Production of Characteristic X Rays by Low-Energy Protons

The cross sections for the production of characteristic x rays emitted by targets bombarded with low-energy 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 $K$-shell ionization are in good agreement with previous measurements for a number of elements. The dependence of the $K$-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 $L$-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 $M$-shell ionization cross sections so only a qualitative explanation of their behavior is presented.