Polarized Neutron Laue Diffraction on a Crystal Containing Dynamically Polarized Proton Spins

We report on a polarized-neutron Laue diffraction experiment on a single crystal of neodynium doped lanthanum magnesium nitrate hydrate containing polarized proton spins. By using dynamic nuclear polarization to polarize the proton spins, we demonstrate that the intensities of the Bragg peaks can be enhanced or diminished significantly, whilst the incoherent background, due to proton spin disorder, is reduced. It follows that the method offers unique possibilities to tune continuously the contrast of the Bragg reflections and thereby represents a new tool for increasing substantially the signal-to-noise ratio in neutron diffraction patterns of hydrogenous matter.Comment: 5 pages, 3 figure

The spin-dependent nd scattering length - a proposed high-accuracy measurement

The understanding of few-nucleon systems at low energies is essential, e.g. for accurate predictions of element abundances in big-bang and stellar fusion. Novel effective field theories, taking only nucleons, or nucleons and pions as explicit degrees of freedom, provide a systematic approach, permitting an estimate of theoretical uncertainties. Basic constants parameterising the short range physics are derived from only a handful of experimental values. The doublet neutron scattering length a_2 of the deuteron is particularly sensitive to a three-nucleon contact interaction, but experimentally known with only 6% accuracy. It can be deduced from the two experimentally accessible parameters of the nd scattering length. We plan to measure the poorly known "incoherent" nd scattering length a_{i,d} with 10^{-3} accuracy, using a Ramsey apparatus for pseudomagnetic precession with a cold polarised neutron beam at PSI. A polarised target containing both deuterons and protons will permit a measurement relative to the incoherent np scattering length, which is know experimentally with an accuracy of 2.4\times 10^{-4}.Comment: 5 pages LaTeX2e, 1 .eps figure. To be published in Nucl. Inst. Methods A as part of the Proceedings of the 9th International Workshop on Polarized Solid Targets and Techniques in Bad Honnef (Germany), 27th - 29th October 200

Time-resolved nuclear spin-dependent small-angle neutron scattering from polarised proton domains in deuterated solutions

Abstract.: We have investigated the process of dynamic proton polarisation by means of time-resolved polarised small-angle neutron scattering (SANS) on frozen solutions of EHBA-CrV molecules in glycerol-water mixtures as a function of the concentration of EHBA-CrV and for different degrees of deuteration of the solvent. In the EHBA-CrV complex, the spins of the 20 protons which surround the paramagnetic CrV can be oriented using the method of dynamic nuclear polarisation (DNP), thereby offering the possibility to create locally a nuclear spin-dependent contrast for SANS. The time constants which describe the build-up of polarisation around the paramagnetic centre and the subsequent diffusion of polarisation in the solvent were determined by analysing the temporal evolution of the nuclear polarisation, which in turn was obtained by fitting a core-shell model to the time-dependent SANS curves. The results on the spin dynamics obtained using the scattering function of a core-shell could be independently confirmed by evaluating the integrated SANS intensity. A thermodynamic one-centre model is presented which is able to reproduce the observed dependence of the proton polarisation times on the proton concentration of the solven

Past millennium hydroclimate variability from Corsican pine tree-ring chronologies

Palaeoclimatic evidence is necessary to place the current warming and drying trends of the Mediterranean region in a long‐term perspective of pre‐industrial variability. Annually resolved and absolutely dated climate proxies that extend back into medieval times are, however, limited to a few sites only. Here we present a network of long ring width chronologies from Pinus nigra tree‐line sites in northern Corsica (France) that cohere exceptionally well over centuries and support the development of a single high‐elevation pine chronology extending back to 974 CE. We apply various detrending methods to these data to retain high‐to‐low frequency ring width variability and scale the resulting chronologies against instrumental precipitation and drought observations to produce hydroclimate reconstructions for the last millennium. Proxy calibration and transfer are challenged by a lack of high‐elevation meteorological data, however, limiting our understanding of precipitation changes in sub‐alpine tree‐line environments. Our new reconstructions extend beyond existing records and provide evidence for low‐frequency precipitation variability in the central‐western Mediterranean from 974–2016 CE. Comparison with a European scale drought reconstruction network shows that regional predictor chronologies are needed to accurately estimate long‐term hydroclimate variability on Corsica

Impact of climate change on larch budmoth cyclic outbreaks

Periodic outbreaks of the larch budmoth Zeiraphera diniana population (and the massive forest defoliation they engender) have been recorded in the Alps over the centuries and are known for their remarkable regularity. But these have been conspicuously absent since 1981. On the other hand, budmoth outbreaks have been historically unknown in the larches of the Carpathian Tatra mountains. To resolve this puzzle, we propose here a model which includes the influence of climate and explains both the 8–9 year periodicity in the budmoth cycle and the variations from this, as well as the absence of cycles. We successfully capture the observed trend of relative frequencies of outbreaks, reproducing the dominant periodicities seen. We contend that the apparent collapse of the cycle in 1981 is due to changing climatic conditions following a tipping point and propose the recurrence of the cycle with a changed periodicity of 40 years – the next outbreak could occur in 2021. Our model also predicts longer cycles

Creating local contrast in small-angle neutron scattering by dynamic nuclear polarization

Low-resolution small-angle neutron scattering measurements can benefit from polarized protons to generate scattering contrast profiles. In a recently developed technique, time-resolved polarized SANS tries to make use of spatial polarization gradients created around paramagnetic centres at the onset of dynamic nuclear polarization. The time constants which describe the build-up of polarization around the paramagnetic centre and the subsequent diffusion of polarization in the solvent were determined by analysing the temporal evolution of the nuclear polarization. The possible use and the limitations of this technique as a spectroscopic tool are discussed

Using machine learning on tree-ring data to determine the geographical provenance of historical construction timbers

Funder: Bavarian Climate Research Network (BayKliF)Funder: Gutenberg Research College; Id: http://dx.doi.org/10.13039/501100004812Dendroclimatology offers the unique opportunity to reconstruct past climate at annual resolution and wood from historical buildings can be used to extend such information back in time up to several millennia. However, the varying and often unclear origin of timbers affects the climate sensitivity of individual tree‐ring samples. Here, we compare tree‐ring width and density of 143 living larch (Larix decidua Mill.) trees at seven sites along an elevational transect from 1400 to 2200 m asl and 99 historical tree‐ring series to parametrize state‐of‐the‐art classification models for the European Alps. To achieve geographical provenance of the historical series, nine different supervised machine learning algorithms are trained and tested in their capability to solve our classification problem. Based on this assessment, we consider a tree‐ring density‐based and a tree‐ring width‐based dataset for model building. For each of these datasets, a general not species‐related model and a larch‐specific model including the cyclic larch budmoth influence are built. From the nine tested machine learning algorithms, Extreme Gradient Boosting showed the best performance. The density‐based models outperform the ring‐width models with the larch‐specific density model reaching the highest skill (f1 score = 0.8). The performance metrics reveal that the larch‐specific density model also performs best within individual sites and particularly in sites above 2000 m asl, which show the highest temperature sensitivities. The application of the specific density model for larch allows the historical series to be assigned with high confidence to a particular elevation within the valley. The procedure can be applied to other provenance studies using multiple tree growth characteristics. The novel approach of building machine learning models based on tree‐ring density features allows to omit a common period between reference and historical data for finding the provenance of relict wood and will therefore help to improve millennium‐length climate reconstructions

Steady-state Spin Contrast Created by Dynamic Nuclear Polarisation

URL: http://www-spht.cea.fr/articles/S03/010Rapport PSI. Scientific Report p. 191 (march 2003)