DESIGN OF A PULSED X-RAY SYSTEM FOR FLUORESCENT LIFETIME MEASUREMENTSWITH A TIMING ACCURACY OF 109 PS

We describe the design of a table-top pulsed X-ray system for measurin g fluorescent lifetime and wavelength spectra of samples in both cryst al and powered form. The novel element of the system is a light-excite d X-ray tube with a tungsten anode at +30 kV potential. The S-20 photo cathode is excited by a laser diode with a maximum rate of 10 MHz, eac h pulse having < 100 ps fwhm (full-width at half-maximum) and > 10(7) photons. In a collimated 2 mm x 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 f luorescent photons are detected by a microchannel PMT with a photoelec tron transit time spread of 60 ps fwhm, a sapphire window, and a bialk ali photocathode (wavelength range 180-600 nm). The combined time spre ad of a laser diode, the X-my tube, and a microchannel tube has been m easured to be 109 ps fwhm. To measure fluorescent wavelength spectra, a reflection grating monochromator is placed between the sample and th e PMT.