A Compton-vetoed germanium detector with increased sensitivity at low energies

The difficulty to directly detect plutonium in spent nuclear fuel due to the high Compton background of the fission products motivates the design of a gamma detector with improved sensitivity at low energies. We have built such a detector by operating a thin high-purity Ge detector with a large scintillator Compton veto directly behind it. The Ge detector is thin to absorb just the low-energy Pu radiation of interest while minimizing Compton scattering of high-energy radiation from the fission products. The subsequent scintillator is large so that forward-scattered photons from the Ge detector interact in it at least once to provide an anti-coincidence veto for the Ge detector. For highest sensitivity, additional material in the line of sight is minimized, the radioactive sample is kept thin, and its radiation is collimated. We will discuss the instrument design, and demonstrate the feasibility of the approach with a prototype that employs two large CsI scintillator vetoes. Initial spectra of a thin Cs-137 calibration source show a background suppression of a factor of ~2.5 at ~100 keV, limited by an unexpectedly thick 4 mm dead layer in the Ge detector.     

On tuples of commuting symmetric,non-selfadjoint operators

Generalizing the Cowen-Douglas-Theory to certain tuples of unbounded symmetric operators we obtain canonical models for such tuples, which are realized in holomorphic functional Hilbert spaces. The results are applied to multidimensional moment problems.     

Rheological behavior of PS-melts containing surface modified PMMA-particles

The rheological properties of polystyrene (PS) compounds containing cross-linked PMMA particles are characterized by oscillatory shear experiments, while the amount of covalently grafted carboxylic-acid terminated polystyrene (CT-PS) on their surface is varied. All samples show an additional relaxation process with a long relaxation time in the frequency range where the matrix flows. The strength of this process depends strongly on the amount of particles, i.e., on the degree of filling and, to the same extent, on the different “coverage” of the particles surface, i.e., the degree of grafting. For the highest degrees of filling and in dependence on the degree of grafting a particle network is formed which is characterized by an evolving equilibrium modulus. Moreover, compatibilization by grafting CT-PS onto the PMMA particles' surface reduces the strength of the additional relaxation process remarkably. This surprising effect is related qualitatively to the balance of the interaction between particles and the interaction between the particles and the matrix due to PS grafts. Some of these results can be understood quantitatively on the basis of the sticky hard-sphere model. Received: 3 January 2000 Accepted: 14 August 2000     

Why legendre made a wrong guess about <Emphasis Type="Bold">π</Emphasis>-(<Emphasis Type="Italic">x</Emphasis>), and how laguerre’s continued fraction for the logarithmic integral improved it

Carl Friedrich Gauβ, in 1792, when he was 15, found by numerical evidence that π(x), the number of primes p such that p ≤ x, goes roughly with x/in x (letter to Encke, 1849). This was, as can be seen from Table 1, a very weak approximation with an error of about 10%. In 1798 and again in 1808,     

8-Oxychinolin als analytisches Reagens Bestimmung von Magnesium, Zink, Aluminium; Trennung des Aluminiums oder Zinks von den Erdalkalien, Magnesium und Alkalien, des Magnesiums von den Alkalien

Ohne Zusammenfassung