A new puzzle for random interaction

We continue a series of numerical experiments on many-body systems with random two-body interactions, by examining correlations in ratios in excitation energies of yrast $J$ = 0, 2, 4, 6, 8 states. Previous studies, limited only to $J$ = 0,2,4 states, had shown strong correlations in boson systems but not fermion systems. By including $J \ge 6$ states and considering different scatter plots, strong and realistic correlations appear in both boson and fermion systems. Such correlations are a challenge to explanations of random interactions.Comment: 4 pages, 4 figure

Radiolysis of water ice in the outer solar system: Sputtering and trapping of radiation products

We performed quantitative laboratory radiolysis experiments on cubic water ice between 40 and 120 K, with 200 keV protons. We measured sputtering of atoms and molecules and the trapping of radiolytic molecular species. The experiments were done at fluences corresponding to exposure of the surface of the Jovian icy satellites to their radiation environment up to thousands of years. During irradiation, O2 molecules are ejected from the ice at a rate that grows roughly exponentially with temperature; this behavior is the main reason for the temperature dependence of the total sputtering yield. O2 trapped in the ice is thermally released from the ice upon warming; the desorbed flux starts at the irradiation temperature and increases strongly above 120 K. Several peaks in the desorption spectrum, which depend on irradiation temperature, point to a complex distribution of trapping sites in the ice matrix. The yield of O2 produced by the 200 keV protons and trapped in the ice is more than 2 orders of magnitude smaller than used in recent models of Ganymede. We also found small amounts of trapped H2O2 that desorb readily above 160 K.Fil: Bahr, D.A.. University of Virginia; Estados UnidosFil: Famá, M.. University of Virginia; Estados UnidosFil: Vidal, Ricardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Baragiola, Raul Antonio. University of Virginia; Estados Unido

Designing an optimal HIV programme for South Africa: Does the optimal package change when diminishing returns are considered?

Abstract Background South Africa has a large domestically funded HIV programme with highly saturated coverage levels for most prevention and treatment interventions. To further optimise its allocative efficiency, we designed a novel optimisation method and examined whether the optimal package of interventions changes when interaction and non-linear scale-up effects are incorporated into cost-effectiveness analysis. Methods The conventional league table method in cost-effectiveness analysis relies on the assumption of independence between interventions. We added methodology that allowed the simultaneous consideration of a large number of HIV interventions and their potentially diminishing marginal returns to scale. We analysed the incremental cost effectiveness ratio (ICER) of 16 HIV interventions based on a well-calibrated epidemiological model that accounted for interaction and non-linear scale-up effects, a custom cost model, and an optimisation routine that iteratively added the most cost-effective intervention onto a rolling baseline before evaluating all remaining options. We compared our results with those based on a league table. Results The rank order of interventions did not differ substantially between the two methods- in each, increasing condom availability and male medical circumcision were found to be most cost-effective, followed by anti-retroviral therapy at current guidelines. However, interventions were less cost-effective throughout when evaluated under the optimisation method, indicating substantial diminishing marginal returns, with ICERs being on average 437% higher under our optimisation routine. Conclusions Conventional league tables may exaggerate the cost-effectiveness of interventions when programmes are implemented at scale. Accounting for interaction and non-linear scale-up effects provides more realistic estimates in highly saturated real-world settings

Towards high performance small animal positron emission tomography

Proceeding of: 2002 IEEE Symposium International on Biomedical Imaging, Washington, D.C., USA, July 7-10, 2002During the last decade increasingly sophisticated positron emission tomography (PET) scanners have been developed for imaging small laboratory animals. These systems often exhibit performance characteristics, e.g. spatial resolution, substantially better than contemporary human PET scanners and are often the first systems to demonstrate new technologies, e. g. avalanche photodiodebased detector modules. Despite these advances, spatial resolution, sensitivity, resolution uniformity and other performance parameters must continue to be improved if accurate general purpose imaging is to be carried out in the most popular research subject, the mouse. Moreover. as these improvements occur, methods must also be devised to minimize the resolution-degrading effects of positron range, the distance a positron travels from the decaying nucleus before encountering and mutually anll1hllattng an electron. Range effects are particularly important for compounds labeled with "non-traditional" positron-emitters such as 1-124 or Tc-94m. In order to illustrate the complex interplay of issues that must be addressed when contemplating such improvements, we describe how we have approached high performance PET imaging in the design and construction of ATLAS (Advanced Technology Laboratory Animal Scanner), a small animal PET scanner now entering servIce at the National Institutes of Health (NIH) in Bethesda, Md

Gamow-Teller transitions and deformation in the proton-neutron random phase approximation

We investigate reliability of Gamow-Teller transition strengths computed in the proton-neutron random phase approximation, comparing with exact results from diagonalization in full $0\hbar\omega$ shell-model spaces. By allowing the Hartree-Fock state to be deformed, we obtain good results for a wide variety of nuclides, even though we do not project onto good angular momentum. We suggest that deformation is as important or more so than pairing for Gamow-Teller transitions.Comment: 8 pages, 5 figures; added references, clarified discussion with regards to stabilit