Background optimization for the neutron Ttime of flight spectrometer NEAT

The neutron time of flight spectrometer NEAT at BER II is currently undergoing a major upgrade where an important aspect is the prevention of parasitic scattering to enhance the signal to noise ratio. Here, we discuss the impact of shielding to suppress parasitic scattering from two identified sources of background the sample environment and detector tubes. By means of Monte Carlo simulations and a modification of the analytical model of Copley et al. [Copley and Cook, 1994], the visibility functions of instrument parts are computed for different shielding configurations. According to three selection criteria, namely suppression of background, transmission and detection limit, the parameters of an oscillating radial collimator are optimized for NEAT s default setup. Moreover, different configurations of detector shielding are discussed to prevent cross talk within the radial detector syste

Triphilic ionic-liquid mixtures: fluorinated and non-fluorinated aprotic ionic-liquid mixtures

We present here the possibility of forming triphilic mixtures from alkyl- and fluoroalkylimidazolium ionic liquids, thus, macroscopically homogeneous mixtures for which instead of the often observed two domainspolar and nonpolarthree stable microphases are present: polar, lipophilic, and fluorous ones. The fluorinated side chains of the cations indeed self-associate and form domains that are segregated from those of the polar and alkyl domains. To enable miscibility, despite the generally preferred macroscopic separation between fluorous and alkyl moieties, the importance of strong hydrogen bonding is shown. As the long-range structure in the alkyl and fluoroalkyl domains is dependent on the composition of the liquid, we propose that the heterogeneous, triphilic structure can be easily tuned by the molar ratio of the components. We believe that further development may allow the design of switchable, smart liquids that change their properties in a predictable way according to their composition or even their environment

From soft harmonic phonons to fast relaxational dynamics in CH3_{3}NH3_{3}PbBr3_{3}

The lead-halide perovskites, including CH$_{3}$NH$_{3}$PbBr$_{3}$, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties. CH$_{3}$NH$_{3}$PbBr$_{3}$ undergoes a series of structural transitions associated with orientational order of the CH$_{3}$NH$_{3}$ (MA) molecular cation and tilting of the PbBr$_{3}$ host framework. We apply high-resolution neutron scattering to study the soft harmonic phonons associated with these transitions, and find a strong coupling between the PbBr$_{3}$ framework and the quasistatic CH$_{3}$NH$_{3}$ dynamics at low energy transfers. At higher energy transfers, we observe a PbBr$_{6}$ octahedra soft mode driving a transition at 150 K from bound molecular excitations at low temperatures to relatively fast relaxational excitations that extend up to $\sim$ 50-100 meV. We suggest that these temporally overdamped dynamics enables possible indirect band gap processes in these materials that are related to the enhanced photovoltaic properties.Comment: (main text - 5 pages, 4 figures; supplementary information - 3 pages, 3 figures

Ready to Use Detector Modules for the NEAT Spectrometer Concept, Design, First Results

The paper presents the detector system developed by Datalist Systems, Ltd. previously ANTE Innovative Technologies for the NEAT II spectrometer at HZB. We present initial concept, design and implementation highlights as well as the first results of measurements such as position resolution. The initial concept called for modular architecture with 416 3He detector tubes organized into thirteen 32 tube modules that can be independently installed and removed to and from the detector vacuum chamber for ease of maintenance. The unalloyed aluminum mechanical support modules for four 8 tube units each also house the air boxes that contain the front end electronics preamplifiers that need to be on atmospheric pressure. The modules have been manufactured and partly assembled in Hungary and then fully assembled and installed on site by Datalist Systems crew. The signal processing and data acquisition solution is based on low time constant 60 ns preamplifier electronics and sampling ADC s running at 50 MS s i.e. a sample every 20 ns for all 832 data channels. The preamplifiers are proprietary, developed specifically for the NEAT spectrometer, while the ADC s and the FPGA s that further process the data are based on National Instruments products. The data acquisition system comprises 26 FPGA modules each serving 16 tubes providing for up to 50 kHz count rate per individual tube and it is organized into two PXI chassis and two data acquisition computers that perform post processing, event classification and provide appropriate preview of the collected data. The data acquisition software based on Event Recording principles provides a single point of contact for the scientific software with an Event Record List with absolute timestamps of 100ns resolution, timing data of 100 ns resolution for the seven discs chopper system as well as classification data that can be used for flexible data filtering in off line analysis of the gathered data. A unique 3 tier system of filtering criteria of events is in operation a hard threshold in the FPGA s to reduce the effect of noise, a pulse shape based classification to eliminate gamma sensitivity and an additional flexible feature based classification to filter out pileup and other unwanted phenomena. This ensures high count rates 50kHz per tube, 1MHz overall while maintaining good quality of measurements e.g. position resolution .The first measurement results show that the delivered detector system meets the initial requirements of 20 mm position resolution along the 2000mm long detector tubes. This is partly due to the innovative event classification system that provides vital pulse shape data that can be used for sophisticated position resolution algorithms implemented on the DAQ computer

Ultracold Neutron Production in a Pulsed Neutron Beam Line

We present the results of an Ultracold neutron (UCN) production experiment in a pulsed neutron beam line at the Los Alamos Neutron Scattering Center. The experimental apparatus allows for a comprehensive set of measurements of UCN production as a function of target temperature, incident neutron energy, target volume, and applied magnetic field. However, the low counting statistics of the UCN signal expected can be overwhelmed by the large background associated with the scattering of the primary cold neutron flux that is required for UCN production. We have developed a background subtraction technique that takes advantage of the very different time-of-flight profiles between the UCN and the cold neutrons, in the pulsed beam. Using the unique timing structure, we can reliably extract the UCN signal. Solid ortho-D$_2$ is used to calibrate UCN transmission through the apparatus, which is designed primarily for studies of UCN production in solid O$_2$. In addition to setting the overall detection efficiency in the apparatus, UCN production data using solid D$_2$ suggest that the UCN upscattering cross-section is smaller than previous estimates, indicating the deficiency of the incoherent approximation widely used to estimate inelastic cross-sections in the thermal and cold regimes

High H2 Sorption Energetics in Zeolitic Imidazolate Frameworks

A combined experimental and theoretical study of H2 sorption was carried out on two isostructural zeolitic imidazolate frameworks ZIFs , namely ZIF 68 and ZIF 69. The former consists of Zn2 ions that are coordinated to two 2 nitroimidazolate and two benzimidazolate linkers in a tetrahedral fashion, while 5 chlorobenzimidazolate is used in place of benzimidazolate in the latter compound. H2 sorption measurements showed that the two ZIFs display similar isotherms and isosteric heats of adsorption Qst . The experimental initial H2 Qst value for both ZIFs was determined to be 8.1 kJ mol 1, which is quite high for materials that do not contain exposed metal centers. Molecular simulations of H2 sorption in ZIF 68 and ZIF 69 confirmed the similar H2 sorption properties between the two ZIFs, but also suggest that H2 sorption is slightly favored in ZIF 68 with regards to uptake at 77 K 1.0 atm. This work also presents inelastic neutron scattering INS spectra for H2 sorbed in ZIFs for the first time. The spectra for ZIF 68 and ZIF 69 show a broad range of intensities starting from about 4 meV. The most favorable H2 sorption site in both ZIFs corresponds to a confined region between two adjacent 2 nitroimidazolate linkers. Two dimensional quantum rotation calculations for H2 sorbed at this site in ZIF 68 and ZIF 69 produced rotational transitions that are in accord with the lowest energy peak observed in the INS spectrum for the respective ZIFs. We found that the primary binding site for H2 in the two ZIFs generates high barriers to rotation for the adsorbed H2, which are greater than those in several metal organic frameworks MOFs which possess open metal sites. H2 sorption was also observed for both ZIFs in the vicinity of the nitro groups of the 2 nitroimidazolate linkers. This study highlights the constructive interplay of experiment and theory to elucidate critical details of the H2 sorption mechanism in these two isostructural ZIF

Spin-state polaron in lightly hole-doped LaCoO_3

Inelastic neutron scattering (INS), electron spin (ESR) and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly hole-doped La_{1-x}Sr_xCoO_3, x=0.002. Both, INS and ESR low temperature spectra show intense excitations with large effective g-factors ~10-18. NMR data indicate the creation of extended magnetic clusters. From the Q-dependence of the INS magnetic intensity we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions.Comment: 10 pages, 3 figure