Conceptual design of a novel instrument for producing intense pulses of 10 ps X-rays for ultra-fast fluorescence measurements

摘要

A novel bench-top device for producing intense, fast pulses of X-rays has been designed with 10 ps fwhm (full-width at half-maximum) X-ray pulse width, 120 keV maximum energy, 100 kHz repetition rate, and 1 A peak current onto the X-ray anode. The device includes three sections: (1) an electron gun that generates 5 ns wide pulses of 120 keV electrons at 100 kHz; (2) solenoidal magnetic lenses and deflection plates that focus the electrons onto an aperture plate and sweep the pulsed beam past the aperture, respectively; and (3) a tungsten anode onto which the post-aperture electrons are focused, producing pulses of X-rays. Using solenoidal magnetic lenses with a current density of 150 A/spl middot/turns/cm/sup 2/ to focus the electron beam, a deflection plate dV/dt of 10/sup 13/ V/s is needed to achieve electron pulse widths of about 10 ps. The design process used EGUN (an electron optics and gun design program) electron trajectory simulations, including calculation of important space charge effects. When built, this instrument will be used to excite scintillator samples in crystal or powdered form, allowing fluorescent lifetimes and spectra to be measured with a microchannel PMT. The very narrow 10 ps X-ray pulse width is necessary for accurate measurements of the risetimes of very fast scintillators (e.g., BaF/sub 2/). In addition, the large X-ray flux (1 A peak current) is advantageous when using a reflection grating monochromator to measure decay times at different wavelengths.