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

We present a new concept for c-ray detector arrays. An example, called GRETA (Gamma-Ray Energy Tracking Array), consists of highly segmented HPGe detectors covering 4p solid angle. The new feature is the ability to track the scattering sequence of incident c-rays and in every event, this potentially allows one to measure with high resolution the energy deposited, the location (incident angle) and the time of each c-ray that hits the array. GRETA will be of order of 1000 times more powerful than the best present arrays, such as Gammasphere or Euroball, and will provide access to new physics. ( 1999 Published by Elsevier Science B.V. All rights reserved.

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GRETA: utilizing new concepts in γ-ray detection

The past decade has seen the rapid development and exploitation of one of the most significant tools of nuclear physics, the semiconductor radiation detector. Applications of the device to the anal...

Semiconductor radiation detectors.

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

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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Entrance Windows in Germanium Low-Energy X-Ray Detectors

The availability of germanium with less than 1010 acceptors/cm3 has enabled us to fabricate and study the performance of germanium detectors of substantially larger volume than has hitherto been possible except by using lithium-drifting techniques Detectors having a sensitive volume of up to 25 cm3 have been made These detectors have a lithium diffused n+ contact and a metal barrier non-injecting back contact, and are totally depleted The energy resolution of these detectors is similar to that of equivalent sized lithium-drifted detectors, but the high-purity detectors are easier to fabricate, and present considerably fewer handling problems

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High-Purity Germanium: Detector Fabrication and Performance

Several important areas for the use of germanium spectrometers would be opened if these detectors could be successfully operated at temperatures considerably in excess of 77°K. Precise measurements on a series of detectors made from different high-purity germanium crystals and on one lithium-drifted germanium detector indicate that the resolution does not degrade significantly until nearly 150°K is reached. Similar-sized detectors, including the lithium-drifted device, exhibited essentially identical temperature-resolution relationships.

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Operational Characteristics of Germanium Detectors at Higher Temperatures

A Li-diffused n+ surface layer is the best n+ contact developed up to the present time. However, the effective dead layer resulting from the Li-diffusion prevents the optimum stacking of detectors for counter telescopes. Diffusion temperatures as low as 160°C have been successfully used to produce effective dead layers as thin as 10 ?. Unfortunately, these dead layers increase significantly when the detector is annealed following radiation damage.

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Lithium-Diffused n+ Contacts on High-Purity Germanium Detectors: How Thin Can They be Made-How Stable Are They?

https://escholarship.org/content/qt26s8n5g9/qt26s8n5g9.pdf?t=p0hhlg

Richard C. Cordi

Papers

Entrance Windows in Germanium Low-Energy X-Ray Detectors

High-Purity Germanium: Detector Fabrication and Performance

Operational Characteristics of Germanium Detectors at Higher Temperatures

Lithium-Diffused n+ Contacts on High-Purity Germanium Detectors: How Thin Can They be Made-How Stable Are They?

Thin-window germanium detectors: Fallacy and fact