Muon spin rotation/relaxation/resonance (μSR) spectroscopy uses highly polarized muons to study the microscopic magnetic structure and dynamics of condensed matter. In addition to the five existing muon facilities, the first Chinese muon source, the Muon station for sciEnce technoLOgy and inDustrY (MELODY), is planned to be constructed in Phase II of the China Spallation Neutron Source (CSNS). It aims to provide intense and pulsed muon beams to conduct μSR applications in multiple disciplines. The group from the University of Science and Technology of China (USTC) participated in the collaboration with the CSNS accelerator group for the construction of the muon source. The USTC group led the research and development (R&D) of the first-generation photomultiplier tube (PMT)-based μSR spectrometer, and the design of the second-generation silicon photomultiplier (SiPM)-based spectrometer. The PMT-based spectrometer is a 128-channel prototype to demonstrate and develop key detector and electronics technologies for the planned MELODY. After several iterative designs and updates of detectors and electronics, the spectrometer prototype achieved a 7-ns dead time, which can record more than 12 positrons per channel per pulse according to the ISIS running experience. Based on the technologies developed from the first-generation spectrometer, the second-generation spectrometer will use SiPMs to accommodate over 2500 detector units to make better use of muons in MELODY. The two generation developments of Chinese μSR spectrometers will greatly advance the construction of MELODY, and provide high-quality data for users to interpret material properties in the near future.

A 100-nm-thick gadolinium layer deposited on a pixelated silicon sensor was activated in a neutron field to measure the internal conversion electron (ICE) spectrum generated by neutron capture products of 155Gd and 157Gd. The experiment was performed at the ISIS neutron and muon facility, using a bespoke version of the HEXITEC spectroscopic imaging camera. Signals originating from internal conversion electrons, Auger electrons, x rays and gamma rays up to 150 keV were identified. The ICE spectrum has an energy resolution of 1.8-1.9 keV at 72 keV and shows peaks from the K, L, M, N+ ICEs of the 79.51 keV and 88.967 keV 2+-0+ gamma transitions from the first excited states in 158Gd and 156Gd, respectively, as well as the K ICEs of the 4+-2+ transitions at 181.931 keV and 199.213 keV from the respective second excited states. Spectrum analysis was performed using a convolution of a Gaussian with exponential functions at the low and high energy side as the peak shaping function. Relative ICE intensities were derived from the fitted peak areas and compared with internal conversion coefficient (ICC) values calculated from the BrIcc database. Relative to the dominant L shell contribution, the K ICE intensity conforms to BrIcc and the M, N, O+ ICE intensities are somewhat higher than expected.

Super-MuSR is the first of the next generation of muon spin spectrometers and is currently being built at the ISIS pulsed source, UK. Funded via the ‘Endeavour Programme’, Super-MuSR is a six million pound development which will provide novel muon techniques at transformational counting rates. The expected counting rate is an order of magnitude greater than the existing instrument, exceeding 10^ 9 •counts•hr -1 . The improved count rate capability comes from the use of silicon photomultipliers (SiPM) to build a high density detector array and the implementation of a new digital signal processing pipeline. The array will exceed 700 independent detector channels in a barrel geometry, roughly 1 m in length and 0.35 m in diameter. Each SiPM is coupled to a plastic scintillator using wavelength shifting fibers. Encapsulation of the ‘tile’ assembly is required for light collection and optical isolation from the environment. Comparison of encapsulation methods, including spray-coatings are reported. Signal digitisation, processing and streaming are provided by Nuclear Instruments DAQ 121, which uses a Xilinx Zynq ® UltraScale +TM ‘system on a chip’ with ADCs capable of 1 GHz sampling. Super-MuSR’s count-rate stability specification, a 0.032% variation, translate into a demanding thermal stability requirement. To hold the array at a fixed temperature a novel temperature control system has been developed. A resistive heater has been embedded inside the readout PCB in a ‘serpentine’ pattern, capable of delivering 400 mW of power, heating the SiPM to ≈45 °C.

The MARI direct geometry time-of-flight neutron spectrometer at ISIS has been upgraded with an m=3 supermirror guide and new detector electronics. This has resulted in a flux gain of ≈6× at λ=1.8 Å, and improvements on discriminating electrical noise, allowing MARI to continue to deliver a high quality science program well into its fourth decade of life.

The next generation of muon spin spectrometers at the ISIS pulsed source are being developed to make efficient use of the increased source intensity. They will provide a transformational improvement in counting rates: ‘Super-MuSR’ will be the first of these instruments, capable of counting at ≈1 G·event·hr −1 . Key to delivering this capability is the development of highly pixelated, high density detector arrays that cover an appreciable solid angle, with each detector element optimised for counting at very high data rates. A series of ‘firsts’ are planned to optimise individual element count rate capability, where analogue waveforms recorded from SiPMs are fully digitised and processed using digital signal processing (DSP) methods at either software or firmware level. Full raw-signal digitisation will be achieved using the Xilinx Zynq ® UltraScale+ TM series of ‘system on a chip’ operating with ADCs capable of 1 GHz sampling, data handling using event streaming technology, and DSP to provide novel data correction techniques. We will discuss our concept and present preliminary results. Our prototype digitising data acquisition system, which is key to implementing a ‘digital data pipeline’ (DDP) is presented.

In this study, we report the first case of design and implementation of a polarized neutron imaging option on the Imaging and Materials Science & Engineering Station (IMAT). This is a significant addition to the capabilities of the station that allows the characterization of advanced magnetic materials for different engineering applications. Combining its time-of-flight feature with a polarized beam yields data that facilitate both quantitative and qualitative analysis of magnetic materials. Using the simple field of an aluminium solenoid, we perform a characterization of the new setup. In addition, we present polarized measurements of additively manufactured (AM) MnAl samples where the magnetic anisotropy due to the fabrication process has been investigated as a first scientific application of the setup. The results indicate that the anisotropy of the material can be engineered through variation of the AM fabrication parameters.

Mantid Imaging has been developed to provide a graphical reconstruction process for users of neutron imaging instruments to eliminate the need to fall back on commercial software. Mantid Imaging builds on algorithms provided by libraries including Astra Toolbox and Tomopy to offer noise reduction, artifact removal, alignment, filtered back projection and iterative reconstruction methods. Extra functionality was added by using algorithms from ALGOTOM for ring removal and from the Core Imaging Library (CIL) for regularised 3D reconstruction. Mantid Imaging 2.4 has recently been released. It is an open source Python GUI, runs under Linux and Windows and can easily be installed on end user systems. Mantid Imaging is aimed at users with no programming background and with little image processing experience. At ISIS Mantid Imaging runs on the ISIS-Data-Analysis-as-a-Service (IDAaaS) platform, which is remotely accessible with any modern web browser and gives users access to sufficient hardware resources to handle large datasets. Extensions of Mantid Imaging for energy-resolved neutron imaging are planned for the future.

The ISIS neutron facility at the Rutherford Laboratory UK, is currently undergoing a major refurbishment of target station 1 (https://www.isis.stfc.ac.uk/Pages/ISIS-First-Target-Station-Project.aspx [1]), which is more than 30 years old. This work will include a new Target, Reflector And Moderator (TRAM) which will change both moderated neutronic time structures and fluxes delivered to the instruments. Potentially this could have serious detrimental effects on instrument resolutions. To mitigate this risk a novel calibration rig was designed and built which has allowed accurate measurements of the absolute fluxes and moderator time structures for nearly every instrument at ISIS. This information was then used to refine the moderators for the new TRAM using neutronic simulations (Skoro and Ansell, 2014 [2]). In this paper details of the calibration rig are presented along with some of the data taken.

Systematic and random errors are often introduced in white-beam neutron Computed Tomography (CT) due to the nature of the neutron source, neutron-matter interactions or limitations in hardware. These errors can cause bias in measured linear attenuation coefficients (LACs) and artifacts in the reconstructed images, reducing image quality and impeding quantitative analysis. Three sources of error in neutron tomography were investigated and quantified that are particularly pertinent to spallation source facilities. These include detector to sample stage misalignment, beam fluctuations and undersampling due to missing CT projections. Quantitative analyses of the grayscale biases and misalignment artifacts were performed on the reconstructed data using ImageJ. Furthermore, the efficacy of classical iterative reconstruction algorithms for the suppression of missing projection artifacts was explored. Several image qualifying metrics were used to compare Filtered Back Projection (FBP) and iterative algorithms quantitatively. We show that beam intensity fluctuations if left uncorrected provide a minimal bias even when exaggerated across a stack of projections. In addition, detector-sample stage misalignment causes a geometrical misalignment blur when not corrected for and severe ring artifacts despite image rotation correction. We suggest that iterative algorithms are more favourable in suppressing missing projection neutron artifacts, however they inaccurately reproduce the LACs for missing-wedge reconstructions compared to FBP.

The IMAT project is now well into its commissioning phase, and a user programme for neutron imaging has started on the new instrument at ISIS TS2. The performance parameters for white-beam tomography and energy-dispersive neutron imaging had been determined earlier. Here we report on a further evaluation of the wavelength-resolving imaging options on IMAT, including selection of neutron wavelength bands using disk choppers as well as energydispersive Bragg edge imaging using time-resolving detectors. We review the instrument parameters of IMAT relevant for energy-resolved imaging, and present one example of residual strain imaging. Introduction Energy-selective and energy-dispersive neutron imaging has been developed over the past decade as discussed at many Neutron Wavelength Dependent Imaging (NeuWave) workshops [1] and as documented by many proof-of-concept and materials science studies [2], [3] (and references therein). Energy-selective neutron imaging using crystal monochromators was explored early on at continuous neutron sources [4, 5], whilst energy-dispersive Bragg edge transmission methods were developed on time of flight (TOF) instruments [6]. Further developments of energy-selective analysis at a pulsed neutron source [7] and the development of an MCP detector with high spatial and time resolution [8] helped to advance the field. Several dedicated TOF neutron imaging beamlines were designed in the past few years, with RADEN [9] and IMAT [3, 10] having started operation already. Here we report on results from the commissioning of the IMAT instrument at the ISIS pulsed neutron source, UK. The basic performance parameters for white-beam tomography and energydispersive neutron imaging have been determined earlier [10], and a more application-related characterisation of the instrument was completed recently [11]. Here we continue to discuss the wavelength-resolving imaging options on IMAT, and report on the evaluation of energyselection related instrument parameters. Neutron Radiography WCNR-11 Materials Research Forum LLC Materials Research Proceedings 15 (2020) 29-34 https://doi.org/10.21741/9781644900574-5 30 IMAT set-up IMAT is installed on the coupled 18 K liquid hydrogen (LH2) moderator of beam port W5 on the 10 Hz pulsed source TS2 of ISIS. Fig. 1 shows the main components; details of the instrument setup are given in [3] and [10]. The flight path length of the beamline from the moderator centre to the centre of the sample positioner is 56 m. Two double-disk choppers, a T0 chopper, five vanadium-foil neutron monitors and different types of TOF detectors use an external trigger indicating at the time of the neutron pulse generation. On IMAT there are three different detector systems available that take advantage of the TOF option. A gated light-intensified CCD camera can be used with the Messina camera box [3]. A microchannel plate detector (MCP) [8] uses a 2×2 array of Timepix readout chips (512×512 pixels, each 55×55 μm) and has a field of view of 28×28 mm. An active pixel sensor (GP2) uses the PImMS-2 CMOS and a gadolinium sheet for converting neutrons to electrons for a pixel size of 70×70 μm and for a field of view of 22×22 mm [12]. For each pixel of a TOF neutron camera (be it the gated-CCD, the MCP or the GP2) the time of the neutron arrival relative to the external trigger is measured, with an electronic time resolution of the order of 10 ns, i.e. well below the instrument resolution δλ. Fig. 1: Schematic overview of the IMAT instrument. The wavelengths of the detected neutrons are calculated from their time of flight by mL T T h ) ( 0 ∆ + = λ = L T T ) ( * 3957 0 ∆ + (1) where λ is the neutron wavelength (in Angstrom), h is Planck’s constant, T is the neutron time of flight (in seconds), ∆T0 is the time offset of the source trigger received by the data processing electronics (in seconds), m is the neutron mass, and L is the flight path from source to detector (in meters). The maximum wavelength band, defined by the need to avoid frame overlap, is about 7 Å for L=56 m and 10 Hz operation [3]. The wavelength resolution of the instrument, on the other hand, is determined by the uncertainty δλ to which a given wavelength can be determined. The width and shape of the neutron pulse for a given wavelength depend on the type, geometry and physical (slowing down, storage) processes occurring within the moderator. The relative uncertainty δλ/λ, as determined recently for IMAT [10], defines the broadening of a Bragg edge or a Bragg dip in a TOF spectrum. It is this resolution function that signifies the Neutron Radiography WCNR-11 Materials Research Forum LLC Materials Research Proceedings 15 (2020) 29-34 https://doi.org/10.21741/9781644900574-5 31 ability of the instrument to discriminate neutron energies, not the width of a time channel (in histogram mode) or uncertainty of a TOF measurement (in event mode). Energy selection on IMAT Energy-selective and energy-dispersive radiography enable image contrast enhancement and the mapping of structure properties, respectively. We use the terms energy-selective/dispersive synonymously with wavelength-selection/dispersion as the kinetic energy and the wavelength of a neutron follow from the measured time-of-flight via Eq 1. We use the term ‘energy-selective’ if one or more (wide or narrow) wavelength bands are engaged, with monochromatic neutron imaging being a special case. Energy-selection is a prerequisite for ‘energy-dispersive’ neutron imaging where histogramming within a wavelength range is performed with a sufficiently fine channel width that is smaller than the instrument resolution, for example by performing a scan across a Bragg edge. The selection of neutron wavelength bands using the two double-disk choppers on IMAT is demonstrated in Fig. 2. The four disks running at 10 Hz can be ‘phased’ to select broader (Fig. 2a) or narrow (Fig. 2b) wavelength bands, up to the width of the white-beam spectrum (red curves). The narrow bands are defined by the condition not to reduce the peak flux (Fig. 2b), yielding relative band widths Δλ/λ ranging from 30% to 10%, for wavelengths from 2 to 6.5 Å, respectively. For example, the bandwidth at the flux maximum of the IMAT spectrum at 2.6 Å is Δλ/λ~22% (blue curve in Fig 2b). Given these values, and that only one energy band is engaged at a time, this method allows energy-selective rather than energy-dispersive measurements. Fig, 2: Examples of wavelength band selection using choppers, for (a) broad and (b) narrow bands. Coarse wavelength bandwidth selection is flexibly and quickly achieved via chopper dephasing from a script without changing beamline components. Experiments with a ‘pink beam’ (i.e. a narrow wavelength band; a term adopted from the synchrotron X-ray imaging community) allow for changing image contrasts without changing the sample set-up and for surveying energy-dependencies of attenuation coefficients for the largest field of view of IMAT (up to 20×20 cm) with any camera system. The neutron flux levels for the narrow wavelength bands in Fig. 2b are reduced to 3-7% of the white-beam flux (depending on wavelength) thus practically precluding tomography studies. Because of the coarse banding, detailed Bragg edge studies are not possible. Another approach to achieve energy discrimination on a TOF instrument is to synchronise the imaging camera with the neutron source. An energy-selective radiography set-up with a gated image-intensified CCD or CMOS camera [7, 13] allows selecting a wide or narrow energy interval out of a white-beam spectrum. The energy discrimination is achieved for the maximum Neutron Radiography WCNR-11 Materials Research Forum LLC Materials Research Proceedings 15 (2020) 29-34 https://doi.org/10.21741/9781644900574-5 32 possible field of view, and in principle for time bins smaller than the instrument resolution. Thus, Bragg edge studies can be performed in principle; however with the gated camera only one channel is selected at a time (different from a high frame-rate camera) and only a fraction of the neutron spectrum is used at a time. Therefore, mapping of Bragg edge parameters is usually not performed on IMAT with this camera system. With the IMAT pixel detectors that accumulate counts into multiple time-slices (3100 for the MCP and 4096 for GP2) performance is improved tremendously, in terms of acquisition times, albeit for small field of views. For a TOF instrument it is important to determine the spectral resolution, i.e. the monochromacity for a given wavelength. An energy-dispersive radiography was collected from a cylindrical CaF2 crystal of 20 mm thickness and 35 mm diameter, at a distance of 25mm from the MCP detector and with an L/D of 250 (Fig. 3a). Fig. 3b displays Bragg dips from the CaF2 single crystal for a selected wavelength range. Fig. 3c shows the wavelength dependence of the FWHM as an indicator of the resolution function. An alternative description was given earlier [10] using the trailing tail (asymmetry τ) of Bragg edges from a CeO2 powder, shown in Fig. 3c for comparison. The asymmetry τ, an indicator of the storage term of the moderator process and reflecting the peak broadening, is preferred to represent the resolution function of IMAT as it is sample-independent and can be determined with a calibration measurement. The FWHM values are smaller by about a factor of two; the scatter of the values is due to the FWHM dependence on wavelength and diffraction angle. Fig. 3: (a) Radiography of a CaF2 crystal; (b) Bragg dip spectrum for selected region of interest; (c) resolution function. “τ” data points taken from [10]. Solid curves are guides to the eye. Table 1 summarises some of the instrument parameters of IMAT relevant for energy selective and energy-dispersive measurements for the different set-up options. The strain resolution value, demonstrating the precision with which a shift of a Bragg edge can be determined, is on the order of 0.01% for a go

We report on the recent developments of the ‘GP2’ detector, highlighting a selection of energy resolved measurements and associated methodology. GP2 is a 100k pixel time-offlight (ToF) neutron camera, which combines a gadolinium converter film and a CMOS (Complementary Metal Oxide Semiconductor) readout sensor. This paper describes an up-todate specification of the detector and its variants, progress that has been made towards integration into the Imaging and Materials Science instrument (IMAT) and an independent review at the ESS test beamline, V20. Two ToF data reduction methods are detailed, namely wavelength dispersive contrast enhancement and ‘wavelength frame multiplication’ (WFM) reduction.

The quality of pulsed riSR spectra are affected by several factors, namely beam time structure, the time width of detector signals, signal processing methods. A Monte Carlo simulation code - "MuSS" (Musr Signal Simulation) has been developed to investigate the influences of all these factors. Simulations show that both beam time width and external transverse field decrease the observable asymmetry. The asymmetry reduces quickly with larger beam time width under strong filed (> 100 Gauss). The deadtime calculated by leading-edge or constant fraction discrimination method shows no big differences. Bias voltage setup in the constant fraction discriminator should be small (in the range (-10, 10) mV). A figure of merit is used to quantify the maximum tolerable event rate of mu SR detectors. Using a proper pole-zero circuit, the deadtime of the Chinese mu SR detector can be reduced from similar to 14 to similar to 7 ns. Therefore the maximum tolerable event rate can be increased from 8 to 12 events/frame/detector.

An experimental muon source (EMuS) to provide a very intense muon beam has been studied at the China Spallation Neutron Source (CSNS) since 2007 and will be operational in the next few years. R&D efforts including a 128-channel muon spin rotation, relaxation, and resonance (μSR) spectrometer prototype are also ongoing. A data acquisition (DAQ) system was been developed to run the μSR spectrometer prototype on the EMuS. It includes front-end electronics (FEEs), time-to-digital converter (TDC) modules, and a DAQ software. The raw data acquired by the DAQ software were converted into a specific format and submitted to specialized analysis software for analysis. The DAQ system was tested on the ISIS muon beam, and the test results proved its performance.

The MAPS direct geometry time-of-flight chopper spectrometer at the ISIS pulsed neutron and muon source has been in operation since 1999, and its novel use of a large array of position-sensitive neutron detectors paved the way for a later generations of chopper spectrometers around the world. Almost two decades of experience of user operations on MAPS, together with lessons learned from the operation of new generation instruments, led to a decision to perform three parallel upgrades to the instrument. These were to replace the primary beamline collimation with supermirror neutron guides, to install a disk chopper, and to modify the geometry of the poisoning in the water moderator viewed by MAPS. Together, these upgrades were expected to increase the neutron flux substantially, to allow more flexible use of repetition rate multiplication and to reduce some sources of background. Here, we report the details of these upgrades and compare the performance of the instrument before and after their installation as well as to Monte Carlo simulations. These illustrate that the instrument is performing in line with, and in some respects in excess of, expectations. It is anticipated that the improvement in performance will have a significant impact on the capabilities of the instrument. A few examples of scientific commissioning are presented to illustrate some of the possibilities.

For the last 30 years, muon experiments at ISIS have been making a significant contribution to a number of scientific fields.However, as a community of researchers, we are always aiming to improve and extend the instruments' capabilities.In this paper, we will review key developments at the ISIS muon facility, the primary beamline upgrade and recent technique developments, before taking a forward look to new projects, such as: the upgrade for MuSR, e-learning, detector development and sample environment.

This paper reports on the development and commissioning of the GP2 detector. GP2 was developed to address the requirement for a high-resolution event-mode imaging detector, for application in energy-resolved neutron radiography. The name GP2 derives from the use of gadolinium as a neutron conversion material, combined with a second-generation mass spectrometry sensor known as PImMS2. Theoretical and measured characteristics of GP2 are compared, with emphasis on the usability and functionality of the detector. The development of the detector has been steered by a design philosophy which was established to ensure that the detector has unique and novel impact. The motivation and consequences of the design philosophy are discussed. The key parameters reported are the neutron detection efficiency (7.5% at 2.5 angstrom), gamma sensitivity (1.5x10(-3)), and a spatial resolution (modulation transfer function at 10%) of 6.4 lp/mm for a 4-mu m-thick natural gadolinium neutron converter film.

A new neutron imaging and diffraction facility, called IMAT, is currently being commissioned at the ISIS pulsed neutron spallation source. IMAT will take advantage of neutron time-of-flight measurement techniques for flexible neutron energy selection and effective energy discrimination. The instrument will be completed and commissioned within the next few months, after neutrons have been recently delivered to the sample area. From 2016 IMAT will enable white-beam neutron radiography and tomography as well as energy-dependent neutron imaging. The facility will offer a spatial resolution down to 50 microns for a field of view of up to 400 cm(2). IMAT will be operated as a user facility for material science applications and will be open for developments of time-of-flight imaging methods.

Until recently a photomultiplier tube (PMT) was the only viable option for photon detection on μSR instruments [1]. A PMT is well suited to this application, as it offers fast rise time, small dead-time (~15ns) and an excellent spectral match to the scintillator emission. They are also low-noise devices and are relatively inexpensive. However, the PMT has certain limitations, such as a strong sensitivity to magnetic fields, prompting this search for new technologies. Specific requirements depend on the measurement technique, the SiSR instrument, and the type of muon source being used. For example, the PSI high field instrument has successfully implemented siliconphotomultiplier technology (SiPM) to construct compact, field-insensitive, fast timing detectors [2]. At PSI the beam timing structure is `continuous' meaning that there is effectively only one muon being measured at a time, negating the need for high count rate capability. While SiPM technology has been used very successfully at the PSI continuous muon source, benefits at a pulsed muon source have yet to be realized.

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.

This paper documents the R&D undertaken jointly by the ISIS Neutron Detector Group and the Oxford University PImMS collaboration. The aim of this project was to develop a high resolution, energy resolved, neutron imaging detector named GP2. This conference record introduces the GP2 detector and lists its key physical properties; however the emphasis here will be on the earlier proof-of-principle work performed with both gadolinium thin films and thick rolled sheets with the prototype PImMS1 sensor and the larger PImMS2 sensor.