The series of NEUWAVE workshops was established in 2008 with its start-up at the Technische Universität München, Germany, followed by next meetings in Abington, UK, in 2009 and at the Hokkaido University, Japan, 2010. The intention of the workshops was and is still the support of neutron imaging facilities at spallation sources by the International Society for Neutron Radiology (www.isnr.de) and discussions about options for energy-selective imaging with neutrons generally by all other interested individuals during the meetings.

Traditionally, the workshops are started with a mountain hike (except for Abington, where due to missing mountains a walk along the Themse river took place) the day before its official start. This gives participants the chance to become familiar with each other, to exchange latest information in neutron imaging and related fields or simply enjoy the beautiful landscapes.

The 2011 NEUWAVE-4 workshop took place October 2^nd - 5^th, 2011, in Gatlinburg, Tennessee, USA and was perfectly organized by the team of Ken Tobin and Hassina Bilheux together with great assistance by Linda Stansberry from the Spallation Neutron Source at the Oak Ridge National Laboratory.

This year's hike went along the Great Smoky Mountain National Park Alum Cave Bluff Trail. Announced was a moderately difficult 4.6 miles round trip. A group of about 20 participants made a rather quick walk enjoying Mother Nature's majesty and power, but making up changing smaller groups having extensive discussions. As one of the original initiators of the workshop series was unable to participate in the hike, the traditional Gipfelbier (translation: having a beer on the top of a mountain) was omitted. Regardless the hike was an excellent introduction to the workshop starting the next day.

The Workshop was subdivided in five sections

• Beam line design and activity at existing facilities
• Advanced Techniques (Polarized, Phase and Fission Imaging)
• Bragg Edge and Energy Selective Imaging
• Detectors and Optics
• Applications

The first day was mainly focused on the status of the (planned) imaging facilities at the four spallation sources SNS, ISIS, J-PARC and ESS. More detailed information on these facilities is presented by individual reports within this newsletter. The participants were informed on the actual running upgrade program of the CONRAD facility at HMI and on some the highlights achieved during the last years (e.g. high resolution, energy selective and magnetic imaging etc.). Another presentation was devoted to measurements at the HFIR CG1D imaging prototype beamline. Information on beam characterization, the use of diffusers and data treatment were given.

A new monochromator for energy-selective imaging at PSI, called TESI, was presented. It enables two- and three-dimensional mapping of material properties with high spatial resolution and a flexible Δλ/λ ‰ˆ 2%-5%. First results were presented. The next talk focused on the investigations of superconductors with polarized neutron radiography. here it was demonstrated that Nb samples suppress the Meissner effect and that their surface treatment apparently influences the local field distribution. For Pb-single crystals the Meissner effect seems to prefer crystals showing no flux pinning, while for high purity Pb-polycrystalline materials a partial Meissner-effect with non-uniform flux pinning shows up. The next presentation on experimental and theoretical considerations on edge enhancement in neutron imaging was extensively discussed by the participants as this topic may bring together the fields of neutron physics and neutron imaging. Further investigations must be supported by improved modeling of the experiments to get best agreement to the experimental results. This is essential for the expanded understanding of this effect. The last presentation of this day reported on some new results on imaging of dynamic processes using fission neutrons.

At the succeeding general discussion on the topic spallation based concepts in neutron imaging participants reported on their experiences on detectors based on new sCMOS devices, which seem to be well suited for time-of-flight measurements giving a great improvement compared to former CMOS detectors, the latter especially for the signal-to-noise ratio. They also enable exposure times of up to 10 minutes showing a stable dark image. The only disadvantage seems to be the somewhat low detectum quantum efficiency (DQE) of about 60 %. Other interesting detection systems reported were the MEDIPIX / TIMEPIX based system as well as the systems build by Anton Tremsin and Volker Dangendorf. The presentations on Bragg edge and energy selective imaging were continued on the second day. Results on investigations using a double crystal monochromator at a steady neutron source were presented demonstrating that this method can provide complimentary information but having the advantage of position sensitive resolved scattering signals. The wavelength resolution of the used double crystal monochromator device enabled to observe residual stresses and texture effects. A comparison with TOF techniques showed the advantage of pulsed neutron beams in case of high wavelength-resolution applications. A method for data evaluation of Bragg edge transmission imaging results by a Rietveld type analysis was presented in the next talk. The results of this development were verified by experiments resulting in the availability of crystalline phase imaging for user measurements. The following presentations were devoted to the application of Bragg edge imaging. First, in-situ strain mappings under multi-axial loading were demonstrated successfully in case studies for plane stress and for a sample with stress concentration, respectively. Next, the applicability of Bragg edge imaging was shown in neutron tomography and diffraction of intact, commercial Lithium-ion polymer batteries. While two-dimensional inspection worked successfully, some more research on three-dimensional analyses is necessary. Investigations and results on strain mapping using Bragg edge imaging were the topic of the next presentation, too. Additionally, the simultaneous imaging of the same material or object using both neutrons and X-rays at multiple energies and resolutions to exploit multi€modality was discussed in this presentation e. g. for carbon fiber vinyl ester composite structural materials. The talk on archaeometallurgical studies at ICON and a closer look at the velocity selector highlighted the suitability of energy resolved imaging for the example of historical Japanese swords. The last presentation of the session closed the circle of Bragg edge and energy selective imaging and of neutron imaging at pulsed sources by reporting on various applications of pulsed neutron TOF imaging.

The session detectors and optics was opened by a talk on high resolution energy resolved neutron imaging and transmission diffraction and resonance absorption imaging with multi-channel plate (MCP) detectors. Here the impressive features of these detectors were presented for different applications. Although the next presentation wanted to show results on their way towards 1µm spatial resolution in neutron radiography, the emphasis of the talk was on a general overview of activities including high resolution imaging developments. The last presentation of the second day was on focusing mirrors for neutron imaging resulting in extensive discussions on the applicability of Wolter mirrors in practice.

Due to the extensive discussions which already took place during and after the individual presentations (and first signs of fatigue of the audience) the final wrap-up discussion was canceled.

On the evening the Workshop Dinner took place at the outstanding Gatlinburg aquarium. Most of the participants enjoyed the beautifulness and diversity of the marine wildlife even a long time after the end of the dinner.

The third day of the workshop was devoted to applications. It was reported on neutron radiography of water in soil and plant systems, on high resolution neutron and X-ray imaging of partially saturated materials and engineering applications, on improving the detection of water in a fuel cell using cold neutrons and finally on the first applications at the HFIR CGID from engineering to plants.

With a guided tour to the Spallation Neutron Source (SNS) and the High Flux Reactor (HFIR) at Oak Ridge the NEUWAVE-4 was completed. Participants, who returned to ORNL after the IAN2006 workshop were impressed by the new installations and the status of the user program. The beam port CG10 at HFIR was found quite useful for dedicated tests for neutron imaging. It should be used further for different purposes until the VENUS project is completed.

Summarizing the few days of the NEUWAVE-4 workshop, an extremely high level in quality of the presentations must be stated, demonstrating the increasing efforts the different neutron imaging groups worldwide put on their research, developments and facility upgrades. New data and evaluations not published yet have been presented and been extensively discussed, often resulting in new ideas for further improvements. Continuing this way, neutron imaging might become even more recognized within the scientific and technological societies.

During the workshop it was decided, that NEUWAVE-5 will take place at Lund, Sweden, in early 2013. It will be interesting to see where the mountain hike will take place.

Thomas Bücherl