An endoscopic detector for ultracold neutrons

A new versatile detector for ultracold neutrons (UCN) has been built and operated which combines multi-pixel photon counters and GS10 lithium-doped scintillators. Such detectors can be very small and can be used to monitor UCN inside storage vessels or guides with negligible influence (of order 10−6 on the UCN intensity itself. We have shown that such detectors can be used in a very harsh radiation environment of up to 200Gy/h via the addition of a 4m long quartz light guide in order to place the radiation-sensitive photon counters outside the hot zone. Additionally we have measured the UCN storage times in situ in this harsh environmen

Random-phase approximation and its applications in computational chemistry and materials science

The random-phase approximation (RPA) as an approach for computing the electronic correlation energy is reviewed. After a brief account of its basic concept and historical development, the paper is devoted to the theoretical formulations of RPA, and its applications to realistic systems. With several illustrating applications, we discuss the implications of RPA for computational chemistry and materials science. The computational cost of RPA is also addressed which is critical for its widespread use in future applications. In addition, current correction schemes going beyond RPA and directions of further development will be discussed.Comment: 25 pages, 11 figures, published online in J. Mater. Sci. (2012

Effect of Temperature on the Adsorption of Short Alkanes in the Zeolite SSZ-13—Adapting Adsorption Isotherms to Microporous Materials

Understanding the diffusion and adsorption of hydrocarbons in zeolites is a highly important topic in the field of catalysis in micro-and mesoporous materials. Especially, the properties of alkanes in zeolites have been studied extensively. A theoretical description of these processes is challenging, because two interactions are involved: the alkane physisorbs to the zeolite wall and chemisorbs weakly to the active centers. At room temperature, the alkane remains physisorbed almost all the time, but the chemical bond to the active sites is regularly broken. In this work, we study this behavior using ab initio molecular dynamics simulations for the adsorption of methane, ethane, and propane in SSZ-13, the zeolite with the smallest unit cell, at temperatures of 250, 275, 325, and 350 K. We find a temperature dependence of the adsorption energy and the probability of the alkane to be close to the active site, which corresponds to chemisorption. We derive a temperature-dependent expression for these probabilities or active site coverages, which have the energy difference between physisorbed and chemisorbed state as the main variable. The methodology derived in this work will be highly useful in correlating static electronic structure calculations to finite temperature coverages, which, following the Sabatier principle, is a key step to understand the performance of catalysts under reaction conditions and a prerequisite to computationally design such materials

An endoscopic detector for ultracold neutrons

A new versatile detector for ultracold neutrons (UCN) has been built and operated which combines multi-pixel photon counters and GS10 lithium-doped scintillators. Such detectors can be very small and can be used to monitor UCN inside storage vessels or guides with negligible influence (of order 10(-6)) on the UCN intensity itself. We have shown that such detectors can be used in a very harsh radiation environment of up to 200 Gy/h via the addition of a 4 m long quartz light guide in order to place the radiation-sensitive photon counters outside the hot zone. Additionally we have measured the UCN storage times in situ in this harsh environment