Simulation of a suite of generic long-pulse neutron instruments to optimize the time structure of the European Spallation Source

We here describe the result of simulations of 15 generic neutron instruments for the long pulsed European Spallation Source. All instruments have been simulated for 20 different settings of the source time structure, corresponding to pulse lengths between 1 ms and 2 ms; and repetition frequencies between 10 Hz and 25 Hz. The relative change in performance with time structure is given for each instrument, and an unweighted average is calculated. The performance of the instrument suite is proportional to a the peak flux and b the duty cycle to a power of approximately 0.3. This information is an important input to determining the best accelerator parameters. In addition, we find that in our simple guide systems, most neutrons reaching the sample originate from the central 3 5 cm of the moderator. This result can be used as an input in later optimization of the moderator design. We discuss the relevance and validity of defining a single figure of merit for a full facility and compare with evaluations of the individual instrument classes

Systematic performance study of common neutron guide geometries

In this report, we present the results from a systematic benchmarking of four different long neutron guide geometries: elliptic, parabolic, ballistic (piecewise linearly focusing/defocusing), and straight, for various wavelength, divergence restriction, and guide length settings. In this work, we mapped relevant parts of the neutron phase space to show where advanced guide geometries have significant transport advantages over simple guide geometries. The primary findings are that the elliptic and parabolic geometries perform almost equally well, and they are considerably superior to the other geometries, except for low-divergence, cold neutrons. In addition, it was observed that transporting thermal neutrons more than 100 m using elliptic guides was possible with only a 10% loss in the phase space density for divergences up to +/- 0.5 degrees, which enables the construction of very long thermal neutron instruments. Our work will allow instrument designers to use tabulated, standard geometries as a starting point for optimising the guide required for the particular instrument. (C) 2012 Elsevier B.V. All rights reserved

Eliminating line of sight in elliptic guides using gravitational curving

Eliminating fast neutrons (lambda < 0.5 angstrom) by removing direct line of sight between the source and the target sample is a well established technique. This can be done with little loss of transmission for a straight neutron guide by horizontal curving. With an elliptic guide shape, however, curving the guide would result in a breakdown of the geometrical focusing mechanism inherent to the elliptical shape, resulting in unwanted reflections and loss of transmission. We present a new and yet untried idea by curving a guide in such a way as to follow the ballistic curve of a neutron in the gravitational field, while still retaining the elliptic shape seen from the accelerated reference frame of the neutron. Analytical calculations and ray-tracing simulations show that this method is useful for cold neutrons at guide lengths in excess of 100 m. We will present some of the latest results for guide optimization relevant for instrument design at the ESS, in particular an off-backscattering spectrometer which utilizes the gravitational curving, for 6.66 angstrom neutrons over a guide length of 300m. (C) 2010 Elsevier B.V. All rights reserved

Equal abundance of summertime natural and wintertime anthropogenic Arctic organic aerosols

Aerosols play an important yet uncertain role in modulating the radiation balance of the sensitive Arctic atmosphere. Organic aerosol is one of the most abundant, yet least understood, fractions of the Arctic aerosol mass. Here we use data from eight observatories that represent the entire Arctic to reveal the annual cycles in anthropogenic and biogenic sources of organic aerosol. We show that during winter, the organic aerosol in the Arctic is dominated by anthropogenic emissions, mainly from Eurasia, which consist of both direct combustion emissions and long-range transported, aged pollution. In summer, the decreasing anthropogenic pollution is replaced by natural emissions. These include marine secondary, biogenic secondary and primary biological emissions, which have the potential to be important to Arctic climate by modifying the cloud condensation nuclei properties and acting as ice-nucleating particles. Their source strength or atmospheric processing is sensitive to nutrient availability, solar radiation, temperature and snow cover. Our results provide a comprehensive understanding of the current pan-Arctic organic aerosol, which can be used to support modelling efforts that aim to quantify the climate impacts of emissions in this sensitive region