G.C. Smith

X-ray position detection in the region of 6 μm RMS with wire proportional chambers☆

We have developed a MWPC system for x-ray detection with a position resolution in the region of 6 ..mu..m RMS (14 ..mu..m FWHM). The performance with argon, krypton or xenon at pressures from 1 to 10 atm is explored for the x-ray energy range 5 to 25 keV. At a resolution of 6 ..mu..m RMS the effects of photoelectron and Auger electron range, electronic noise, avalanche spread, lateral electron diffusion, as well as x-ray beam collimation, become of comparable magnitude. Their limiting effects on avalanche centroid fluctuation, and hence on position resolution, are investigated. The position resolution achieved in this work compares favorably with that of solid state devices. 7 refs., 6 figs.

A portable gamma-ray spectrometer using compressed xenon

An ionization chamber using compressed xenon has been designed and built for gamma-ray spectrometry. The device is based on signal measurement from a parallel plate detector, with the gas enclosure constructed specifically for packaging into a portable instrument; thus, appropriate engineering practices using ASME codes have been followed. The portable system comprises two small containers that can be setup for operation in just a few minutes. Its sensitivity is 100 keV to over 1 MeV, with a resolution at 662 keV of 2.5% FWHM for uniform irradiation, and 2% FWHM for collimated irradiation, comparable to the best ever with compressed xenon. It also exhibits greater specificity than most scintillators, such as NaI. The device is insensitive to neutron damage and has a low power requirement.

Photoelectron Range Limitations to the Spatial Resolution for X-Rays in Gas Proportional Chambers

Measurements have been made, for x-ray energies from a few keV to 18 keV, of the limiting spatial resolution caused by the finite range of the photoelectron, or electrons, created when an x-ray is absorbed in the gas of a proportional chamber. In hydrocarbon gases such as methane and ethane, where the photoelectron receives the bulk of the x-ray energy, the limiting spatial resolution is found to vary as a power law of x-ray energy. In argon and xenon, at an x-ray energy approximately twice that of the AK edge and the XeL edge respectively, the measured limiting resolution is better than expected from an equivalent power law behavior.

Applications and instrumentation advances with the Stony Brook scanning transmission x-ray microscope

Scanning transmission x-ray microscopes (STXM) are well matched to the optics of high resolution monochromators, offer a variety of imaging modes and can minimize radiation damage to the specimen. We describe the Stony Brook STXM at the NSLS. This microscope is used for a variety of studies by many users; we briefly outline its use for studies of hydrated colloidal system and for dark field microscopy on immunogold labeled specimens as examples. In order to keep pace with developments in zone plate optics, spectroscopy and a variety of imaging modalities, the microscope is being redesigned and its characteristics are discussed. Its preliminary x-ray detector will be a new multiware proportional counter with high count rate capability.

High-performance, imaging, thermal neutron detectors

Abstract Existing and planned spallation neutron sources require two-dimensional detectors for many experiments. Unlike the requirements for steady-state neutron sources, it is essential that these detectors possess good time resolution to determine neutron energy. A range of detectors based on gas proportional chambers, with low-noise encoding electronics, has been fabricated at this laboratory, with properties well suited for use at spallation sources. These high-performance detectors possess outstanding qualities in terms of dynamic range and stability of both recorded neutron positions and response (efficiency), in addition to normal attributes such as good position resolution, high detection efficiency and insensitivity to γ-rays. We review here some of the major characteristics of the detectors, describe recent advances, and illustrate their high level of performance with neutron scattering results. While relatively few such detectors are required world wide, specialized efforts are required for their development. The additional opportunities provided by new spallation sources will need continued advances in detector performance.

High Precision Thermal Neutron Detectors

Two-dimensional position sensitive detectors are indispensable in neutron diffraction experiments for determination of molecular and crystal structures in biology, solid-state physics and polymer chemistry. Some performance characteristics of these detectors are elementary and obvious, such as the position resolution, number of resolution elements, neutron detection efficiency, counting rate and sensitivity to gamma-ray background. High performance detectors are distinguished by more subtle characteristics such as the stability of the response (efficiency) versus position, stability of the recorded neutron positions, dynamic range, blooming or halo effects. While relatively few of them are needed around the world, these high performance devices are sophisticated and fairly complex; their development requires very specialized efforts. In this context, we describe here a program of detector development, based on {sup 3}He filled proportional chambers, which has been underway for some years at Brookhaven. Fundamental approaches and practical considerations are outlined that have resulted in a series of high performance detectors with the best known position resolution, position stability, uniformity of reliability over time of this type.

A large, high performance, curved 2D position-sensitive neutron detector ☆

Abstract A new position-sensitive neutron detector has been designed and constructed for a protein crystallography station at LANLs pulsed neutron source. This station will be one of the most advanced instruments at a major neutron user facility for protein crystallography, fiber and membrane diffraction. The detector, based on neutron absorption in 3He, has a large sensitive area of 3000 cm 2 , angular coverage of 120°, timing resolution of 1 μs , rate capability in excess of 10 6 s −1 , position resolution of about 1.5 mm FWHM, and efficiency >50% for neutrons of interest in the range 1– 10 A . Features that are key to these remarkable specifications are the utilization of eight independently operating segments within a single gas volume, fabrication of the detector vessel and internal segments with a radius of curvature of about 70 cm , optimized position readout based on charge division and signal shaping with gated baseline restoration, and engineering design with high-strength aluminum alloy.

Study of GEM characteristics for application in a MicroTPC

The Gas Electron Multiplier (GEM) may provide a convenient method for obtaining significant electron multiplication over large detector areas. An important potential application of the GEM is for readout of microTPCs. We are conducting a study of a multi-GEM structure with particular emphasis on the following characteristics: gain uniformity/stability, ion feedback and position readout. In particular, we present the first experimental results of interpolating anode pad readout. Initial results provide encouragement that the GEM application in microTPCs may be realized.

A method for reduction of parallax broadening in gas-based position sensitive detectors

A description is given of the principle and experimental verification of a new method which significantly reduces broadening of the position response, due to parallax, for radiation incident on a detector at finite angles of incidence. The technique, which can be implemented in gas-based position-sensitive detectors with planar geometry, substantially improves position resolution for scattering experiments using X-rays and neutrons, and will permit larger angular coverage than has previously been possible. An improvement of nearly a factor four in rms position resolution is predicted, which is confirmed by measurements using a gas proportional X-ray detector with delay-line position encoding.

A LARGE 2D PSD FOR THERMAL NEUTRON DETECTION

Abstract A 2D PSD based on a MWPC has been constructed for a small angle neutron scattering instrument. The active area of the detector was 640 × 640 mm2. To meet the specifications for neutron detection efficiency and spatial resolution, and to minimise parallax, the gas mixture was 190 kPa 3He plus 100 kPa CF4, and the active volume had a thickness of 30 mm. The design maximum neutron count rate of the detector was 105 events per secod. The (calculated) neutron detection efficiency was 60% for 2 A neutrons and the (measured) neutron energy resolution on the anode grid was typically 20% (fwhm). The location of a neutron detection event within the active area was determined using the wire-by-wire method: the spatial resolution (5 × 5 mm2) was thereby defined by the wire geometry. A 16-channel charge-sensitive preamplifier/amplifier/comparator module has been developed with a channel sensitivity of 0.1 V/fC, noise line width of 0.4 fC (fwhm) and channel-to-channel cross-talk of less than 5%. The Proportional Counter Operating System (PCOS III) (LeCroy Corp, USA) was used for event encoding. The ECL signals produced by the 16 channel modules were latched in PCOS III by a trigger pulse from the anode and the fast encoders produce a position and width for each event. The information was transferred to a UNIX workstation for accumulation and online display.