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

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 is described. 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 and >10⁷ photons. In a collimated 2 mm×2 mm beam spot 40 mm from the anode, >1 X-ray per pulse is expected. A sample is exposed to these X-rays and fluorescent photons are detected by a microchannel PMT (photomultiplier tube) 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 the laser diode, the X-ray tube, and a microchannel tube has been measured to be 109 ps FWHM. To measure wavelength spectra, a reflection grating monochromator is placed between the sample and the PMT