Neil A. Schaknowski

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.

A position-sensitive ionization chamber for thermal neutrons

A position-sensitive neutron detector based on /sup 3/He is being developed for a national crystal backscattering spectrometer (CBS) at a pulsed neutron source. It comprises a single gas volume, of constant depth, in the form of an annulus that is also part of a truncated cone. Charge from each event is collected on one or two anode strips, out of a total 336 around the annulus, yielding an angular resolution of about 1/spl deg/. Low-noise signal processing, in the form of a monolithic preamplifier/shaping amplifier connected to each anode strip, provides electronic noise that is small enough to permit detector operation in an ionization mode. In addition, the segmented anodes ensure near complete induced charge collection. This new detector therefore has significant advantages over previous detectors used in CBS instruments, which primarily consisted of arrays of individual /sup 3/He filled cylindrical counters with dead regions and nonuniform detection efficiency. The optimization of gas mixture/pressure, drift field, and shaping time is significant in achieving successful performance in the new detector.

A new thermal neutron detector for protein crystallography

A new position-sensitive detector is being developed for protein crystallography studies at a spallation source. Based on eight, independent, wire proportional chamber segments housed in a curved pressure vessel, the device covers a scattering angle of 120 degrees, and has a collecting area of 1.5 m by 20 cm. The position resolution will be about 1.3 mm FWHM, with a total counting rate in excess of one million per second. Timing resolution, essential for a spallation source application, is of order 1 /spl mu/s and provides neutron energy determination that is well suited for crystallography. Advanced features of this device include a digital centroid finding scheme, a seamless readout between segments, and a wire array design that minimizes anode modulation. Details of the mechanical design are given, together with digital centroid measurements that illustrate accurate, uniform response.

Thermal neutron detectors with discrete anode pad readout

A new two-dimensional thermal neutron detector concept that is capable of very high rates is being developed. It is based on neutron conversion in 3He in an ionization chamber (unity gas gain) that uses only a cathode and anode plane; there is no additional electrode such as a Frisch grid. The cathode is simply the entrance window, and the anode plane is composed of discrete pads, each with their own readout electronics implemented via application specific integrated circuits. The aim is to provide a new generation of detectors with key characteristics that are superior to existing techniques, such as higher count rate capability, better stability, lower sensitivity to background radiation, and more flexible geometries. Such capabilities will improve the performance of neutron scattering instruments at major neutron user facilities. In this paper, we report on progress with the development of a prototype device that has 48 × 48 anode pads and a sensitive area of 24cm × 24cm.

Scientific Reviews: Classical 3He Gas Detectors

The normal component in gas-filled neutron detectors was for many years boron trifluoride, BF3, but in recent years 3He has become the converter gas of choice. Although 3He is expensive (about €90 per liter), it is a better proportional counter gas, particularly with respect to electron multiplication, and has a higher conversion efficiency per molecule than BF3

A sealed, UHV compatible, soft X-ray detector utilizing gas electron multipliers

An advanced soft X-ray detector has been designed and fabricated for use in synchrotron experiments that utilize X-ray absorption spectroscopy in the study a wide range of materials properties. Fluorescence X-rays, in particular CK at 277eV, are converted in a low pressure gas medium, and charge multiplication occurs in two gas electron multipliers, fabricated in-house from glass reinforced laminate, to enable single photon counting. The detector satisfies a number of demanding characteristics often required in synchrotron environments, such as UHV compatibility, compactness, long-term stability, and energy resolving capability.

Two-dimensional, /sup 3/He neutron detectors with pad readout for high rates

We describe initial results from a new technical approach aimed at providing advanced, two-dimensional, thermal neutron detectors for high rate applications at new facilities such as the Spallation Neutron Source. The concept is based on neutron conversion in 3He. The gas mixture is contained in a grid-less ionization chamber, with the conversion depth bounded by an entrance window and an anode plane comprising discrete pixels, or pads, that are read-out in parallel. Weighting field calculations show that a ratio of gas depth to anode pad spacing of around 7:1 or greater will permit observation of almost all of the primary ionization on one pad, or a small group of adjacent pads. We illustrate with prototype detector measurements that it should be possible to construct large 1 m2 devices that are capable of sustaining rates up to 108 n s-1, with position resolution of a mm or so. Operating with unity gain, this technique is extremely reliable and should provide advanced solutions to small angle scattering and reflectometry instruments

A new pad-based neutron detector for stereo coded aperture thermal neutron imaging

A new coded aperture thermal neutron imager system has been developed at Brookhaven National Laboratory. The cameras use a new type of position-sensitive 3He-filled ionization chamber, in which an anode plane is composed of an array of pads with independent acquisition channels. The charge is collected on each of the individual 5x5 mm2 anode pads, (48x48 in total, corresponding to 24x24 cm2 sensitive area) and read out by application specific integrated circuits (ASICs). The new design has several advantages for coded-aperture imaging applications in the field, compared to the previous generation of wire-grid based neutron detectors. Among these are its rugged design, lighter weight and use of non-flammable stopping gas. The pad-based readout occurs in parallel circuits, making it capable of high count rates, and also suitable to perform data analysis and imaging on an event-by-event basis. The spatial resolution of the detector can be better than the pixel size by using a charge sharing algorithm. In this paper we will report on the development and performance of the new pad-based neutron camera, describe a charge sharing algorithm to achieve sub-pixel spatial resolution and present the first stereoscopic coded aperture images of thermalized neutron sources using the new coded aperture thermal neutron imager system.