A positron remoderator for the high intensity positron source NEPOMUC

A remoderator for the high intensity positron source NEPOMUC was developed and installed at the beam facility. A beam of remoderated positrons could be produced with different energies and a diameter of less than 2 mm was obtained. The efficiency of the remoderation setup was determined to be 5%. Due to the brilliance of the remoderated beam, the measurements at the coincidence Doppler broadening spectrometer (CDBS) and at the positron annihilation induced Auger electron spectrometer (PAES) could be improved. The setup and functionality of the remoderation device is presented as well as the first measurements at the remoderator, CDBS and PAES.     

Digital positron lifetime spectroscopy at EPOS

As the digital equipment to measure positron lifetimes gets cheaper and more widely used, it is decided that EPOS, the ELBE positron source will sample the signals from the photomultipliers directly and evaluate it online or offline by digital means.Still using isotope sources, the EPOS lifetime spectrometer results in a timing resolution of around 170 ps (with ⁶⁰Co), which compares good to analog equipment. A distinct improvement is expected when a coincidence setup will be used at ELBE. However, also the software needs further improvement: while one of the goals is of course to achieve the best time resolution, there is also the aspect of runtime and expandability. Results of evaluations will be presented and compared with results from other groups.     

A study of defects in deformed FeSi alloys using positron annihilation techniques

Steels with high amounts of silicon are used in electrical applications due to their low magnetostriction, high electrical resistivity and reduced energy losses, but they exhibit poor formability. The slow positron beam of Gent is used to investigate defects in different deformed FeSi alloys. It was found that the concentration of defects for the alloys deformed at high temperatures are different from the ones related to the alloys deformed at room temperature. These results are correlated to the results of positron annihilation lifetime spectroscopy (PALS).     

基于空芯光子晶体光纤的微小型非本征光纤法布里-珀罗干涉应变传感器

报道了一种用空芯光子晶体光纤制作的法布里-珀罗腔体,利用光纤熔接方法将该法布里-珀罗腔体和两根普通通信单模光纤熔接起来构成的微小型光纤法布里-珀罗干涉应变传感器。这种干涉传感器制作过程仅应用了切割和熔接手段,光纤材料单一,因此受温度变化的影响小。另外,该类型传感器的干涉腔长度对干涉信号强度影响不大,其干涉腔长度可至数厘米。因此,这类传感器将在大容量、准分布式传感系统中具有极大的潜在应用价值。     

10 GHz全光帧时钟提取的理论和实验研究

提出了一种新型的10 GHz全光帧时钟提取的提取方案。采用低精细度的法布里-珀罗(F-P)滤波器直接提出帧时钟,保证帧时钟的快速建立和快速消失。利用半导体光放大器(SOA)的自增益调制效应(SGM)对法布里-珀罗滤波器提取的帧时钟进行整形,降低帧时钟的幅度噪声。并推导了法布里-珀罗滤波器的精细度和半导体光放大器的自增益调制效应对帧时钟建立时间、消失时间的影响。通过理论分析表明,利用半导体光放大器的自增益调制效应可以降低帧时钟的幅度噪声并加快帧时钟建立速度。实验中,用上述方案对10.075 GHz的帧信号进行了全光帧时钟提取,得到了建立时间为8个信号周期,消失时间为22个信号周期,抖动为2.35 ps的帧时钟信号。     

Phragmén-Lindel(o)f and continuous dependence type results in a Stokes flow

This paper investigates the asymptotic behavior of end effects for a Stokes flow defined on a three-dimensional semi-infinite cylinder.With homogeneous Dirichlet conditions of the velocity on the lateral surface of the cylinder,solutions either grow or decay exponentially in the distance from the finite end of the cylinder.In the case of decay,the effect of perturbing the equation parameters is also investigated.     

A Statistical Mechanical Analysis on the Bound State Solution of an Energy-Dependent Deformed Hulthén Potential Energy *

In this article, we investigate the bound state solution of the Klein Gordon equation under mixed vector and scalar coupling of an energy-dependent deformed Hulthén potential in D dimensions. We obtain a transcendental equation after we impose the boundary conditions. We calculate energy spectra in four different limits and in arbitrary dimension via the Newton-Raphson method. Then, we use a statistical method, namely canonical partition function, and discuss the thermodynamic properties of the system in a comprehensive way. We find out that some of the thermodynamic properties overlap with each other, some of them do not.     

Locally risk-minimizing hedging strategies for unit-linked life insurance contracts under a regime switching L��vy model

This paper extends the model and analysis in that of Vandaele and Vanmaele [Insurance: Mathematics and Economics, 2008, 42: 1128–1137]. We assume that parameters of the Lévy process which models the dynamic of risky asset in the financial market depend on a finite state Markov chain. The state of the Markov chain can be interpreted as the state of the economy. Under the regime switching Lévy model, we obtain the locally risk-minimizing hedging strategies for some unit-linked life insurance products, including both the pure endowment policy and the term insurance contract.     

Derivations with large separating subspace

In his famous paper The image of a derivation is contained in the radical, Marc Thomas establishes the (commutative) Singer-Wermer conjecture, showing that derivations from a commutative Banach algebra to itself must map into the radical. The proof goes via first showing that the separating subspace of a derivation on must lie in the radical of . In this paper, we exhibit discontinuous derivations on a commutative unital Fréchet algebra such that the separating subspace is the whole of . Thus, the situation on Fréchet algebras is markedly different from that on Banach algebras.

     

Fréchet-Urysohn spaces in free topological groups

Let and be respectively the free topological group and the free Abelian topological group on a Tychonoff space . For every natural number we denote by () the subset of () consisting of all words of reduced length . It is well known that if a space is not discrete, then neither nor is Fréchet-Urysohn, and hence first countable. On the other hand, it is seen that both and are Fréchet-Urysohn for a paracompact Fréchet-Urysohn space . In this paper, we prove first that for a metrizable space , () is Fréchet-Urysohn if and only if the set of all non-isolated points of is compact and is Fréchet-Urysohn if and only if is compact or discrete. As applications, we characterize the metrizable space such that is Fréchet-Urysohn for each and is Fréchet-Urysohn for each except for . In addition, however, there is a first countable, and hence Fréchet-Urysohn subspace of () which is not contained in any (). We shall show that if such a space is first countable, then it has a special form in (). On the other hand, we give an example showing that if the space is Fréchet-Urysohn, then it need not have the form.