In October 2017, most European countries reported unique atmospheric detections of aerosol-bound radioruthenium (106Ru). The range of concentrations varied from some tenths of µBq·m−3 to more than 150 mBq·m−3. The widespread detection at such considerable (yet innocuous) levels suggested a considerable release. To compare activity reports of airborne 106Ru with different sampling periods, concentrations were reconstructed based on the most probable plume presence duration at each location. Based on airborne concentration spreading and chemical considerations, it is possible to assume that the release occurred in the Southern Urals region (Russian Federation). The 106Ru age was estimated to be about 2 years. It exhibited highly soluble and less soluble fractions in aqueous media, high radiopurity (lack of concomitant radionuclides), and volatility between 700 and 1,000 °C, thus suggesting a release at an advanced stage in the reprocessing of nuclear fuel. The amount and isotopic characteristics of the radioruthenium release may indicate a context with the production of a large 144Ce source for a neutrino experiment.

Inelastic neutron scattering instruments require very low background; therefore the proper shielding for suppressing the scattered neutron background, both from elastic and inelastic scattering is essential. The detailed understanding of the background scattering sources is required for effective suppression. The Multi-Grid thermal neutron detector is an Ar/CO 2 gas filled detector with a 10 B 4 C neutron converter coated on aluminium substrates. It is a large-area detector design that will equip inelastic neutron spectrometers at the European Spallation Source (ESS). To this end a parameterised Geant4 model is built for the Multi-Grid detector. This is the first time thermal neutron scattering background sources have been modelled in a detailed simulation of detector response. The model is validated via comparison with measured data of prototypes installed on the IN6 instrument at ILL and on the CNCS instrument at SNS. The effect of scattering originating in detector components is smaller than effects originating elsewhere. IntroductionInelastic neutron scattering is a very powerful technique for exploring atomic and molecular motion, as well as magnetic and crystal field excitations [1]. Timeof-Flight (ToF) spectrometers allow a broad phase space to be measured in a single setting; this is typically achieved with a large area detector array [2]. In typical state-of-the-art neutron instruments [2][3][4][5][6][7][8], this detector array can be 10-50 m 2 . One of the main performance criteria of these spectrometers is typically defined by the Signal-to-Background Ratio (SBR), therefore understanding and enhancing the latter is important for the instrument optimisation. In particular, scattered neutrons have a significant contribution to the SBR. The estimation of the SBR is done currently on a series of prescriptions based on observations of historical instrument installation.As a consequence of the recent restructuring of the 3 He market [9], a need for cost effective 3 He-replacing detector solutions is raised [10], especially for inelastic neutron scattering instruments, where large area detectors with high SBR are required. A potent new solution for this type of instruments is the Multi-Grid detector [11; 12], which will be used for the three Time-of-Flight chopper spectrometers at ESS [13][14][15][16]. The Multi-Grid design was invented at the Institut Laue-Langevin (ILL) [17; 18], and the detector now is jointly developed by the ILL and the ESS within the CRISP [19] and BrightnESS [20] projects.The Multi-Grid detector is an Ar/CO 2 -filled proportional chamber with a solid boron-carbide ( 10 B 4 C) neutron converter, enriched in 10 B [21][22][23]. The basic unit of the Multi-Grid detector is the grid, an aluminium frame; thin aluminium lamellas, coated on their both sides with boron-carbide, the so called blades are placed in this frame, parallel with each other and the entrance window of the grid, dividing the grid into cells. In the detector the grids are structured into columns, and this way the cells ...