Digital x-ray processing electronics for fluorescence EXAFS and spectroscopy

Abstract

We have developed a digital x-ray processor (DXP) for x-ray fluorescence spectroscopy, implemented in a 4-channel CAMAC module, which accepts inputs of either polarity from reset or tail preamplifiers, and outputs one spectrum per channel. Digital trapezoidal shaping and efficient pileup rejection are implemented in dedicated logic, with programmable peaking times from 0.5 to 20 msec. The energy resolution is comparable to good analog units at equivalent peaking times. A maximum input count rate (ICR) of 500,000 cps per channel can be accomodated at a peaking time of 0.5 msec. A digital signal processor on each channel is used to collect the data, apply corrections, and update the spectrum. The capabilities of the DXP prototype at high rates was tested at SSRL. Using an Ortec single-element germanium detector, the resolution was seen to degrade somewhat with increasing ICR above 150,000 cps, due to effects that we are still investigating. Collaborating with Hewlett-Packard and SSRL, the DXP was also used with a Kevex Si(Li) detector for trace element detection on silicon wafers in comparison with Kevex readout electronics. At 4 msec peaking time, DXP’s resolution was slightly worse (10–15 eV) due to some excess noise pickup, though the background levels in the spectra were essentially identical in the two systems and the DXP’s maximum count rate was several times higher.