A multielement Ge detector with complete spectrum readout for x‐ray fluorescence microprobe and microspectroscopy (abstract)

Abstract

Multielement Ge and Si(Li) detectors have been used in recent years to improve the increase count rate capability and to improve the solid‐angle efficiency in fluorescence x‐ray absorption spectroscopy (XAS). Such systems have typically been equipped with one or more single‐channel analyzers (SCAs) for each detector element. Such SCA‐based electronics are sufficient when only the counts in one or two well‐resolved peaks are of interest. For the fluorescence (XRF) microprobe at beamline X‐26A at the NSLS, SCA‐based electronics were not a satisfactory solution for two reasons: (1) for XRF experiments, the entire fluorescence spectrum is required; (2) for micro‐XAS studies of trace elements in complex systems, the fluorescence peak often sits on a significant background or partially overlaps another fluorescence peak, requiring software background subtraction or peak deconvolution. An electronics system which permits collection of the entire fluorescence spectrum from each detector element has been designed. The system is made cost‐effective by the use of analog multiplexors, reducing the number of analog‐to‐digital converters (ADCs) and multichannel analyzers (MCAs) required. The system was manufactured by Canberra Industries and consists of: (1) a 13 element Ge detector (11 mm diameter detector elements), (2) 13 NIM spectroscopy amplifiers with programmable gains, (3) four analog multiplexors with maximum of eight inputs each, (4) four ADCs with programmable offsets and gains and 800 ns conversion time, and (5) two MCAs with Ethernet communications ports and two ADC inputs each.The amplifiers have shaping times which are adjustable from 0.5 to 12 μs. The analog multiplexors were modified to perform pileup rejection. The analog multiplexing does not significantly reduce the count rate capability of the system, even at the shortest amplifier shaping times. The average detector resolution is 170 eV at 12 μs shaping time and 200 eV at 4 μs shaping time. The maximum aggregate count rate is 400 kHz with 0.5 μs shaping time. The system is controlled by software based upon a package from Canberra and another commercial package (IDL), both running on a VAXstation 4000/90. The software automatically adjusts the gains of the amplifiers and offsets of the ADCs so that the spectra from each detector have identical calibrations and can be added channel for channel. The overhead to read a 1024 channel spectrum from each of the 13 elements and sum them is about 2 s. The software allows a range of options for data storage, from saving the complete spectrum for each of the 13 detectors elements (≳50 000 bytes/point) to saving only the net counts under a single fluorescence peak summed over all the detector elements (4 bytes/point). These data can be stored at each pixel in an elemental map or at each point in a monochromator scan. The system has been commissioned and is being used for XRF and micro‐XAS studies.