At low concentrations, recording X-ray absorption spectra in fluorescence excitation mode is more sensitive than transmission mode. For dilute samples, the fluorescence signal is often obscured by scattered X-rays, and matrix and filter fluorescence. To discriminate against this background, while maintaining a large angular acceptance and high count rate capability, we have constructed a new detection system based on an array of intrinsic Ge detectors. The device uses 13 individual 11 mm diameter Ge detectors, clustered in a 1:3:5:3:1 pattern on a common cryostat, combined with Soller slits and filters to reduce the background signals. Pulsed optical feedback preamplifiers are followed by Gaussian-shaping amplifiers having fast discriminators to register the incoming count rate (ICR). Correction for dead time using the ICR signal allowed operation in the vicinity of 75 kHz per channel, with a 1 μs shaping time at 6 keV. For lower count rate applications, an average resolution of 160 eV at 5.9 keV was obtained with 8 μs shaping. Recent experience with this detector at the Stanford Synchrotron Radiation Laboratory is presented. The performance is illustrated using spectra obtained from phosphorus compounds and a thin iridium foil. The performance of this device is compared with previous fluorescence detection schemes, such as those using filter/slit combinations or barrel monochromators.

A 13-element Ge detector for fluorescence EXAFS

The authors report on the performance of high-purity Ge radiation detectors with amorphous Ge (a-Ge) contacts fabricated using RF sputtering techniques. Electrical contacts formed using sputtered a-Ge films on high-purity Ge crystals, both n- and p-type, were found to exhibit good blocking behavior with low leakage currents, with the contact biased under either voltage polarity. The a-Ge contacts have thin dead layers associated with them, and can be used in place of lithium-diffused, ion-implanted, or Schottky barrier contacts on Ge radiation detectors. The use of such contacts allows fabrication of multi-electrode detectors by means of simple processing steps. >

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Amorphous Ge bipolar blocking contacts on Ge detectors

Synchrotron radiation from the electron storage ring BESSY has been used to determine the absolute detection efficiency (DE) and the response function of a windowless Si(Li) detector in the 0.1–5 keV photon energy range. The determination of the DE above 0.4 keV was based on an energy dispersive measurement of the undispersed calculable radiation emitted from the storage ring, which is a primary radiation standard. A new method based on the measurement of the detector photocurrent for a known monochromatic spectral photon flux was used to measure the DE down to 60 eV. We have introduced a model for the absolute DE including all parts of the response function of a Si(Li) detector which expresses the DE only as an absorption effect in the gold contact and contamination layers. The agreement between the measured efficiency and the model calculation is better than 2%. A model that allows lineshapes from 0.1 keV upwards to be simulated and which agrees well with our measurements in the range up to 5 keV is presented.

Characterization of a windowless Si(Li) detector in the photon energy range 0.1-5 keV

SUMMARY

The Si(Li) detector, with its relatively high collection solid angle, has brought X-ray microanalysis within the operating current limits of both SEMs and transmission microscopes. At present, the main deficiencies arise in the low-energy region of the spectrum, below about 2 keV. Here, detection efficiency falls off because of absorption in the entrance window, peaks invariably overlap and pile-up rejection is less effective. High-energy photons generated in transmission microscopes are responsible for new spectral artefacts and problems of overload. While computer processing of spectra can correct for most artefacts, background and peak overlaps, accuracy is fundamentally limited by counting statistics and the overall stability of the spectrometer. The technique will therefore benefit from improved design of both electronic circuitry and components. For example, advances in FET design may bring a substantial reduction in electronic noise which will not only improve resolution at low energies, but will also improve discrimination between small pulses and noise so that pile-up rejection can be more effective. This will be most important when a very thin entrance window is used to permit observation down to the C Kα energy (282 eV) although the hazards involved in such ‘windowless' operation still limit its application. The stability of detection efficiency with time has been exploited in methods which attempt to remove the need for repetitive standardization of the spectrometer and new techniques have been developed for the analysis of thin films, particles and rough surfaces which make use of information contained in the bremsstrahlung continuum.

X-ray microanalysis with Si(Li) detectors

An improved Si(Li) detector response function model over the range from 5 to 60 keV has been developed. The values of the model parameters are obtained by least-squares fitting of the pulse-height spectra from a number of pure-element samples excited by 109 Cd or 241 Am sources. Simple functional forms of the separable parts of the detected X-ray pulse-height spectrum as functions of incident energy are summed to produce the generalized response function. Compared to previous work [Nucl. Instr. and Meth. A243 (1986) 121, ref. [1]] the following changes and improvements have been made: (1) the entire useful energy range of the Si(Li) detector from 5 to 60 keV is treated rather than the previous upper limit of about 20 keV, (2) a Compton-scattering continuum function from both the detector and the surrounding material has been added to account for the additional features observed with high-energy X-rays, (3) consideration of radiative Auger satellite X-ray lines has been found to account for the previously observed discrepancy in the model parameters for K α and K β X-rays, (4) the resulting generalized detector response function has a much simpler and more consistent form than the previous one, ref. [1] and (5) the new model parameters expressed as functions of energy are more consistent with physical phenomena than before and are not just fitted polynomials. Applications of the new Si(Li) detector response function are discussed.

An improved Si(Li) detector response function

We have found experimentally that high-purity Ge low-energy X-ray detectors have a relatively thick entrance window which renders them practically useless below ~ 2.3 KeV. A simple X-ray fluorescence experiment establishes clearly that the window is physically in the Ge material itself. Experiments with detectors made from different Ge crystals, and with Schottky barrier contacts of different metals indicate that the effect is due to a basic property of the transport of electrons near a surface. Theoretical considerations and a Monte Carlo calculation show that the window is caused by the escape of warm electrons which are the end product of a photo event. The mean free path of the electrons becomes longer as they lose energy by optical phonon collisions and they can be trapped at the surface before they are picked up by the electric field.

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J. Llacer

Papers

Entrance Windows in Germanium Low-Energy X-Ray Detectors