John D. Lipp

A gas microstrip wide angle X-ray detector for application in synchrotron radiation experiments

Abstract The Gas Microstrip Detector has counting rate capabilities several orders of magnitude higher than conventional wire proportional counters while providing the same (or better) energy resolution for X-rays. In addition the geometric flexibility provided by the lithographic process combined with the self-supporting properties of the substrate offers many exciting possibilities for X-ray detectors, particularly for the demanding experiments carried out on Synchrotron Radiation Sources. Using experience obtained in designing detectors for Particle Physics we have developed a detector for Wide Angle X-ray Scattering studies. The detector has a fan geometry which makes possible a gas detector with high detection efficiency, sub-millimetre spatial resolution and good energy resolution over a wide range of X-ray energy. The detector is described together with results of experiments carried out at the Daresbury Laboratory Synchrotron Radiation Source.

Studies of the gain properties of gas microstrip detectors relevant to their application as X-ray and electron detectors

The microstrip gas counter (MSGC) makes an excellent planar (position-sensitive) amplifier of incident electron clouds because both the anodic and cathodic gain-defining elements are produced lithographically on the same rigid substrate. We have studied the dependence of the gas gain and pulse-height resolution of the plate as a function of various geometric and gas parameters. The results show that an MSGC can be made very insensitive to. the shape of the drift electrode, allowing it to be used in a wide variety of applications. An example of an electron-yield XAFS study is given. The aim of the work reported in this paper is to produce a well-defined technology platform from which to build detectors that meet the requirements of high-flux synchrotron radiation and neutron facilities, both of which are key CLRC facilities.

Structure-film thickness relationship study of sputtered NiO∕Ni bilayers using depth profiling and atomic force microscopy techniques

NiO∕Ni thin film bilayers have been grown on Si (100) substrates using low temperature dc reactive magnetron sputtering. The samples were grown under identical process conditions but with different amounts of NiO deposited in order to determine film quality as a function of thickness. In order to investigate the structural properties of the NiO overlayers a synchrotron detector has been developed to make energy-resolved electron yield x-ray absorption spectroscopy measurements at ambient pressure. From these studies we have been able to construct a complete depth profile of the NiO∕Ni bilayers and, by modeling of the electron multiplication/propagation processes within the films, extract quantitative information about them. In combination with atomic force microscopy measurements we have determined that there exists a clear variation in the structural and morphological properties of the NiO thin films as a function of thickness. The densest overlayers with the most conformal surface are observed for film ...

A gas microstrip detector for XAS studies in the photon energy region 300–1500 eV

The ability to perform X-ray absorption spectroscopy (XAS) in the 300-1500 eV energy range allows measurements to be made on transition metal compounds. This paper describes a detector and the technique used to perform fluorescent measurements on such materials. A variety of test sample results are shown to illustrate the low energy and energy-resolving capabilities of the detector (based on gas microstrip technology). Two possible applications are also demonstrated. The first shows how the detector can be used to gather X-ray absorption spectra for the L edges of transition metals and K edges of light elements (C, O and N). The other shows how the magnetic immunity of the detector can be exploited to study the magnetic properties of materials.

A gas microstrip X-ray detector for application in magnetic circular dichroism experiments

X-ray magnetic circular dichroism studies of magnetic 3d transition-metal samples require the recording of high-quality absorption scans in high magnetic fields using circularly polarized soft X-rays of energies in the range 0.5–1 keV. Normally this is performed by electron yield measurements in vacuum. This technique is rendered problematic by the influence of the high magnetic field on the motion of the electrons emitted. Detection of the fluorescent X-rays avoids this problem and eases the constraints of sample preparation and environment. However, the specifications required for a successful X-ray detector are severe, requiring an insensitivity to magnetic fields up to 4 T (for hysteresis curve measurements), a large dynamic range, detection of soft X-rays with good efficiency and signal to noise and containment of the detector structure within a space of a few cm3. Such a detector has been developed using gas microstrip technology and tests show that these requirements can be met.

Energy-resolved electron-yield XAS studies of nanoporous CoAlPO-18 and CoAlPO-34 catalysts

Energy-resolved electron-yield X-ray absorption spectroscopy is a promising technique for probing the near-surface structure of nanomaterials because of its ability to discriminate between the near-surface and bulk of materials. So far, the technique has only been used in model systems. Here, the local structural characterization of nanoporous cobalt-substituted aluminophosphates is reported and it is shown that the technique can be employed for the study of open-framework catalytically active systems. Evidence that the cobalt ions on the surface of the crystals react differently to those in the bulk is found.

A Programmable Analogue Front-End ASIC for Gas Micro-Strip Detectors having a wide range of Input Capacitance

The FREDA ASIC is a new mixed signal ASIC for the HOT series gas micro strip detectors. These detectors have 512 channels each having different capacitance ranging from 2pF-80pF. This new ASIC is an upgrade to the previous programmable version of FREDA with new signal processing capabilities. It is a 16 channel ASIC each of which consists of a Peak Detect and De-randomiser, 8bit counter, 12 bit hybrid SAR ADC with multiplexed LVDS outputs in addition to a preamplifier, a programmable CR-RC shaper and a programmable gain amplifier. The peak detect de-randomiser is a 3 bit analogue peak hold pipeline to store randomly arriving pulses at an average data rate of 1MHz. It also allows baseline measurements of the shaper output during low periods to activity to enable correlated double sampling. A 100MHz, 8 bit counter is used to generate a timestamp which is controlled by the peak detect signal. The 12 bit hybrid SAR ADC generates its 4MSBs from a resistor string and the rest 8 bits from a capacitor bank, it uses a 20MHz clock and takes 800ns for a single data conversion. This ASIC has been manufactured using a 0.35m CMOS process. The ASIC architecture is described including simulated and measured results.