from one calculation to another.In summary this paper reports the first complete determinations of the partial cross sections for capture into each of the L, M L states for the n = 3 manifold. With some modifications in the design of the apparatus the precision of the measurements can be improved. In particular the measurement of the absolute cross section can be increased by an order of magnitude. We plan PACS numbers: 34.70. + e According to the additivity rule the value of a quantity of interest for a molecular target is equal to the sum of the values of these quantities for the constituent atoms in the molecule. The uncontested utility of this rule is marred by questions as to the range of its validity. As an ex-to use this method with an atomic hydrogen target to measure the cross sections for protons incident on hydrogen atoms and thus make an unambiguous test of the theoretical calculations. . These measurements covered the range of 80-400 keV and were extrapolated to 49 keV to obtain the cross sections quoted in Table I. ample, the stopping power of a compound is often determined as the sum of stopping powers for its elements (Bragg rule). 1 The validity of this additivity rule outside the high-velocity limit becomes, however, questionable. 2 Significant deviations from the additivity rule were seen in The measured total electron-capture cross sections per number of carbon atoms in C m H" (m = 1,2,3,4), v c /m 9 decrease with increasing m. This decrease is largest at the lowest velocities of 0.8-3 MeV }H + ions, and diminishes in the limit of high velocities where the strict additivity of atomic cross sections in a molecular target is approached. The breakdown of the additivity rule in the present data is primarily attributed to, and accounted for in terms of intramolecular electron loss processes.
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