Neutron Counting Detector Research Articles

Energy-resolved neutron imaging options at a small angle neutron scattering instrument at the Australian Center for Neutron Scattering.

Energy-resolved neutron imaging experiments conducted on the Small Angle Neutron Scattering (SANS) instrument, Bilby, demonstrate how the capabilities of this instrument can be enhanced by a relatively simple addition of a compact neutron counting detector. Together with possible SANS sample surveying and location of the region of interest, this instrument is attractive for many imaging applications. In particular, the combination of the cold spectrum of the neutron beam and its pulsed nature enables unique non-destructive studies of the internal structure for samples that are opaque to other more traditional techniques. In addition to conventional white beam neutron radiography, we conducted energy-resolved imaging experiments capable of resolving features related to microstructure in crystalline materials with a spatial resolution down to ∼0.1 mm. The optimized settings for the beamline configuration were determined for the imaging modality, where the compromise between the beam intensity and the achievable spatial resolution is of key concern.

Investigation of image distortion due to MCP electronic readout misalignment and correction via customized GUI application

The MCP-based neutron counting detector is a novel device that allows high spatial resolution and time-resolved neutron radiography and tomography with epithermal, thermal and cold neutrons. Time resolution is possible by the high readout speeds of ∼ 1200 frames/sec, allowing high resolution event counting with relatively high rates without spatial resolution degradation due to event overlaps. The electronic readout is based on a Timepix sensor, a CMOS pixel readout chip developed at CERN. Currently, a geometry of a quad Timepix detector is used with an active format of 28 × 28 mm2 limited by the size of the Timepix quad (2 × 2 chips) readout. Measurements of a set of high-precision micrometers test samples have been performed at the Imaging and Materials Science & Engineering (IMAT) beamline operating at the ISIS spallation neutron source (U.K.). The aim of these experiments was the full characterization of the chip misalignment and of the gaps between each pad in the quad Timepix sensor. Such misalignment causes distortions of the recorded shape of the sample analyzed. We present in this work a post-processing image procedure that considers and corrects these effects. Results of the correction will be discussed and the efficacy of this method evaluated.

Counting neutrons with a commercial S-CMOS camera

It is possible to detect individual flashes from thermal neutron impacts in a ZnS scintillator using a CMOS camera looking at the scintillator screen, and off line image processing. Some preliminary results indicated that the efficiency of recognition could be improved by optimizing the light collection and the image processing. We will report on this ongoing work which is a result from the collaboration between ESS Bilbao and the ILL. The main progress to be reported is situated on the level of the on-line treatment of the imaging data. If this technology is to work on a genuine scientific instrument, it is necessary that all the processing happens on line, to avoid the accumulation of large amounts of image data to be analyzed off line. An FPGA-based real-time full-deca mode VME-compatible CameraLink board has been developed at the SCI of the ILL, which is able to manage the data flow from the camera and convert it in a reasonable “neutron impact” data flow like from a usual neutron counting detector. The main challenge of the endeavor is the optical light collection from the scintillator. While the light yield of a ZnS scintillator is a priori rather important, the amount of light collected with a photographic objective is small. Different scintillators and different light collection techniques have been experimented with and results will be shown for different setups improving upon the light recuperation on the camera sensor. Improvements on the algorithm side will also be presented. The algorithms have to be at the same time efficient in their recognition of neutron signals, in their rejection of noise signals (internal and external to the camera) but also have to be simple enough to be easily implemented in the FPGA. The path from the idea of detecting individual neutron impacts with a CMOS camera to a practical working instrument detector is challenging, and in this paper we will give an overview of the part of the road that has already been walked.

Non-Destructive Study of Bulk Crystallinity and Elemental Composition of Natural Gold Single Crystal Samples by Energy-Resolved Neutron Imaging

Energy-resolved neutron imaging enables non-destructive analyses of bulk structure and elemental composition, which can be resolved with high spatial resolution at bright pulsed spallation neutron sources due to recent developments and improvements of neutron counting detectors. This technique, suitable for many applications, is demonstrated here with a specific study of ~5–10 mm thick natural gold samples. Through the analysis of neutron absorption resonances the spatial distribution of palladium (with average elemental concentration of ~0.4 atom% and ~5 atom%) is mapped within the gold samples. At the same time, the analysis of coherent neutron scattering in the thermal and cold energy regimes reveals which samples have a single-crystalline bulk structure through the entire sample volume. A spatially resolved analysis is possible because neutron transmission spectra are measured simultaneously on each detector pixel in the epithermal, thermal and cold energy ranges. With a pixel size of 55 μm and a detector-area of 512 by 512 pixels, a total of 262,144 neutron transmission spectra are measured concurrently. The results of our experiments indicate that high resolution energy-resolved neutron imaging is a very attractive analytical technique in cases where other conventional non-destructive methods are ineffective due to sample opacity.

Energy-dispersive neutron imaging and diffraction of magnetically driven twins in a Ni2MnGa single crystal magnetic shape memory alloy

Single crystals of a partially twinned magnetic shape memory alloy, Ni2MnGa, were imaged using neutron diffraction and energy-resolved imaging techniques at the ISIS spallation neutron source. Single crystal neutron diffraction showed that the crystal produces two twin variants with a specific crystallographic relationship. Transmission images were captured using a time of flight MCP/Timepix neutron counting detector. The twinned and untwinned regions were clearly distinguishable in images corresponding to narrow-energy transmission images. Further, the spatially-resolved transmission spectra were used to elucidate the orientations of the crystallites in the different volumes of the crystal.

Investigation of microstructure within metal welds by energy resolved neutron imaging

The recent development of bright pulsed neutron sources and high resolution neutron counting detectors enables simultaneous acquisition of a neutron transmission spectrum for each pixel of the image. These spectra can be used to reconstruct microstructure parameters within welds, such as strain, texture and phase composition through Bragg edge analysis, and in some cases elemental composition through resonance absorption analysis. In this paper we demonstrate the potential of energy-resolved neutron imaging to study the microstructures of two steel welds, where the spatial distribution of residual strain within the welds, as well as some information on the texture, are obtained with sub-mm spatial resolution. A friction stir weld of two steel plates and a conventional weld of two steel pipes were studied at pulsed neutron facilities, where a Δλ/λ resolution as low as 0.2% can be attained over a wide range of neutron wavelengths ranging from 0.5 Å to 8 Å.

Spatially resolved remote measurement of temperature by neutron resonance absorption

Deep penetration of neutrons into most engineering materials enables non-destructive studies of their bulk properties. The existence of sharp resonances in neutron absorption spectra enables isotopically-resolved imaging of elements present in a sample, as demonstrated by previous studies. At the same time the Doppler broadening of resonance peaks provides a method of remote measurement of temperature distributions within the same sample. This technique can be implemented at a pulsed neutron source with a short initial pulse allowing for the measurement of the energy of each registered neutron by the time of flight technique. A neutron counting detector with relatively high timing and spatial resolution is used to demonstrate the possibility to obtain temperature distributions across a 100µm Ta foil with ~millimeter spatial resolution. Moreover, a neutron transmission measurement over a wide energy range can provide spatially resolved sample information such as temperature, elemental composition and microstructure properties simultaneously.

Time-resolved neutron imaging at ANTARES cold neutron beamline

In non-destructive evaluation with X-rays light elements embedded in dense, heavy (or high-Z) matrices show little contrast and their structural details can hardly be revealed. Neutron radiography, on the other hand, provides a solution for those cases, in particular for hydrogenous materials, owing to the large neutron scattering cross section of hydrogen and uncorrelated dependency of neutron cross section on the atomic number. The majority of neutron imaging experiments at the present time is conducted with static objects mainly due to the limited flux intensity of neutron beamline facilities and sometimes due to the limitations of the detectors. However, some applications require the studies of dynamic phenomena and can now be conducted at several high intensity beamlines such as the recently rebuilt ANTARES beam line at the FRM-II reactor. In this paper we demonstrate the capabilities of time resolved imaging for repetitive processes, where different phases of the process can be imaged simultaneously and integrated over multiple cycles. A fast MCP/Timepix neutron counting detector was used to image the water distribution within a model steam engine operating at 10 Hz frequency. Within <10 minutes integration the amount of water was measured as a function of cycle time with a sub-mm spatial resolution, thereby demonstrating the capabilities of time-resolved neutron radiography for the future applications. The neutron spectrum of the ANTARES beamline as well as transmission spectra of a Fe sample were also measured with the Time Of Flight (TOF) technique in combination with a high resolution beam chopper. The energy resolution of our setup was found to be ∼ 0.8% at 5 meV and ∼ 1.7% at 25 meV. The background level (most likely gammas and epithermal/fast neutrons) of the ANTARES beamline was also measured in our experiments and found to be on the scale of 3% when no filters are installed in the beam.Online supplementary data available from stacks.iop.org/jinst/10/P07008/mmedia. The videos are given as supplementary material linked to the main article.

Imaging of dynamic magnetic fields with spin-polarized neutron beams

Precession of neutron spin in a magnetic field can be used for mapping of a magnetic field distribution, as demonstrated previously for static magnetic fields at neutron beamline facilities. The fringing in the observed neutron images depends on both the magnetic field strength and the neutron energy. In this paper we demonstrate the feasibility of imaging periodic dynamic magnetic fields using a spin-polarized cold neutron beam. Our position-sensitive neutron counting detector, providing with high precision both the arrival time and position for each detected neutron, enables simultaneous imaging of multiple phases of a periodic dynamic process with microsecond timing resolution. The magnetic fields produced by 5- and 15-loop solenoid coils of 1 cm diameter, are imaged in our experiments with ∼100 μm resolution for both dc and 3 kHz ac currents. Our measurements agree well with theoretical predictions of fringe patterns formed by neutron spin precession. We also discuss the wavelength dependence and magnetic field quantification options using a pulsed neutron beamline. The ability to remotely map dynamic magnetic fields combined with the unique capability of neutrons to penetrate various materials (e.g., metals), enables studies of fast periodically changing magnetic processes, such as formation of magnetic domains within metals due to the presence of ac magnetic fields.

Bright flash neutron radiography capability of the research reactor at the McClellan Nuclear Research Center

The capability to produce a bright, short neutron pulse at the McClellan Nuclear Research Center (MNRC) can be very attractive for some neutron imaging applications. Complementary to conventional thermal neutron radiography conducted at the reactor, operating at the average power of 1MW, a short pulse of ~25ms FWHM duration can be produced at MNRC with the peak power exceeding 350MW. Combination of a fast thermal neutron counting detector with a short neutron pulse at MNRC, enables high-resolution stroboscopic imaging to complement conventional neutron radiography. The results presented in this paper demonstrate the MNRC capabilities for conducting conventional thermal neutron radiography, demonstrating imaging spatial resolution below 100μm, as well as bright flash neutron radiography with multiple nearly simultaneous events detected with microsecond timing resolution.

Neutron resonance transmission spectroscopy with high spatial and energy resolution at the J-PARC pulsed neutron source

The sharp variation of neutron attenuation at certain energies specific to particular nuclides (the lower range being from ~1eV up to ~1keV), can be exploited for the remote mapping of element and/or isotope distributions, as well as temperature probing, within relatively thick samples. Intense pulsed neutron beam-lines at spallation sources combined with a high spatial, high-timing resolution neutron counting detector, provide a unique opportunity to measure neutron transmission spectra through the time-of-flight technique. We present the results of experiments where spatially resolved neutron resonances were measured, at energies up to 50keV. These experiments were performed with the intense flux low background NOBORU neutron beamline at the J-PARC neutron source and the high timing resolution (~20ns at epithermal neutron energies) and spatial resolution (~55µm) neutron counting detector using microchannel plates coupled to a Timepix electronic readout. Simultaneous element-specific imaging was carried out for several materials, at a spatial resolution of ~150µm. The high timing resolution of our detector combined with the low background beamline, also enabled characterization of the neutron pulse itself – specifically its pulse width, which varies with neutron energy. The results of our measurements are in good agreement with the predicted results for the double pulse structure of the J-PARC facility, which provides two 100ns-wide proton pulses separated by 600ns, broadened by the neutron energy moderation process. Thermal neutron radiography can be conducted simultaneously with resonance transmission spectroscopy, and can reveal the internal structure of the samples. The transmission spectra measured in our experiments demonstrate the feasibility of mapping elemental distributions using this non-destructive technique, for those elements (and in certain cases, specific isotopes), which have resonance energies below a few keV, and with lower resolution for elements with relatively high resonance energies in the 1–30keV range.

High-Resolution Strain Mapping Through Time-of-Flight Neutron Transmission Diffraction

The spatial resolution of time of flight neutron transmission diffraction was recently improved by the extension of photon/electron counting technology to imaging of thermal and cold neutrons. The development of novel neutron sensitive microchannel plates enables neutron counting with spatial resolution of ~55 um and time-of-flight accuracy of ~1 us, with efficiency as high as 70% for cold and ~40% for thermal neutrons. The combination of such a high resolution detector with a pulsed collimated neuron beam provides the opportunity to obtain a 2-dimensional map of neutron transmission spectra in one measurement. The results of our neuron transmission measurements demonstrate that maps of strains integrated along the beam propagation direction can be obtained with ~100 microstrain accuracy and spatial resolution of ~100 um providing there are sufficient neutron events collected. In this paper we describe the capabilities of the MCP neutron counting detectors and present the experimental results of 2-dimensional strain maps within austenitic steel compact tension (CT) crack samples measured at the ENGIN-X beamline of the ISIS pulsed neutron source.

High Resolution Neutron Resonance Absorption Imaging at a Pulsed Neutron Beamline

The existence of resonance peaks in neutron absorption spectra in the epithermal range of energies enables unique non-destructive testing techniques. The deep penetration of epithermal neutrons provides the opportunity to perform a compositional analysis of a sample which is opaque to X-rays and thermal neutrons. The neutron resonances in the transmission spectra constitute a characteristic pattern for many isotopes, which can be used to identify the isotope and to map the distribution of the isotope in a sample. The neutron transmission spectra can be measured with the time of flight (TOF) technique using a pulsed neutron source. Combining this method with a high resolution neutron counting detector enables substantial improvements of spatial resolution of neutron resonance transmission imaging. Such a detector has been developed to register neutrons with 55 µm spatial and 10–1000 ns temporal resolution Our proof-of-principle experiments at the ISIS pulsed neutron spallation source demonstrate that compositional analysis of multi-element samples can now be performed with ∼150 µm spatial resolution. Images of a test mask consisting of &#60;200 µm thick foils of Au, Ag, In and Gd were collected in the 1–100 eV energy range. The experimental results demonstrate the potential for compositional analysis via resonance absorption transmission with high spatial resolution. In-bulk temperature measurement through Doppler broadening analysis will also benefit from this technique.

High resolution neutron counting detectors with microchannel plates and their applications in neutron radiography, diffraction and resonance absorption imaging

The event counting detection technology based on signal amplification in microchannel plates (MCPs) was initially developed for the applications with relatively low fluxes of photons, electrons and...

Refraction contrast imaging and edge effects in neutron radiography

This paper reports on the edge enhancement and refraction/scattering effects in neutron radiography measured at thermal and cold neutron beams with a high resolution ( ∼ 55 μm) microchannel plate neutron counting detector. These effects in some cases can enhance the contrast of certain features in the neutron radiographic images. At the same time, the edge effects introduce image distortions, as in case of tomographic reconstructions. The edge effects in radiographies of several steel and aluminum objects are shown for different beam divergences and sample orientations relative to the beam. It is also demonstrated how novel microcapillary neutron collimators can enable refraction and scattering contrast imaging in some cases, where the refraction and scattering angles are relatively large. These collimators can also be used to reduce some refraction artifacts, namely remove bright edges in the transmission images.

High‐Resolution Strain Mapping Through Time‐of‐Flight Neutron Transmission Diffraction with a Microchannel Plate Neutron Counting Detector

Abstract: Conventional neutron radiography can be strongly enhanced by obtaining Bragg‐edge information spatially correlated with the attenuation coefficient. This can now be achieved through time‐of‐flight techniques at pulsed neutron sources, utilising a neutron counting detector with high‐spatial and high‐temporal resolution. In these measurements, the positions of Bragg edges can in principle be obtained for each 55 × 55 μm2 pixel of the radiographic image. The combination of both Bragg‐edge and attenuation information enables high spatial resolution studies to be carried out on material composition, phase transitions, texture variations, as well as residual strain mapping. In this article, we present the results of high‐resolution strain maps of a ferritic steel cantilever sample measured at different loads by both transmission and conventional diffraction modes, as well as strains in an austenitic steel compact‐tension (CT) crack sample. The proof of principle experiments performed on the ENGIN‐X beamline on a bent cantilever arrangement resulting in a uni‐axial stress field verified that the strain values measured in diffraction and transmission mode are in good agreement. The characteristics of the transmission mode detector as well as the measured strain maps and future possibilities of this technology are discussed.

Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

Results of neutron transmission Bragg edge spectroscopic experiments performed at the SNAP beamline of the Spallation Neutron Source are presented. A high resolution neutron counting detector with a neutron sensitive microchannel plate and Timepix ASIC readout is capable of energy resolved two dimensional mapping of neutron transmission with spatial accuracy of ∼55 μm, limited by the readout pixel size, and energy resolution limited by the duration of the initial neutron pulse. A two dimensional map of the Fe 110 Bragg edge position was obtained for a bent steel screw sample. Although the neutron pulse duration corresponded to ∼30 mÅ energy resolution for 15.3 m flight path, the accuracy of the Bragg edge position in our measurements was improved by analytical fitting to a few mÅ level. A two dimensional strain map was calculated from measured Bragg edge values with an accuracy of ∼few hundreds μistrain for 300s of data acquisition time.

High-resolution neutron microtomography with noiseless neutron counting detector

The improved collimation and intensity of thermal and cold neutron beamlines combined with recent advances in neutron imaging devices enable high-resolution neutron radiography and microtomography, which can provide information on the internal structure of objects not achievable with conventional X-ray imaging techniques. Neutron detection efficiency, spatial and temporal resolution (important for the studies of dynamic processes) and low background count rate are among the crucial parameters defining the quality of radiographic images and tomographic reconstructions. The unique capabilities of neutron counting detectors with neutron-sensitive microchannel plates (MCPs) and with Timepix CMOS readouts providing high neutron detection efficiency (∼70% for cold neutrons), spatial resolutions ranging from 15 to 55 μm and a temporal resolution of ∼1 μs—combined with the virtual absence of readout noise—make these devices very attractive for high-resolution microtomography. In this paper we demonstrate the capabilities of an MCP–Timepix detection system applied to microtomographic imaging, performed at the ICON cold neutron facility of the Paul Scherrer Institute. The high resolution and the absence of readout noise enable accurate reconstruction of texture in a relatively opaque wood sample, differentiation of internal tissues of a fly and imaging of individual ∼400 μm grains in an organic powder encapsulated in a ∼700 μm thick metal casing.

Improved efficiency of high resolution thermal and cold neutron imaging

Spatial resolution and detection efficiency of neutron imaging detectors are the crucial parameters defining the ultimate resolution in neutron radiography, micro-tomography and other non-destructive testing techniques. The most widely used neutron imaging systems use a neutron-sensitive scintillation screen lens coupled to a CCD or a CMOS sensor. The highest spatial resolution in those devices is achieved using very thin scintillation screens which reduce image blurring due to light spreading in the scintillator. Thus high spatial resolution and high detection efficiency usually could not be achieved in the same device. Neutron counting detectors using neutron-sensitive microchannel plates (MCPs) overcome that deficiency by providing a long neutron absorption path (∼mm scale) and preserving events within a single sub-10 μm pore. At the same time, each neutron in those detectors can be time-tagged with ∼1 μs accuracy. The latest 10B and gadolinium-doped MCPs developed by Nova Scientific were tested at the cold neutron facility ICON of Paul Scherrer Institute and proved to be capable of imaging cold neutrons with 70% detection efficiency. Neutron radiography and micro-tomography confirmed the high spatial resolution capabilities of the latest generation of high efficiency microchannel plates. The high spatial and temporal resolution of our neutron counting detectors allows for high resolution micro-tomography, novel studies of material composition, texture, phase and strain through Bragg edge imaging as well as dynamic studies of magnetic fields inside of thick samples, exploiting the neutron spin interactions with magnetic fields.

High resolution Bragg edge transmission spectroscopy at pulsed neutron sources: Proof of principle experiments with a neutron counting MCP detector

The high spatial and temporal resolution of a neutron counting detector using microchannel plates (MCPs) combined with Medipix2/Timepix readout can substantially improve the spatial resolution of neutron transmission spectroscopy, as shown in our proof-of-principle experiments. Provided that the neutron fluence and data acquisition time are sufficient, transmission spectra can be acquired in each 55×55 μm 2 pixel of the detector, allowing high spatial resolution mapping of Bragg edge positions. Our first experiment demonstrates that energy resolution as high as Δ E/ E<1% or Δ E<4 mÅ can be achieved. Variation of the residual strain in a well-characterized VAMAS round robin shrink-fitted Al ring-and-plug sample was measured with ∼200 microstrain resolution through an accurate mapping of the first (1 1 1) Bragg edge. The measured stress profile agrees well with the expected values for that particular sample. More developments on the detector processing electronics are required in order to reduce the data acquisition times by enabling simultaneous measurements of spectra in a wide energy range covering multiple Bragg edges.