Design of a pulsed X-ray system for fluorescent lifetime measurements with a timing accuracy of 109 ps

We describe the design of a table-top pulsed X-ray system for measuring fluorescent lifetime and wavelength spectra of samples in both crystal and powdered form. The novel element of the system is a light-excited X-ray tube with a tungsten anode at +30 kV potential. The S-20 photocathode is excited by a laser diode with a maximum rate of 10 MHz, each pulse having <100 ps fwhm (full-width at half-maximum) and >10⁷ photons. In a collimated 2 mm×2 mm beam spot 40 mm from the anode we expect >1 X-ray per pulse. A sample is exposed to these X-rays and fluorescent photons are detected by a microchannel PMT with a photoelectron transit time spread of 60 ps fwhm, a sapphire window, and a bialkali photocathode wavelength range 180-600 nm. The combined time spread of a laser diode, the X-ray tube, and a microchannel tube has been measured to be 109 ps fwhm. To measure fluorescent wavelength spectra, a reflection grating monochromator is placed between the sample and the PMT