N.J. Rhodes

Application of the microhole and strip plate detector for neutron detection

We introduce the microhole and strip plate (MHSP) detector as a micropattern detector for the detection of thermal and epithermal neutrons. Detection sensitivity is obtained by filling these detectors with /sup 3/He at high pressures. We propose the use of argon-xenon penning mixtures as the stopping gas as opposed to the usual carbon based stopping gases. These argon-xenon mixtures provide suitable gas gains for the high pressure/high resolution neutron detector applications. With these mixtures it is possible to obtain a sealed detector with only rare-gas filling which is simple to purify and not subject to ageing. An MHSP gas detector filled with a 3-bar argon/50-mbar xenon/6-bar helium mixture can achieve gains above 2/spl times/10/sup 3/. This mixture allows neutron detection efficiencies of about 70% at 1 /spl Aring/ for a 2.5-cm thick absorption region and intrinsic position resolution (full-width at half-maximum) of about 1.8 mm. The sensitivity to /spl gamma/-rays of the present mixture will be the same when compared to that of 2.6-bar CF/sub 4/.

Single-Electron Response Using a GEM-MIGAS Electron Multiplier

A gas electron multiplier with a micro-induction gap amplifying structure (GEM-MIGAS) is formed when a conventional GEM is operated with a short induction gap, typically set at 50 mum. Experimental studies were carried out to investigate the single-electron response of a GEM-MIGAS, using a He/iso-C4H10 (85/15%) gas mixture operated in a flow mode at atmospheric pressure. The additional charge multiplication in the induction gap results in a gain increase up to one order of magnitude when compared to the GEM mode operation alone. A series of measurements were undertaken to examine the pulse height distributions induced by single-electrons under a wide range of bias voltages applied across the GEM holes, 100 to 550 V, and for a wide range of electric fields in the induction gap, 0.6 to 100 kV/cm. It was possible to sustain effective charge gains in excess of 3times105 B and multiplication relative variances around 0.4 over a large range of GEM voltages, enabling us to demonstrate single-electron detection efficiencies above 98%.

Neutron detectors for the ESS diffractometers

The ambitious instrument suite for the future European Spallation Source whose civil construction started recently in Lund, Sweden, demands a set of diverse and challenging requirements for the neu ...

Adaptive algorithms of position and energy reconstruction in Anger-camera type detectors: experimental data processing in ANTS

The software package ANTS (Anger-camera type Neutron detector: Toolkit for Simulations), developed for simulation of Anger-type gaseous detectors for thermal neutron imaging was extended to include a module for experimental data processing. Data recorded with a sensor array containing up to 100 photomultiplier tubes (PMT) or silicon photomultipliers (SiPM) in a custom configuration can be loaded and the positions and energies of the events can be reconstructed using the Center-of-Gravity, Maximum Likelihood or Least Squares algorithm. A particular strength of the new module is the ability to reconstruct the light response functions and relative gains of the photomultipliers from flood field illumination data using adaptive algorithms. The performance of the module is demonstrated with simulated data generated in ANTS and experimental data recorded with a 19 PMT neutron detector. The package executables are publicly available at http://coimbra.lip.pt/~andrei/

Operational characteristics of a GEM-MSGC system for X-ray detection

This report outlines a recent study undertaken at CCLRC Rutherford Appleton Laboratory to evaluate the performance of a gas electron multiplier (GEM) coupled with a microstrip gas counter (MSGC). The parameters investigated during this study were effective gain, effective gain stability and energy resolution at 8.05 keV using Cu-K X-rays. These parameters were studied as a function of drift field, induction field, potential differences across the GEM holes and that across the MSGC anodes and cathodes. This report demonstrates that a single stage GEM can sustain effective gains up to 6000 whilst retaining adequate X-ray energy resolution. By utilising the MSGC as well as the GEM amplification these gains easily exceed 100,000 and allow the MSGC operation at much lower voltages. This report also demonstrates that the introduction of the GEM preamplification to the MSGC enables the operation of the latter at much higher effective gains (30,000) before any degradation in the X-ray energy resolution.

A 2-D MSGC-based imaging detector for neutrons

The development and testing of a two-dimensional (2-D) prototype detector based on a microstrip gas chamber (MSGC) is reported using a gas mixture of 2.5 bar /sup 3/He and 2.5 bar CF/sub 4/. The second coordinate is obtained by utilising a plane of wires as pick up electrodes. The detector is operated with the wire plane at such a potential so as not to induce any gain around the wires. This means that the high tolerances normally associated with wire planes in multi wire proportional counters are not mandatory. The detector comprises of 48 individually instrumented channels in both X (MSGC strips) and Y (orthogonal wire plane). A specially designed encoding module has been constructed which feeds digital addresses for each event to the ISIS Data Acquisition Electronics (DAE) system. An intrinsic detector resolution of /spl sim/1 mm full-width at half-maximum has been measured for both dimensions (in experimental exposures on the ROTAX beamline at ISIS) which is degraded slightly by the digital resolution for the overall system. This readout method is shown to be very tolerant of a poor signal to noise ratio in the readout channels (unlike traditional analogue wire chamber readout systems) and permits the operation of the MSGC at low avalanche gains (/spl sim/10) which helps to maximize the rate and lifetime performance of the detector as well as permitting data capture rates in the megahertz range. The event timing resolution is comfortably submicrosecond and is, therefore, suitable for applications on spallation neutron sources.

Operational performance characteristics of the WISH detector array on the ISIS spallation neutron source

The performance of the position sensitive neutron detector array of the WISH diffractometer is discussed. WISH (Wide angle In a Single Histogram) is one of the seven instruments currently available for users on the second target station (TS2) of the ISIS spallation neutron source, and is used mainly for magnetic studies of materials. WISH is instrumented with an array of 10 detector panels, covering an angular range of 320o, orientated in two semi-cylindrical annuli around a central sample position at a radius of 2.2m. In total the 10 detector panels are composed of 1520 3He based position sensitive detector tubes. Each tube has an active length of one metre, a diameter of 8mm and is filled with 3He at 15 bar. The specification for the WISH detectors included a neutron detection efficiency of 50% at a neutron wavelength of 1A with good gamma rejection. A position resolution better than 8 mm FWHM along the length of the tubes was also required which has been met experimentally. Results obtained from the detector arrays showing pulse height and positional information both prior to and post installation are shown. The first 5 of the 10 detector panels have been operational since 2009, and comparable diffraction data from powder and single crystal samples taken from the remaining 5 panels (installation completed in 2013) shows that we have a detector array with a highly stable performance which is easily assembled and maintained. Finally some real user data is shown, highlighting the excellent quality of data attainable with this instrument.

A 2D gas scintillation detector for thermal neutrons

A 2D position sensitive gas scintillation detector for thermal neutrons is under development as part of the European FP7 NMI3 JRA program (Project 226507). The aim of the project is to have a detector with: sub-millimetre position resolution, high rate capability (>1 MHz), high efficiency (>50% for 1 Angstrom neutrons) and an active area of 200×200 mm2. A detector with these characteristics is of interest for retlectometry and micro-focusing SANS instruments at the neutron scattering facilities. The detector under development is a gas scintillation proportional counter (GSPC) that uses the light emitted in the gas avalanche to determine the position of neutrons absorbed inside the detector using the Anger camera principle. The device for the gas multiplication is an MSGC and the gas is a mixture of 3He and CF4, Despite the worldwide shortage of 3He, it still represents a valid option for small detectors that require high efficiency like the one under development in this project. The status of the project will be reported in the following sections.

ANTS — a simulation package for secondary scintillation Anger-camera type detector in thermal neutron imaging

A custom and fully interactive simulation package ANTS (Anger-camera type Neutron detector: Toolkit for Simulations) has been developed to optimize the design and operation conditions of secondary scintillation Anger-camera type gaseous detectors for thermal neutron imaging. The simulation code accounts for all physical processes related to the neutron capture, energy deposition pattern, drift of electrons of the primary ionization and secondary scintillation. The photons are traced considering the wavelength-resolved refraction and transmission of the output window. Photo-detection accounts for the wavelength-resolved quantum efficiency, angular response, area sensitivity, gain and single-photoelectron spectra of the photomultipliers (PMTs). The package allows for several geometrical shapes of the PMT photocathode (round, hexagonal and square) and offers a flexible PMT array configuration: up to 100 PMTs in a custom arrangement with the square or hexagonal packing. Several read-out patterns of the PMT array are implemented. Reconstruction of the neutron capture position (projection on the plane of the light emission) is performed using the center of gravity, maximum likelihood or weighted least squares algorithm. Simulation results reproduce well the preliminary results obtained with a small-scale detector prototype. ANTS executables can be downloaded from http://coimbra.lip.pt/~andrei/.

A 2 dimensional GMSD based imaging detector for neutrons

The development and testing of a 2-D prototype detector based on a gas microstrip detector (GMSD) is reported using a gas mixture of 2.5 bar /sup 3/He and 2.5 bar CF/sub 4/. The second coordinate is obtained by utilising a plane of wires as pick up electrodes. The detector is operated with the wire plane at such a potential so as not to induce any gain around the wires. This means that the high tolerances normally associated with wire planes in multi wire proportional counters are not mandatory. The detector comprises of 48 individually instrumented channels in both X (GMSD strips) and Y (transverse wire plane). A specially designed encoding module has been constructed which feeds digital addresses for each event to the ISIS data taking electronics system (DAE). An intrinsic detector resolution of /spl sim/1 mm FWHM has been measured for both dimensions (in experimental exposures on the ROTAX beamline at ISIS) which is degraded slightly by the digital resolution for the overall system. This readout method is shown to be very tolerant of a poor signal to noise ratio in the readout channels (unlike traditional analogue wire chamber readout systems) and permits the operation of the GMSD at low avalanche gains (/spl sim/10) which helps to maximise the rate and lifetime performance of the detector as well as permitting data capture rates in the MHz range. The event timing resolution is comfortably sub microsecond and is therefore suitable for applications on spallation neutron sources.