Sensitivity and parameter-estimation precision for alternate LISA configurations

We describe a simple framework to assess the LISA scientific performance (more specifically, its sensitivity and expected parameter-estimation precision for prescribed gravitational-wave signals) under the assumption of failure of one or two inter-spacecraft laser measurements (links) and of one to four intra-spacecraft laser measurements. We apply the framework to the simple case of measuring the LISA sensitivity to monochromatic circular binaries, and the LISA parameter-estimation precision for the gravitational-wave polarization angle of these systems. Compared to the six-link baseline configuration, the five-link case is characterized by a small loss in signal-to-noise ratio (SNR) in the high-frequency section of the LISA band; the four-link case shows a reduction by a factor of sqrt(2) at low frequencies, and by up to ~2 at high frequencies. The uncertainty in the estimate of polarization, as computed in the Fisher-matrix formalism, also worsens when moving from six to five, and then to four links: this can be explained by the reduced SNR available in those configurations (except for observations shorter than three months, where five and six links do better than four even with the same SNR). In addition, we prove (for generic signals) that the SNR and Fisher matrix are invariant with respect to the choice of a basis of TDI observables; rather, they depend only on which inter-spacecraft and intra-spacecraft measurements are available.Comment: 17 pages, 4 EPS figures, IOP style, corrected CQG versio

Use and Abuse of the Fisher Information Matrix in the Assessment of Gravitational-Wave Parameter-Estimation Prospects

The Fisher-matrix formalism is used routinely in the literature on gravitational-wave detection to characterize the parameter-estimation performance of gravitational-wave measurements, given parametrized models of the waveforms, and assuming detector noise of known colored Gaussian distribution. Unfortunately, the Fisher matrix can be a poor predictor of the amount of information obtained from typical observations, especially for waveforms with several parameters and relatively low expected signal-to-noise ratios (SNR), or for waveforms depending weakly on one or more parameters, when their priors are not taken into proper consideration. In this paper I discuss these pitfalls; show how they occur, even for relatively strong signals, with a commonly used template family for binary-inspiral waveforms; and describe practical recipes to recognize them and cope with them. Specifically, I answer the following questions: (i) What is the significance of (quasi-)singular Fisher matrices, and how must we deal with them? (ii) When is it necessary to take into account prior probability distributions for the source parameters? (iii) When is the signal-to-noise ratio high enough to believe the Fisher-matrix result? In addition, I provide general expressions for the higher-order, beyond--Fisher-matrix terms in the 1/SNR expansions for the expected parameter accuracies.Comment: 24 pages, 3 figures, previously known as "A User Manual for the Fisher Information Matrix"; final, corrected PRD versio

Optimierte Umwandlung von Plutonium und Americium in Druckwasserreaktoren

The world energy requirement will dramatically increase during this century, because of an increasing world population on the one hand and an increasing per capita energy consumption on the other hand. Future prospects of primary energy requirement, basing on different assumptions of the future technological, economical and ecological developments, predict a multiple of todays nuclear energy production capabilities worldwide. A worldwide phasing out of the nuclear energy option is not foreseeable. To the greatest possible extend today's nuclear fuel cycle is based on the usage of uranium-based fuels in light water reactors. At this accumulating residues contain typically per tonne heavy metal beside fission products and surpluses of uranium also 11 kg transuranium elements (TRU). The TRU will dominate the hazardous potential for a very long time span in a future repository because of their long half-life and radiotoxicity whereas the fission products dominate only at the very beginning. Some of the transuranium isotopes are very good fissionable and open due to this a proliferation risk additionally. This paper analyses a possible evolution of todays fuel cycle, considering on the one hand the requirements of a sustainable waste disposal strategy and on the other hand the highest safety properties of today's western nuclear power plants. The aim is to deposit only the nuclear residuals which hazardous potential will fall below the natural hazardous potential of fresh nuclear fuel during a time period in which engineered barriers guarantee a safe encapsulation. By pin-cell, assembly and full-core simulations of the neutronics of a thoriumbased pressurised water reactor with different calculational tools the possibility to operate the reactor with (Th,Pu)O_2-fuel in a four-batch-mode up to a burn-up of 60 MWd/kg was shown. The nuclear stability of the system for all burn-up conditions was proven for the full core and especially for the void coefficients the pin-cell and assembly calculations were shown to be conservative. Under the term of minimal modifications of today's established systems further optimisation approaches with the aim of increasing the actinide consumption were performed. Beside the adjustment of moderation ratio the possibility to use spectral shifts within the pellet by a two layer approach and enlargement of the pellet diameter were analysed for different fuels of the second recycling step. Furthermore effects of hardening the neutron spectra by a cadmium filter between the inner and outer fuel zone were studied. Repercussions of those special designed fuel pins within the whole fuel assembly, the maximal fuel temperatures, heat surface stress and helium production were also part of the analysis. Very favourable is the heterogenous (Th,Am,U)O_2 fuel, because without blending with fresh fuel an advantageous americium isotopic composition was found even up to the third recycling step. (orig.)SIGLEAvailable from TIB Hannover: RA 831(4131) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman