Control of the chemical state change of sulfur in solid compound targets during high-resolution PIXE measurements

Abstract

A high-energy-resolution wavelength-dispersive (WD) X-ray spectrometer in the Johansson geometry, which allowed energy resolution below the natural linewidth of the Kα lines was employed in measurements of the proton-induced Kα X-ray emission spectra for six typical sulfur compounds (CdS, Na₂SO₃, Na₂ S₂O₅, NaHSO₃, (NH₄)₂SO₄, and Na₂SO₄) to investigate the chemical state change during 2.4-MeV proton irradiation with a current density of 7.5 nA/mm². We found that the chemical state change of each compound depended on the various factors affecting the surface temperature increase, such as target thickness, mounting method, and existence of active cooling during the measurement. The chemical state of sulfur on the target surface of S⁴⁺ compounds was gradually changed into S⁶⁺ without exception through irradiation under poor cooling conditions. Sulfur compounds of the S⁰ and S⁶⁺ states with closed shell structures were proven to be chemically stable against proton bombardment, as expected. However, (NH₄)₂SO₄ was found to be most sensitive to proton irradiation among the sulfur compounds, and S⁰, one of the reaction products, became a major element at doses higher than 3 × 10⁸ Gy. If thick targets were mounted by using a carbon adhesive tape, chemical state change could be observed in some cases even with lowtemperature cooling down to −80 °C, however, the chemical state change seemed to be remarkably suppressed by using very thin targets mounted with a silver paste even without active cooling. In conclusion, the chemical states of sulfur compounds could be preserved without significant change for an accumulated dose of about 3 × 10⁷ Gy, equivalent to a typical high-resolution PIXE scanning period, by adopting a proper target preparation scheme to discharge proton-induced thermal energy effectively from the irradiated target surface.