1,987 research outputs found

    Semimicroscopical description of the simplest photonuclear reactions accompanied by excitation of the giant dipole resonance in medium-heavy mass nuclei

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    A semimicroscopical approach is applied to describe photoabsorption and partial photonucleon reactions accompanied by the excitation of the giant dipole resonance (GDR). The approach is based on the continuum-RPA (CRPA) with a phenomenological description for the spreading effect. The phenomenological isoscalar part of the nuclear mean field, momentum-independent Landau-Migdal particle-hole interaction, and separable momentum-dependent forces are used as input quantities for the CRPA calculations. The experimental photoabsorption and partial (n,γ)(n,\gamma)-reaction cross sections in the vicinity of the GDR are satisfactorily described for 89^{89}Y, 140^{140}Ce and 208^{208}Pb target nuclei. The total direct-neutron-decay branching ratio for the GDR in 48^{48}Ca and 208^{208}Pb is also evaluated.Comment: 19 pages, 5 eps figure

    Description of double beta decay within continuum-QRPA

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    A method to calculate the nuclear double beta decay (2νββ2\nu\beta\beta- and 0νββ0\nu\beta\beta-) amplitudes within the continuum random phase approximation (cQRPA) is formulated. Calculations of the ββ\beta\beta transition amplitudes within the cQRPA are performed for ^{76}Ge, ^{100}Mo and ^{130}Te. A rather simple nuclear Hamiltonian consisting of phenomenological mean field and zero-range residual particle-hole and particle-particle interaction is used. The calculated M^{2\nu} are almost not affected when the single-particle continuum is taken into account. At the same time, a regular suppression of the 0νββ0\nu\beta\beta-amplitude is found that can be associated with additional ground state correlations due to collective states in the continuum. It is expected that future inclusion of the nucleon pairing in the single-particle continuum will somewhat compensate the suppression.Comment: 20 pages, 1 figure, published versio

    Dynamic avalanche breakdown of a p-n junction: deterministic triggering of a plane streamer front

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    We discuss the dynamic impact ionization breakdown of high voltage p-n junction which occurs when the electric field is increased above the threshold of avalanche impact ionization on a time scale smaller than the inverse thermogeneration rate. The avalanche-to-streamer transition characterized by generation of dense electron-hole plasma capable to screen the applied external electric field occurs in such regimes. We argue that the experimentally observed deterministic triggering of the plane streamer front at the electric field strength above the threshold of avalanche impact ionization but yet below the threshold of band-to-band tunneling is generally caused by field-enhanced ionization of deep-level centers. We suggest that the process-induced sulfur centers and native defects such as EL2, HB2, HB5 centers initiate the front in Si and GaAs structures, respectively. In deep-level free structures the plane streamer front is triggered by Zener band-to-band tunneling.Comment: 4 pages, 2 figure

    Impact ionization fronts in Si diodes: Numerical evidence of superfast propagation due to nonlocalized preionization

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    We present numerical evidence of a novel propagation mode for superfast impact ionization fronts in high-voltage Si p+p^+-nn-n+n^+ structures. In nonlinear dynamics terms, this mode corresponds to a pulled front propagating into an unstable state in the regime of nonlocalized initial conditions. Before the front starts to travel, field-ehanced emission of electrons from deep-level impurities preionizes initially depleted nn base creating spatially nonuniform free carriers profile. Impact ionization takes place in the whole high-field region. We find two ionizing fronts that propagate in opposite directions with velocities up to 10 times higher than the saturated drift velocity.Comment: 3 pages, 4 figure

    The 0nbb-decay nuclear matrix elements with self-consistent short-range correlations

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    A self-consistent calculation of nuclear matrix elements of the neutrinoless double beta decays (0nbb) of 76Ge, 82Se, 96Zr, 100Mo, 116Cd, 128Te, 130Te and 130Xe is presented in the framework of the renormalized quasiparticle random phase approximation (RQRPA) and the standard QRPA. The pairing and residual interactions as well as the two-nucleon short-range correlations are for the first time derived from the same modern realistic nucleon-nucleon potentials, namely from charge-dependent Bonn potential (CD-Bonn) and the Argonne V18 potential. In a comparison with the traditional approach of using the Miller-Spencer Jastrow correlations matrix elements for the 0nbb-decay are obtained, which are larger in magnitude. We analyze the differences among various two-nucleon correlations including those of the unitary correlation operator method (UCOM) and quantify the uncertainties in the calculated 0nbb-decay matrix elements.Comment: 11 pages, 5 figure

    Numerical studies of variable-range hopping in one-dimensional systems

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    Hopping transport in a one-dimensional system is studied numerically. A fast algorithm is devised to find the lowest-resistance path at arbitrary electric field. Probability distribution functions of individual resistances on the path and the net resistance are calculated and fitted to compact analytic formulas. Qualitative differences between statistics of resistance fluctuations in Ohmic and non-Ohmic regimes are elucidated. The results are compared with prior theoretical and experimental work on the subject.Comment: 12 pages, 12 figures. Published versio

    Discrete conformal maps and ideal hyperbolic polyhedra

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    We establish a connection between two previously unrelated topics: a particular discrete version of conformal geometry for triangulated surfaces, and the geometry of ideal polyhedra in hyperbolic three-space. Two triangulated surfaces are considered discretely conformally equivalent if the edge lengths are related by scale factors associated with the vertices. This simple definition leads to a surprisingly rich theory featuring M\"obius invariance, the definition of discrete conformal maps as circumcircle preserving piecewise projective maps, and two variational principles. We show how literally the same theory can be reinterpreted to addresses the problem of constructing an ideal hyperbolic polyhedron with prescribed intrinsic metric. This synthesis enables us to derive a companion theory of discrete conformal maps for hyperbolic triangulations. It also shows how the definitions of discrete conformality considered here are closely related to the established definition of discrete conformality in terms of circle packings.Comment: 62 pages, 22 figures. v2: typos corrected, references added and updated, minor changes in exposition. v3, final version: typos corrected, improved exposition, some material moved to appendice

    Tunneling-assisted impact ionization fronts in semiconductors

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    We propose a novel type of ionization front in layered semiconductor structures. The propagation is due to the interplay of band-to-band tunneling and impact ionization. Our numerical simulations show that the front can be triggered when an extremely sharp voltage ramp (∼10kV/ns\sim 10 {\rm kV/ns}) is applied in reverse direction to a Si p+−n−n+−p^+-n-n^+-structure that is connected in series with an external load. The triggering occurs after a delay of 0.7 to 0.8 ns. The maximal electrical field at the front edge exceeds 106V/cm10^6 {\rm V/cm}. The front velocity vfv_f is 40 times faster than the saturated drift velocity vsv_s. The front passes through the n−n-base with a thickness of 100μm100 {\mu m} within approximately 30 ps, filling it with dense electron-hole plasma. This passage is accompanied by a voltage drop from 8 kV to dozens of volts. In this way a voltage pulse with a ramp up to 500kV/ns500 {\rm kV/ns} can be applied to the load. The possibility to form a kilovolt pulse with such a voltage rise rate sets new frontiers in pulse power electronics.Comment: 12 pages, 6 figure

    What is happening to the health of the Croatian population?

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    AIM: To describe the problems in the interpretation of Croatian mortality data and explore possible reasons for the recorded increase in mortality in the 1990-1999 period, particularly related to different methods of collection and estimation of data on deaths and population. METHODS: Numbers of recorded deaths and population estimates were first obtained from the Croatian Institute for Public Health and examined in detail. The Institute used population estimates supplied by the Croatian Statistics Bureau, which included de jure population data (including all Croatian citizens wherever they live) until 1996 and de facto population data (including only population living in Croatia at least for a year, irrespective of citizenship) since 1996. A different set of population estimates based on de facto estimates since 1992 was obtained from the Croatian Bureau of Statistics. We examined trends in age- and sex-specific death rates from major causes in 1990-1999 period, using the mortality data from the Croatian Institute for Public Health and both sets of population estimates. Lung cancer as a cause of death was examined in more detail, since it is relatively stable over short periods of time. Interviews were undertaken with key informants to identify the reasons for any discrepancies. RESULTS: In Croatia, relatively stable death rates from lung cancer in men ranged from 84/100,000 in 1990 to 79/ 100,000 in 1995. In 1996, a marked discontinuity appeared in the Croatian data, with a 14% increase compared to 1995 (from 79/100,000 to 91/100,000) and a further increase in 1999 (94/100,000), which is not credible on the basis of the natural history of lung cancer. Analysis of mortality rates with de facto population estimates showed more gradual increase from 1992-1996. Methods used to estimate population and mortality during the 1990s were inconsistent and misleading. At present, it is impossible to be certain about the true level of mortality in Croatia during 1990s, as the numerator (deaths) and denominator (population) were incompatible until 1998. CONCLUSION: Major problems in data collection would have been identified if the investigation of unexpected mortality trends in Croatia in the 1990s had been done. Systematic analysis of health patterns should be done as soon as data from the 2001 census become available. Capacities in public health should be strengthened to make this possible. This issue has received little recognition from the international donor organizations, particularly those that use health data
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